Caused by Dirofilara immitis, heartworms are a potentially fatal parasitic worm living in the pulmonary arteries, lungs and hearts of cats. Heartworms are nematodes, a type of roundworm, they are several inches long, thin and white.
Heartworms are a common parasite in dogs, although cats do become infected and may develop disease. However, while cats are more resistant to heartworm infestations than dogs, cats in particular are extremely vulnerable to heartworm and even a small number can lead to death.
The disease is spread from mosquito to animals when the mosquito feeds from its host. This is known as vector borne.
Life cycle of heartworms:
In an infected animal, the adult heartworms produce their young, known as microfilaria, which swim around the bloodstream. Microfilaria require an intermediate host in the form of the mosquito. When the mosquito bites an infected animal, it takes up some of these microfilaria circulating in the animal's blood. Once inside the mosquito, they undergo further maturation, which takes 10 - 14 days, in which time they become infective larvae. When the mosquito feeds from a cat or dog, these infective larvae are injected into the animal.
Once in the cat, they take around 8 months to mature to adult worms. When they have matured they make their way to the heart (although other organs can also be infected with heartworms). Due to their resistance, cats are usually only infected with a small number of heartworms (usually between 1-3 worms), whereas in dogs numbers are generally higher. However, cats do not tolerate heartworm infection as well as dogs and even one or two heartworms can cause death.
Heartworms live in dogs for around 5-7 years and in cats for around 2-3 years. Cats are commonly found to have only one sex of heartworm, and it is unusual for cats to have microfilaria in their bloodstream. It is uncommon for heartworms in cats to produce microfilaria, due to the low number of worms found in cats.
What are the symptoms of heartworms in cats?
Symptoms of heartworms are often non-specific in cats. Some symptoms include:
Diagnosis of heartworms in cats is often difficult and not always 100% reliable. There are several tests which your veterinarian may perform, including;
Antibody test: Detects antibodies made by the cat, to adult heartworm antigen. This may give false positive results if the cat has had a prior heartworm infection which has cleared up. Also, it is possible for the cat to have had microfilaria in the blood and removed this without them developing into adult worms.
Antigen test: Detects the presence of heartworm antigen in the blood. This relies on the cat being infected with an adult female heartworm, so may give false negatives if the cat is infected with male only or immature heartworms.
Echocardiogram (ultrasound reading of the heart): To detect the presence of heartworms.
Radiography: May detect enlarged pulmonary arteries possibly with ill defined margins and an enlarged right hand side of the heart and lung changes.
Microfilarial Tests: This tests for the presence of microfilaria in the blood. Less than 20% of cats will have microfilaria in the blood. This may be due to several reasons. As cats often only have one or two heartworms, they may male only or female only, which would rule out mating of worms producing microfilaria. Also, the cat's immune system may be attacking and destroying any microfilaria present. Therefore a negative blood test will not rule out the presence of heartworms.
How are heartworms treated?
There are no approved methods to treat heartworm in cats. The treatments which are available are themselves dangerous. A single dead worm can be fatal in cats as it can break away and cause a blockage of the pulmonary artery (pulmonary embolism.
If there are no clinical symptoms your vet may decide not to treat the cat and wait for it to clear the parasite in its own time. As stated earlier, heartworms live for around 2-3 years in cats. If this is the chosen method, your veterinarian will want to monitor your cat every 6-12 months for signs of complications.
If the cat is displaying symptoms of heartworm disease supportive therapy may be recommended. Prednisone may be given to the cat to reduce the inflammation and reaction to the worm.
Cats with severe symptoms may require additional supportive therapy such as a bronchodilator to open the airways, oxygen therapy and intravenous fluids.
Adulticide treatment may be recommended for cats with clinical signs who are not responding to supportive care. Caparsolate is the drug used and kills the adult worms. This carries risks, as a dead worm can result in a pulmonary embolism. Around 1/3rd of cats receiving treatment will face life threatening complications as a result of the dying worms. Confinement will be necessary for a few weeks after treatment. Either way, if you choose to let nature take its course and hope that the worm lives out its lifespan within the cat, or if you use an adulticide there are risks. These must be weighed up by your veterinarian before a decision is made. Surgical removal of the worms has been used in some cases.
How are heartworms prevented?
There are several products on the market which can be used to prevent heartworm. These are administered monthly. It is recommended that your veterinarian perform the necessary testing for heartworm infection prior to using these products. Heartgard®, RevolutionTM and Interceptor®.
Can I catch heartworm from my cat?
You can not catch heartworm directly from your cat, but it is possible to become infected via a mosquito bite. Heartworm infections in humans are extremely rare and almost all cases the larvae are unable to develop into adult heartworms.