What are
heartworms?
Caused by Dirofilara immitis, heartworms are a potentially fatal parasitic worm living in the pulmonary arteries, lungs & hearts of cats. Heartworms are nematodes, a type of roundworm, they are several inches long, thin & white.
Heartworms are a common parasite in dogs, although cats do become infected & may develop disease. While cats are more resistant to heartworm infestations than dogs, cats in particular are extremely vulnerable to heartworm & even a small number can lead to death.
The disease is spread from mosquito to animals when the mosquito feeds from it's 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. The
mosquito bites an infected dog & takes up some of these
microfilaria circulating in the dogs 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 & even one or two heartworms
can cause death.
Heartworms live in
dogs for around 5-7 years & 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:
How are heartworms
diagnosed?
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
& 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 & an enlarged right hand side of the heart & 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 & 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 & cause a blockage of
the pulmonary artery (pulmonary embolism.
If there are no
clinical symptoms your vet may decide not to treat the cat &
wait for it to clear the parasite in it's 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 & reaction to the worm.
Cats with severe
symptoms may require additional supportive therapy such as a
bronchodilator to open the airways, oxygen therapy & intravenous
fluids.
Adulticide treatment may be recommended for cats with clinical signs who are not responding to supportive care. Caparsolate is the drug used & 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 it's course & hope that the worm lives out it's 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
&
Interceptor®.
Can I catch
heartworm?
Yes, but not from
your cat, but it is possible to become infected via a mosquito
bite. Heartworm infections in humans are extremely rare.
Please note: The medical articles on this site have not been written by a veterinarian & should not be considered a replacement for a veterinarian visit. The articles are provided for informative purposes only. Always seek immediate veterinary advice for any problems (health or behavioural) in your pets. While great care has been made in the creation of these articles, we cannot guarantee the accuracy or omissions on these pages. If in any doubt whatsoever, seek professional medical advice.
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