Also known as 'dirofilariasis', heartworm is a serious parasitic infection caused by the nematode Dirofilaria immitis which lives in the pulmonary arteries, lungs, and hearts of cats. Heartworms are a type of roundworm, they are several inches long, thin and white.
Heartworms are a common parasite in dogs, although cats can also become infected and may develop disease. While cats may be more resistant to heartworm infection than dogs, cats, in particular, are extremely vulnerable to heartworm and even a small number can cause death.
There is no age or breed predisposition, males have been shown to be easier to infect experimentally. Outdoor cats are at greater risk due to their increased exposure to mosquitoes.
How do cats become infected with heartworm?
Cats become infected with heartworm when a mosquito carrying microfilaria (baby heartworms) feeds on a cat. This is known as vector-borne transmission.
Life cycle of heartworms:
In an infected animal, the adult heartworms produce their young, known as microfilaria, which swims around the bloodstream. Microfilaria requires 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, at 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). Heartworms live primarily in the right-hand side of the heart and in the blood vessels going to the lungs. 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.
Pathophysiology of heartworm infection in cats:
Worms in the heart can interfere with the action of the heart, cause inflammation and a hardening of the arteries.
In some cases, juvenile heartworms can migrate to other sites ('aberrant migration') such as the eyes, spinal cord, an artery in the leg and brain which can lead to unusual symptoms such as blindness, seizures, limping, paralysis and sudden death.
Caval Syndrome (or Vena Cava Syndrome) is caused when heartworms move from the pulmonary artery to the vena cava which is a large vessel that carries deoxygenated blood to the right-hand side of the heart from the lower body. When heartworms obstruct this vessel, life-threatening consequences occur. Red blood cells are destroyed as they flow through the mass of worms. Liver and kidney dysfunction occur along with reduced cardiac output and in some cases disseminated intravascular coagulation. There is a high mortality rate with this condition.
Symptoms of heartworms are often non-specific in cats. The death of juvenile or adult heartworms can result in a condition known as HARD which stands for Heartworm Associated Respiratory Disease.
What are the symptoms of heartworms in cats?
Heartworm disease can manifest in many different forms with a wide range of symptoms.
Common symptoms of heartworm include:
- Difficulty breathing
- Heart murmur
- Anorexia (loss of appetite)
- Weight loss
- Intermittent vomiting
- Weight loss
How are heartworms diagnosed?
Diagnosis of heartworms in cats is often difficult and not always 100% reliable therefore diagnosis usually requires a combination of tests.
- This blood 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 them without them developing into adult worms. Up to 25% of cats with adult heartworm infection are antibody-negative.
- 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.
- May detect enlarged pulmonary arteries possibly with ill-defined margins and an enlarged right-hand side of the heart and lung changes.
- 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 in cats?
There are no approved medications 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 already noted, 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. Corticosteroids (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 or Immiticide are the drugs used and kills the adult worms in cats. Neither has been approved for use in cats and treatment does carry risks. A dead worm can result in a pulmonary embolism (blockage in one of the pulmonary arteries in the lungs). 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 in cats?
There are several products on the market which can be used to prevent heartworm. These are administered monthly.
|Imidacloprid and Moxidectin||Advantage Multi/Advocate|
Keeping cats indoors reduces their exposure to mosquitoes although doesn't completely eliminate the risk.
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.