Ectoparasite and Riakettsia tsutsugamushi

Studies in Thailand

Principal Investigators: Richard G. Andre, MAJ, MSC*Pan ita Lakshana Tanskul~ M.Sc.Br~~e A. Harrison, MAJ, MSCAlexander L. Dohany p MAJ ~ MSC**Aki Shirai, Ph.D.**

Associate Investigators: Michael C. Callahanp SFCInkam InlaoVand~e NongngorkVichit Phunkitchar

OBJECTIVE: To establish and describe the chiggers and ticks that are vectorsor potential vectors of human pathogens in Thailand, and to dete~mine the geo-graphical distribution of Rickettsia tsutsugamushi in natural populations ofchiggers in Thailand,

BACKGROUND: This is a continuation of previously reported studies (1). Earlierwork during the period of 1961-1972 served as the basis for a number of publica-tions, in which Lakshana (2) and Lekagul and MCNeely (3) have established a firmtaxonomic base on which epidem1ological-ecological studies on Rickettsia tsutsu-gamushi in Thailand can proceed. Recent emphasis has shifted to the distributionof strains of R. tsutsugamushi that occur in vector chiggers in Thailand. How-ever, problems still exist in identifying chigger specimens collected in Thailandsbence taxonomic studies on chiggers are continuing.

METHODS: Ectoparasites are collected from live trapped rodents and other smallmammals by removal with forceps, by scraping or by holding the animals aliveover a pan of water and allowing engorged ectoparasites to drop into the water.Engorged chiggers are normally preserved in alcohol and mounted on s11des forstudy. Chiggers used for R. tsutsugamushi isolation attempts are preferablyunengorged.

Unengorged chiggers are usually found in leaf litter, on rotten logsand other favorable habitats frequented by rodents and other small mammals, andare easily collected by using 5" x 5" formica black plates. Collected unengorgedchiggers are placed and kept alive in vials of water, which are then shipped toUSAMRU-Kuala Lumpur for rickettsia studies. A technique using dire.ct immuno-fluorescence has recently been developed to detect rickettsia in naturally'infected mites (4). Using this technique,. the internal contents of each unen-gorged chigger can be screened for nine different strains of R. tsutsugamushi~i.en, Karp, Gilliam, Kato TC 586, TA 678, TA 686, TA 716, TA 763 and TH 1817.After the internal contents of the chigger has been t~sted for rickettsia, itsexoskeleton is mounted in Hoyer's mounting media on a slide for identification.

~ 804 Gaskill Dr., Ames, Iowa 50010*1* USAMRU-Kua1a Lumpur, Institute for Medical Research, Kuala Lumpur, Malaysia

32

RESULTS: Between 25 Oct. and 21 Dec. 1978,1,379 unengorged chiggers of 12species were collected in Thailand by the black plate method and sent to USAMRU-Kuala Lumpur for rickettsia detection and subsequent identification (Table 1).These specimens were collected in 2 provinces, Chiang Mai and Nakhon R.atchasima(two areas -Pak Chong and Sakaerat). The five most commonly collected specieswere L. (L.) soutellare~ L. (L.) deliense~ L. (L.) miaulum arvinum~ Odontacarussp, a~.d L. (T.) paniaulatum~ in descending.order of abundance. Four new species,BelenicuZa sp., L. (L.) sp. D, L. (L.) sp. G and Odontacarus were also identifiedduring this period, and represented 7.6% of the total specimens collected. Ofthe 12 species collected, 828 specimens of 10 species were screened in KualaLumpur for Rickettsia tsutsugamushi, and 6% (50/828) were positive (Table 2). Atotal of six species were positive for R. tsutsugamushi from the two provinces,with L. (L.) deZiense and scutellare positive in both provinces, while L. (L.)miaulum arvinum was positive only in Chiang Mai and E. wichmanni~ L. (T.)paniculatum and Odontacarus sp. positive only in Nakhon Ratchasima Province. Atotal of 5.1% (20) of chiggers screened from Chiang Mai were positive while 6.8%(30) of chiggers screened from Nakhon Ratchasima were positive.

The sou~'ces and habitats for the chiggers screened for R. tsutsugamushifrom Thailand during this period are shown in Table 3. Initially it would appearthat the primary foci for infected chiggers are primary and secondary evergreen

forests. However, only a few black plate collections were made from the otherthree basic habitats early regenerating evergreen, Pa Daeng (dry dipterocarp)forest and grass. Additional collections from all the habitats are neededbefore infection rates per habitat can be properly interpreted, however, four ofthe five habitats analyzed here contained infected chiggers. Probably most, ifnot all, of the basic habitats in Thailand contain chigger species haboring R.tsutsugamushi.

Several chigger species appear to be highly seasonal and found only duringthe cool season in Thailand. These species, HeZeniauZa sp., L. (L.) scuteZZ~,and adontacarus sp., have been collected in Thailand only between the months ofNovember to,February. Of these, only HeZenicuZa sp. was not found infected withR. tsutsugamushi. Leptotrombidium (L.) sauteZZare is a known vector of R.tsutsugamushi in other areas of Southeast Asia, but previous to this study, hasnot been found infected in Thailand.

Table 4 summarizes the chigger species captured in black plate collectionsin Thailand and screened for R. tsutsugamushi since this project was initiatedin June 1977. Included are chigger-rickettsial data from the Pak Chong studydescribed elsewhere in this Annual Report. At that time only Lo (L.) deZien5ehad been incriminated as a vector of Ro tsutsugam4shi in Thailand. One and halfyears later 8.2% (146) of 1,778 chiggers screened forlR. tsutsugamushi infectionshave been found positive. Furthermore, 9 of 15 species screened containedrickettsial infections, indicating a much larger number of potential vectors ofthis human pathogen in Thailand than previously suspected. The role of at leastseven of the nine infected species "found here in the zoonotic maintenance ofthis pathogen is currently unknown. Species of the subgenus Leptrombidium areusually considered of primary importance in the transmission of this pathogen toman (5) and five of the infected species found in this study belong in this sub-genus. However, infected species of 3 other genera, EutrombiauZa, MicrotrombicuZa

33

and Odontacarus, and one species in the subgenus f Leptotpombidium (TrombiauZin-dus~ were also found. Generally, the species found infected with R. tsut8~a-mushi during this study fall into Nadchatram's (6) ecological group I, i.e.,those species which effect man or have a better opportunity of coming in contactwith him" Ac..:ording to the current interpretation of Nadchatram's groupings,only Lept. (T.) paniauZatum and Miaro~ombiauZa ahamZongi possibly would notfall into group I. However, these 2 species were not included in Nadchatram'sstudy.

The large number of R. tsuts'ugamushi isolations made during this study wereo~ly possible because of the development of new techniques (4). Based on thea~tached data, the advantages of the MDFA technique in conjunction with the col-l~ction of unengorged chiggers on black plates are very obvious. Although thecurrent project is scheduled for termination, these data suggest an obvious needfor the continuation of similar studies.

A total of 7 newspecies of chiggers were identified during this study, ofwhich 3 were found infected with R, tsutsugamushi. Currently, the descriptionsand illustrations for~2 new species of the genus Leptotrombidium (Lept.J havebeen completed (including one new species found in this study) and those for 12additional species are being completed in preparation for publication. Theother 6 new species found in this study will be described and prepared for pub-lication in the future.

A checklist of the Ticks of Thailand, containing 54 species in 10 genera isn~aring completion. Of those, 23 have not been reported previously from ThailandA host list is also included.

The biological and taxonomic efforts of this study are continuing and addi-ttonal rickettsial isolation studies are anticipated.

REFERENCES:

1.. Andre,R.G., P.L. Tansku1, BoAo Harrison, A.L. Dohany and A. Shirai. 1978Ectoparasite and Rickettsia tsutsugamushi studies in Thailand. AFRIMS.Annual Progress Report, October 1977-September 1978, pp. 25-8.

2~ LakshaDa, P. 1973. A checklist of the trombicu1id mites of Thailand(prostigmata: Trombiculidae). U.S. Army Med. Comp.-SEATO~ Bangkok, Thailand,44p. .

3 ~. Lekagul, B. and J.A. McNeely.Wildlife, Bangkok. 758 p.

1977.

Mammals of Thailand.

Assoc.

Conser.

4..

Dohany,

A.L., Ao Shirai, D.M. Robinson, S. Ram and D.L. Huxsoll. 1978.Identification and antigenic typing of Rickettsia tsutsugamushi in naturallyinfected chiggers (Acarina: Trombiculidae) by direct immunofluorescence.Amo J. Trop. Med. Hyg. 27: 1261-4.

5~ Traub~ R. and C.L. Wisseman, Jr. 1974. The ecology of Chiggerbornerickettsj.osis (scrub typhus). J. Med. Entomo1. 11: 237-303.

34

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