Heartworm Disease in Cats
Heartworm disease, so named because the adult worms live in the right side of
the heart, is common in dogs, less so in cats. In fact, cats may be
accidental hosts only, and certainly they are less perfect hosts for this
parasite than dogs are.
Heartworm Life Cycle
A knowledge of the life cycle of this parasite (Dirofilaria immitis) is
needed to understand how to prevent and treat it. Infection begins when
L3 infective larvae in the mouthparts of a mosquito enter the cat’s
skin at the site of a bite. The larvae burrow beneath the skin and undergo two
molts that eventually lead to the development of small, immature worms. The
first molt (L3 to L4) occurs 1 to 12 days after the cat
is bitten by the mosquito. The larvae remain in the L4 stage for 50
to 68 days, and then molt into the L5 stage (immature worms).
Immature worms make their way into a peripheral vein and are carried to the
right ventricle and the pulmonary arteries. In cats, the larvae may become
disoriented and migrate into body cavities and the central nervous system.
Approximately six months after entering the cat’s body, they mature into
adults. Adults can grow from 4 to 12 inches (10 to 30 cm) long and live up to
two to three years.
In dogs, mature heartworms produce larvae,
called microfilaria, that circulate in the bloodstream. This is much less
common in cats, possibly because the cat’s immune system removes the
microfilaria or because low numbers of adult worms or same-sex worms actually
prevent the production of microfilaria to begin with.
Because of the small size of the cat’s heart, one or two worms may be enough
to cause serious heart trouble or even sudden death. Signs of heartworm
infestation include a cough made worse by exercise, lethargy, loss of weight
and coat condition, and bloody sputum. At this point, it may appear that the
cat has asthma or allergic bronchitis. The cat’s pulmonary
artery response to heartworms is much more severe than is the dog’s.
Cats who pass through this phase of infection may be relatively fine until
the adult heartworms start to die in two to three years. Labored breathing and
mild, low-grade, chronic respiratory signs may go on for a while. Congestive
heart failure, along with heart murmurs, loss of condition and appetite, and
intermittent vomiting may all appear late in
the disease. Worms may be discovered at autopsy following sudden, unexplained
Diagnosis is generally done by blood tests looking for the heartworm
antigens or antibodies produced to fight them. Both types of tests are valuable
before starting treatment for a suspected infection. X-rays of the chest and
the use of echocardiography can be especially helpful in diagnosing heartworms