Although heartworm disease is as preventable as any of the infectious diseases of dogs and cats, infections with D. immitis
continues to be a common occurrence. Because of the awareness of owners and the efforts on education, the veterinary profession
at times has over simplified this disease and has lead to some of the confusion over feline heartworm disease.
Heartworm disease in cats must also be considered a potential health risk by veterinarians. Clinical signs can often be overlooked
or misdiagnosed as another disease.
A little history
We have historically told cat and dogs owners that D. immitis has a six-month life cycle, which is true relative to time of
infection until possibility of producing microfilaria and production a host that serve as a source for mosquitoes to initiate
the next infection. However, from the animal's point of view, heartworms have a three-month disease cycle, especially in the
cat. After a dog or cat is exposed to mosquitoes with L3, as early as 90 to 100 days later, small young adult worms (L5s,
2-5 cm in length) arrive in the right ventricle and are found in the terminal pulmonary arteries (especially the caudal arteries).
Three months after the infection, there is disease being produced and radiographic and histologic changes are evident. Because
the early part of this three-month disease cycle is typically asymptomatic in dogs, it has largely been ignored. In the cat,
the arrival of L5 as early as three months after infection is associated with an acute inflammatory reaction including peribronchial
infiltration and eosinophilic interstitial and alveolar reactions. In addition, this early phase is considered more dramatic
because of a higher mortality of immature adult worms during the three- to six-month time following infection compared to
the dog. On radiographs, an enlarged caudal pulmonary artery can be viewed. However it is actually the pulmonary artery and
an extensive periarteritis, which can have the same radiographic tissue density. The enlarged pulmonary artery and associated
periarterial reaction often resolve over several months leading to the false impression that the heartworms are gone. The
truth is that once adult heartworms have developed, the cat and heartworms often exist without obvious clinical disease and
the lung pattern radiographically is dominated by the bronchial disease without any cardiac and/or pulmonary arterial changes.
Based on several clinical studies, we have come to the understanding that feline heartworm disease (FHD) is dynamic over time
as compared to the dog. The clinical and radiographic changes can vary from month to month, usually dictated by how the host
reacts to that particular stage of the heartworm life cycle. Because cats with adult heartworms often have no clinical signs,
acute signs associated with the death of the heartworm may be a complete surprise to the owner and veterinarian alike. In
a study of spontaneous death associated with dyspnea of cats in practice, we found cats that died from heartworm disease always
had dead or dying heartworms in them. However, one cannot predict how each cat will react to the death of the heartworm and
most cats survive the adult heartworm infection. Because of the much greater incidence of heartworm disease associated with
the three-month disease cycle, heartworms as a potential fatal event should be given very little emphasis. Heartworms as a
differential for coughing and dyspenic cats should be considered a much more crucial message.
Diagnostics and testing related to initiation of preventative medications continues to complicate feline heartworm disease.
Because antibody testing is evaluating how the host reacts to the parasite and each antibody test measures antibody against
different target proteins, it is of no surprise that in each point in the life cycle of the heartworm, disparate results can
occur. More germane is that most antibody tests detect antibodies which can be detected as a result of immature heartworms
migrating towards the heart. Thus, a cat which is antibody positive is considered to be a cat which has been successfully
infected, the worms have lived typically two or three months and may or may not have developed into adult heartworms. Nevertheless,
they have been successfully infected and typically some have developed into L5 in the distal pulmonary arteries. To consider
a cat that gets immature adults, develops an inflammatory lung disease, and has all the worms die over a one- to two-month
period a "self-cure" would not be appropriate.
The ventrodorsal radiographic view is best to view enlarged caudal pulmonary arteries and associated perivasculitis which
are usually associated with the 4-6 month stage after infection. In addition, extensive peribronchial densities and diffuse
alveolar patterns can beidentified in most areas of the lung. Typically, cats with heartworms do not have cardiac changes.
If an adult cat is presented for the initiation of heartworm preventative, testing becomes an issue. First, there is the question
of can the exact heartworm status of a cat be evaluated? A cat that is heartworm antibody positive has been successfully infected
and may or may not have adult worms. Some cats that have adult heartworms are negative on all current heartworm antibody tests.
Therefore, antibody testing does not establish whether the cat has heartworms at the initiation of therapy. Because antigen
tests are only positive in cats with fully mature adult female heartworms, all cats with immature stages (three-month disease
cycle) and most cats with adult heartworms are negative. A positive antigen test does indicate an active adult heartworm status
but even most cats with heartworms are antigen negative. A negative antigen result does not confirm a negative heartworm status.
Next, one must consider the safety issue. Because a cat with heartworms - even with microfilaria - can be placed on monthly
heartworm preventatives without acute adverse effects, there is no significant safety issue involved with testing before the
initiation of heartworm preventatives. Thus, currently no heartworm tests are performed before the initiation of preventatives
in adult cats. Microfilaria are rarely present is cats with heartworms and even when present, transient in circulation.