The cat is considered a resistant, yet susceptible host for Dirofilaria immitis. Worm burdens are much lower in cats than in dogs (average 15 worms in dogs and 1-3 in cats in endemic areas) and about 1/3
of feline infections involve worms of the same sex. Feline heartworm (HW) was first described in the 1920s; awareness has
increased greatly since the introduction of Heartgard for cats in 1997 and the associated marketing campaign. Feline HW remains
a difficult to diagnose, yet fully preventable disease.
Cats are infected with HW in the same way as dogs, but far fewer larvae mature to adulthood. It is difficult to estimate prevalence
of feline HW for several reasons – there is no ideal test, inapparent infections go unnoticed, and some cats die acutely without
a diagnosis. The prevalence of immature infections is higher than the adult infection rate. Based on necropsy surveys of shelter
cats, feline HW is thought to be present at about 5-15% of the canine rate in endemic areas.1 Certainly wherever canine heartworm is found, feline HW is present as well.
Our understanding of feline heartworm infection has progressed rapidly in recent years, especially with regard to the pathophysiology
of the respiratory tract signs associated with various stages of infection.1-3 Many cats will have no clinical signs of HW disease and they will spontaneously eliminate the infection without incident.
Other cats may have clinical signs associated with infection at two possible time points:
1) Upon arrival of immature worms (L5s) in the pulmonary arteries and arterioles in the 3 to 6 month post-infection
period. The high mortality of immature worms stimulates a severe vascular and parenchymal inflammatory response. Pulmonary
lesions may be long-lasting. The clinical response in the cat is termed HARD because respiratory signs predominate (dyspnea,
tachypnea, and cough). The clinical signs may be transient or intermittent. Clinical signs subside as the worms mature. Many
cats with HARD are misdiagnosed as having asthma or bronchitis.
2) Upon death of adult worms, with release of antigens and toxins leading to pulmonary inflammation and thromboembolism.
Clinical signs include rapid onset of respiratory compromise or sudden death (occurs in 10% or more of HW infected cats).
Even the death of 1 adult worm can be lethal by causing circulatory collapse and respiratory failure. Adult worms are able
to suppress pulmonary intravascular macrophage activity and so actually induce little inflammation until they die in 1 to
Nonspecific clinical signs associated with feline HW include chronic vomiting (present in 25-33% of cases), lethargy, anorexia,
and weight loss.4 Less common signs due to aberrant migration include ascites, pneumothorax, chylothorax, neurological signs (ataxia, seizures,
syncope, collapse, blindness, vestibular signs), and hemoptysis. Signs of cardiac disease or failure are very uncommon in
cats with HW. Sudden death occurred in 10% of infected cats in one study.4
Our understanding of the role of Wolbachia, an endosymbiont Rickettsial bacterium found in D. immitis in feline HW disease is evolving. HW infected cats may be exposed to Wolbachia when larvae or adult worms die, and probably at other times in the worm's life cycle. A strong antibody response against
Wolbachia surface protein has been demonstrated in HW infected cats.5
Wolbachia may play a role in the inflammatory response seen in HARD; raising the possibility that treatment with antibiotics such as
doxycycline may help reduce clinical signs in cats with HARD. Research is currently underway to define the exact relationship
between Wolbachia and pulmonary inflammation in cats.6
Diagnosis of feline HW may be difficult. Cats are rarely microfilaremic so filtration or IFA testing for microfilaria is not
recommended. No single diagnostic test can detect feline HW at all life stages of the worm. Combining antigen and antibody
testing achieves higher sensitivity than either test alone7-8, but may generate more false positives.
A positive HW antibody test documents exposure to early stage infection but not necessarily current infection, and a negative
test does not rule out infection. The different tests available also vary widely in sensitivity and specificity, as each brand
may detect a different stage of larval development. Interestingly, up to 30% of cats on HW prevention may become antibody
positive although they are not at risk for HARD.