Heartworm Disease in Cats and Dogs

Dr. Stephanie Everidge

Is heartworm disease new to the United States?

The answer is actually no.  Heartworm disease has been detected in all 50 states at this time with the most prevalent areas being in the southeast and the Mississippi Delta region due to the warm temperatures and the amount of mosquitoes found in these regions.  Heartworm disease was discovered in dogs about 100 years ago as well as the first case for cats was detected in the 1920’s.  

How does my pet become infected with heartworms?

Heartworm disease is transmitted by mosquitoes and can be a silent disease in many pets in the early stages.  The mosquito must first bite an infected cat or dog with baby heartworms called microfilariae and the baby heartworms start to grow in the mosquito to a new stage called larvae.  This same mosquito will then bite a healthy pet and the larvae will then infect the pet with heartworm disease.  These larvae will then start to mature in the cat or dog to adult heartworms, which can start to reproduce.  The larvae can take up to 7 months to turn into adult heartworms in dogs or cats.  These heartworms develop in the heart and lung vessels.  Cats can have a smaller number of heartworms than dogs in the body.

 What are the signs that my pet may be infected with heartworms?

The signs in both cats and dogs can vary.  One of the most common signs can be a cough in dogs or cats.  However, other signs such as lethargy/weakness, gagging, reduced appetite, blood in urine, and even weight loss can occur with a heavy infection of heartworms in dogs.  Dogs can also go into heart failure with a severe heartworm infection.  Cats can have a different set of symptoms such as wheezing, trouble breathing, vomiting, as well as weight loss.  This can mimic feline asthma or allergies in cats due to the larvae moving into the arteries of the feline patient.

How do we test for a Heartworm infection?

There is a blood test that requires a very small amount of blood that test for the adult heartworm usually female in the body.  The heartworm has a special part called an “antigen” that the test will look for in the body of the pet.  This test is completed after 7 months of age since this is how long the life cycle of the heartworm takes to mature.  Also, we usually look for the microfilariae or “baby” heartworms on a separate test for infection.  There are radiographs/x-rays as well as ultrasound methods that can be used to try to detect the heartworms in a patient but usually this is a secondary step after the first tests indicate infection.

How can we prevent a Heartworm infection?

There is a special class of drugs that are called Macrocyclic Lactones that help prevent heartworm disease. These drugs help to prevent the heartworms from developing in the pet’s body and are usually very easy to administer as a once a month preventive orally or even as a topical treatment monthly.  This class of drugs also helps to prevent intestinal parasites such as roundworms and hookworms that can also infect both your pet as well as humans including children.  There are several different products that have this preventative and we can help you choose the best product to help your pet at Animal Medical Hospital.  It is very important to know that your pet is heartworm negative before starting heartworm preventative since there can be side effects if a pet is heartworm positive and starts a preventative.

How do we treat Heartworm infection?

A dog that is infected with heartworm disease can be treated with a medication that will kill the adult heartworms.  There is one product on the market that is FDA approved to kill adult heartworms for dogs and requires a series of injection. The treatment for heartworm disease in dogs can be costly (over $1,000) and requires strict cage rest after treatment to prevent side effects.  If the pet is having heart failure, usually the heartworms need to be removed manually at a specialty hospital.

Unfortunately, there is not an approved treatment to kill adult heartworms for cats at this time.  Cats can rid themselves of infection spontaneously and have been shown to be more resistant to heartworm infections than dogs.  However, cats can also have a more severe reaction to dying heartworms than dogs and this can be fatal.

In conclusion, both cats and dogs can become infected with heartworm disease in our area.  Therefore, the recommendation from the American Heartworm society is for pets to be on heartworm prevention year-round to help protect against heartworm disease as well as they have the added benefit of preventing intestinal parasites.