Recovery of the black-footed ferret (Mustela nigripes) is one of the most ambitious breeding and reintroduction efforts in conservation. Rescued from the brink of extinction with only 18 animals remaining, this North American carnivore now has ∼850 individuals in the combined ex-situ and in-situ population. Black-footed ferrets have been reintroduced to the western Great Plains, yet success has varied among sites. Some populations have expanded, while others have produced limited numbers of wild-born kits and rely on yearly augmentation from the ex-situ breeding program. A multi-disciplinary biomedical survey of wild black-footed ferrets was conducted to assess health, disease, immunology, genetics, and reproduction. From 2002 through 2006, 253 wild black-footed ferrets (134 males, 119 females) were captured at reintroduction sites in South Dakota (n=129), Montana (n=2), Wyoming (n=33), Mexico (n=12), Utah (n=19), Colorado (n=6), and Arizona (n=52). Each animal was trapped and anesthetized for a health exam and blood collection, then returned to the capture site. In this study, we report a serologic survey to assess antibodies to sylvatic plague (Yersinia pestis), tularemia (Francisella tularensis), toxoplasmosis (Toxoplasma gondii), and canine distemper virus in reintroduced black-footed ferrets and/or wild-born descendants at various reintroduction sites. Blood samples also were evaluated for heartworms (Dirofilaria immitis) using a commercially available, enzyme-linked immunosorbent assay (ELISA) for detection of adult D. immitis antigen in plasma or serum (DiroCHEK®, Synbiotics, Inc., San Diego, CA, USA), as well as blood smears to detect circulating microfilaria.
All ferrets lacked antibodies to sylvatic plague. Positive titers to tularemia were detected in 11 animals in South Dakota (n=4), Wyoming (n=2), and Utah (n=5). Only two animals in South Dakota had positive titers to toxoplasmosis. Antibodies to canine distemper virus were not detected in unvaccinated, wild-born animals. Following vaccination, protective titers were observed for at least 6 months after an injection of the recombinant vaccine PUREVAX® (Merial, Inc., Athens, GA, USA). For heartworm assessment, 36 of 233 (15.5%) blood samples tested positive for D. immitis antigen by ELISA (antigen+) and/or presence of microfilaria (microfilaria+). Of the 36 positive samples from 33 individuals, 13 were antigen+/microfilaria+; 17 were antigen+/microfilaria-; and six were antigen-/microfilaria+. All 33 positive ferrets appeared healthy with no evidence of dyspnea, and three ferrets were recaptured up to 1 year later with no symptoms of heartworms. Although DiroCHEK® claims to be a highly specific, sensitive test for D. immitis (detecting female uterine antigen in the filarid nematode), we proceeded to validate the test by molecular methods. Using DNA extracted from the positive ferret blood samples, filarid DNA was amplified, sequenced, and identified as a filarid species that was not closely related to D. immitis. The species of filarid could not be identified in an extensive filarid DNA sequence database. The anatomic location of adult worms in the ferrets remains undetermined.
To date, these results demonstrate the susceptibility of the reintroduced and wild-born black-footed ferret populations to the potentially fatal sylvatic plague and show the need for distemper vaccinations in wild ferrets. Data also indicate that the commercial, antigen-based ELISA kit results in false positives and is not adequate for heartworm surveillance in the black-footed ferret. This comprehensive biomedical survey illustrates the need to understand factors influencing survival of endangered species after reintroduction. This collaborative, multi-disciplinary approach also demonstrates the benefits of collaboration among veterinarians, scientists, and wildlife managers.
This project is dedicated to the memory of Dr. Elizabeth (Beth) Williams, a prominent wildlife pathologist, disease scientist, colleague, and friend. Beth and her husband, Dr. Tom Thorne, had remarkable accomplishments in life as wildlife veterinarians and made substantial scientific contributions to the black-footed ferret recovery program.