Feline heartworm disease is more common than previously believed, often fatal and completely preventable. It is now estimated
that 26% of cats may be infected with heartworm disease. This is very significant especially when compared to 5% who are
FeLV positive and 6% who have FIV. Cats get heartworm (HW) disease at a rate of about 15% of that of dogs wherever HW is
prevalent. Currently 59% of American households regularly administer HW preventative to dogs but only 5% to cats.
Mechanism of feline HW disease
The feline HW life cycle begins when a microfilaria infected mosquito bites the host injecting stage 3 (L3) larvae into the
skin. Within a few days the larvae develop into L4 which set up housekeeping over the next 2-3 months in the SQ connective
tissue of the abdomen or thorax. The 5th stage, L5 larvae migrate to the pulmonary circulation where damage begins. Initially the larvae are attacked by pulmonary
intravascular macrophages (PIMs) in a phagocytic process that results in significant bronchopulmonary inflammation. Some larvae
will be killed in this process inducing further inflammatory response. Acute lung pathology and even death can occur at this
stage. Cats with acute heartworm associated pulmonary signs are often misdiagnosed as asthmatic or allergic. Parasiticides
like selemectin (Revolution®, Pfizer Animal Health) prevent the development of larvae to stage 5.
After this initial phase some cats will appear to "self cure". That is, their symptoms abate and immune response is suppressed
allowing cats to tolerate their HW infection. However in the second phase which can occur up to 2 years later when the heartworms
begin to die devastating pulmonary injury can occur. At the second phase pulmonary thromboembolism and sudden death are not
uncommon. Cats that die suddenly may appear clinically normal up to 1 hour before death. Postmortem examination has revealed
as few as two worms in cats that have died suddenly.
Why we have missed it
There are a number of reasons cat heartworm disease has eluded the veterinary community and the public. We know that HW disease
in dogs is caused by adult heartworms. Cats are resistant but susceptible hosts for adult heartworms. Feline immune systems
are frequently able to kill and remove heartworm larvae before they reach adulthood therefore it seemed unlikely that cats
would develop symptoms of disease. Only recently has it been unquestionably proven that the major cause of HW disease in
cats occurs from the larval stage. Furthermore feline HW disease significantly affects the pulmonary rather than the cardiac
Heartworm disease has been considered regional; thought to occur only in limited areas in the United States. Recently and
especially after the redistribution of HW positive dogs following Hurricane Katrina, HW disease has been seen in 49 states
with Alaska as the only exception.
Five myths persist that continue to inhibit our ability to convince veterinarians, technicians and pet owners that feline
heartworm exists at all or that it is as prevalent as we now realize.
Myth #1 Heartworm is a disease of dogs only
Heartworm disease is transmitted by a mosquito that has bitten a HW positive dog. While cats are not direct hosts for HW
disease, they need only to be bitten to contract HW disease. Many types of mosquitoes feed happily on both dogs and cats.
In any area where HW preventative is thought appropriate for dogs it is equally if not more appropriate for cats as HW disease
in cats is potentially more serious than in dogs.
Myth # 2 Heartworm can not affect indoor only cats
Heartworm is carried by mosquitoes. Culex pipiens, or northern house mosquito is the most common species of mosquito found
in urban areas. Culex mosquitoes are painful and persistent biters which prefer to attack at dusk and after dark, and readily
enter dwellings for blood meals. They prefer to feed on domestic animals over man. The culex is considered a weak flyer but
still easily covers 2 miles (some mosquitoes can fly 20 miles). Since mosquitoes do get indoors quite frequently indoor cats
are at risk as well as outdoor cats. A recent study in North Carolina showed that 28% of cats diagnosed with HW were described
by owners as indoor only.