Abstract

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 historic 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 an 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 historic range of the species. Because the extant populations at Caswell State Park 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 California State University Stanislaus’ Endangered Species Recovery Program to design and implement a controlled propagation and reintroduction program (plan available at www.esrp.csustan.edu) (VIN editor: Original link not accessible 02–02–2021).

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 screening of all founding adult breeders captured at Paradise Cut before translocation into the controlled propagation pens at the start of the breeding season
• Health screening of all progeny born in the controlled propagation pens before reintroduction into San Joaquin National Wildlife Refuge
• Disease monitoring in the captive and reintroduced populations via complete necropsies and histopathologic evaluations of all dead rabbits for which sufficient remains are recovered and by serologic surveys for select pathogens
• Disease screening of other lagomorph species at the reintroduction site prior to reintroduction of riparian brush rabbits
• Opportunistic disease screening in sympatric mammals
• Individual animal treatment and care as needed

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, ultrasonography, ophthalmologic examinations, and cytology and biopsies of superficial masses.

As of the end of 2003, 26 rabbits have been brought into captivity to serve as founding breeders; 340 offspring have been produced; and 243 rabbits have been reintroduced to the wild at San Joaquin National Wildlife Refuge. As of March 15, 2004, 63 necropsies have been performed on rabbits for which sufficient remains were available. Major causes of mortality have included: predation; parasitic encephalitis (presumed Baylisascaris), necrotizing typhlitis, trap-related trauma (including conspecific aggression), bacterial sepsis, inanition/starvation (in neonates), and lymphoproliferative disease. Principle causes of morbidity which have required therapeutic intervention have included: ocular disease (keratitis, uveitis, conjunctivitis), wounds related to radio collars, and miscellaneous traumatic injuries, wounds, and abscesses.

Thirty rabbits from the 2003 breeding season were screened for antibodies to Encephalitozoon cuniculi and Treponema cuniculi. All 30 rabbits were seronegative for Treponema, and 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. We have not seen clinical or pathologic evidence of disease due to either pathogen in the captive or reintroduced populations.