All You Need to Know About Heartworm Disease

Most dog owners are familiar with administering a monthly medication to help prevent heartworm disease. This often comes in the form of a brown chewable, called Heartgard. Veterinarians frequently receive questions in regards to why this monthly chewable is required and why yearly blood testing is necessary if this treatment is vigilantly administered every month. We want to briefly outline the basics of heartworm disease, prevention, diagnosis and treatment.

What is heartworm disease?

Heartworm disease is transmitted by mosquitoes and dogs generally serve as the definitive hosts for the heartworm, Dirofilaria immitis. Cats can also be affected by heartworm disease, but more frequently serve as a limited source of infection for mosquitoes. The mosquitos carry a microfilaria (a neonatal larval stage) that transforms inside the mosquito’s body into different stages of larva. When the mosquito bites an animal it is then able to transmit these infected larva through their saliva. Development in the new host continues to molt or transform until it can travel through the animal’s tissues and eventually reach the heart, specifically the pulmonary arteries.

It has been reported that female heartworms can grow to be as large as 14 inches long; males, meanwhile, are generally shorter. These heartworms will continue to grow in size and can cause a severe blockage for normal blood flow, leading to severe health problems.


A monthly heartworm preventative is recommended year-around. These preventatives, such as Heartgard or Sentinel contain ivermectin and milbemycin oxime. The topical liquid formulations contain moxidectin and selamectin in products such as Revolution.

Preventive therapy in dogs is recommended beginning at 6–8 weeks of age and testing is not required at this age because it takes at least 6 to 7 months for the heartworm to be mature enough to be detected by an antigen test. If starting heartworm prevention after 7 months of age it is strongly recommended to perform an antigen test prior to administration of preventative medication.

Many pet owner’s wonder why their pet needs to be tested every year for heartworm disease if they are on prevention every single month. There are several reasons for this with the first being that product failure is a possibility, even though the preventatives are excellent. Second, there is always the chance that the preventative was accidentally not administered one month or was not swallowed by the pet (spit out or dropped in another location and not retrieved). Lastly, there is the possibility of a serious reaction if a heartworm positive dog is administered a preventative because of the rapid killing of microfilaria.


The majority of diagnoses for heartworm disease are made by a heartworm antigen test, which detects a protein that is secreted mainly by the adult female heartworm. The American Heartworm Society states that the tests are nearly 100% specific but subtle differences in sensitivity exist especially in cases with low worm burdens. There are also tests present that screen for microfilaria or immature heartworm.


The only treatment approved by the FDA for heartworm treatment is Melarsomine dihydrochloride (immiticide). The recommended treatment that is endorsed by the Heartworm Society is a three shot protocol, which consists of one intramuscular injection followed a month later by two separate intramuscular injections 24 hours apart. The most essential component following treatment with immiticide is exercise restriction. This is to prevent migration of dying worm components that can migrate to the pulmonary arteries or specific areas of the lungs leading to risk of thromboembolism and potential right-sided heart failure. Immiticide therapy is recommended to be frequently accompanied with Prednisone and Doxycycline. Prednisone is a steroid that is used at an anti-inflammatory dose to help control clinical signs of irritation in the pulmonary arteries. Doxycycline in an antibiotic utilized to help reduce Wolbachia bacteria that are present with heartworm disease.

Presently, there is no satisfactory treatment approach to heartworm infections in cats. Infection often is lethal, and a safe and effective immiticide protocol has not yet been developed. Therefore, cats should be administered preventatives in areas known for having high incidences of heartworm disease.

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