Pathogenic species in dogs include:
- Dirofilaria immitis - heartworm disease
- Dirofilaria repens - subcutaneous panniculitis
- Dirofilaria hongkongensis
Transmission of D. immitis by mosquitoes is strictly dependent on a suitable climate allowing the larval development in the vectors, thus environmental temperatures are the main factors favoring the life cycle of this parasite. Species of mosquitoes include Culex spp, Aedes spp, Ochlerotatus spp, Anopheles spp and Mansonia spp.
Infective larvae (L3) are transmitted during feeding by mosquitoes and interstitial migration results in larval growth and migration into the pulmonary vein. Eventual maturity of L5 and adult Dirofilaria results in stable populations residing in the pulmonary arterial branches, right ventricle and atrium, and on rare occasions, the vena cava. Ectopic migration is relatively common, with reports of D. immitis found in the brain, abdominal cavity and spine. Sex disequilibrium exists in D. immitis at lower worm burdens in dogs, with female worms predominating. However, as numbers of adults increase, this disequilibrium disappear.
Maturation of the dirofilaria larva depends upon a number of factors including host immunity to both the filariid and Wolbachia spp endosymbionts. Worm burdens in dogs range from 1 to 250 worms, with worm numbers directly correlated with cardiopathy. Continued growth and death of worms leads to shedding of cuticular proteins which cause a significant percentage of inflammatory disease within the pulmonary tree and concurrent clinical signs.
The life span of the worms in dogs appears to be about 5 to 7 years. The average prepatent period (the time elapsed from when the larvae enter the host until the adult female worms begin to produce microfilariae) in dogs is about 6 - 7 months.
Microfilaremia is relatively common in dogs but occult infections occur as a result of single sex infections, host immune responses affecting the presence of circulating microfilariae and the administration of heartworm preventives.
Although D. immitis is the most common parasite of this group and is associated with heartworm disease, D. repens causes dermal swelling and subcutaneous nodules in dogs in tropical and subtropical countries.
A differential diagnosis is isolated cases of coughing must include other respiratory parasites such as Filaroides osleri, Aelurostrongylus spp, Crenosoma vulpis, Angiostrongylus vasorum, Paragonimus kellicotti and Eucoleus aerophilus.
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