Antibody response of endangered riparian brush rabbits to vaccination against rabbit hemorrhagic disease virus 2

Abstract

Rabbit hemorrhagic disease virus 2 (RHDV2; Caliciviridae, Lagovirus europaeus), the cause of a highly transmissible and fatal lagomorph disease, has spread rapidly through the western United States and Mexico, resulting in substantial mortality in domestic and wild rabbits. The disease was first detected in California in May 2020, prompting an interagency/zoo/academia/nonprofit team to implement emergency conservation actions to protect endangered riparian brush rabbits (Sylvilagus bachmani riparius) from RHDV2. Prior to vaccinating wild rabbits, we conducted a vaccine safety trial by giving a single SC dose of Filavac VHD K C+V (Filavie) vaccine to 19 adult wild riparian brush rabbits captured and temporarily held in captivity. Rabbits were monitored for adverse effects, and serum was collected before vaccination, and at 7-10, 14-20, and 60 d post-vaccination. Sera were tested using an ELISA to determine antibody response and timing of seroconversion. Reverse-transcription quantitative real-time PCR (RT-qPCR) was performed on rectal swabs to evaluate infection status. No adverse effects from the vaccine were observed. Before vaccination, 18 of 19 rabbits were seronegative, and RHDV2 was not detected by RT-qPCR on any rectal swabs. After vaccination, all rabbits developed an antibody response, with titers of 1:10-1:160. Seroconversion generally occurred at 7-10 d. The duration of antibody response was ≥60 d in 12 of 13 rabbits. Sixteen animals were released and 4 were recaptured several months later, offering a glimpse into longer duration immune response. Our study has informed vaccination strategies for this species and serves as a model for protecting other vulnerable lagomorphs against RHDV2.

Keywords: endangered species; rabbit hemorrhagic disease; serology; vaccination.

Figure 1.

Rabbit hemorrhagic disease virus 2 (RHDV2)-specific antibody titers in the serum of 13 vaccinated riparian brush rabbits (Sylvilagus bachmani riparius). Blood samples were collected at day 0, then at 7–10, 14–20, and ~60 d post-vaccination (dpv). The rabbit sera were tested using four 4-fold serial dilutions of 1:10, 1:40, 1:160, and 1:640. Rabbit 6 is not shown because it was seropositive before being vaccinated. Rabbits 3, 5, 13, 14, and 18 are not shown because they were not sampled at 60 dpv; however, rabbit 3 followed a pattern similar to rabbits 16 and 17; rabbit 5 followed a pattern similar to rabbit 10; rabbit 13 followed a similar pattern to rabbits 1, 9, and 19; rabbits 14 and 18 followed a pattern similar to rabbits 7, 8, and 11. An asterisk indicates no detectable RHDV2 antibody response at a given time.

Figure 2.

Rabbit hemorrhagic disease virus 2 (RHDV2)-specific antibody titers in the serum of 4 vaccinated riparian brush rabbits (Sylvilagus bachmani riparius). Blood samples were collected at day 0, then at 7–10, 14–20, and ~60 days post-vaccination. In addition, these rabbits were released into the wild and subsequently recaptured 2–3 times over the next 1.5 y (spring 2021, fall 2021, spring 2022). At each capture event, blood samples were collected, and a booster vaccine was administered. Rabbits 16, 17, and 19 were vaccinated 3 times (2 booster vaccines), and rabbit 18 was vaccinated 4 times (3 booster vaccines). The time between vaccinations ranged from 5–8 mo. An asterisk indicates no detectable RHDV2 antibody response at a given time.