Diseases, Internal Parasites and Deworming

Heartworm in Dogs and Cats

Heartworm is a potentially devastating disease that can occur in both dogs and cats. Clinical signs may not appear until it’s in advanced stages, which means screening and prevention is vital for protecting your pets from heartworm.

How Does a Dog Get Heartworm?

Heartworm is caused by the parasite Dirofilaria immitus, and it’s carried from host to host by infected mosquitoes. Heartworms cannot be transmitted through direct contact. When an infected mosquito bites your pet and injects the heartworm larvae, the larvae travel through the bloodstream, develop into adult worms and become lodged in the heart and blood vessels. As they grow into adults, the larvae cause inflammation and impede blood flow, causing severe damage to the heart and other organs. Left undiagnosed and untreated, this can lead to death.

Heartworm Symptoms in Dogs and Cats

Even though adult heartworms can grow to be six to 14″ long, the symptoms aren’t noticed until months to years after the initial infection. Dogs may have a soft cough, difficulty breathing, weakness, fainting or weight loss. Cats with heartworm may also experience vomiting or gagging. If not treated, the animal can die of heart, liver or kidney failure.

Heartworm Treatment in Dogs and Cats

Treatment for heartworm in dogs and cats includes eliminating all forms of the parasite—adult, larvae and microfilaria (prelarval stage). Multiple injections are often used to kill the adult heartworms, antibiotics to make the parasite more vulnerable to treatment kill, and steroids to reduce inflammatory changes as the parasites die. Prior to treatment to kill the adult heartworms, a heartworm preventive is often used to kill the microfilaria.

The animal must be closely monitored with restricted activity for six weeks after treatment while the dying heartworms are removed from the bloodstream. Restricted activity is to prevent the dying worms from breaking off in pieces that could plug the arteries of the lungs. Heartworm treatment is tricky depending on the level of infestation. Currently, there is no approved treatment for heartworm in cats, so only the symptoms can be treated.

What Can I Give My Dog to Prevent Heartworms?

Treatment is expensive and difficult, so prevention is key to managing heartworms. There are a variety of safe monthly preventatives available with a veterinarian’s prescription, including Heartgard® and Iverhart®. Most heartworm preventatives work by killing the immature larvae, which prevents them from maturing into adults. Before prevention is started, your pet should be blood tested to be sure he or she is free of existing heartworm. Your pet should also be annually re-tested for heartworm.

One word of caution for dog breeds that have the MDR1 gene concerns. Many heartworm preventatives contain ivermectin. Studies have shown breeds that have MDR1 genes such as collies, Shelties and breeds that have collie and Sheltie genetics in them, have a higher sensitivity to the effects of ivermectin. The ivermectin dose in commercially marketed heartworm preventives is low enough to be safe for MDR1 positive dogs. However, some owners of these breeds may consider using Sentinel, selamectin or ProHeart 12 if you’d rather avoid giving medications containing ivermectin.

Can I Give my Pregnant Dog Heartworm Medicine?

The answer is almost all of the oral heartworm medications for dogs are labeled as safe during pregnancy and in breeding dogs. The exception is Trifexis, which has not been tested in breeding animals. Even the 12 month injectable ProHeart 12 is labeled as safe.

That being said, avoiding any drug during the first trimester (1st three weeks) is always a good idea. This the time period that fetal structural development occurs. This is when birth defects caused by chemicals can occur. To be on the safe side, time the dam’s dose of heartworm medication to a few days before she is bred, then repeating it on the four week/30 day schedule will allow you to bypass this critical fetal development period.

In many parts of the country, heartworm disease is a serious threat so eliminating the use of all heartworm preventives during pregnancy can put your breeding female at risk of developing heartworm disease and other intestinal parasites. So with careful timing, you can provide heartworm protection as well as using it to keep your upcoming litter safe.

Your pet’s monthly heartworm prevention is important in keeping them healthy!

If you have more questions on how do you prevent heartworms in dogs and cats, call us at 800.786.4751.

Written by: Marty Greer, DVM

Director of Veterinary Services

Marty Greer, Doctor of Veterinary Medicine, has 40+ years’ experience in veterinary medicine, with special interests in canine reproduction and pediatrics. She received her Doctor of Veterinary Medicine from Iowa State University in 1981. She’s served as Revival’s Director of Veterinary Services since 2019. In 2023, Dr. Greer was named the Westminster Kennel Club Veterinarian of the Year.

If you need help, call us at 800.786.4751.