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Disease monitoring in captively-propagated and reintroduced riparian brush rabbits (Sylvilagus bachmani riparius) in California

Presented at joint meeting of:

American Association of Zoo Veterinarians, American Association of Wildlife Veterinarians, and Wildlife Disease Association, San Diego, CA, 2004.

Kirsten V.K. Gilardi, DVM, Dipl ACZM,1* Karen A. Terio, DVM, PhD, Dipl ACVP,2 Laurissa P. Hamilton, BS3, Elizabeth V. Williams, MS3 and Daniel F. Williams, PhD3.

1Wildlife Health Center, UC Davis School of Veterinary Medicine, 1 Shields Ave, Davis, CA 95616 USA; 2University of Illinois Zoological Pathology Program, LUMC Bldg 101, Rm 0745, 2160 S First St, Maywood, IL 60153 USA; 3Endangered Species Recovery Program, California State University - Stanislaus, Department of Biological Sciences, Turlock, CA 95382 USA


The riparian brush rabbit (Sylvilagus bachmani riparius) is a state- and federally-listed endangered species. It is native to riparian communities in the northern San Joaquin Valley of California. Riparian habitat in the San Joaquin Valley has been reduced to less than 1% of its historical extent, primarily due to clearing of natural vegetation, irrigated agriculture, livestock grazing, impoundment of rivers, and stream channelization. At the time of state and federal listing of the species, there were only two known remnant populations of riparian brush rabbits in California, one in Caswell State Park along the Stanislaus River, and another along a overflow channel of the San Joaquin River (Paradise Cut). The size of both populations was too low to provide sufficient captures to estimate population sizes with capture-recapture population estimator models. To recover riparian brush rabbits, the US Fish and Wildlife Service set a goal of establishing three or more self-sustaining populations outside of Caswell Memorial State Park within the historical range of the species. Because the extant populations at Caswell SP and Paradise Cut were isolated from other suitable sites that are currently uninhabited, it was determined that reintroduction of individuals derived from existing populations would be required to achieve this goal. The USFWS contracted with CSU Stanislaus’ Endangered Species Recovery Program to design and implement a controlled propagation and reintroduction program (plan available at www.esrp.csustan.edu).

The UC Davis Wildlife Health Center (WHC) drafted guidelines for monitoring and maintaining the health of the captive and reintroduced riparian brush rabbit populations, and has provided veterinary input on all aspects of the program since its inception. Veterinary oversight has generally been in the form of:

Health screens typically consist of a physical examination under gas anesthesia, and blood collection for complete blood count, serum chemistry, and serum banking. Ectoparasites are collected opportunistically. Fecal analysis for gastrointestinal parasites in live rabbits is not performed routinely, except when disease due to parasite infections is suspected in clinically ill rabbits. Additional diagnostics performed on several rabbits have included radiography, ultrasonagraphy, ophthalmologic examinations, and cytology and biopsies of superficial masses

As of Spring 2004, 30 rabbits have been brought into captivity to serve as founding breeders, and 275 rabbits have been reintroduced to the wild at San Joaquin NWR. For cases in which a cause of mortality has been determined, major causes of death have included: predation; parasitic encephalitis (presumed Baylisascaris), necrotizing typhlitis, trap-related trauma (including conspecific aggression), bacterial sepsis, inanition/starvation (in neonates) and lymphoproliferative disease (in one case of lymphoma, a PCR for herpesvirus was positive, but this could not be confirmed as herpesvirus sylvilagus). Principle causes of morbidity which have required therapeutic intervention have included: ocular disease (keratitis, uveitis, conjuncitivits), wounds related to radiocollars, and miscellaneous wounds and abscesses.

Thirty rabbits from the 2003 breeding season were screened for antibodies to Encephalitozoon cuniculi, and to Treponema cuniculi . All 30 rabbits were seronegative for Treponema. One rabbit was weakly seropositive for Encephalitozoon. This rabbit was trapped in the wild at Paradise Cut for screening as a founding breeder, treated for a subcutaneous mass, and returned to the wild prior to testing.

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