Disease risks for the travelling pet: heartworm disease

Prevalence, diagnosis and treatment of heartworm disease in dogs and cats

source: Luca Ferasin
In Practice vol 26 no 7, July/August 2004 starts p 350,
7 pages long

Heartworm infection is caused by a roundworm, or nematode, called Dirofilaria immitis. Heartworm is found in regions where there are a large number of dogs and mosquitoes, and it is warm enough for long enough for heartworm larvae to develop in mosquitoes. It is usually too cool for the larvae to mature in the UK in summer, but pets that have been abroad, such as dogs, cats and ferrets, can be affected. The disease mainly affects heart and lungs. Pulmonary arteries may be blocked, and the worms may migrate to the heart, leading to right-sided heart failure. Cats generally suffer from lung infections, and are more likely to show ectopic infections than are dogs. Dogs may cough and have trouble exercising, though most cats show no symptoms. Microfilariae may be seen in fresh blood, though ELISA antigen tests are a more reliable form of diagnosis.

Treatment for heartworm depends on the severity of signs. Though adult heartworms can be killed, there are risks of pulmonary thromboembolism, especially with cats. Cats can recover when the parasites die a natural death, so killing adult heartworms is best carried out only when cats are stable, and do not respond to help with symptoms, such as cage rest, bronchodilators, fluid therapy and steroids. Seriously ill dogs should have just one dose of adulticide, to reduce risks from dead worms, with later doses after a month. Ivermectin is one method of eliminating microfilariae, after adult worms have died. Prophylactic drugs, like milbemycin oxide (Novartis Program Plus) and Pfizer's Stronghold can be given monthly to dogs at risk from infection by mosquitoes where the disease is prevalent. Milbemycin oxide and Ivermection can be toxic to collies at high doses.