Uncovering feline heartworm 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.
No indicatons 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.
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.