The nematode Dirofilaria immitis, commonly called heartworm, is an important canine endoparasite, together with other nematodes like roundworms, hookworms and whipworms. It further is classified as a member of the order Spirurida and within that of the family Onchocercidae. The main representative of the genus Dirofilaria is D. immitis, but another closely related Dirofilaria species, D. repens, is also known to infect dogs (subcutaneous dirofilariosis).
Like all other members of this family of nematodes, the development of D. immitis requires both an arthropod and a mammalian host. First, adult female heartworms release their offspring, called microfilariae, into an animal's bloodstream. Then, mosquitoes become infected with microfilariae while taking a blood meal from the infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage, a third-stage larva (often abbreviated as L3) within the mosquito’s Malpighian tubules.
After that time, the infective larvae make their way into the head and mouthparts of the mosquito and then enter another host – e.g. another dog, cat or other susceptible animal – when the mosquito feeds. Seventy days after infection, migration and exsheathment to the adult stage inside the vertebrate host is complete. Approximately 6 months after infection, female adult worms contain mature microfilariae.
This generation of microfilariae cannot mature into adult heartworms without first passing through a mosquito. Patent infections, i.e. infections with detectable microfilariae in the blood, are observed typically 7 months post infection; when patency is measured using the presence of circulating antigens (glycoproteins made by a female worms), these moieties may be found in the blood as early as 5 months after infection.
The ubiquitous presence of one or more species of vector-competent mosquitoes makes transmission possible wherever a reservoir of infection and favourable climatic conditions coexist.
A pivotal prerequisite for heartworm transmission to occur is a climate that provides adequate temperature and humidity to support a viable mosquito population, and also sustain sufficient heat to allow maturation of ingested microfilariae to infective, third-stage larvae (L3) within this intermediate host. In mosquitoes under optimal temperatures above 26°C, infective larvae can develop in around 10 days to two weeks. When mosquitoes are chilled, development slows, but when these mosquitoes are again at the elevated temperatures, the larvae will continue in their development. Since development in the mosquito is temperature related, the peak months for heartworm transmission in the Northern Hemisphere are July and August.
In areas with low prevalence, a nidus of heartworm infection may be detected which usually represents both a focal spread of infection, and heightened awareness through increased testing. Once a reservoir of microfilaremic domestic and feral canids is established beyond the reach of veterinary care, eradication becomes much more difficult.
- Current Canine Guidelines for the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Dogs (revised January, 2012), Executive Board of the American Heartworm Society (AHS)