Dirofilaria infections in animals and

humans

New insights in the prevention and treatment of

canine and feline dirofilarial infections

Claudio Genchi, Med Vet, PhD, EVPC Dipl

Dept of Veterinary Science and Public Health, Università

degli Studi di Milano

Bucharest, June 14th, 2012

Searching for an effective

chemotherapy and prevention

Until the first half of 900’s, no effective treatment:

• Essential turpentine oil

• Intravenous injections of 0.40% formalin

• Antimony salts: till 11 injections

G. Zanotti: Of the degree of receptiveness, diagnosis and, antimony therapy

of canine filariosis. Profilassi 16: 79-87, 1943

The use of antimony salts was promptly given up for the

severe side effects

In 1947 Otto and Maren suggested the use of

thiacetarsamide (TCA) against heartworm infection.

They were the first who experimented the efficacy of

arsenical salts in HW infected dogs as a model for

treatment of Wuchereria brancoftii human infection. At

that time, Wuchereria infection was quite frequent in

American soldiers fighting in the Pacific area.

Otto GF, Maren TH. Possible use of an arsenical compound in threatment of

heartworm in dogs. Vet Med 1947, 42: 128

Searching for an

effective chemotherapy

and prevention

TCA [Caparsolate]: histolesivity, difficultly to manage

(2.2 mg/kg bw iv, 12 hours apart for 2 consecutive days),

toxicity and discontinuity in efficacy limited an effective

control of canine heartworm disease, mostly for severe

infections.

When melarsomine dihydrocloride was put on the

market (1997), the production of tiacetarsamide was

stopped.

Searching for an effective

chemotherapy and

prevention

TCA vs Melarsomine [Immiticide®]

TCA:

high affinity for red cells [Dirofilaria worms are not aematophagous]; the efficacy varies with the age and the sex of worms.

high content of arsenic

histolesivity, hepato- and nephrotoxicity

Melarsomine dihydrocloride :

higher affinity for blood plasma

less content of arsenic, les hepato- and

nephrotoxicity

Max blood concentration within about 150 min, 2-3

higher than TCA

The idea that it would be possible to prevent HW

larval development to adult parasite preventing canine

HW disease was developed at the beginning of

1960’s.

Kume et al. (1962) confirmed the concept of HW

prevention through daily administration of

diethylcarbamazine (DEC).

In the 90’s, macrocyclic lactones prevention was

extended to both feline HW infection and to

subcutaneous filarial infections (Dirofilaria repens).

Searching for an effective chemotherapy

and prevention

… before the discovery of macrocyclic lactones

and of its efficacy against Dirofilaria larval stages:

2 options

1. Diethylcarbamazine (DEC) citrate: 1.25 mg DEC

base/4.5 kg bw/day.

To be effective, the treatment must begin shortly before

the start of the transmission season and continue daily

for 2 months after the end of the transmission season.

The omission of only 2 or 3 consecutive doses can

compromise efficacy and cause a lapse of protection.

Side effects: vomiting, diarrhea, male infertility, severe

anaphylactic reaction in microfilaremic dogs.

In 1980, John W. McCall presented the first results on

the preventive efficacy of the ivermectin and concluded

… in terms of efficacy, safety and infrequency of

dosing, ivermectin looks promising as a

chemoprofilactic agent against canine heartworm.

25 years after Kume’s paper (1962), the introduction on the market of ivermectin (1987), milbemycin oxime (1990) and, more recently, of selamectin and moxidectin, made prevention possible through monthly administration of a preventive drug.

From 2002, an injectable moxidectin formulation is on the veterinary market in some European countries. The drug is able to protect dogs against HW infection throughout the entire transmission season following a single injection.

Searching for effective

chemotherapy and prevention

Nowadays, several macrocyclic lactones products such

as ivermectin, milbemycin oxime, selamectin and

moxidectin are available in Europe for the prevention

and control of canine and feline Dirofilaria infections.

IVM and MBO formulations are; SLM formulation is for

topical treatment (spot-on); moxidectin formulations are

oral, topical (spot on) and injectable.

All are safe [including ivermectin-sensitive dogs], a full

efficacy is requested by regulatory authorities worldwide

and they have retroactive activity [ML safe net or reach-

back effect].

Do not forget: we say prevention but it is

a larvicidal treatment: MLs monthly or

injectable

Treatment is effective against L3 and L4 larvae

Treatment should begin within 1 month from the

beginning of transmission by infected

mosquitoes and the last dose should be given 1

month after transmission ceases

Dose of preventative drugs for

canine HW prophylaxis

Macrocyclic lactone Min-max dose

Ivermectin

Milbemycin oxime

Moxidectin (tablets)¹

Moxidectin/imidacloprid spot on

Moxidectin injectable³

Selamectin (spot-on)²

6 - 12 µg/kg bw

0.5 - 0.99 mg/kg bw

3 - 6 µg/kg bw

2.5 – 6.25 mg/kg bw

0.17 mg/kg bw

6 - 12 mg/kg bw

¹, ² the efficacy lasts 60 days; ³ full risk season efficacy

Dose of preventive drugs for the

feline HW prophylaxis

Macrocyclic lactone Minimum dose

Ivermectin

Milbemycin oxime

Moxidectin/imidacloprid spot on

Selamectin (pour-on)¹

24 - 71.7 µg/kg bw

2 - 4 mg/kg bw

1 - 2 mg/kg bw

6 - 12 mg/kg bw

1 the efficacy lasts 60 days

Efficacy

Preventative

Dog Cat

IVM MBO MOXI tablets MOXI pour on MOXI inject SLM

D. immitis

D. immitis, Toxocara, Toxascaris,

Ancylostoma, Trichuris D. immitis

D. immitis, D. repens, Toxocara, Toxascaris, Ancylostoma,

Trichuris, Sarcoptes, Otodectes, Demodex (control)

D. immitis, D. repens D. immitis, D. repens, Toxocara,

Toxascaris, Ancylostoma, Cyenocephalides, Sarcoptes,

Otodectes

D. immitis, Toxocara, Toxascaris,

Ancylostoma D. immitis, Toxocara, Toxascaris,

Ancylostoma,

D. immitis, D. repens, Toxocara, Toxascaris, Ancylostoma,

Otodectes

D. immitis, D. repens, Toxocara, Toxascaris, Ancylostoma,

Cyenocephalides, Otodectes

Parasites

age (mths)

No. of

treatmentsEfficacy

%

Vitality/motility of

recovered worms

IVM 6µg/kg/mth

MBO 0.5 mg /kg/mth

SLM 6 mg/top/mth

3

4 - 5

7

8

3 - 4

3

adulti

13

12-31

29

16

12-14

12

18

98

95-99

95

56

41-97

98.5

39

abnormal

abnormal

abnormal

abnormal

normal

abnormal

abnormal

from Guerrero, McCall e Genchi, 2002

Reach-back efficacy of macrocyclic lactones

HW adulticide treatment: melarsomine

dihydrocloride

• Two intramuscular injections of 2.5

mg / kg 24 hours apart (standard

regimen, not advised)

► Strongly advised

two step treatment by

giving one injection and

then administering the

standard pair of injections

at least 60 days later

One administration of melarsomine at the dose of 2.5 mg/kg kills about 90% of male worms and 10% of female worms resulting therefore in 50 % reduction of the worm burden (which is safer in terms of embolism and shock).

A 10% death rate can be

expected in dogs with High

risk of thromboembolic

complications when 2

doses are given initially.

Given strictly intramuscularly, deep in the lumbar

(back) muscle. Do not administer at any other site. No

more than 2 ml in the same site. Alternate sides. Use

thin needles (21-22 gauge). Change the needle for the

injection (to avoid subcutaneous tissues contamination)

Pulmonary thromboembolism is an inevitable

consequence of a successful adulticide therapy. If

several worms die widespread pulmonary thrombosis

frequently develops. Mild thromboembolism may be

clinically unapparent, but in severe cases life

threatening respiratory distress can occur.

Watch for:

Coughing, Dyspnoea

Fever

Hemoptysis, Lipothymias

The most critical time is 7-10 days following a melarsomine

treatment but pulmonary thromboembolism can occur

anytime in the following month

ANCILLARY THERAPY

• REST

• Calcium heparine

• Prednisolone

• Aspirine

REST• After treatment, the patient

must be strictly confined for

one month following the

final treatment.

• No walks, no running

around.

• The dog must live the

indoors (or in cage in

selected cases). The

reason for this is that it is

the most important thing to

minimize embolism-related

problems.

Calcium heparin

50-100 UI s.c. 3 times daily starting 1-2

weeks prior to and continuing for 4-6 weeks

after adulticidal treatment

NO

Prednisolone

0.5 mg/kg every other day from 5 th to

10 th day post adulticide treatment

Even though not recommended, IVM administered orally

at the prophylactic dose of 6 µg/kg monthly throughout the

year for a period of at least 2-2.5 years has been shown to

kill adult parasites.

To note that throughout this period the infection would

persist and pathology continues to worsen. Furthermore

the long-term use of a macrocyclic lactone in heartworm

positive dogs is the potential for selection of resistant sub-

populations of heartworms.

This regimen should be restricted to selected cases,

excluding active dogs, working dogs and heavily infected

dogs. X-ray examination should be performed every 4-5

months throughout the treatment period to monitor the

pulmonary patterns.

What about the use of MLs as an adulticide

drug?

Recently, it has been shown that a combination of

ivermectin at 6 µg/kg given every 15 days for 180 days

and doxycycline at 10 mg/kg once daily for 30 days is

well tolerated, has good adulticide efficacy and reduces

the risk of thromboembolism. Exercise should be rigidly

restricted for the duration of the treatment process. An

antigen test should be performed every 6 months and the

combination treatment continued until two consecutive

negative heartworm antigen tests have been obtained.

Anecdotal reports on other macrocyclic lactones with

adulticidal properties suggest similar results but no

confirmatory studies have been published.

IVM plus Doxy

Surgical heartworm removal via jugular

vein in the dog

Why, when and wherefore…

Surgical intervention is advised when several worms

have been displaced into the right cardiac chambers

producing sudden onset of caval syndrome.

It can be accomplished under general anesthesia with

flexible alligator forceps introduced via the jugular vein

aided by fluoroscopic guidance which gives access

not only to the right cardiac chambers but also to the

major pulmonary arteries.

Whenever …

Caval Syndrome

(compulsory!)

Dogs harboring large

worm burden at

ECHO examination

(advised)

Flexible Alligator Forceps® Fujinon

SURGICAL TOOLS

flexible Alligator Forceps

fluoroscopy

basic suturing tools

2 haemostatic clips

Anaestetic protocol

Ketamin hydrocloride 5 mg/kg i.v.

Diazepam 0,25 mg/kg i.v.

Oxigen (mask)

technique

main pulmonary artery

Surgical technique

Surgical technique

Surgical technique

Surgical technique

EFFICACY

negativization

of Ag test four months after surgery

Caval Syndrome 78%

Chronic Cases 65 %

Average of removed heartworms 90%

In cats, adultice therapy with melarsomine is not advised.

If a cat does not display overt clinical signs despite

radiographic evidence of pulmonary vascular/interstitial

lung disease consistent with HARD, it may be advised to

allow time for self-cure. Subclinical cases can be

monitored periodically (6- to 12-month intervals) by

antibody and antigen re-testing and thoracic xR.

In those cats destined to recover, regression of xR signs

and especially seroconversion of a positive antigen test to

negative status provide evidence that the period of risk

probably has passed.

Adulticide treatment of feline HW infection

Prednisone in diminishing doses is often an effective

medical support for infected cats with radiographic

evidence of lung disease, whether or not they are

symptomatic. Also, this should be initiated whenever

antibody-and/or antigen-positive cats display clinical signs.

An empirical oral regimen is 2 mg/kg body weight/day,

declining gradually to 0.5 mg/kg every other day by 2

weeks and then discontinued after an additional 2 weeks.

At that time, the effects of treatment should be

reassessed based on the clinical response and/or thoracic

radiography.

This treatment may be repeated in cats with recurrent

clinical signs

Adulticide treatment of feline HW infection

Compound

Formulation

Dose

Safety

Efficacy

Melarsomine dihydrochloridre

Injectable

deep in the muscles

2.5 mg 24 hrs apart

Toxicity,

possible side effects if the

compounds is injected in connective

tissues

NO efficacy

D. repens adulticide

treatment, is it possible?

Adult worms can be removed using a 19 Gauge

needle, connected to a vacuum syringe

Compound

Formulation

Dose

Safety

Ivermectin Selamectin Moxydectin

plus Doxycycline

tablets/ chawable

topical topical

Per os

6 mcg/kg 12-24 mths

60 mg/kg 12-24 mths 1 mg/kg 6-12 mths

10 mg/kg daily 6-8 weeks/intervals

2-3 weeks

Good

Possible idiosyncratic intolerance in single

subjects

Because D. repens severe clinical

signs can be as a consequence of

Mfs sensitization

Compound

Formulation

Dose

Reference

Selamectin Ivermectin Moxidectin

Topical

Tablet per os

Chewable

Injectable SR

6 mg/kg bw

6 mcg/kg bw

0.17 mcg/kg bw

Genchi et al., 2002 Fok et al., 2009 Marconcini et al., 1986 Pollono et al., 1998 Rossi et al., 2004

Dirofilaria repens prevention in dogs

MLs [larvicidal treatment]: field data from owned dogs

observed comparing the treatment season with previous

data of prevalence

Formulation

Dose

Efficacy

Reference

IVM MOX

Tablet per os

Injectable SR Topical

6 mcg/kg bw 12 mcg/kg bw

0.17 mcg/kg bw 2.5 mg/kg bw

87% 93%

100% 100%

Cancrini et al,1989

Genchi et al, 2010 Genchi et al, 2012

Dirofilaria repens prevention in dogs

MLs [larvicidal treatment]: experimental data

MOXI is more lipophilic than IVM and it is stored in the

fat, which may act as a drug reservoir. Compared to IVM,

MOXI has a higher distribution volume and a longer half-

life elimination. This may facilitate its distribution from the

blood stream to different tissues and longer residence

time for the drug in the body.

D. repens larval stages and the adult worms are

permanent residents in subcutaneous tissues, which are

rich of connective and fat tissues and the high lipophilic

nature of MOXI are probably the reason of the full

efficacy.

D. repens prevention: MOXI vs IVM

Canine and feline HW patent infection can be safely and

effectively prevent by the use of macrocyclic lactones, bot

throughout a monthly administration in the risk season or

throughout moxidectin sustained release formulation

injection.

From WAAVP, FDA and EMEA guidelines of veterinary

medicinal the efficacy of such a treatments must be

100%

Background

What is new about HW?

Canine HW infection: 2005

Most failures have been reported in HW-endemic states ... it is

unclear whether these are representative of the rare occurrences

of failure that have been in existence for a long time, but not

reported regularly, or whether there is a true increase

ineffectiveness … Results suggest that more comprehensive

reporting will provide FDA/CVM more accurate surveillance

information regarding efficacy. It will permit to better interpret both

incidence and severity of ineffectiveness and possible emerging

resistance and to convey this in any necessary updated labelling.

It also indicates that practitioners should return to a more

conservative testing schedule.

2011

A controlled lab study was conducted to evaluate the efficacy of 4

commercial products. On Day −30, dogs were infected with 100 D.

immitis L3. On Day 0, each dog group was treated with

ivermectin/pyrantel (IVM 6.6 mcg/kg) or milbemycin oxime (500 mcg/Kg)

orally, or selamectin (60 mg/kg) solution or moxidectin/imidacloprid

topically (MOX 100mcg/kg). Group 5 dogs remained nontreated.

At necropsy (Day 120), all 8 dogs in the no treated group were infected

with adult D. immitis (34–70 worms/dog). One or more adult D. immitis

and/or worm fragments were recovered from dogs in each treatment

group (efficacy 95.6, 95.4 and 95.5% efficacy). No worms were

recovered from any of the 8 dogs in Group 4 resulting in 100% efficacy.

Results: Thirteen of 14 control dogs had adult HW detected at necropsy

with a geometric mean worm count of 22.3. One HW was found in 1 dog

in each of the MBO and IVM treatment groups.

Conclusions and Clinical Importance: Two currently approved

macrocyclic lactone HW preventives used at their labelled dose rates

were < 100% effective against a recent HW field isolate, supporting the

hypothesis that the effectiveness of a single dose of these preventives

can vary. This is important in guiding clients on expectations of product

effectiveness.

Experimental studies with a prouved lab resistant strain

The combination product of spinosad (anti-flea) and MBO

provides effective control of canine heartworms. A single

treatment at 30 days post infection showed high but incomplete

effectiveness against a HWs isolate that had been shown to be

partially refractory to treatment with marketed monthly

heartworm preventives. Three consecutive monthly treatments

provided complete control, providing support to the

recommendation that heartworm prophylaxis should be

maintained year round for optimal effectiveness.

Experimental studies with a prouved lab resistant strain

Resistant field strain

Knott mf always

positive

IVM 50-200 µg

MB 0.75-2 mg x

8 gg

D. immitis has exhibited a lower than expected heterozygosis and

worm populations were found to be relatively homogeneous. For

instance, the Japanese field samples were found to share several

common combined genotypes with US field samples. The

situation differs when laboratory isolates are compared with field

isolates from the same country or with field strains from other

countries (i.e. from US lab and US field and US lab and

Japanese field).

To note that the genetic polymorphism of D. immitis populations is

a consequence of many factors both of the mammalian host and

the invertebrate intermediate hosts (mosquito); however, all

previous data were obtained from dogs in areas that have been

intensively exposed to macrocyclic lactone preventatives for over

20 years (US, Japan and Italy). Nevertheless, the different field

isolates studied by did show identical gene sequences.

Towards resistance in Europe?

HW: last recommendations

Before the seasonal HW preventive treatment:

- Knott microfilariae, Ag test kit

Before and after adulticide HW treatment:

- Knott microfilariae, Ag test kit to assess the efficacy

The presence of resistant microfilariae must be carefully

checked to avoid the infection of mosquitoes and the

spreading of resistance

Although Dirofilaria does not appear entirely

dependent on its bacterial symbiont Wolbachia, which

can be killed by a prolonged antibiotic treatment, the

clearing of bacteria from circulating microfilarie seems

to prevent infective larvae which develop in mosquitoes

from continuing their development in dogs.

The combination of heartworm preventatives with

products designed to prevent mosquito blood-feeding

activity (repellents) during the heartworm transmission

season could be useful in protecting dogs from

infection and from ectoparasite infestations that often

occur in the same season.

HW: last recommendations

Dirofilaria repens: last recommendations

Before seasonal D. repens preventive treatment:

- Knott microfilariae

[No Ag test kit for filarial subcutaneous infections]

Microfilaraemic dogs must be treated against mfs with

macrocyclic lactones at the same dose to prevent patent

D. immitis infection for 3-12 moths [Knott examination until

negative results] to control the spreading of the infection

and preserve human health.

D. repens human infection is an emergent zoonosis in

Europe and human cases are increasing.

Mf clerance from blood stream

Ivermectin 6 mcg/kg Milbemicine ossime 0.5 mg/kg Selamectin 6 mg/kg Moxidectin 2.5 mg/kg

Low efficacy (4-6 mth adm), mild reaction as a consequence of the death of some mfs Very effective, hypersensibility reactions in dogs with high microfilaraemia Low effecacy, no side effect after treatment in microfilaraemic dogs Efficacy after 4-6 moth administrations, no side effects

Microfilaria treatment [mf clearance]

Before the prophylactic treatment with MBO products, dogs must be Knott

examined for circulating Mfs

Interceptor, Sentinel, [MBO + lufenuron], Trifexis [MBO + spinosad]

Starting and ending HW temperature

suitability in some European localities

2008

6-7 mth

treatment

May-Oct/Nov