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15 JANUARY/FEBRUARY 2018 ● TVPJOURNAL.COM ESSENTIALS Infections with D immitis in ferrets are described in laboratory settings with a susceptibility of 100%. 7 Multiple cases of HWD in client-owned ferrets have been reported; however, compared to cats and dogs, there is not as much data regarding prevalence and susceptibility for HWD in ferrets. 8 Housing a ferret outside is not recommended in areas where heartworm prevalence in cats and dogs is high. Even housing a ferret indoors 100% of the time is not a guarantee that it will avoid infection. One study found that about 25% of indoor cats have HWD. 9 CLINICAL SIGNS The clinical signs described in ferrets depend on the stage of HWD but are often associated with heart failure ( FIGURE 2 ). Signs include dullness, anorexia, coughing, dyspnea, systolic heart murmur, pleural effusion, ascites, anemia, intravascular hemolysis, and acute renal and hepatic failure. 1 Bilirubinuria is a frequently described sign. In a study of clinical observations of naturally occurring HWD in ferrets, bilirubinuria was observed in 83% of the cases. 10 A rare case of an aberrant larval migration infesting the subdural space of the cranial cavity was reported in Europe in 2010. 11 HWD may be misidentified in ferrets. A study in 2008 showed HWD was diagnosed in 11 black-footed ferrets using an antigen-based enzyme-linked immunosorbent assay (ELISA). Polymerase chain reaction (PCR) amplification of the 5S spacer region of rDNA of the filarial sequences showed only 76% conformity with D immitis; it was reported that the population was infected with an undescribed filarial species. 12 Pathogenesis The life cycle of D immitis is 210 to 270 days. Female mosquitos can be infected with microfilariae during feeding on an infected host. After ingestion, the microfilariae transform into first-stage larvae (L1). Depending on temperature, L1 molt into infective third-stage larvae (L3) within about 10 to 14 days. The infective L3 are deposited on the skin of a potential host by the feeding mosquito. They enter the subcutaneous tissue of the host through the bite wound and, after a few days, transform into fourth-stage larvae (L4). The L4 migrate through the subcutaneous and muscle tissue toward the thorax, where they molt to juvenile worms 50 to 70 days postinfection. The immature worms enter the bloodstream via peripheral veins and finally infest the pulmonary vasculature. A mean duration of 70 days after inoculation has been described for this migration. The juvenile worms reach maturity in the pulmonary arteries 180 days after infection. Mating takes place in the pulmonary arteries, and microfilariae are found in circulation at approximately 180 to 210 days after initial infection. 3 Ferrets can be severely affected by a low heartworm burden of 1 to 2 adult worms. 13 DIAGNOSTICS Results of tests using antigen detection of a glycoprotein secreted by female heartworms will be falsely negative in the case of a male-only infection. Screening by blood test is not reliable in ferrets; symptomatic animals should be radiographed to evaluate the cardiac silhouette as the first step ( FIGURE 3 ). A biochemistry profile and a complete FIGURE 2. A necropsy of a ferret affected with Dirofilaria . Note the globoid shape of the heart. Image courtesy of pathology service at UGA. FIGURE 3. Lateral thoracic radiograph of the thorax of a ferret. A globoid cardiac silhouette is a common finding in thoracic radiographs of affected animals. Image courtesy of radiology service at UGA.