Ferret Friendly Facts and Advice by Erika Matulich, Ph.D.

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Heartworm in Ferrets

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Have a Heart: Protect Your Ferrets from Heartworm

© Erika Matulich, Ph.D.

Just one little mosquito bite: That's what infected my ferret Tigger with the single heartworm that took her life at the tender age of 3.

The survival rate of ferrets infected with heartworm is poor; prevention is the key to saving your ferret's life. And prevention is very easy! If you live in an area where even one mosquito could enter your home, your ferret should be on heartworm medication. In some parts of the world, this means monthly medication during the mosquito, or wet, seasons. But where I live, in Florida, a year-round medication regimen is a necessity.

The heartworm cycle
Heartworms are a type of roundworm parasite called Dirofilaria immitis. This parasite is common in all sorts of species, including dogs, cats, ferrets, raccoons, skunks, otters, horses, pigs, and even people. Of all these critters, only the canines (either wild, like wolves and coyotes, or domestic, like our pet dogs) are “definitive” hosts, or hosts where the heartworm can reproduce. All other animals are “dead-end” hosts, meaning the heartworm can infect them and live in them, but can't breed.

When a mosquito bites a canine infected with heartworms, it sucks up the microscopic heartworm larvae. These larvae continue to develop in the mosquito, and the mosquito deposits the parasite into its next victims. Mosquitoes can travel several miles from their hatch site, biting and infecting many animals as they go.

Keeping your ferrets indoors can somewhat reduce the risk of heartworm infection, but it can't eliminate the risk, because mosquitoes do get past those screens! In fact, nearly half the cats and almost all the ferrets infected with heartworms are strictly indoor pets, as Tigger was.

Once the heartworm larvae enter the new victim, they travel through the blood vessels, taking between three and six months to reach the heart. Once in the heart, the parasites lodge in the right ventricle and begin growing into adult worms that can reach a length of 12 inches. The worms can also invade the lungs and block blood supply to the liver. Because of the small size of the ferret's heart and lungs, even a single heartworm is enough to kill a ferret.

Symptoms, diagnosis, treatment
A ferret may show several different signs of heartworm, depending on how many worms there are, how big they are, and where they are growing. The most common “symptom” for a ferret is sudden death from heart failure, and this is what happened to Tigger.

If your ferret does show signs of infection, these may include reduced activity and tiring easily during exercise; coughing; rapid breathing; fainting; development of a pear-shaped stomach (because of fluid buildup); and a bluish color to the tongue, gums, and lips. Unfortunately, some of these signs resemble the symptoms of other ferret diseases (such as insulinoma, lymphoma, and heart disease). But if you have any suspicion of heartworm, don't delay in seeking professional treatment; a wait-and-see attitude could kill your ferret.

To check for heartworms, your veterinarian should use an “occult heartworm” test. Because these tests measure the amount of a specific protein found on the heartworm's skin, they can detect the presence of hidden infections.

For dogs, vets commonly take a blood sample and look for heartworm microfilariae (larvae) under the microscope, but in ferrets these are not present (or are not present in large enough numbers to be detected). Deborah Kemmerer, D.V.M., a noted ferret heartworm specialist, recommends the Snap® Heartworm PF Antigen Test (made by IDEXX) as the most dependably accurate heartworm test for ferrets. A set of X-rays and an echocardiogram can also help support a blood-test diagnosis by showing any changes in the heart and lungs.

There are a number of treatments for animals infected with heartworm, but not all work well for ferrets. For example, in larger animals, adult worms can be removed surgically or killed with the drug Immiticide. But these procedures have not shown a high success rate in ferrets.

Kemmerer recommends the drug Caparsolate. This drug must be administered intravenously by a vet in a series of six injections (two shots 12 hours apart, wait four weeks, and then four shots 12 hours apart). Ideally, your ferret should be boarded with the veterinarian during this time, because extra activity can cause a fatal embolism (blood-vessel blockage) from the dying adult worms. Ferrets who undergo this treatment show about a 60 percent survival rate.

It's much easier (and less expensive) to prevent heartworm than to treat it after the fact. A monthly oral heartworm preventive is the way to go. One option is to use the monthly Heartgard pills for cats. Get the product for cats weighing up to 5 pounds, and cut the pill in half. (Most ferrets weigh 2 to 3 pounds, so half the cat dose is plenty.) Then crush the half pill into a powder and mix with Ferretone or Nutrical. There are also chewable tablets, which some ferrets will eat like treats. Again, get the product for cats up to 5 pounds; ferrets can take the whole cat dose for chewable tablets.

My favorite monthly heartworm medication is a liquid made up by my vet of 0.3 cubic centimeters of 1 percent, injectable ivermectin mixed with an ounce of propylene glycol. This solution is inexpensive (ideal for multiferret households!) and has a long shelf life. Each ferret gets a dose equal to 0.1 cubic centimeter per pound of body weight. On the first day of each month, we set up the postal scale, weigh each ferret (and record weights on a chart), and use a little oral syringe to squirt medicine in each mouth. The medicine apparently doesn't taste that great, so we follow up with a liberal dose of Linatone as a treat.

So what happened to Tigger? Although she had been receiving heartworm medication while I owned her, she apparently had gotten her single worm from a mosquito bite before I rescued and adopted her. The heartworm preventive will stop any new adults from attaching to the heart and growing, but it won't necessarily kill existing adult worms. After Tigger's surprising demise, I was quick to test all my other ferrets for heartworm. Fortunately, everyone else was fine. And with the easy monthly prevention regimen, they'll stay heartworm-free!