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Analysis of the Spread of Rabbit Calicivirus from Wardang Island through Mainland Australia - Field evaluation of RCD under quarantine

Project start date: 01 January 1993
Project end date: 01 October 1996
Publication date: 01 October 1996
Project status: Completed
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The escape of RCD from Wardang Island was associated with an experiment which began on l3th September 1995. The presence of infected rabbits and increased insect activity, associated with warm weather from 18th - 20th September, probably facilitated the spread of RCD from experimental sites to sentinel sites within the quarantine facility on Wardang Island. The light westerly winds at the time were consistent with the easterly direction of spread. Accidental spread into sentinel sites by personnel involved in the project can be ruled out. Scavenging birds also seemed unimportant. An infected rabbit was collected on the north-east coast of War dang Island, outside the quarantine area, on 29th September 1995. A similar focus was detected on the north coast a short while later. The disease spread rapidly; dead rabbits were found up to 300 metres from the foci within a few days. It was subsequently shown that blowflies, Calliphora stygia, collected on 24th September 1995 from traps placed immediately outside the quarantine area on Wardang Island, were positive for rabbit calicivirus. The testing was done at AAHL using PCR techniques. No rabbit calicivirus was detected on C. nociva or bushflies, Musca vetustissima, collected at the same time. A source of virus was available on Wardang Island from 13th September until at least the 19th October 1995 when the last active case of RCD was recorded on the island. However, because rabbits in the experimental sites within quarantine were quickly killed and removed, infected rabbit carcases would have been most common after 29th September 1995 when the first infected rabbits were discovered outside the quarantine area. Rabbit carcases may have provided an increasing source of virus at that time but this would have rapidly declined once the majority of rabbits were killed by poisoning, ripping and fumigation as part of the emergency protocol. Rabbits infected with rabbit calicivirus were found on Point Pearce, on the mainland adjacent to Wardang Island, from 12th October onwards, but were first recorded at Yunta and Gum Creek in north-eastern South Australia on 28th October. Nucleotide sequences of virus samples show that virus recovered from Yunta was identical to those recovered from Wardang Island but differed slightly from the strain recovered from Point Pearce. While such information must be treated carefully in view of natural variation in populations of viruses, the simplest explanation is that virus was transferred directly from Wardang Island to Yunta. Working on the basis of the rate of spread of the virus from the initial focus at Yunta and from the age-structure of surviving rabbits at Gum Creek (assuming rabbits less than 6 weeks old suffered lower mortality than older rabbits) it seems likely that the disease arrived in north-eastern South-Australia in the first two weeks of October 1995. On this basis, the projections of insect movements from Wardang Island (Wardhaugh and Rochester 1996) are extremely interesting, indicating that movement of day-flying insects on 12th, 13th and 14th October 1995, could account for the spread of virus to Yunta and indeed the broad distribution of virus eventually recorded in South Australia during late 1995. Despite reasonable evidence that insects are involved in the transmission of RCD, Wardhaugh and Rochester (1996) state that the slowing in the rate of spread and the small number of new cases during the sUn1mer of 1995 - 96 could not be explained by a lack of vectors. Furthermore, field samples collected at sites where RCD had been widespread confirmed that substantial numbers of flies and mosquitoes persisted throughout the summer. Factors other than the presence of vectors must affect the spread of the virus. Temperature is likely to be the most important factor. Laboratory tests at AAHL show that survival of the virus declines rapidly with increasing temperature, and normal summer-time temperatures in inland Australia would be expected to reduce potential for the virus to survive. In the 1995-96 summer, new foci of RCD were mainly observed in coastal areas of South Australia where mean daily temperatures were generally below 27oC. Continuing, slow spread of the virus during the winter suggests that low temperature might be important, possibly by reducing the activity of flying insects. A simple model is being developed to explore the seasonal behaviour of RCD in Australia. This can be tested against the seasonal timing of disease outbreaks in other countries. It predicts that within southern Australia outbreaks of RCD should occur mostly in spring and autumn whereas in inland Australia winter outbreaks would be more common

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Project manager: David Beatty
Primary researcher: CSIRO