Apr. 3, 2004 12:00 AM

QUESTION: My ferret died of heartworm disease. I thought this affected only dogs.

ANSWER: The heartworm Dirofilaria immitis can infect many families within the order Carnivora other than canines. Infections have been found in ferrets, cats, bears, seals and raccoons to name a few. Occasionally, heartworm has infected humans and horses. Whereas most domestic dogs can carry large number of adult worms without showing signs of heart disease, in ferrets even a single adult worm can be life threatening.

Heartworms depend on mosquitoes to spread from animal to animal. The larva of the heartworm is known as a microfilaria. If an infected dog has enough microfilariae circulating in its bloodstream, some may be sucked up by mosquitoes. Once in the mosquito, the microfilariae pass through a couple of larval stages before they can infect a dog. At this stage, known as L3, the heartworms live near the mosquito's mouthparts so that they can rapidly be passed into a dog when the opportunity arises. To feed, a mosquito saws through a dog's skin with its sharp mouthparts, stabbing about as it tries to find a rich source of blood such as a capillary. Once the mosquito strikes blood, it spits anti-coagulant containing saliva into the area so its blood meal doesn't coagulate and become too thick to be sucked up. During this period of backwash, the L3 heartworms get flushed into the dog. Once inside the dog, the larvae migrate through the fat and subcutaneous tissue and undergo more molts. Eventually, the mature heartworm sets up housekeeping in the dog's heart, usually in the right ventricle or adjacent pulmonary artery. Within about six months, adult heartworms are producing microfilariae of their own that float about the bloodstream waiting for a mosquito to suck them up and carry them to a new host.

Occasionally, the microfilariae get injected into the wrong host, something other than a dog. If the host is too different, the microfilariae die. In other hosts, there's enough similarity to dogs for the microfilariae to migrate and molt successfully, reaching adulthood and settling in to the right ventricle. Unfortunately, the small size of a ferret's heart compared to a dog's means that even one heartworm is enough to cause damage.

A ferret with heartworm disease may cough a lot and become sleepy and unwilling to play. If it does anything mildly strenuous, it gets out of breath and may collapse. About 50 percent of infected ferrets have microfilariae detectable in their blood, but usually an antigen test that detects adult female worms will be positive. However, because ferrets can be quite sick from a single worm, if that worm is male then the antigen test will also be negative.

Treatment is expensive and requires hospitalization. Ferrets in heartworm-risk areas (i.e., where mosquitoes live) should be on heartworm preventive, especially if they spend time outside.

Wright is director of conservation, science and sanctuary at the Phoenix Zoo. Write to him at DoctorKevin@thephxzoo.com or at 455 N. Galvin Parkway, Phoenix, AZ 85008.

 

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