Help Protect Your Pet from Heartworm Disease

You get the reminder card from your veterinarian that your dog, Otis, is due for his annual physical examination and heartworm test. You give Otis the recommended monthly heartworm preventative, more or less on time—sometimes you miss a month…or two. So, you make the appointment, Otis's blood is drawn to test for heartworms, and hopefully you receive the good news that no heartworms are detected. Does this scenario sound familiar? Are you protecting your pet from getting heartworms?

An estimated one million or more pets in the United States have heartworm disease, and the Southeast has the highest incidence. Mississippi has the dubious honor of having the highest percentage of heartworm prevalence in dogs. Almost 10% of dogs tested in our state have heartworms. We even beat Louisiana and Alabama! The percentage of dogs in Mississippi animal shelters that test positive for heartworms is even higher.

Heartworm infection is a serious, potentially fatal, widespread condition caused by a parasitic roundworm, Dirofilaria immitis. Heartworms are transmitted from animal to animal by mosquitos. Although heartworms are known to cause disease primarily in dogs, cats, and ferrets, they also infect wild animals. Even infection in humans has been reported, but it's very rare-so don't panic.

Heartworms can infect your pet if it has been bitten by a mosquito that carries immature heartworms, called larvae. Be assured that not every mosquito that bites your pet carries heartworm larvae. However, in instances where the they do, the tiny larvae continue to mature and increase in size as they travel throughout your pet's body. They eventually end up in the heart, lungs, and surrounding blood vessels. Adult heartworms can grow to over a foot in length within a few months after entering your pet's body.

These adult heartworms mate and produce offspring, called microfilariae, over their several-year life spans. The microfilariae circulate in your pet's bloodstream, and a mosquito can pick up the microfilariae when it feeds on an infected animal. With proper environmental conditions, the microfilariae develop into infective larvae within the mosquito and can be later passed along to another animal the mosquito bites.

Dogs are natural hosts for heartworms and serve as the major reservoir of infection. They can have several hundred adult worms and circulating microfilariae, although the average number of adults is usually far less. Cats are atypical hosts and usually have just one or two heartworms, if infected, and seldom have circulating microfilariae. In an otherwise healthy cat, the immune system detects heartworms and destroys them before they mature into adult worms. Heartworms can migrate to other parts of a cat's body, including the eyes, brain, and spinal cord.

How will your pet act if it has heartworms? This depends upon many factors, including your pet's age and overall health, and the number of worms present. Your pet may act completely normal and not show any, or very subtle, signs of illness, especially early in the disease. Some animals, with even large numbers of heartworms, don't show signs of infection. But, as time passes and the number of heartworms increases or the damage to your pet's heart and lung increases, your pet may show more pronounced signs of illness.

A dog with heartworms may cough, tire easily, show less interest in food, and lose weight. In severe cases, dogs may suffer heart failure and their abdomens may fill with fluid, giving them a pot-bellied appearance. Accumulations of heartworms can block blood vessels in the heart and lungs and cause difficulty breathing. This complication is often fatal unless the heartworm blockage is immediately removed.

Cats with heartworms may show signs of respiratory disease similar to asthma and will cough or wheeze. They may have less interest in food, vomit, or lose weight. In some cases, a cat may appear to faint, or have a seizure, or have difficulty walking. They can also collapse or die without having shown any previous signs of illness.

How will you know whether your pet has heartworms? Your dog can usually be diagnosed with a quick, simple blood test in your veterinarian's office. Since cats usually have fewer heartworms and microfilariae than dogs do, determining if your cat is infected can be a lot more challenging and often requires additional blood tests and chest x-rays and/or ultrasounds.

Treatment of heartworms involves several months of staged medications and strict exercise restriction. Heartworm infection in an otherwise healthy dog is often successfully treated. Generally, the fewer number of worms and the earlier the disease is diagnosed, the better the prognosis. However, the damage heartworms leave behind may be permanent.

Unfortunately, the medication that effectively clears adult heartworms in dogs cannot be used in cats. Instead, treatment is focused on helping prevent additional heartworm infection and providing supportive care.

The good news is that you can help prevent your dog, cat, or ferret from developing heartworm disease through regular, year-round administration of an FDA approved heartworm preventative. Options include chewable flavored tablets, topical solutions, and 6- or 12- month injections. Some heartworm preventatives products are conveniently combined with flea, tick, or intestinal parasite preventatives.

Heartworm preventative is available from your veterinarian by prescription. We recommend you start giving preventative at your puppy or kitten's first visit. Pets seven months of age and older need a heartworm test prior to starting preventative. A heartworm test is repeated annually in dogs. You will need to repeat the test at different intervals if your pet has missed doses, has just started preventative, or has switched types of preventative.

There is a risk of a harmful reaction if a heartworm positive pet receives heartworm preventative. This is one of the several reasons we test your pet for heartworms every year.

Heartworm preventative works very well to prevent heartworm infection so long as your pet receives it on a regular, year-round basis. It won't work well if you are late or skip even a single dose. Then, like Otis's owner, your pet is at risk of getting heartworms. If you use oral preventative, make certain your pet actually swallows it. We have known pets to spit it out, leave it in the food bowl, hide it behind the couch, or bury it in the backyard. Use topical preventative? Don't let your pet rub or groom it off or jump in the pool or pond before the medication has time to absorb and dry.

It is not uncommon for even the best-intentioned pet owners to forget to give heartworm preventative regularly. Tablets or topical solutions need to be given at monthly intervals—unless your cat uses the new topical preventative that lasts two months. Pick a date that is easy for you to remember. We find it easiest to remember to give our three dogs their preventative on the first day of the month. If you find it difficult to give monthly preventative, talk to your veterinarian about whether a heartworm preventative injection, which can be repeated every six or twelve months might be an option for your dog.

Think your pet doesn't need heartworm preventative because it spends all day sleeping in the house? Sorry, you're not off the hook. Your pet still needs preventative. Mosquitos come indoors, so your pet is still it at risk of being bitten by a mosquito carrying heartworms.

How else can you reduce the threat of your pets from getting heartworms? Reduce their contact with mosquitos. You can start by removing sources of stagnant water around your property. Unused birdbaths, plastic and metal tubs, old tires, empty flowerpots, clogged drain gutters, play pools, and areas with tall grass and weeds are popular spots for mosquitos to breed and hang out. Try to keep your pet indoors during the times of day when mosquitoes are most actively feeding. You can discourage mosquitos from biting your dog through use of an Epa-registered topical mosquito product labeled to repel and kill mosquitos in dogs. We are not aware of any available Epa-registered similar products approved for use in cats.

We would love to see a decrease in the number of animals diagnosed with heartworm infections. Your veterinarian can test your pet for heartworms and discuss which preventative or treatment options may be best suited.

We invite you to email us topics or general questions about animal care that you would like us to consider addressing in our monthly, “Ask the Vets” column. We are not able to privately respond to your questions and encourage you contact your regular local veterinarian if you have questions about your pet's medical care. Those of you interested in learning more about heartworms can also visit the American Heartworm Society website, www.heartwormsociety.org