Rev. sci. tech. Off. int Epiz., 1992, 11 (3), 713 733

Epidemiology of bovine anaplasmosis and babesiosis in Latin America and the Caribbean *

M. ALONSO( 1 ), C. ARELLANO-SOTA(2), V.H. CERESER(3), C.O. CORDOVES(4), A.A. GUGLIELMONE(5), R. KESSLER(6),

A.J. MANGOLD( 5 ), A. NARI( 7 ), J.H. PATARROYO(8 ), M.A. SOLARI( 7 ), C.A. VEGA( 9 ),

O. VIZCAÍNO( 1 0 ) and E. CAMUS(11)

Summary: The various parameteis which interact in the epizootiology of babesiosis and anaplasmosis and which it is necessary to obtain in a survey are analysed and the two diseases compared. Where no data are available from Latin America and the Caribbean, the references from Australia and the United States of America are discussed.

The two principal objectives of a study on the epidemiology of babesiosis and anaplasmosis are:

— to determine the risk of occurrence

to understand the relevant factors leading to outbreaks.

The first objective is relatively simple to achieve for both diseases by calculating the animal inoculation rate (h) at a determined age and the critical value of h for enzootic stability.

The second objective requires exhaustive and complex studies. For babesiosis epidemiology, studies of the tick infection rate, cattle infestation rate, genetic composition of cattle, acaricides, stocking rate, climatic data and other factors are necessary. Anaplasmosis epidemiology cannot be satisfactorily explained by considering it purely as a tick-borne disease. Difficulties to be overcome include the great diversity of haematophagous Diptera m Larm America, and the scarcity of data on the capacity of these insects as anaplasmosis vectors and their feed preferences on cattle.

KEYWORDS: Anaplasmosis - Babesiosis Caribbean - Cattle Epidemiological surveys - Epidemiology - Latin America.

This article is the work of the United Nations Food and Agricultural Organisation Committee of Experts in Haemoparas i tes for Latin America and the Caribbean.

1) Centro Nacional de Salud Animal , San José de Las Lajas, Havana , Cuba 2) United Nat ions Food and Agriculture Organisat ion, Casilla 10095, Santiago, Chile 3) Instituto de Pesquisas Veterinarias, Br 116, Km 291, El Dorado de Sul, R10 Grande do Sul, Brazil. 4) Instituto Médico Veterinario, Calle 15 n° 1011, Havana , Cuba . 5) Instituto Nacional de Tecnología Agropecuar ia , CC 228, 4400 Salta, Argentina. 6) Empresa Brasileira de Pesquisa Agropecuar ia , Cx P154, CEP 79100, Malo Grosso do Sul, Brazil. 7) Centro de Investigaciones Veterinarias, CC6577, Montevideo, Uruguay. 8) Universidad Federal de Viçosa, 36570, Viçosa, Minas Gerais, Brazil. 9) Instituto Nacional de Investigaciones Pecuarias, Aptdo Postal 206, CIVAC, CP 62500, Morélos, Mexico.

10) Instituto Co lombiano Agropecuar io , AA 29743, Bogota, Colombia. 11) Institut d 'Elevage et de Médecine Vétérinaire des Pays Tropicaux, BP 1232, 97185 Pointe à Pitre,

Guadeloupe.

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I N T R O D U C T I O N

In L a t i n A m e r i c a a n d the C a r i b b e a n , as in o t h e r r e g i o n s of t h e w o r l d , a n u m b e r of d i f ferent bov ine h a e m o p a r a s i t i c d iseases o c c u r .

T h e r e is a consensus o n the e c o n o m i c i m p o r t a n c e o f b a b e s i o s i s ( B a b e s i a bovis a n d / o r B. bigemina) a n d a n a p l a s m o s i s (Anaplasma marginale).

T h e vec tor of B. bovis a n d B. bigemina for t he w h o l e r e g i o n is t h e c o m m o n cat t le t ick , Boophilus microplus, t h o u g h in M e x i c o it sha res th i s ro le wi th Boophilus annulatus (30, 116). B. microplus is a l so ab l e t o t r a n s m i t A. marginale ( 2 1 , 96 , 129, 132, 133). H o w e v e r , in s o m e reg ions of La t i n A m e r i c a , t h e g e o g r a p h i c a l d i s t r i b u t i o n o f a n a p l a s m o s i s is m o r e w i d e s p r e a d t h a n p r e s e n c e o f Boophilus, i n d i c a t i n g the existence of o the r vectors (7). A grea t var ie ty of h a e m a t o p h a g o u s D i p t e r a , t icks o the r t h a n B. microplus a n d ru r a l f a r m i n g p rac t i ces w e r e all f ac to r s i nvo lved in the t ransmiss ion of A. marginale (144), b u t i n f o r m a t i o n re levan t t o these vec tors is scarce for some zones in the reg ion (1). In C u b a , C o r d o v e s a n d co l leagues (23) a n d P o s t o i a n a n d col leagues (94) l isted t h e m o s t c o m m o n h a e m a t o p h a g o u s D i p t e r a suspec ted as vectors of a n a p l a s m o s i s . S o m e of these (e .g . T a b a n i d a e ) c o u l d p l a y a n i m p o r t a n t ro le in the t r ansmi s s ion of A. marginale in t he w h o l e r e g i o n .

The various pa ramete r s which interact in t he ep idemiology of babesiosis and anaplasmosis a n d which it is necessary t o ob ta in in a survey will be analysed and the two diseases compa red . W h e n da ta f rom Lat in A m e r i c a a n d the C a r i b b e a n are lacking, references from Aust ra l ia a n d the Uni ted States of A m e r i c a (USA) will be discussed.

B A B E S I O S I S

T h e ep izoot io logy of babes ios is can b e s tud ied in t w o w a y s : t h r o u g h inves t iga t ion of the Boophi lus-Babesia-cat t le sys tem a n d t h r o u g h analysis of t he va r ious pa rame te r s involved .

The Boophilus-Babesia-cätÜe system: babesios is forecast ing

In an ideal s i t ua t i on , t he c o m p o n e n t s of t he Boophilus-Babesia-catÜe system in te rac t to m a i n t a i n a b a l a n c e ensu r ing the i r p r o p a g a t i o n . E x c e p t i o n a l l y , Babesia of cat t le can t a k e refuge in o the r he rb ivores (17, 104). T h e Boophilus vec to r s of Babesia are cons idered as specific t o ca t t le , t h o u g h r a r e i n fe s t a t ions c a n b e o b s e r v e d in other ungula tes (42). F o r the purposes of this discussion, the Boophilus Babesia-cattle system will be cons ide red to be w i t h o u t o t h e r h o s t s .

Babesia

Babesia is t he weakes t po in t of t h e sys tem, since it n e e d s Boophilus a n d catt le t o m a i n t a i n itself; then fol lows Boophilus w h i c h d e p e n d s o n l ives tock; a n d finally the la t ter , ac t ing as s imple s u p p o r t for t he o t h e r t w o c o m p o n e n t s a n d d e p e n d e n t on ne i ther .

T h e weak pos i t ion of Babesia in the b io log ica l sys t em is m o r e ev iden t w h e n one considers that 74 -90% of larvae fixed o n suscept ible cat t le d o no t c o m p l e t e their cycle; these pe rcen tages reach 9 4 - 9 9 % in B. microplus-resistant ca t t l e (122) . M o r t a l i t y of

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the larvae o c c u r s m a i n l y in t h e first 24 h o u r s af ter t ick f ixa t ion o n the hos t (60, 105). Moreover , c l ima t i c f a c t o r s m a y resu l t in la rva l m o r t a l i t y a n d p r e v e n t ov ipos i t i on o r normal d e v e l o p m e n t o f t h e eggs (40, 81) whi le ca t t le c a n a lso av o i d p laces wi th h igh larval in fes t a t ion (123) . T h e r e f o r e , in o r d e r t o b e m a i n t a i n e d in t he s u r r o u n d i n g env i ronment , Babesia m u s t g e n e r a t e t ick in fec t ion ra tes h igh e n o u g h t o exceed t h e tick m o r t a l i t y r a t e a n d infect ca t t l e . I n ca t t l e , Babesia m u s t o v e r c o m e the immunologica l r e a c t i o n o f a n i m a l s a n d m a i n t a i n a sufficient p a r a s i t a e m i a t o infect ticks be fo re t h e y d r o p f r o m t h e h o s t .

Cattle

A l t h o u g h , in s o m e s i t u a t i o n s , babes ios i s is cons ide r ed m o r e p a t h o g e n i c in calves (69, 86), Babesia i n f ec t ion is genera l ly m o r e severe in ca t t le over n ine m o n t h s o ld (100, 114). T h i s w a s c o n f i r m e d in A u s t r a l i a , w h e r e 7 6 % of t h e m o r t a l i t y i n d u c e d by B. bovis o c c u r r e d in ca t t l e over o n e year of age (11), a n d in A r g e n t i n a , w h e r e 84% of B. bovis a n d B. bigemina cases c o r r e s p o n d e d t o th i s a g e g r o u p (118) . T h e recovered ca t t le a r e p r o t e c t e d aga ins t a n o t h e r cha l l enge f rom Babesia for a b o u t four years (50). T h e g r e a t e r r e s i s t ance of y o u n g ca t t le l eads t o t he c o n c e p t s of enzoo t i c stability a n d ins tab i l i ty . I n t h e case of enzoo t i c s tabi l i ty , t he in te rac t ion of Boophilus, Babesia a n d loca l ca t t l e is s igni f icant e n o u g h to p r o d u c e an ear ly in fec t ion of ca t t le and prevent cases of babes io s i s . I n a n u n s t a b l e s i tua t ion , howeve r , m a n y calves r each critical age w i t h o u t i n f ec t ion a n d express babes ios i s la te r .

Ticks

Tick in fec t ion t a k e s p l ace d u r i n g t h e r a p i d p h a s e of feeding c o r r e s p o n d i n g t o t he last 24 h o u r s o f t h e f e m a l e cycle o n ca t t l e (18) . A l t h o u g h p a r a s i t a e m i a is obv ious ly necessary t o infec t B. microplus, t h e r e is n o di rec t r e l a t i on b e t w e e n the level of parasi taemia a n d t h e t ick in fec t ion r a t e (25), a s i tua t ion which h a s also been observed in ticks fed o n Bos taurus b r e e d s ( H e r e f o r d a n d Cr io l l a ) w i th va r ious n a t u r a l f o r m s of pa r a s i t a emia i n v o l v i n g Babesia (25) . H o w e v e r , t icks fed o n B. indicus (Nelore) showed a lower infect ion ra te t h a n their coun te rpa r t s fed on B. taurus; this was related to the low p a r a s i t a e m i a f o u n d in N e l o r e (36) .

The sexual cycle of Babesia beg ins in t h e t ick gu t (78) a n d resul ts in kinetes w h i c h , via the h a e m o l y m p h , i n v a d e ovar ies a n d o t h e r o r g a n s . In fec ted eggs f r o m ovar ies produce infected l a r v a e wh ich ensu re t h e survival o f Babesia if they succeed in feeding on cat t le . B. bovis is i n o c u l a t e d b y l a rvae 48 t o 72 h o u r s af ter f ixa t ion (100) whi le B. bigemina is t r a n s m i t t e d m a i n l y b y n y m p h s (99) a n d poss ib ly by ma le s (24) a n d females of B. microplus (31) .

Husbandry management

The i n t e r a c t i o n b e t w e e n Boophilus, Babesia a n d ca t t l e gives r ise t o a w ide r a n g e of s i tua t ions d u e t o d i f f e ren t h u s b a n d r y m a n a g e m e n t sys t ems . F o r i n s t a n c e , t he in t roduct ion o f E u r o p e a n b r e e d s i n t o t r o p i c a l a n d s u b t r o p i c a l r eg ions of S o u t h America o f t en fa i led b e c a u s e of h i g h losses d u e t o t icks a n d babes ios i s (139) . T h e use of aca r i c ides , a l o n e o r in c o m b i n a t i o n wi th p a s t u r e spel l ing, was r e spons ib l e for enzootic in s t ab i l i t y (49) . M o r e o v e r , D a l l w i t z a n d co l leagues (25) obse rved t h a t t ick-resistant ca t t l e c a n b e r e a r e d w i t h o u t cl inical b a b e s i o s i s . H o w e v e r , M a h o n e y a n d colleagues (74) i n d i c a t e d t h a t t i ck - r e s i s t an t ca t t l e c a n n o t r e a c h enzoo t i c s tabi l i ty as easily as suscep t ib l e ca t t l e . S p a t h a n d co l l eagues (120) o b s e r v e d a direct r e l a t i on

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be tween instabil i ty a n d cross-bred B. taurus x B. indicus ca t t le wh ich were cons idered t i ck- res i s tan t . T h e o b s e r v a t i o n m a d e by Da l lwi t z cou ld b e e x p l a i n e d by a h igher res is tance t o babes ios i s a m o n g t i ck - res i s t an t ca t t l e (26, 106). N e v e r t h e l e s s , this res is tance to babes ios i s is no t a b s o l u t e ; R o g e r s a n d co l l eagues (107) f o u n d t h a t 2 5 % of babes ios i s o u t b r e a k s in Q u e e n s l a n d af fec ted t i ck - re s i s t an t ca t t l e (B. taurus x B. indicus). M o r e o v e r , P a r k e r a n d co l l eagues (89) f o u n d t h a t t i ck - r e s i s t an t ca t t le were less suscept ible t o B. bigemina t h a n n o n res i s tan t ca t t l e , whi le D a l y a n d Ha l l (26) were u n a b l e t o find any di f ference b e t w e e n B. bigemina i n fec t ions in t ick res i s tan t a n d non- res i s tan t ca t t le . T h e r e f o r e , ev idence of a l ink be tween res i s t ance t o t icks a n d enzoo t i c s tabi l i ty is no t conc lus ive .

Mathematical model

With a be t te r u n d e r s t a n d i n g of t he i n t e r a c t i o n s b e t w e e n Boophilus, Babesia a n d cat t le , m a t h e m a t i c a l mode l s were deve loped t o expla in s o m e re l a t ions of t he sys tems, s imula te the ep izoo t io logy of babes ios i s (29, 7 1 , 7 3 , 113) a n d p r e d i c t t he risk of occu r r ence .

T o this end , the key p a r a m e t e r is t he i n o c u l a t i o n r a t e (h ) . If t h e va lue of h is h igh e n o u g h , the ma jo r i t y of calves will be infec ted b e f o r e n ine m o n t h s of age , a n d the p robab i l i t y of babes ios i s will be low. T h e r isk of babes ios i s is a l so low wi th a low value of h , bu t h igh wi th an i n t e r m e d i a t e v a l u e .

T h e value of h for B. bovis is ca lcu la ted by the fo l l owing f o r m u l a (73):

h m a b

where :

m n u m b e r of vec tor b i tes per hos t pe r d a y

a - p r o p o r t i o n of t he vec tor p o p u l a t i o n infec ted wi th Babesia

b p r o p o r t i o n of bi tes by infected t icks w h i c h successful ly infect t he hos t .

Fo r ca t t le , b 1 a n d the re fo re h m a .

In this m o d e l , t he infect ion r a t e (I) - 1 e ht w h e r e t is t he age of an ima l s . This can also be rep resen ted as fo l lows: I 1 e( m a ) t .

T h u s , t he fol lowing f o r m u l a e can be o b t a i n e d :

h 1 lîLiLJl a n d a = - I n (1 I) .J_ t t m

(In: N a e p i e r i a n l o g a r i t h m ) .

Given k n o w l e d g e of the pe r cen t age of se ropos i t ive a n i m a l s (I) of a d e t e r m i n e d age, it is poss ib le to ca lcula le h , a a n d m . H o w e v e r , t h e r e is no t a lways a good cor re la t ion be tween ca lcu la ted a n d obse rved va lues for a, as s h o w n in T a b l e I.

M o r e o v e r , M a h o n e y and col leagues (74) obse rved t h a t t h e Boophilus microplus coun t on B. taurus x B. indicus ca t t le u n d e r e s t i m a t e d t h e n u m b e r of t icks involved m the t r ansmis s ion of Babesia bovis a n d finally o v e r e s t i m a t e d t he va lue of a. There a re few reasons to use the m o d e l p r o p o s e d by M a h o n e y a n d co l l eagues (74) in cali le wi th B. indicus genes .

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T A B L E I

Calculated and observed rates of vector infection (a) with Babesia

(74)

Calculated a Observed a

0.0039 0.0037 0.0075 0.0050 0.0028 0.0000 0.0020 0.0000

In s u m m a r y , t h e e s t i m a t i o n of a va lues b y ca l cu la t ion does n o t seem su i t ab le . But the direct o b s e r v a t i o n o f a is t i m e - c o n s u m i n g w o r k , r e q u i r i n g t h e m i c r o s c o p i c observation of t h o u s a n d s of B. microplus l a rvae t o see on ly a few infec ted wi th Babesia. M o d e r n b i o t e c h n o l o g y m a y faci l i ta te such o b s e r v a t i o n .

Table II s h o w s r a t e s of in fec t ion of l a rvae w i t h Babesia s p p . obse rved in t w o studies. In s a m p l e 4 , t h e l a r v a e c a m e f r o m B. indicus x B. taurus ca t t le . A c c o r d i n g to the ca lcu la t ion of a, t h e r e s h o u l d h a v e b e e n r o u g h l y five infec ted l a rvae .

T A B L E I I

Detection of larvae positive to Babesia spp. in two studies (72, 74)

Sample No. of observed larvae

No. positive to B. bovis

No. positive to B. bigemina

1 500 0 1 2 " 1,050 1 2 3 - 1,060 0 3 4 =,> 2,305 0 ND

" Mahoney and Mirre (72) *Mahoney and colleagues (74)

ND not detected

M o r e o v e r , Da lg l i e sh a n d S t e w a r t (24) d e m o n s t r a t e d t h a t t he e x p o s u r e of l a rvae at 14°C over t h r e e t o five w e e k s i nc rea sed in fec t ion wi th B. bovis a n d , t o a lesser extent, wi th B. bigemina. C o n s i d e r i n g t he t e m p e r a t u r e fluctuations, it w o u l d be necessary t o t a k e s a m p l e s of l a rvae every t w o or t h r ee weeks a t different t e m p e r a t u r e s to know the in fec t ion r a t e . T h i s m a y be t o o diff icult t o p e r f o r m .

The low re l iab i l i ty o f c a l c u l a t e d a va lues does n o t necessar i ly affect t he val idi ty of h es t ima ted w i t h I a n d t .

The va lue o f h c a n e x p r e s s t h e n u m b e r o f Babesia i n fec t ions received by a h e r d during a cer ta in t ime w i t h o u t cons ide r ing the complex cond i t ions leading to infect ion.

Smith a n d co l l e agues (115) c o n c l u d e d in Brazi l a n d U r u g u a y t h a t t h e r isk of babesiosis c o u l d n o t b e p rec i se ly d e d u c e d f r o m t h e o b s e r v e d infec ted l a r v a e , u n l i k e the infect ion r a t e of e ight t o t e n - m o n t h - o l d ca t t l e .

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T h e F A O (30) a lso po in t ed ou t t h a t t he t w o key ep idemio log ica l p a r a m e t e r s which m a y be used t o c o n t r o l t i c k - b o r n e diseases a r e :

- t he va lue of h ca lcu la ted f r o m t h e a n i m a l i n f ec t ion r a t e a t a d e t e r m i n e d age

- t h e cri t ical va lue of h for e n z o o t i c s tab i l i ty .

Analytic epidemiology

Tick infection rate

T o u n d e r s t a n d the ep idemio logy of babes ios i s , it is essent ia l t o k n o w the infect ion r a t e of B. microplus.

T h e de tec t ion of Babesia k ine tes in e n g o r g e d B. microplus f ema le s is easier t h a n the inspec t ion of l a rvae . F e m a l e s w h i c h h a d fed o n ca t t l e w i t h l o w p a r a s i t a e m i a (not detectable on b l o o d smears) 24 h o u r s be fo re t ick d r o p p i n g , h a d infected h a e m o l y m p h 8, 9 a n d 10 days af ter d r o p p i n g (34). M a h o n e y a n d M i r r e (72) c o n s i d e r e d t h a t this technique unde re s t ima ted the n u m b e r of infected females , b u t t h e y e x a m i n e d the ticks on ly unt i l day seven af ter d r o p p i n g . I n a n e p i d e m i o l o g i c a l su rvey of t h r e e different s i tuat ions , Gug l i e lmone a n d col leagues (35) o b t a i n e d c o h e r e n t a n d s imilar results with t he h a e m o l y m p h t e c h n i q u e a n d the sero logica l t e c h n i q u e .

T h e h a e m o l y m p h t e c h n i q u e can b e pa r t i a l l y q u a n t i f i e d b y c o u n t i n g t h e n u m b e r of k inetes per m i c r o s c o p i c field (34, 35) . H o w e v e r , t h e r e l a t i o n b e t w e e n t h e n u m b e r of k inetes a n d the p e r c e n t a g e of in fec ted l a rvae is n o t yet k n o w n .

A l t h o u g h J o h n s t o n (48) d is t inguished be tween kinetes of B. bovis a n d B. bigemina ob ta ined f rom the field, the differences be tween the t w o a re n o t a lways easy t o detect; l ength c a n n o t p r o v i d e re l iable cr i te r ia , as t he re is s o m e o v e r l a p in size, a n d 1 9 % of the B. bovis spec imens o b t a i n e d in A r g e n t i n a d o n o t h a v e a c u r v e d ta i l , which is otherwise characterist ic of this species a n d useful in di f ferent ia t ing it f rom B. bigemina. M o r e o v e r , a m o n g h ighly infec ted t icks ( > 2 0 Babesia k ine te s pe r m i c r o s c o p e field of h a e m o l y m p h ) atypical specimens can be observed (Gug l i e lmone , u n p u b l i s h e d data) .

A n o t h e r aspect of the Boophilus-Babesia i n t e r a c t i o n c o n c e r n s t h e pa tho log ica l effect of Babesia o n t he t ick . Th i s effect was o b s e r v e d w i t h B. bovis (27 , 100), with B. bigemina (99) a n d wi th mixed in fec t ions (34) in B. microplus. S imi la r effects were observed in B. annulatus (88) a n d B. decoloratus (145). T h e p a t h o l o g i c a l effects were n o t e d af ter expe r imen ta l in fec t ion ; G u g l i e l m o n e a n d co l l eagues (36) obse rved no de t r imen t a l effect o n n a t u r a l l y infected B. microplus, even w i t h h i g h h a e m o l y m p h infect ion. There could be an adap ta t ive to le rance be tween local Boophilus a n d Babesia. T h e r e is n o need for a p a t h o g e n i c effect of Babesia o n t i cks t o b e cons ide red in ep idemio log ica l surveys at p r e sen t .

Cattle infestation rate

Unless t he ra te of t ick in fes ta t ion in ca t t le is k n o w n , n o p r a c t i c a l use c a n be m a d e of the tick infect ion ra te . A coun t of the female t icks 4.5 t o 8 m m in length ( " s t a n d a r d f e m a l e s " ) gives the n u m b e r of t icks r each ing the e n d of the i r cycle , b e c a u s e they will d r o p wi th in a day (137). A c c u r a t e d a t a o n th is n u m b e r a n d o n t h e effect of the e n v i r o n m e n t on free stages a re essent ia l in d e v e l o p i n g m a t h e m a t i c a l m o d e l s for the Boophilus p o p u l a t i o n . D a t a on free stages a r e a l r eady ava i l ab le for v a r i o u s countr ies

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of Lat in A m e r i c a a n d t h e C a r i b b e a n , e .g . A r g e n t i n a (45 , 46 ) , Brazi l (70, 117), Colombia (12) , C u b a (20) , J a m a i c a (98) a n d U r u g u a y (19, 81).

In a d d i t i o n t o t h e e s t i m a t i o n of t h e n u m b e r of a n i m a l s se ropos i t ive t o B. bovis and B. bigemina, t h e t ick i n f e s t a t i o n r a t e a n d the t ick in fec t ion r a t e , t he fo l lowing informat ion is n e c e s s a r y t o m a k e a n exhaus t ive s t u d y of t he ep i zoo t i o logy of babesiosis.

Genetic composition of cattle

Catt le w i t h B. indicus genes i n d u c e lower h va lues t h a n B. taurus, b e c a u s e Boophilus microplus i n f e s t a t i o n is inversely p r o p o r t i o n a l t o t he p r o p o r t i o n of B. indicus genes (37 , 66 ; C o r d o v e s a n d V i t o r t e , u n p u b l i s h e d d a t a ) a n d b e c a u s e parasi taemia w i t h Babesia bovis is l ower in B. indicus ca t t le (2). G u g l i e l m o n e a n d colleagues (36) o b s e r v e d f e m a l e in fec t ion r a t e s of 2 8 % , 2 3 % a n d 5 % respect ively in Creole {B. taurus), H e r e f o r d (B. taurus) a n d Ne lo re (B. indicus) b reeds . T h e Creole breed has a be t t e r r e s i s t ance t o Boophilus microplus i n fes t a t ion t h a n o t h e r B. taurus breeds; bu t t h e d i f f e rence is m o r e i m p o r t a n t b e t w e e n B. taurus a n d B. indicus b r eeds (75, 134). Th i s d i f f e rence b e t w e e n t h e t ick in fec t ion r a t e in B. taurus a n d B. indicus has a ma rked inc idence o n the ep izoot io logy of babesiosis . A n y survey shou ld consider the p ropor t ion of B. indicus genes in herds a n d the reason why they were incorpora ted .

Acaricides: type, application method, rhythm and season

The m a j o r i t y of A m e r i c a n c o u n t r i e s u s e o r g a n o p h o s p h a t e s , f o r m a m i d i n e s a n d pyrethroids as a c a r i c i d e s . B u t in M e x i c o a n d C u b a the on ly a u t h o r i s e d p r o d u c t s a re one o r g a n o p h o s p h a t e a n d o n e p y r e t h r o i d ; t h e o t h e r s a re kep t for cases of t ick resistance (130; C o r d o v e s , u n p u b l i s h e d d a t a ) .

With r e g a r d t o t h e e p i d e m i o l o g y of babes io s i s , t h e biggest d i f ference be tween acaricides is t he i r r e s i d u a l effect , w h i c h p r e v e n t s t he t r a n s m i s s i o n of Babesia. Pyrethroids c a n pers i s t for 15-21 d a y s (76, 125) whi le o r g a n o p h o s p h a t e s persis t for only three d a y s (10) . I v e r m e c t i n a t a single d o s e of 0.2 µ g / k g p ro t ec t s ca t t le for 21-28 days (83 , 92 , 111); th is is a sys temic aca r i c ide , inges ted wi th t h e b l o o d by t icks , which m a y n o t p r e v e n t t r a n s m i s s i o n of Babesia (especial ly B. bovis). Never the less , the tick p o p u l a t i o n is p rog re s s ive ly r e d u c e d b y t h e aca r i c ide . T a y l o r a n d co l leagues (124) obse rved th i s p h e n o m e n o n w i t h B. divergens t r a n s m i t t e d by Ixodes ricinus.

Acaricides a r e a p p l i e d (in dec reas ing o rde r ) by d ipp ing , s t a t i o n a r y sp ray ing (spray race) a n d m a n u a l s p r a y i n g . S o m e p y r e t h r o i d s c a n b e used as d o r s a l " p o u r o n " wi th the same eff iciency as s t a t i o n a r y s p r a y i n g . T h e y d o n o t a p p e a r t o h a v e a sys temic effect (82). Su rveys s h o u l d t a k e a c c o u n t of t he n u m b e r of a n n u a l t r e a t m e n t s a n d the season of t r e a t m e n t . T h e effect o n Babesia i n o c u l a t i o n decreases a c c o r d i n g to the m o d e of t r e a t m e n t : p r o p h y l a c t i c , t h r e s h o l d c o n t r o l o r o p p o r t u n i s t (84).

Stocking rate and feed resources

The s tock ing r a t e di rect ly inf luences t he p robab i l i t y of B. microplus l a rvae mee t ing a host, a n d a l so t h e i n o c u l a t i o n r a t e o f Babesia.

The g raz ing s y s t e m is a l so i m p o r t a n t . I n s o m e r o t a t i o n a l g raz ing wi th Chloris gayana, for i n s t a n c e , p a s t u r e s c a n r e m a i n w i t h o u t ca t t l e for t w o t o t h r e e m o n t h s , thus r educ ing t h e B. microplus p o p u l a t i o n (140) . I r r i g a t i o n , h o w e v e r , f avou r s t h e

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B. microplus cycle d u r i n g t he d r y s e a s o n ; s o m e p a s t u r e s (Medicago sativa) for dairy catt le are i r r igated in no r th -wes t e rn A r g e n t i n a a n d cent ra l Bol ivia . T h e t ime necessary t o es tabl ish a n n u a l g r az ing is l o n g e n o u g h t o destroy t h e l a r v a e p r e v i o u s l y p re sen t . If infes ted cat t le a re i n t r o d u c e d , l a rvae will a p p e a r j u s t a f ter t h e r e m o v a l of cat t le . In some regions of A r g e n t i n a w h e r e c l ima t i c c o n d i t i o n s a l low t h e d e v e l o p m e n t of t h r e e g e n e r a t i o n s of B. microplus (46) , i m p o r t a n t d i f fe rences in t h e n u m b e r of B. microplus were observed be tween f a r m s , a p p a r e n t l y i n d u c e d by di f ferent pract ices o f a n n u a l g raz ing ( 1 0 1 , 118). De ta i l s o f t h e f eed ing sys t em s h o u l d b e i n c l u d e d in a survey o n babes ios i s .

Climatic data

T h e c l imate inf luences t he longev i ty of l a r v a e a n d t h e n u m b e r of a n n u a l gene ra t ions of B. microplus (122) . T r a n s o v a r i a n t r a n s m i s s i o n of Babesia (99) a n d the pe r cen t age o f infec ted l a rvae a r e a l so a f fec ted b y c l i m a t i c f a c t o r s (24) . A s p a r t of an ep idemio log ica l s tudy on babes io s i s , such d a t a as t h e t e m p e r a t u r e , m i n i m u m and m a x i m u m relative humid i ty a n d daily ra infal l at g r o u n d level in t he reg ion should b e col lected.

His tor ica l i n f o r m a t i o n is c o n s i d e r e d suff icient in o r d e r t o p r e d i c t t h e p r o b a b i l i t y of babes ios is o c c u r r e n c e .

Other factors

S o m e grasses , such as Melinis minutiflora a n d Stylosanthes scabra, possess larvicidal p rope r t i e s aga ins t Boophilus (47 , 123). H o w e v e r , t h e y a r e b o t h repel lent (85, 108), which m a y coun te rba lance the first effect, except in p u r e cul t iva t ion . Studies to evaluate anti- t ick grasses have no t clarified t he s i tua t ion . T h o m p s o n a n d colleagues (129) c o n c l u d e d t h a t M. minutiflora a n d Andropogon gayanus h a v e a n anti- t ick ac t ion , b u t n o stat is t ical analys is was p e r f o r m e d . A p p a r e n t l y Cynodon dactylon was be t te r t h a n A. gayanus a n d p e r h a p s even be t t e r t h a n M. minutiflora. In a s tudy by A y c a r d i a n d col leagues (9), t h e effect of t h e s t o c k i n g r a t e c o u l d n o t b e separa ted f rom the effect of an t i - t ick g ra s s . W i l s o n a n d S u t h e r s t (143) d e m o n s t r a t e d tha t B. microplus l a rvae d o no t s tay clear of S. scabra w h e n t h e b r a n c h e s lack exuda te , du r ing t he vegeta t ive g r o w t h or af ter d isease o r g r a z i n g . H o w e v e r , t h e l a rvae will h a v e difficulty r each ing a hos t if t h e b r a n c h e s a re s u r r o u n d e d wi th y o u n g exuda te b r a n c h e s . A legume such as S. scabra is a very g o o d feed a n d will c o n t i n u e to be used m o r e widely (32). H o w e v e r , it seems t o o ear ly t o i nc lude a re levan t effect of grasses on Boophilus p o p u l a t i o n in an epizoot io logica l survey. In t he s a m e way , other fac tors a p p e a r t o have a l imi ted effect o n t he t ick p o p u l a t i o n :

in 1973, t he cat t le egret Bubulcus ibis, a na t ive of Af r i c a , was o b s e r v e d in Rio G r a n d e d o Sul (Brazil) a n d n o r t h e r n A r g e n t i n a for t h e first t i m e ; f o u r b i rds killed in Brazil h a d ingested a t o t a l of 132 t icks (5) b u t , in C u b a , C o r d o v e s (unpub l i shed da ta ) observed t h a t t he impac t of the cat t le egret on B. microplus was n o t significant;

- Pheidole megacephala an t s eat engo rged B. microplus f ema le s , b u t m o r e t h a n 1,000 an t s are necessary t o eat o n e female (135) a n d they d o n o t affect t he na tu ra l p o p u l a t i o n of B. microplus ( C o r d o v e s a n d V i t o r t e , u n p u b l i s h e d d a t a ) ;

tick pa ras i t e s have a low p a t h o l o g i c a l inc idence (10) .

H o w e v e r , Su thers t a n d col leagues (123) p o i n t e d ou t t h a t p i e d a t i o n of engorged females of B. microplus was a m a j o r f ac to r a f fec t ing t he d y n a m i c s of the tick p o p u l a t i o n in A u s t r a l i a .

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Conclusion

To d e t e r m i n e t h e r i sk o f o c c u r r e n c e o f babes io s i s , a s imple sero logica l survey is sufficient.

Howeve r , a n u n d e r s t a n d i n g of t he re levan t f ac to r s l ead ing t o o u t b r e a k s r equ i r e s complex s tudies of t ick i n fec t ion r a t e , ca t t l e i n fe s t a t ion r a t e , gene t i c c o m p o s i t i o n of cattle, aca r i c ides , s t o c k i n g r a t e , c l imat ic d a t a a n d o t h e r f a c t o r s .

A N A P L A S M O S I S

The causa l a g e n t o f a n a p l a s m o s i s is a r icke t t s ia (136) t a x o n o m i c a l l y d i s t an t f rom the p r o t o z o a n s w h i c h p r o d u c e b a b e s i o s i s . H o w e v e r , b o t h agen t s sha re s o m e epizootiological c h a r a c t e r i s t i c s .

Similarities with babes ios is

The m o s t s igni f icant a spec t is t h e s imi lar g e o g r a p h i c a l d i s t r i b u t i o n of babes ios i s and a n a p l a s m o s i s in A u s t r a l i a (112) , S o u t h Afr ica (95) a n d U r u g u a y (14). H o w e v e r , in the U S A babes io s i s is an exot ic d i sease , whi le a n a p l a s m o s i s has an i m p o r t a n t economic i m p a c t ( 3 , 146).

In A r g e n t i n a , babes ios i s occu r s over a n a r ea c o n t a i n i n g a p p r o x i m a t e l y 12 mil l ion head of ca t t l e , w h e r e a s o u t b r e a k s of a n a p l a s m o s i s occu r in a n a r ea c o n t a i n i n g 18 mill ion h e a d (33 , 38) .

In areas w h e r e b o t h d iseases a r e p r e s e n t , t h e sea sona l d i s t r i bu t i on is s imilar , t he anaplasmosis p e a k a p p e a r i n g a few weeks after the babesiosis peak (119). Y o u n g cattle are cons idered m o r e r e s i s t an t t o a first in fec t ion b y a n a p l a s m o s i s (89, 103). H o w e v e r , Hosokama (43) a n d O t t e a n d P e r e z (87) ind ica ted t h a t a n a p l a s m o s i s c a n cause severe disease in calves in s o m e r eg ions of S o u t h A m e r i c a .

After t h e in i t ia l i n f e c t i o n , a sol id i m m u n i t y is d e v e l o p e d , las t ing t h r o u g h o u t t he economic life of ca t t l e . T h e c o n c e p t of e n z o o t i c s tabi l i ty u s e d for babes ios i s c an , therefore, a l so b e a p p l i e d t o a n a p l a s m o s i s (142) .

Pa rke r a n d co l l eagues (89) o b s e r v e d a mi lde r a n a p l a s m o s i s infect ion in B. indicus cattle t h a n in B. taurus, wh i l e o t h e r a u t h o r s d id n o t f ind a n y d i f fe rence (8 , 6 3 , 141). Paras i taemia w i t h A. marginale is m o d i f i e d by t he g e n o t y p e , B. indicus ca t t le generating a l o w e r i n o c u l a t i o n r a t e t h a n B. taurus ( 1 , 6) .

A sizeable i n t e rva l b e t w e e n r o t a t i o n a l g r a z i n g p e r i o d s , in a d d i t i o n t o d ipp ing , affected t he i n o c u l a t i o n r a t e of B. bovis, B. bigemina a n d a lso A. marginale in Queensland (49) .

The similari t ies b e t w e e n babes ios is a n d a n a p l a s m o s i s a re also reflected in t he close correlation b e t w e e n t h e s e r o p r e v a l e n c e of B. bovis a n d A. marginale (r - 0.78) observed in t w o m i lk s h e d s in n o r t h e r n A r g e n t i n a (39) a n d M e x i c o (126) , w h e r e b o t h diseases a p p e a r t o h a v e h a d t h e s a m e r e a c t i o n s t o a s imi lar e n v i r o n m e n t .

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Differences from babesios is

T h e r e a re a lso ev ident d i f fe rences b e t w e e n t h e t w o d i seases . I n A r g e n t i n a , an o u t b r e a k of a n a p l a s m o s i s was recen t ly d i a g n o s e d a t a l a t i t u d e of 3 5 ° 5 (Ga l le to , p e r s o n a l c o m m u n i c a t i o n ) , a n d in c o u n t r i e s s u c h as E l S a l v a d o r (91) a n d C u b a (23) a n a p l a s m o s i s a p p e a r s t o b e of m a j o r i m p o r t a n c e c o m p a r e d t o b a b e s i o s i s .

It is a s ton i sh ing t h a t for a d isease of i nc reas ing i m p o r t a n c e w h i c h affects b o t h deve loped a n d deve lop ing c o u n t r i e s , t h e r e h a s b e e n n o q u a n t i f i c a t i o n of t h e mos t re levant epidemiological fac tors for a n a p l a s m o s i s . Th i s is d u e t o a lack of i n fo rma t ion o n a n a p l a s m o s i s vec to rs a n d the m e a n s of t r a n s m i s s i o n a n d in fec t ion o f t h e vec to r s . A signif icant e x a m p l e is p r o v i d e d b y Da l lw i t z a n d co l l eagues (25) w h o c l a imed tha t t he t ick infec t ion r a t e wi th A marginale in e n z o o t i c a r e a s c a n r e a c h 1 0 0 % , w i t h o u t ind ica t ing t h e or ig in of the i r o b s e r v a t i o n s .

Transmission modalit ies

In fact , t he c o n d i t i o n s a n d i m p o r t a n c e of t i ck t r a n s m i s s i o n of A. marginale have no t been fully eva lua t ed .

Boophilus

Boophilus s p p . a p p e a r t o be a n i m p o r t a n t v e c t o r of Anaplasma. T r a n s o v a r i a l t r a n s m i s s i o n h a s long b e e n cons ide r ed i m p o r t a n t (16, 64 , 109, 127); even recent ly , Olivier a n d col leagues ( u n p u b l i s h e d d a t a ) r e p o r t e d th is in C u b a , as d id Vizca íno and L o p e z (personal c o m m u n i c a t i o n ) in C o l o m b i a . H o w e v e r , o the r s tudies have indicated t he absence of this p h e n o m e n o n (15, 22 , 65 , 67 , 9 5 , 110, 128, 131). P e r h a p s some s t ra ins of A. marginale c an b e t r a n s m i t t e d on ly a t h i g h t e m p e r a t u r e s (109) .

M o r e o v e r , i n t r a s t ad i a l (95) a n d t r a n s - s t a d i a l t r a n s m i s s i o n ( 2 1 , 95 , 110, 128, 132, 133) of A. marginale by Boophilus a l so o c c u r .

Conne l l (21) ind ica ted t h a t B. microplus was ab le t o t r a n s m i t t h e in fec t ion unti l a t least t w o h o u r s after a n i n t e r r u p t i o n in its cycle . T h i s f ac to r w a s u s e d t o ob ta in p u r e s t ra ins of A. marginale f r o m cat t le w i t h m i x e d in fec t ions (24). A l t h o u g h B. microplus is a s ingle-host t ick , p r o v i d i n g n o o p p o r t u n i t y for i n t r a s t a d i a l or t r ans -s tadia l t r ansmi s s ion of A. marginale u n d e r n a t u r a l c o n d i t i o n s , excep t i ons h a v e been r eco rded (13, 22 , 77); however , t he i m p o r t a n c e of these excep t ions was n o t eva lua ted . In A r g e n t i n a , eleven B. microplus infes ted calves a n d six t ick- f ree calves were m a i n t a i n e d for eight days o n a t ick-free p a s t u r e : t he t o t a l i n f e s t a t i on of t he infested g r o u p was 2 ,450 females (4.5 t o 8 m m ) a n d 2 ,450 m a l e s . In t h e t ick-f ree g r o u p , one calf was infested by a single B. microplus on the sixth day (Gug l i e lmone a n d Mango ld , u n p u b l i s h e d d a t a ) . N u m e r o u s o the r t ick species were i nvo lved in t h e expe r imen ta l t r ansmis s ion of A. marginale. S o m e , such as Argas persicus (44) , a r e s e l d o m linked wi th ca t t le . H o w e v e r , subs t an t i a l a d v a n c e s h a v e b e e n m a d e r e g a r d i n g k n o w l e d g e on the deve lopment a n d t ransmiss ion of A. marginale in t icks of t he genus Dermacentor, presen t in N o r t h A m e r i c a .

Dermacentor

Initially, A. marginale was d e m o n s t r a t e d in the midgu t of t w o Dermacentor species and was different iated from symbiot ic r icket ts iae (52). T h e infectivity of A. marginale was observed in catt le (51) and the m o r p h o l o g y in adul t t icks (54) a n d n y m p h s . Similar

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colonies were d e m o n s t r a t e d in S o u t h A f r i c a n Rhipicephalus simus (96). A. marginale is not t r a n s m i t t e d t r a n s o v a r i a l l y in Dermacentor (53) . A d i rec t r e l a t i on was p r o v e d between p a r a s i t a e m i a of ca t t l e a n d t ick in fec t ion (53 , 55) . A. marginale c an d e v e l o p not only in t h e g u t o f D. andersoni (57) b u t a l so in t h e h a e m o l y m p h (56) a n d is observed in t h e sa l iva ry g l a n d s (121) of t h e t i ck . T h e n u m b e r of co lon ies in t h e t ick midgut decreases wi th t i m e (59) b u t colonies persist for twelve m o n t h s in overwinter ing D. variabilis (68) .

It m u s t b e s t ressed t h a t n o t every A. marginale i so la te c a n b e t r a n s m i t t e d b y Dermacentor (58) , a n d t h a t t h e p r e s e n c e o r a b s e n c e of a n a p p e n d i x does n o t i nd i ca t e the capac i ty t o infect t i cks o r n o t (138) .

These i m p o r t a n t d i scover ies h a d n o i m p a c t o n t h e q u a n t i t a t i v e aspec t s of anaplasmosis e p i d e m i o l o g y . T h e use of D N A p r o b e s (4) will s o o n a l low the de tec t ion of A. marginale in ca t t l e a n d t i cks ( b o t h Dermacentor a n d Boophilus).

H o w e v e r , o t h e r vec to r s a r e a l so i m p l i c a t e d in t r a n s m i s s i o n of A. marginale.

Haematophagous Diptera

H a e m a t o p h a g o u s D i p t e r a a r e c o n s i d e r e d r e l evan t vec to rs of A. marginale. I n Texas, A lde r ink a n d Die t r i ch (3) l inked t a b a n i d s t o t he anap la smos i s o u t b r e a k s which occurred in s u m m e r , a n d m o s q u i t o e s t o t h e o u t b r e a k s in a u t u m n .

A c o n s i d e r a b l e n u m b e r of h a e m a t o p h a g o u s D i p t e r a seem ab le t o t r a n s m i t A. marginale t o c a t t l e , b u t t h e r e is a c o n s e n s u s t o cons ide r T a b a n i d a e as t h e m a j o r vectors (144). H o w e l l a n d col leagues (44) indica ted t h a t th i r teen Tabanus oklahomensis bites were neces sa ry t o infect e a c h suscep t ib le h e a d of ca t t l e , a n d a s imi lar n u m b e r was observed b y H a w k i n s a n d co l l eagues (41) , w h o s tud ied fou r Tabanus species . The fo rmer a u t h o r s o b s e r v e d t h a t t h e m a x i m u m in te rva l b e t w e e n t w o bi tes was five minutes, b u t t h e l a t t e r r e p o r t e d t h e m a x i m u m t i m e as t w o h o u r s . D i p t e r a p r o b a b l y play an i m p o r t a n t ro l e in a n a p l a s m o s i s t r a n s m i s s i o n , even in a r ea s w h e r e t icks a re considered vec to r s . T h e a n n u a l inc idence r a t e of A. marginale in n o r t h e r n Queens l and fluctuated be tween 1 0 % a n d 7 0 % (61 , 90) ind ica t ing t h a t t he presence of B. microplus alone c a n n o t e n s u r e a h i g h r a t e of A. marginale t r a n s m i s s i o n (142). P e r e z a n d colleagues (93) p o i n t e d o u t t h e l ack o f c o n c o r d a n c e b e t w e e n the p o p u l a t i o n of B. microplus a n d o u t b r e a k s of a n a p l a s m o s i s in C o s t a Rica .

The close r e l a t i o n b e t w e e n s e r o p r e v a l e n c e of B. bovis a n d A. marginale co u l d mean t h a t A. marginale h a s d i f fe ren t vec to r s f r o m B. bovis b u t t he s a m e seasona l variations. R ios a n d co l l eagues (102) d e t e r m i n e d s imi lar A. marginale in fec t ion ra tes in two a d j a c e n t f a r m s : B. microplus w e r e p r e s e n t a l m o s t t h e w h o l e yea r r o u n d o n one of t he f a r m s , wh i l e t h e o t h e r was a l m o s t en t i re ly free f r o m t i cks .

Other aspects

I a t rogen ic t r a n s m i s s i o n s h o u l d a l so b e c o n s i d e r e d (28, 80) .

M o r e o v e r , u n l i k e b a b e s i o s i s in ca t t l e , v a r i o u s w o o d l a n d v e r t e b r a t e s seem to b e important n a t u r a l r e se rvo i r s o f A. marginale in s o m e reg ions (62): wa t e r b u f f a l o , bison, A f r i c a n a n t e l o p e , g n u , b l e s b o k , d u i k e r , A m e r i c a n dee r , Vi rg in ia whi te - t a i l ed deer, elk, c a m e l a n d w i l d e b e e s t . Ver t i ca l t r a n s m i s s i o n of A. marginale f r o m c o w t o calf is c o n s i d e r e d t o h a v e a n e p i d e m i o l o g i c a l i m p o r t a n c e b y P o t g i e t e r a n d V a n Rensburg (97) a n d C o r d o v e s ( u n p u b l i s h e d d a t a ) .

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Conclus ion

It seems unjus t i f ied t o a t t e m p t t o u n d e r s t a n d t h e e p i d e m i o l o g y of a n a p l a s m o s i s as a t i c k - b o r n e d isease . It w o u l d b e a c o n s i d e r a b l e t a s k t o i n c o r p o r a t e i n t o such a s tudy all t he p a r a m e t e r s wh ich seem t o b e invo lved in t h e e p i d e m i o l o g y o f t h e disease.

F o r i n s t ance , a s tudy on m o s q u i t o e s in an a r e a of A r g e n t i n a w h i c h par t ia l ly co inc ided wi th t he r eg ion in fec ted wi th a n a p l a s m o s i s r e s u l t e d in t h e d e t e r m i n a t i o n of e ighteen di f ferent species (79) . T h e diff icul t ies t o b e o v e r c o m e in u n d e r s t a n d i n g a n a p l a s m o s i s ep idemio logy inc lude t h e g rea t d ivers i ty of h a e m a t o p h a g o u s D ip t e r a in La t i n A m e r i c a a n d the scarci ty of d a t a o n the i r ro l e as a n a p l a s m o s i s vec to rs and their hos t preferences . A s a first s t ep , a p i lo t survey s h o u l d b e u n d e r t a k e n in a l imited a r e a .

F o r a n a p l a s m o s i s , un l i ke b a b e s i o s i s , t h e lack of bas ic i n f o r m a t i o n p r even t s an u n d e r s t a n d i n g of t h e n a t u r a l causes wh ich d e t e r m i n e o c c u r r e n c e of t h e d isease .

H o w e v e r , t he risk of a n a p l a s m o s i s o c c u r r e n c e can b e p r e d i c t e d , as for babes ios is , f r o m t h e ca lcu la t ion of h va lues f r o m I a n d t va lues g a t h e r e d b y s imple serological surveys (87, 126).

C O N C L U S I O N

T h e t w o di f ferent objec t ives of a s t u d y o n t h e e p i d e m i o l o g y of babes ios i s and a n a p l a s m o s i s a r e :

— to d e t e r m i n e t h e r isk of o c c u r r e n c e

t o u n d e r s t a n d t h e re levan t f ac to r s l ead ing t o o u t b r e a k s .

T h e first objec t ive is re la t ively s imple t o ach ieve for b o t h d iseases b y ca lcula t ing t he i n o c u l a t i o n r a t e , h . Th i s s h o u l d be d o n e in every c o u n t r y w h e r e babes ios i s and anaplasmosis pose a threa t , in order t o a d a p t t ick a n d disease con t ro l t o every si tuation.

T h e second objec t ive r equ i re s exhaus t ive a n d c o m p l e x s t ud i e s . It shou ld be r e c o m m e n d e d first in l imi ted reg ions or o n i s lands w i t h a l imi ted n u m b e r of species of h a e m a t o p h a g o u s D i p t e r a (in t h e case of a n a p l a s m o s i s ) . B i o t e c h n o l o g i c a l too ls , such as D N A p r o b e s a n d the p o l y m e r a s e cha in r e a c t i o n , will c e r t a in ly faci l i ta te these inves t iga t ions .

ÉPIDÉMIOLOGIE DE L'ANAPLASMOSE ET DE LA BABÉSIOSE CHEZ LES BOVINS EN AMÉRIQUE LATINE ET AUX CARAÏBES. - M. Alonso, C. Arellano-Sota, V.H. Cereser, C.O. Cordoves, A.A. Guglielmone, R. Kessler, A.J. Mangold, A. Nari, J.H. Patarroyo, M.A. Solari, C.A. Vega, O. Vizcaíno et E. Camus.

Résumé: Les auteurs analysent les différents paramètres qui interviennent dans l'épidémiologie de la babésiose et de l'anaplasmose, et qui doivent être évalués lors de toute élude. Ils comparent les deux maladies en Amérique Latine et aux Caraïbes; lorsqu'aucune information n'est disponible pour cette légion, ils se servent des données provenant d'Australie et des Etats-Unis d'Amérique.

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Les deux principaux objectifs d'une enquête épidémiologique sur la babésiose et l'anaplasmose sont:

la détermination du risque d'infestation,

l'identification des facteurs qui conduisent à l'apparition de foyers.

Le premier objectif est relativement simple à atteindre pour les deux maladies, en calculant le taux d'infestation (h) à un âge déterminé et la valeur critique de h qui assure la stabilité de l'enzootie.

Le second objectif nécessite des études exhaustives et complexes. Pour l'épidémiologie de la babésiose, il est indispensable de disposer d'informations sur les populations de tiques, le taux d'infestation des bovins, leurs caractéristiques génétiques, les acaricides, la densité du cheptel, les conditions climatiques et d'autres facteurs. L'épidémiologie de l'anaplasmose ne peut pas être considérée exclusivement comme une maladie transmise par les tiques. Parmi les difficultés, il faut citer la grande diversité des diptères hématophages en Amérique Latine, la rareté des informations concernant leur rôle de vecteur dans l'anaplasmose et leur habitude de se n o u r r i r sur les bovins.

MOTS-CLÉS : Amérique Latine - Anaplasmose - Babésiose - Bovins - Caraïbes -Enquêtes épidémiologiques Epidémiologie.

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EPIDEMIOLOGíA DE ANAPLASMOSIS Y BABESIASIS BOVINAS EN AMÉRICA LATINA Y EL CARIBE. - M. Alonso, C. Arellano-Sota, V.H. Cereser, C.O. Cordoves, A.A. Guglielmone, R. Kessler, A.J. Mangold, A. Nari, J.H. Patarroyo, M.A. Solari, C.A. Vega, O. Vizcaíno y E. Camus.

Resumen: Los autores analizan los diversos parámetros que interactúan en la epizootiología de la babesiasis y la anaplasmosis, y que se deben obtener cuando se realiza una encuesta. Comparan además ambas enfermedades. Cuando no se dispone de datos procedentes de América Latina o el Caribe, se hace referencia a los de Australia o los Estados Unidos de América.

Los dos objetivos principales de un estudio de epidemiología de babesiasis y anaplasmosis son:

determinar el riesgo de ocurrencia,

comprender los factores pertinentes que provocan los brotes.

El primer objetivo es relativamente sencillo de alcanzar para ambas enfermedades calculando la tasa de inoculación (h) en el animal a determinada edad y el valor crítico de h para la estabilidad enzoótica.

El segundo objetivo requiere estudios exhaustivos y complejos. Para la epidemiología de la babesiasis, es necesario hacer estudios sobre las tasas de infección de garrapatas y de la infestación del ganado, la composición genética del ganado, los acaricidas, la tasa de población, los datos climáticos y otros factores. La epidemiología de la anaplasmosis no puede comprenderse únicamente como una enfermedad transmitida por las garrapatas. Las dificultades que hay que superar incluyen la gran diversidad de dípteros hematófagos en América Latina y la escasez de datos sobre su actuación como vectores de la anaplasmosis y sus preferencias alimentarias en lo referente al ganado.

PALABRAS CLAVE: América Latina - Anaplasmosis Babesiasis Bovinos Caribe Epidemiología - Estudios epidemiológicos.

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