Heartworms (Dirofilaria
immitis)
Heartworm (Dirofilaria immitis) is a parasitic roundworm
that is spread from host to host through the bites of mosquitoes. The heartworm
is a type of filaria, a small thread-like worm, that causes filariasis. The definitive
host is the dog, but it can also infect cats, wolves, coyotes, foxes and other
animals, such as ferrets, sea lions and even, under very rare circumstances,
humans. The parasite is commonly called "heartworm"; however, adults
often reside in the pulmonary arterial system (lung arteries) as well as the
heart, and a major effect on the health of the animal is a manifestation of
damage to the lung vessels and tissues. Occasionally, adult heartworms migrate
to the right heart and even the great veins in heavy infections. Heartworm
infection may result in serious disease for the host, with death typically as
the result of congestive heart failure.
Heartworms go throughout several life stages before they
become adults infecting the pulmonary artery of the host animal. The worms
require the mosquito as an intermediate stage to complete their life cycles.
The rate of development in the mosquito is temperature-dependent, requiring
about two weeks of temperature at or above 27°C (80°F). Below a threshold temperature
of 14°C (57°F), development cannot occur, and the cycle will be halted. As a
result, transmission is limited to warm months, and duration of the
transmission season varies geographically. The period between the initial
infection when the dog is bitten by a mosquito and the maturation of the worms
into adults living in the heart takes six to seven months in dogs and is known
as the "prepatent period".
After infection, the third-stage larval heartworms (L3)
deposited by the mosquito grow for a week or two and molt to the fourth larval
stage (L4) under the skin at the site of the mosquito bite. Then, they migrate
to the muscles of the chest and abdomen, and 45 to 60 days after infection,
molt to the fifth stage (L5, immature adult). Between 75 and 120 days after
infection, these immature heartworms then enter the bloodstream and are carried
through the heart to reside in the pulmonary artery. Over the next three to
four months, they increase greatly in size. The female adult worm is about 30
cm in length, and the male is about 23 cm, with a coiled tail. By seven months
after infection, the adult worms have mated and the females begin giving birth
to live young, called microfilariae.
The microfilariae circulate in the bloodstream for as long
as two years, waiting for the next stage in their life cycles in the gut of a
bloodsucking mosquito. When ingested by a mosquito, the microfilariae undergo a
series of molts to the infective third larval stage, and then migrate to the
salivary glands of the mosquito, where they wait to infect another host. The
incubation period required to reach the stage where the microfilariae become
transmittable to another host can be as little as two weeks or as long as six
weeks, depending on the warmth of the climate, and the larval life cycle ceases
entirely if the ambient temperature drops below 14°C (57°F).