ABC-Animal - Parasitology Chronicle
Heartworm Disease
Definition
Heartworm disease or dirofilariosis is caused by a parasite transmitted by a mosquito from one animal to another. Dirofilariosis is serious and can cause an animal’s death.
What is a Heartworm
The heartworm is a roundworm from the Dirofilaria genus. The adult worm, also called filaira, resembles a very fine strand of whitish-coloured spaghetti measuring up to 30 centimetres in length. It lodges mostly in the pulmonary arteries, the right heart and the vena cava. A severely infected dog could harbour up to 500 adult worms! The female gives birth to miniscule babies, called larvae (stage L1) or microfilariae, which circulate freely in the blood. The Dirofilaria genus is made up of approximately 27 species. In North America, the following species are found: Dirofilaria immitis, D. subdermata, D. tenuis, D. striata and D. ursi. To this group are added the Dipetalonema and Brugia genera, which are responsible for the same types of problems.
How is the Disease Transmitted
The mosquito spreads the parasite from one animal to another.
Let’s take the example of 2 dogs, Buffy, a dog infected by heartworm disease, and Kari, a healthy dog.
The female adult heartworm sends its microfilariae into Buffy’s blood circulation. The mosquito bites Buffy and ingests her blood along with the microfilariae. For the next 2 to 4 weeks, with an outdoor temperature of 18ºC to 27ºC, this mosquito becomes in intermediate host, meaning that it allows the immature worms (L1) to transform into infectious larvae (L3).
After this incubation period, this contaminated mosquito bites Kari, leaving behind a drop of saliva containing a few infectious larvae which penetrate the skin through the wound. Once in Kari’s tissues, the larvae continue their development (L4), migrate to the heart where they reach the adult stage, reproducing and releasing microfilariae into the blood. This cycle takes 190 days after the initial infection.
Kari is now infected and is a reservoir of contamination for the entire neighbourhood!
Host
Definitive Host
* The dog is the usual host and all other races are susceptible to becoming infected.
* Approximately 21 other species of domestic and wild Canidae,
such as the fox, coyote and wolf, become infected and can play the role of reservoir.
These wild animals are important since they travel long distances,
compared to the mosquito which travels short distances.
Intermediate Host
* Mosquitos from the Culex, Aedes and Anopheles genera.
* The flea cannot be a vector.
Abnormal Host
An abnormal animal host can be infected but probably not play any significant role in the transmission of the disease. These are species such as
* Felidae, such as the domestic cat and a dozen other species,
* seals and otters,
* wild animals such as the beaver, the rabbit, the mink, the raccoon, the bear,
* Equidae, such as the horse,
* Monkeys (several species),
* And the human. For this reason, heartworm disease is considered a zoonosis, a disease that can be transmitted to people.
Geographical Distribution
On a world scale, the parasite lives in regions with tropical and subtropical climates, mainly in coastal and fluvial zones. The enzootic zones or problem areas where the parasites cause numerous cases of infections are located in
* North and South America (particularly coastal and fluvial zones)
* Asia * Australia
* Southern Europe * North Africa
In Canada, the parasite is found in the southern belt of the provinces of
* Québec * Manitoba
* Ontario * and the Okanagan Valley in British Columbia
In Québec, using meteorological data from the past 30 years, it has been concluded that the transmission of the parasite is mostly possible in July and August (from July 2 to September 8). In 1984, an epidemic of dirofilariosis was declared. In 2007, the enzootic zone has become
* the entire southern part of the province,
* the northern boundary from the east to the north of Québec City.
* But close to half of the cases are reported north of Laval Island between L'Assomption and Lachute.
The Animal's Symptoms or Signs of the Disease
* Chronic cough * Heart failure
* Difficulty breathing or dyspnea * Passive or apathetic
* Fatigue after effort * Kidneys not functioning as well
* Collapse after effort * Death
* Weight loss despite good appetite
Heartworms obstruct blood circulation, damage the heart, lungs, liver and kidneys. Unfortunately, when signs of the disease become apparent, several organs have already suffered irreversible damage.
Diagnosis
Screening for the disease is done with a blood test to look for the presence of microfilariae or adults, but no test is 100% reliable (Difil, modified Knott test, blood smear, blood test).
Treatment of the Disease
Yes, it is possible to treat a dog with heartworms, but there are risks. The animal must be kept calm, with a minimum of exercise since the medication kills adult worms, possibly causing further damage to already weakened organs.
Prevention? The Ideal!
Test your pet as recommended by your vet. A blood test at the beginning of the season ensures that there are no microfilariae (negative test) and then the dog will immediately be placed on a prevention program with a medication. If a mosquito bites the animal and transmits microfilariae, the medication will stop the infectious larvae from developing, thus interrupting the heartworm’s lifecycle.
http://www.medvet.umontreal.ca/portail/specialistes/consulterReaction.asp?ID=17
When to Treat
It depends on what part of the world your pet is living in! In Québec, we recommend treating your dog continuously in the spring, summer, and fall, or from June to November. The important thing is to begin treatment one month after the mosquitoes arrive and to terminate it one week after mosquito season ends.
Below is a summary of the main medications available to treat heartworm disease or Dirofilaria immitis.
|
Medication
Commecial Name
|
Company
|
Medication
Generic Name
|
Parasite
|
|
Species
|
Minimum age for Treatment
|
| Filiricid / Adulticid |
|
|
|
|
|
|
|
Advantage-Multi
|
Bayer
|
Imidacloprid
+ Moxidectin
|
Dirofilaria immitis, Ancylostoma spp, U. stenocephala, T. canis, T. cati, Toxascaris leonina, T. vulpis, Flea (Ctenocephalides felis), Ear Mites (Otodectes cynotis), Sarcoptic Mange (Sarcoptes scabei), Demodex canis
|
Topical
Monthly
|
Ca
Fe
|
Ca = 7 wks
Fe = 8 wks
|
Filaribits
Filaribits Plus
|
|
Diéthylcarbamazine
Diéthylcarbamazine
+ Oxibendazole
|
D. immitis, Ascaridés
D. immitis, A. caninum
T. canis, T. vulpis
|
|
Ca
|
8 wks
|
|
|
|
Ivermectin
Ivermectin
+ Pyrantel
|
D. immitis
D. immitis, T. canis, T. leonine, A. caninum, U. stenocephala
|
Cube to chew
Monthly
|
Ca
Fe
|
6 wks
|
|
Immiticide
|
Merial
|
Mélarsonamine
|
D. immitis
|
Injection
|
Ca
|
ND
|
|
Interceptor
|
Novartis
|
Milbemycine
|
D. immitis, A. caninum, T. canis, T. leonina, T. vulpis
|
Tablet
Monthly
|
Ca
Fe
|
Ca = 4 wks
si + 0,9 kg
Fe = 6 wks
si + 0,68 kg
|
|
Milbemax
|
|
Milbémycine
+ Praziquantel
|
D. immitis, A. tubaeforme, D. caninum, Taenia spp, T. taeniaeformis, E. multilocularis,, T. vulpis, C. vulpis, A. vasorum, Echinococcus spp, Mesocestoides spp, T. cati,
|
|
|
|
|
|
|
|
D. immitis, Sarcoptes, Otodectes, Capillaria aerophila, Oslerus osleri, Pneumonyssus, Aelurostrongylus, Strongyloides, T. canis, T. cati, T. vulpis, A. caninum, U. stenocephala, Cheyletiella, Demodex
|
Injection
Oral
Injection
|
|
|
|
|
Fort Dodge
|
Moxidectin
|
D. immitis, A. caninum, T. canis*, T. leonina*, U. stenocephala*
|
Injection
Seasonal
FDA recall
|
|
|
|
Revolution
|
Pfizer
|
Sélamectin
|
D. immitis, Flea, Ticks, Ear Mites, Sarcoptic Mange, T. canis, T. cati, A. tubaeforme, Dermacentor sp
|
|
Ca
Fe
|
|
|
Sentinel
|
Novartis
|
Milbemycine
+ Lufenuron
|
D. immitis, A. caninum, T. canis, T. leonina, T. vulpis, Flea
|
Tablet
Monthly
|
Ca
|
Ca = 2 wks
si + de 1 kg
|
|
Mosquitoes Repulsive
|
|
|
|
|
|
|
|
Active 3
|
|
|
Fleas, ticks, mosquitoes, flies
|
|
|
|
|
|
|
DEET(Diethyl-3-
methylbenzamide
Diethyl-m-
toluamide)
|
Mosquitoes, flies, fleas, ticks and trombiculidae
|
|
UD
|
|
|
Defend,
Zodiac
|
|
|
|
|
|
|
Key
* = not approved, Ca = Canine or domestic dog, Fe = Feline or domestic cat,
wks = weeks, UD = Undetermined, + 1 kg = Weight superior to 1 kilogram
In Conclusion
The best treatment is prevention!
Useful links
American Heartworm Society www.heartwormsociety.org
CAPC or Companion Animal Parasite Council www.capcvet.org
Caroline Fortin, D.V.M.
Veterinarian
www.ABC-Animal.com
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