Date: Fri, 3 Oct 1997 13:02:33 +1000 (EST)
Tony Robinson,CSIRO, wrote

"In response to a request for further information regarding possible Australian arthropod vectors that may be transmitting RHD virus (called rabbit calicivirus disease (RCD) virus in Australia), I can provide the following information."

"Our current view is that insects are the main means of natural spread of RCDV. The 300 km or so spread from Wardang Island to Yunta and the Flinders Ranges in South Australia over a period of a few weeks and then the rapid spread to other regions of Australia over the succeeding months was a reasonable indication that insects were important and, although some later spreading may have been deliberate, the magnitude of the event suggested that spread by people in four-wheel-drives was a minor component. Scavenging by mammals and birds may also play a part by opening carcasses and exposing large amounts of virus in blood and organs to flies. There appears to be very little virus shed by infected rabbits prior to death and most virus is found in the liver and blood. Some virus is found in urine in animals found dead and this could also be a source (Gelmetti _et al_. 1992, J. Appl. Rabbit Res. 15: 1435-1471). Also after animals die there is frequently blood stained fluid coming from the nasal cavity, which is presumably a post mortem change and probably contains a great deal of virus. This could serve as a source of virus for flies feeding on an unopened carcass. Contact spread may play a role in spreading virus in warrens. As the virus can retain infectivity after traversing the alimentary tract of dogs (del Carmen Simon _et al_. 1994, Rec Med Vet, 170: 841-845) it may also be able to retain infectivity through avian predators and scavengers, allowing a wider dispersal of virus, although this has not been investigated."

"A number of insect species have been shown to transmit the disease in the laboratory and, with other species, have sometimes been found in the field to be contaminated with virus. In the laboratory, a species of mosquito commonly found in association with rabbits and a major vector of myxoma virus (_Culex annulirostris_), the European rabbit flea (_Spilopsyllus cuniculi_), the Spanish rabbit flea (_Xenopsylla cunicularis_) and the Australian bush fly (_Musca vetustissima_) have been shown to be capable of transmitting the virus."

"In the case of the mosquito, 100 female _C. annulirostris_ were allowed a brief interrupted feed from the ear or shaven flank of a RCD virus-infected rabbit 16 and 20 hours post infection (p.i.). Mosquitoes were transferred to two susceptible rabbits and allowed to continue feeding. Both acceptor rabbits died of RCD 42 and 52 hours p.i. Virus (strictly speaking, viral genomes) was also found in mosquitoes by PCR. Regarding the fleas, 100 Spanish or 100 European rabbit fleas were allow to feed on an infected rabbit from 16 hours p.i. until death. For each species of flea, fleas were transferred to two susceptible rabbits and both died of RCD within 4 days. Again, virus could be found in the fleas by PCR. There is no evidence that the virus replicates in these insect species and it is proposed that transmission is mechanical. There is no information on how long virus will persist on or in these insects. In these experiments the insects were transferred from infecteds to susceptibles within a few hours (Lenghaus _et al._, 1994, in "Rabbit haemorrhagic disease: issues in assessment and control", Munro and Williams eds. Bureau of Resource Sciences, Canberra. 104-129)."

"With bush flies, the experimental design was somewhat different because of the biology of the species. The bush fly breeds mainly in cattle and horse dung in outback Australia and each summer and autumn migrate from their breeding grounds to cover most areas of Australia. They feed on secretions from eyes and mouth of man and animals to obtain moisture and the protein required for egg laying. They will also obtain moisture and protein from carcasses. They are not biting insects like fleas or mosquitoes although, like a number of other flies, they do have prestomal "teeth" surrounding the prestomal cavity and they can utilise these to scrape surfaces, sometimes resulting in the sensation of a "bite". In laboratory experiments, different groups of approximately 20 food and water-deprived bush flies were allowed access to infected rabbits at different times after inoculation and later to unopened and opened carcasses. After various times, flies were transferred to susceptible rabbits. Although there were complications with the experiment in that at early times flies were allowed [to remain in contact] too long with infected rabbits which made them less interested in feeding on the susceptible rabbits, the experiments clearly showed that transmission could occur from rabbit carcasses both intact and opened but not from the living infected rabbits (CSIRO Divisions of Animal Health and Wildlife and Ecology, 1997, Field evaluation of RCD under quarantine. Project CS.236 report. Meat Research Corporation, Sydney)."

"Direct evidence of insect transmission in the field has not been possible but evidence from PCRs done on insects trapped in the field have shown that a number of insect species become contaminated with virus (CSIRO Divisions of Animal Health and Wildlife and Ecology, 1997, Field evaluation of RCD under quarantine. Project CS.236 report. Meat Research Corporation, Sydney). In one case it was shown that the virus was viable. Species that have been shown to be contaminated with virus in the field are:

_Musca vetustissima_ bush fly
_Calliphora stygia_ large brown blowfly
_Calliphora novica_ and _C.augur_ lesser brown blowflies
_Chrysomyia rufifacies_ secondary green blowfly
_Chrysomyia varipes_ small green blowfly
(positive by inoculation into rabbits)
_Aedes postspinaculosis_ mosquito
_Aedes notoscriptus_ mosquito

It is becoming clear that the virus contaminates a wide range of insect species in the field and there is no doubt that the list will grow. Again, there is no evidence that there is an intrinsic cycle in any of these insects."

"Although all these insects have been found to be contaminated this does not mean that they are important or even involved in transmission. Experimental transmission in the laboratory shows the potential for the two fleas, the mosquito _C. annulirostris_, and the bush fly to be important in the field. The blowflies pose a different problem. It is not surprising that they become contaminated in the field but can they transmit? One possibility is that they leave their faeces or regurgitated gut contents on vegetation which could be consumed by rabbits (B. Cooke, pers comm.). The virus is infectious by the oral route. With bush flies it is perhaps easier to see a mechanism of transmission directly from carcasses to the eye of a rabbit."

"The work is continuing on the role of insects in transmission of RCD virus. It will always be difficult to determine precisely which insects are involved in any particular transmission event. However, by piecing together information from virology, entomology and climatology we should be able to develop a fairly accurate picture."

Any inquiries re the above, contact Tony Robinson
CSIRO Wildlife and Ecology


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