An epidemiological study of the spread of rabbit haemorrhagic disease virus amongst previously non-exposed rabbit populations in the North Island of New Zealand.
ABSTRACT To monitor the initial releases of rabbit haemorrhagic disease virus (RHDV) into previously unexposed rabbit populations in the North Island of New Zealand.
The study programme consisted of pre-release spotlight counts of rabbits on the study farms, pre-release serological samples to check for prior exposure to RHDV, a farmer-completed questionnaire and post-release spotlight counts to measure any change in rabbit numbers following the release of RHDV. In total, 23 sites within the lower North Island where RHDV was released during the period November 1997 to June 1998, were monitored. The most common release method involved the spreading of chopped carrot bait laced with a solution of virus-infected material obtained from dead rabbits.
Eighty percent of farmers thought that the disease had spread away from the release sites to areas where virus had not been liberated, although only 27% reported finding dead rabbits more than 300 m away from release locations. Seventy-three percent of farmers were satisfied with the overall effectiveness of rabbit haemorrhagic disease (RHD) as a means of reducing rabbit numbers, but 56% indicated they would modify the way they released the virus in the future. Average pre-release night spotlight counts per property ranged from 2.2 rabbits/km to 36.9 rabbits/km, the median being 12.8 rabbits/km. The time interval from initial release to when the first dead rabbit was seen which the farmer believed to have died from RHD varied from 3 to 21 days, the mean being 7.4 days and the median 7 days. The median change in night spotlight counts per site at 3 weeks after release, expressed as a percentage relative to pre-release counts, was -15.5% (range +18.9% to -76.9%) and at 6 weeks was -49.7% (range 0% to -76.9%). The time of the estimated peak of the disease epidemic ranged from 1 to 7 weeks after release of RHDV, the mean being 3.1 and the median 3 weeks.
Rabbit haemorrhagic disease reduced rabbit numbers on the majority of farms where the virus was released, and appears to be an effective measure for controlling rabbit populations in New Zealand.
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ABSTRACT: A study was conducted to investigate the persistence of rabbit haemorrhagic disease virus (RHDV) in the environment. Virus was impregnated onto two carrier materials (cotton tape and bovine liver) and exposed to environmental conditions on pasture during autumn in New Zealand. Samples were collected after 1, 10, 44 and 91 days and the viability of the virus was determined by oral inoculation of susceptible 11- to 14-week-old New Zealand White rabbits. Evidence of RHDV infection was based on clinical and pathological signs and/or seroconversion to RHDV. Virus impregnated on cotton tape was viable at 10 days of exposure but not at 44 days, while in bovine liver it was still viable at 91 days. The results of this study suggest that RHDV in animal tissues such as rabbit carcasses can survive for at least 3 months in the field, while virus exposed directly to environmental conditions, such as dried excreted virus, is viable for a period of less than 1 month. Survival of RHDV in the tissues of dead animals could, therefore, provide a persistent reservoir of virus, which could initiate new outbreaks of disease after extended delays.Epidemiology and Infection 09/2005; 133(4):719-30. · 2.84 Impact Factor