Prevention beats treatment for heartworms

By Dr. Beth Leermakers

More than a million pets in the United States have heartworms. As their name indicates, heartworms are parasitic organisms (worms) that live in the chambers of the heart and adjacent blood vessels. As the worms move through the animal’s body, they damage many organs, including the heart, lungs, liver and kidneys. In severe cases, heartworms can lead to heart failure and death.

April is National Heartworm Awareness Month. The American Heartworm Society (AHS) urges pet parents to educate themselves about heartworm prevention to help reduce the number of cases seen in the U.S.

Test your knowledge about heartworms by answering these True/False statements:

1. Only dogs (not cats) can get heartworms. 

2. Heartworms can be 10 to 12 inches long, and a single dog can have 250 worms. 

3. Heartworms can live for two years. 

4. Heartworms reproduce inside the host animal. 

5. Your dog can catch heartworms by playing with another dog that has them.

6. There are no symptoms of heartworm disease. 

7. Treating heartworm disease costs the same as preventing it. 

8. You should start testing your dog for heartworms when she’s seven months old. 

9. You should give heartworm prevention pills year-round. 

10. Arsenic is used to treat heartworm disease. 

Answers:

1. False. Cats, ferrets and other mammals (e.g., foxes and coyotes) can get heartworms too. Heartworms do not typically reach maturity in cats, and affected cats usually have only two or three immature worms. These immature worms can still damage your cat’s heart and lungs, but the presence of heartworms can be harder to detect in cats.

2. True. The large worms, Dirofilaria immitis, take 6-7 months to complete their life cycle and become mature adults.

3. False. Heartworms can live for 5-7 years inside your dog’s heart and blood vessels. They can cause significant damage during that time.

4. True. At seven months of age, the adult female worms become sexually mature, mate and produce microfilariae [young, immature heartworms (larvae)]. 

5. False. Transmission of heartworms cannot occur from one dog to another. Heartworms are transmitted by mosquitos. When mosquitoes bite and take a blood meal from an infected host (the heartworm positive dog), they ingest circulating microfilariae. The microfilariae mature inside the mosquito (two weeks). 

When that same mosquito bites a heartworm negative dog, the larva is deposited on the dog’s skin and then migrates into his body. As it matures, the worm travels throughout the dog’s tissues and bloodstream, winding up in the heart. 

6. False. Early in the disease course, your pet may be asymptomatic, making heartworm disease tricky to diagnose. 

However, as the disease progresses, your dog may experience several symptoms, including persistent coughing, fatigue, reluctance to exercise and/or exercise intolerance (i.e., shortness of breath when walking or playing), collapse and sudden death. Some heartworm positive dogs may also show weight loss, difficulty breathing and even excessive panting. Left untreated, dogs may develop right-sided heart failure and abdominal bloating (caused by fluid buildup). As the heart becomes clogged with worms, there’s less blood it can push out to the rest of the body, so heart failure can occur. 

7. False. Heartworm treatment for a large dog usually costs $350-$1000 or more (depending on the vet clinic, treatment protocol, and diagnostic tests, which can include extensive blood work and X-rays). Heartworm prevention pills for a 50-to-100-pound dog cost about $10 per month. Heartworm treatment is very hard on dogs and requires 30 days of restricted activity after each injection. Left untreated, heartworm disease can be fatal. Prevention is definitely the way to go! 

8. True. All pets should undergo annual heartworm testing. Your vet will take a blood sample to conduct an antigen test. This test detects proteins produced by the parasite within your dog’s bloodstream. Puppies that are under seven months of age can be started on a heartworm preventive without needing a blood test. Dogs older than seven months should be tested before beginning any heartworm preventive regimen. That’s because heartworm tests detect antigens produced by female adult heartworms (that mature at seven months). 

9. True. Because the heartworm season is difficult to predict, AHS recommends year-round administration of heartworm preventives. 

10. True. Heartworm treatment involves the administration of two or three injections of the arsenic-based compound melarsomine. Some veterinarians administer two melarsomine injections (on two consecutive days).  

Keep in mind that mosquitos travel indoors, so even “inside” dogs need year-round heartworm prevention. If you’re likely to forget to give that essential monthly pill, ask your vet about an injectable heartworm preventive that is injected under your dog’s skin every six or 12 months by your vet.