Rabbit haemorrhagic disease: advantages of cELISA in assessing immunity in wild rabbits (Oryctolagus cuniculus)

Rabbit haemorrhagic disease (RHD) is an acute fatal disease of domestic and wild European rabbits (Oryctolagus cuniculus) caused by RHD virus (RHDV). Accurate assessment of immunity is of great importance for the conservation and control of wild rabbits. We evaluated a competitive ELISA (cELISA) against isotype ELISAs for assessing the protective immunity against the disease by challenging 50 wild-caught rabbits with a lethal dose of RHDV. Death or survival to the challenge was used as a criterion to determine the performance characteristics of the assay for the assessment of immunity in rabbits. At 1:10 dilution, a serum exhibiting ≥ 25% inhibition (1:10(25)) was regarded as the presence of RHDV-specific antibodies. Eleven of 16 (68.8%) rabbits with antibodies at 1:10(25) (<1:40) died of RHD. When the cut-off was moved from 25% to 50% inhibition (1:10(50)) at 1:10 serum dilution, the assay sensitivity, specificity and accuracy for the protective immunity were improved from 84%, 54.2% and 69.4% to 84%, 100% and 91.8%, respectively. We also demonstrated at the epitope amino acid sequence level why the presence of the RHDV-cross reactive benign rabbit calicivirus, which interfered with isotype ELISAs, had little impact on the specificity of the cELISA for the diagnosis of RHDV infection. The presence of RHDV-specific antibody at 1:10(50) by the cELISA is a reliable indicator for the protective immunity. In contrast to isotype ELISAs, the cELISA is a valuable specific tool for monitoring the herd immunity to RHD for the conservation and management of wild rabbits in the field.

Antibody status and survival of Australian wild rabbits challenged with rabbit haemorrhagic disease virus

In Australia, the epidemiology of rabbit haemorrhagic disease virus (RHDV) is complicated by non-pathogenic forms of calicivirus (bCV) co-circulating with RHDV and providing variable protection from RHDV. Currently no bCV virus-specific antibody tests exist; however, a series of four ELISAs used to detect antibodies to RHDV provided an indirect means to detect antibodies to bCV, enabling antibody categories of seronegative, maternal RHDV, RHDV or bCV to be determined. Rabbits (188) from four locations were challenged with RHDV and logistic regression models determined that, for rabbits <15 months old, survival was dependent on antibody titres alone and the relationship did not vary with age, capture site, gender, liveweight or reproductive status. All rabbits survived challenge after reaching 15 months of age, irrespective of their antibody titres. Where bCV antibodies were prevalent in young rabbits, the bCV category did not adequately summarise all information about rabbit survival that can be obtained from antibody titres. Within antibody categories, 95% of rabbits with RHDV, 33% with bCV, 40% with maternal RHDV and 22% with seronegative antibodies survived. The high survival rate of adults implies that natural outbreaks or controlled releases of RHDV will have little impact on adult breeding rabbits. Therefore, where RHDV and bCV are endemic, conventional rabbit-control programs targeting the immune breeding populations should provide the most predictable outcome for long-term maintenance of low rabbit populations.

Temporal dynamics of rabbit haemorrhagic disease virus infection in a low-density population of wild rabbits (Oryctolagus cuniculus) in New Zealand

A longitudinal capture–mark–recapture study was conducted to determine the temporal dynamics of rabbit haemorrhagic disease (RHD) in a European rabbit (Oryctolagus cuniculus) population of low to moderate density on sand-hill country in the lower North Island of New Zealand. A combination of sampling (trapping and radio-tracking) and diagnostic (cELISA, PCR and isotype ELISA) methods was employed to obtain data weekly from May 1998 until June 2001. Although rabbit haemorrhagic disease virus (RHDV) infection was detected in the study population in all 3 years, disease epidemics were evident only in the late summer or autumn months in 1999 and 2001. Overall, 20% of 385 samples obtained from adult animals older than 11 weeks were seropositive. An RHD outbreak in 1999 contributed to an estimated population decline of 26%. A second RHD epidemic in February 2001 was associated with a population decline of 52% over the subsequent month. Following the outbreaks, the seroprevalence in adult survivors was between 40% and 50%. During 2000, no deaths from RHDV were confirmed and mortalities were predominantly attributed to predation. Influx of seronegative immigrants was greatest in the 1999 and 2001 breeding seasons, and preceded the RHD epidemics in those years. Our data suggest that RHD epidemics require the population immunity level to fall below a threshold where propagation of infection can be maintained through the population.