News 99N General News RHD after One Year in New The Foundation for Research, Science and 40 days. Carcasses were found over the Zealand Technology, the Ministry of Agriculture whole baited area soon after the baiting. and Forests, and regional governments At the natural epidemic site, the daily initiated a research programme in death rate peaked at day 20 and new Last year BNI [18, 100N-101N] reported August 1997. The programme is led by carcasses were still being found up to 80 on the illegal introduction of the rabbit Landcare Research working on the field days after the first death was recorded. calicivirus disease RCD (now reverted epidemiology of RHD, but includes a to its original name of Rabbit Haemor- consortium of other research agencies rhagic Disease or RHD) into New Zea- Sera from shot rabbits were tested for (AgResearch, the Rural Futures Trust, land. Readers will recall that in July antibodies to RHD using a competitive and Massey, Auckland and Lincoln uni- 1 1997 the Ministry of Agriculture had ELISA test developed in Italy . We used versities) investigating virology, vector a 1:40 dilution and assumed ‘inhibition’ declined an application to import the behaviour, predation effects, epidemi- virus as a biological control for rabbits, levels above 50% indicated immunity to ology and modelling. Some results of RHD. No rabbits (out of 60) were largely because of the lack of certainty Landcare Research’s work to date are immune on the natural site before the about its benefits and risks. The virus described here. epidemic, but this increased to 31% (n = was imported by persons unknown, 62) immediately after the epidemic. probably before the Ministry decision Status of RHD in New Zealand Eight per cent of rabbits (n = 60) were had been made, and released and immune on the biocide site before the spread by farmers in late August 1997 The disease has been spread, by people epidemic (presumably because the using a variety of bait concoctions. This and naturally, over most of the country farmer did some spot baiting before blatant breach of New Zealand’s border with variable effects on the rabbits. mass biociding) and this increased to biosecurity system caused considerable Mortality rates, where measured, have 43% (n = 60) immediately after. There anger in government agencies and varied from zero to 94% with reduc- were no differences in the levels of among many members of the public, tions of around 60-70% being common. immunity between these two sites, but exacerbated by the cavalier attitudes of Generally, natural epidemics have been more consistently successful than the other studies have shown higher levels some farmers. However, I am happy to various attempts at using RHD on baits. of immunity after biociding than after report that the initial stand-off between Largely to avoid the haphazard use of natural epidemics. The proportion of farmers and government reported in concoctions of virus obtained from antibody-positive rabbits among the the earlier article has been ameliorated dead rabbits in the field, a known lethal cohort that was alive before the spring somewhat by their common need to strain of the virus is now commercially 1997 epidemics has since declined on find out how the disease has worked. available to farmers in New Zealand. both sites, although the levels of anti- Everyone wanted to know whether bodies in those that were positive RHD, rabbits, and conventional control Field Epidemiology of RHD remained high. One explanation for this could be managed, or at least the out- might be that rabbits that survive infec- comes of the disease predicted, so that We compared the behaviour of RHD at tion have higher mortality rates than two sites in Central Otago, one where benefits could be maximized and risks rabbits that were never infected, i.e. the RHD was released by mass aerial minimized. disease is not without cost even if the baiting (biociding), and the other where animal lives. Challenge trials indicate it arrived naturally. Indices of rabbit New Zealand has major exotic verte- that loss of antibodies does not neces- abundance declined by 67% (from 68 brate pest problems and invests about sarily mean loss of immunity to further and 35 rabbits per spotlight kilometre, NZ$100 million a year on their control challenge. and on research. However, most of the respectively) on both sites during the spring 1997 epidemic. Rabbit abun- impacts of these pests remain unre- dance remained static for the next three Fresh rabbit carcasses appeared in a solved and biological control offers the months and then declined at a rate down-wind direction on both sites, at a only sustainable widespread solution greater than expected for that time of rate of about 100 m/day. A number of for many of these problem animals. The year to a low of ten and three rabbits per fly species were carrying RHD virus, use of RHD is the first modern attempt spotlight kilometre, respectively, in and preliminary work by AgResearch in New Zealand at biological control of June 1998. Numbers have begun to showed some rabbits became infected a vertebrate pest, and it would be a 2 increase again with the start of a new and died when exposed to flies . Scav- great pity if the unfortunate way it was breeding season, and reached 16 and engers presumably play a role in dis- introduced blighted future considera- five per kilometre in August 1998. seminating virus by opening carcasses tion of other biocontrol agents. This, and exposing infected tissues to flies. and the need to understand how it has At the biocided site, the daily death rate More fresh rabbit carcasses were scav- worked, has overridden some of the (indicated by the presence of fresh enged during the natural epidemic anger at its origin and brought many of rabbit carcasses along fixed transects) (41%, n = 157) than during the biocide the stakeholders together in a common peaked three days after the biociding, (18%, n = 127). Predicting the timing cause. and few new carcasses were found after and intensity of epidemics will partly Are we on your mailing list? BiocontrolNews and Information is always pleased to receive news of research, conferences, new products or patents, changes in personnel, collaborative agreements or any other information of interest to other readers. If your organization sends out press releases or newsletters, please let us have a copy. In addition, the editors welcome proposals for review topics. 100N BiocontrolNews and Information 1998 Vol. 19 No. 4 depend on understanding the role of whether the factor is similar to the sodium monofluoroacetate (Com- vectors. European non-pathogenic virus, and (c) pound 1080)7. This compared with only whether it imparts any immunity to 23% ± 4% egg loss (averaged from 12 A unique symptom among seropositive rabbits challenged with RHD virus. sites) during subsequent breeding sea- survivors, observed only in New Zea- sons when no rabbit control was con- land, is that a small proportion have lost In March 1998, we captured 64 rabbits ducted. Preliminary data from the their ears. from areas of New Zealand where RHD breeding season during the 1997 RHD had not been reported. Serum from epidemic indicated that 56% ± 10% Antibody Status of Other Species each was taken at capture, and one and (averaged from four sites) of eggs were six months after challenge with RHD. Feral cats, ferrets, harrier hawks, and to lost to predators where rabbit abun- Each sample was tested at four dilu- dance was originally high (up to 50 rab- a lesser extent hedgehogs, use rabbits as tions (1:10, 1:40, 1:160 and 1:640) using a food source either by scavenging or bits per spotlight kilometre) and both the competition ELISA specific for population declines were pronounced predation. By eating rabbits that have RHD and a less-specific indirect ‘sand- died of, or are infected with, RHD they (up to 90%). This is a similar predation wich’ ELISA used by the Applicant rate to that reported after rabbit poi- may produce antibodies in response to Group to measure the presence of any the virus, as occurs in foxes3. Our objec- soning. The longer-term implications caliciviruses. All rabbits were orally for dotterel populations, and for other tive was to determine whether any dosed after the first sample of serum predators, scavengers, or hares pro- native prey, are unknown. Continued was taken and the survivors again after monitoring during subsequent breeding duced an antibody response when six months. Each dose was 50 LD s of exposed to rabbits with RHD. 50 seasons will quantify the longer-term the Czech-strain of RHD virus, obtained effects of RHD on these native bird pop- We collected serum samples from pred- from the Elizabeth MacArthur Institute ulations. ators and scavengers, from an area of in Victoria. 1 mass biociding and from spot-baited Capucci, L.; Frigoli, G.; Rønsholt, L.; We found that: 14 rabbits survived chal- Lavazza, A.; Brocchi, E.; Rossi, C. (1995) areas, during February and May 1998. lenge including one that was seroposi- The samples were tested for RHD anti- Antigenicity of the rabbit hemorrhagic tive before the first challenge and one disease virus studied by its reactivity bodies using the competitive ELISA test that did not sero-convert at the first at 1:40 dilution. We found that: 53% (n = with monoclonal antibodies. Virus challenge but died at the second chal- Research 37, 221-238. 51) of cats, 10% (n = 51) of ferrets, 11% (n lenge; all but one of the survivors were = 18) of hawks and 3% (n = 30) of hedge- positive to ‘factor x’, the negative sur- 2 Barratt, B.