Heartworm

(Dirofilaria immitis)

Dirofilaria immitis is a filarial nematode of domestic and wild canids that can also infect other hosts, including cats. It rarely causes zoonotic infections.

Distribution

Tropics and subtropics, where heartworm is known to occur in dogs. The prevalence of feline adult heartworm infection is estimated to be 5-20% the rate of dogs ^[1].

Clinical signs

In cats, clinical signs develop either due to arrival of heartworms in the pulmonary arteries (3-4 months post-infection) or due to death of adult worms. The predominant clinical signs in cats are wheezing, coughing, dyspnoea and respiratory distress. Other clinical signs include vomiting and neurological deficits. As the clinical signs of involvement of the lower respiratory tract is more common than that of the heart, heartworm associated respiratory disease (HARD) is often used to describe the disease in cats. In some cats, disease can be peracute and present as sudden death, often associated with the death of adult worms.

Diagnosis

Dirofilaria immitis infections in cats can be difficult to confirm. Cats typically present with low worm burdens (often a single worm) and without circulating microfilariae. In the same way, many infected cats will not present circulating heartworm antigens or anti-heartworm antibodies. A lateral flow immunoassay for the detection of IgG antibodies to D. immitis is commercially available (HESKA Solo Step FH, Heska Corporation, Loveland, USA). A positive heartworm antibody test would increase ‘suspicion’ of D. immitis, but is not in itself diagnostic. Radiography and echocardiography are reputed to be useful for diagnosing feline heartworm disease ^[2,3]. Heat treatment of blood prior to antigen testing also can be helpful in diagnosing heartworm in cats suspected of infection ^[4]. Bronchioalveolar wash may reveal eosinophil infiltrates which can be confused with allergic bronchitis e.g. feline asthma or Aelurostrongylus abstrusus infection.

Treatment

In contrast to treatment of canine heartworm disease, adulticides (e.g. melarsomine) are not recommended in cats. There is no approved drug for the treatment of heartworm infection in cats. There is no recommendation to treat asymptomatic cats with confirmed heartworm infection; however, owners should be advised that clinical signs may develop and sudden death is a possible outcome. Sick cats should receive supportive care, according to the clinical signs present. Cats in acute respiratory distress should receive corticosteroids (prednisolone at 1 mg/kg BID or dexamethasone at 0.01-0.16 mg/kg IV or SC daily for 3 days), bronchodilators (e.g. terbutaline at 0.1-0.2 mg/kg PO BID), aminophylline at 6.6 mg/kg PO BID or theophylline at 4 mg/kg PO BID) and oxygen supplementation ^[1]. Prednisolone (1 mg/kg BID, tapering dose) is recommended for symptomatic infected cats with radiographic evidence of lung disease. All heartworm positive cats should be given chemoprophylaxis using a macrocyclic lactone. Surgical removal of heartworms from right atrium of cats has been successfully performed, but the owners should be advised that this is a very risky procedure.

Control

Monthly chemoprophylaxis is recommended for cats living in areas where canine heartworm is endemic. For preventative treatment options, refer to Table 4.

Table 4. Routes of administration and dosage of commonly utilised preventatives against heartworm infection in cats ^[1,2].

Public health considerations

Although D. immitis infection is rare in humans, it can cause respiratory manifestations such as coughing, chest pain and haemoptysis. Granulomas in the lungs resembling ‘coin-like’ lesions have also been detected on radiographs of infected humans. Ocular infections have also been reported. As cats are not the natural definitive host for D. immitis, they are unlikely reservoirs for zoonotic infection.

References

[1] Labuc R, Korman R and Traub R. Australian Guidelines for Heartworm Prevention, diagnosis and management in cats. Vet Australia, 2018 (www.vetsaustralia.com.au/heartworm).

[2] Bowman DD, Hendrix CM, Lindsay DS, Barr SC. Feline Clinical Parasitology. Iowa State University Press, Ames, USA. 2002.

[3] Venco L, Calzolari D, Mazzocchi D, Morini S, Genchi C. The use of echocardiography as a diagnostic tool for the detection of feline heartworm (Dirofilaria immitis) infections. Feline Pract. 1998;26:6-9.

[4] Little SE, Raymond MR, Thomas JE, Gruntmeir J, Hostetler JA, Meinkoth JH, Blagburn BL. Heat treatment prior to testing allows detection of antigen of Dirofilaria immitis in feline serum. Parasit Vectors. 2014;7:1.