Heartworm in Dogs and Cats

Last updated: August 2, 2016

Heartworm is a potentially devastating disease that can occur in both dogs and cats. It can be difficult to diagnose until it's in advanced stages, which means screening and prevention is vital for protecting your pets from heartworm.

What Is Heartworm?

Heartworm is caused by the parasite dirofilaria immitus, and it's carried from host to host by infected mosquitoes. Heartworm cannot be transmitted through direct contact. When an infected mosquito bites your pet and injects the heartworm larvae, the larvae travel through the bloodstream 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. This usually leads to death.

Heartworm Symptoms in Dogs and Cats

Even though 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 may also experience vomiting or gagging. If not treated, the animal usually dies of heart failure.

Heartworm Treatment

Treatment includes eliminating all forms of the parasite—adult, larvae and microfilaria (prelarval stage). Multiple injections are often used to kill the adult heartworms, and antibiotics to prevent secondary pneumonia. After treatment kills 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; it may be difficult to prevent more harm than good. Currently, there is no approved treatment for heartworm in cats, so only the symptoms can be treated.

Prevention and Protection

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 is free of existing heartworm. Your pet should also be periodically re-tested.

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. For these breeds, consider using Sentinel if you'd rather avoid giving medications containing ivermectin.

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

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-The Revival Education Team