Dirofilariasis (Heartworm Disease)

Case history.

A 4-year old castrated male ferret which had recently arrived from Florida presented with signs of lethargy and mild exercise intolerance. A routine CBC was normal. Thoracic radiographs showed marked cardiac enlargement and edema in the hilar area of the lungs. Based on the history, an occult heartworm test was performed at a regional veterinary laboratory and was reported as negative. The animal showed poor response to lasix and digitalis therapy and was euthanized two weeks later.

Discussion.

Dirofilariasis, or heartworm disease is an uncommon disease in ferrets, but one which is certainly on the rise. Currently, the majority of cases in the country are seen in the southern states, especially Florida. Due to the small size of the ferret heart, ferret are unable to carry a load of more than 3-5 worms, and as little as two may precipitate heart failure.

Clinical signs of heartworm infection in ferrets may include lethargy, anorexia, dyspnea, pale mucous membranes, and ascites. Auscultation may reveal systolic murmurs, arrhythmias, or heart sounds may be muted as a result of pleural effusion. Clinical pathology is most often unremarkable in these cases.

Because of the low heartworm load in affected ferrets, the occult heartworm test is the test of choice. However, many tests will return false negative findings as demonstrated in this case. Dr. Deborah Kemmerer of West End Animal Hospital, Newberry FL, highly recommends the Idex Snap Test for heartworms.

(The following information kindly provided by Dr. Deborah Kemmerer)

Prevention.

Heartworm prevention may be accomplished by either use of prepackaged ivermectin tablets or by compounded liquid ivermectin. The American Heartworm Society and Merck recommend use of Heartguard in ferrets. 0.5-1 tablet for small dogs is recommended. However, due to the inherent difficulty of getting the tablets into ferrets, Dr. Kemmerer uses a liquid formulation of 100 mcg/suspension of Ivomec in propylene glycol (in an amber bottle) given at a dose of 1/10 ml per pound body weight (or 10 mcg/lb). Ferrets over six months of age should be tested prior to beginning heartworm prevention.

Treatment.

Thiacetarsemide should be given at 0.12cc/lb via a cephalic catheter. Four doses are given at twelve-hour intervals. Starting on the same day as the first injection, administer prednisone orally at 0.25 mg/lb once daily. Prednisone must be administered for three months following thiacetarsemide therapy, and tapered off over a two-week period.

One key to treatment of ferrets is the prevention of emboli. Ferrets must remain caged during the entire treatment period. The natural instinct of the ferret to run and play may result in a fatal embolism.

Following treatment, occult heartworm tests may be repeated at monthly intervals. Antigen tests may remain positive for up to six months, but the average is about four months.