HEARTWORM—What do I need to know to protect my pet?


Right now, the biggest threat you’re probably facing with your dog is all this rain! But the same moisture that is turning your yard into a swamp leads to another threat … mosquitoes. And for pet owners, mosquitoes mean heartworm, which is one of the most dangerous parasites a dog can get.

All of the parasites your vet talks about at your pet’s annual preventive care exam can get downright confusing. It’s pretty easy to figure out where heartworms end up, but how do they get there and how can you prevent infection in your own pet? We’ve outlined our most frequently asked questions regarding heartworm and hope to clear the air on this deadly disease.

How does my dog get heartworms?

Simply put, a mosquito that sucks the blood of a heartworm-infected animal will swallow microscopic baby heartworms, called microfilariae. These microfilariae transform inside the mosquito so that when that same mosquito bites another animal about 2 weeks later- heartworm larvae are injected into that uninfected animal. Once injected, the larvae travel through the skin and body and undergo further changes for about six months. The juvenile heartworms end their journey as mature worms in the animal’s heart and lungs. Heartworms can live up to seven years and reach 12” long. These adult worms will continually produce microfilariae that will infect more mosquitoes and the cycle continues.

How do I know if my dog is infected with heartworms?

Most dogs with heartworm disease do not show any signs that they’re sick. What typically happens is that more and more heartworms clog the heart and lungs over time until these vital organs fail to work properly. At that point, the dog may cough because of inflamed lungs, or get tired or out of breath because their heart can’t effectively pump oxygenated blood throughout the body. It is possible for a dog to collapse and die suddenly without ever having shown symptoms, which is what makes heartworm so dangerous. A simple blood test can be used to determine if your dog has been infected.

If my dog has heartworms, is there treatment?

While heartworm disease is treatable the treatment is done over a period of months. In conjunction with oral antibiotics and steroids the dog will receive a series of arsenic-based injections into the muscles near the spine. It is painful, dangerous and expensive – around $1000 by the time it’s complete (that’s the equivalent of about 12 years worth of heartworm preventives!). Additionally, treatment cannot reverse the permanent damage heartworms do inside the body.

What can I do to prevent my dog from getting heartworm disease?

The American Heartworm Society recommends thatall dogs be on heartworm preventative 12 months of the year. Here’s why:

  • Predicting “heartworm season” in Illinois is difficult. In 2012 we had an unseasonably warm winter, and mosquitoes were active in February. Not surprisingly, we saw more heartworm positive dogs at South Town Animal Hospital that year.
  • Some mosquito species have adapted to the cooler conditions and can hibernate as adults in protected places over winter. They awaken as soon as it’s warm – this could be a sunny wall of your house, even if the outside air temperature is still cool.
  • Heartworm is so prevalent it infects over 30 species of animals including coyotes, foxes, and cats.
  • Most people do not realize that heartworm preventives work backwards. If you give your dog his last Heartgard on September 1st, it will work on microfilariae that infected him in August leaving him vulnerable in the fall and winter months.
  • If you travel with your dog over winter, you’ll pack their bowl, leash, bed … but what about their heartworm preventives? Mosquitoes are active all year in some states, and forgetting can be deadly to your dog.
  • Lastly, heartworm preventives also kill common intestinal worms (these are the ones in the poop!), which dogs can pick up at any time of the year. These worms can be transmitted to the human members of the family and cause disease.


Can my cat get heartworm disease?

Yes, cats are susceptible to heartworm disease, however it is difficult to diagnose. Currently there is no treatment for cats with heartworm, only supportive care to minimize the effects. Although the lifespan of heartworms is a bit different in cats as compared to dogs it is transmitted the same way; by mosquitoes. According to the American Heartworm Society, “a relatively high percentage of cats considered by their owners to be totally indoor pets also become infected.” If your cat has any exposure to the outdoors including sitting on the patio or going for short walks outside, then year round heartworm preventative is recommended.

How about some good news?

Thanks to responsible pet owners who give their dogs regular heartworm preventives, heartworm disease is far less common than it used to be. When South Town’s owner and veterinarian Dr. Crittenden first started practicing at a rural animal hospital over 20 years ago, her clients rarely used heartworm preventives for their pets and she would diagnose over 50 cases each year. These days, most suburban veterinary practices typically only see a few cases per year. But for those few, diagnosis and treatment is heart-wrenching.

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