The Eurasian red squirrel, Sciurus vulgaris, is under severe environmental stress in Britain, as the grey squirrel, Sciurus carolinensis, and an associated virus wipe it out in England and Wales. In parts of Ireland and Scotland, the species still resists the invader(s) successfully, but will the virus alone wipe it out in more of its territory?
Dr Neil Reid and his colleagues from Queen's University, Belfast the Moredun Research Institute, the Animal Health and Veterinary Laboratories Agency and the University of Lincoln have tried to solve the mystery of the virus's transmission. He explains, "our work suggests that the devastating effect of the disease may be down to the apparent ease with which it spreads via urine, parasites and faeces. Moreover, unlike many viruses which peak in winter our experiments suggest squirrelpox persists best in spring and summer when squirrels are more active and likely to encounter the disease."
The virus concerned is known as squirrelpox virus or SQPV and is generally fatal. The nearest known relative of this virus is Orf virus in sheep. In the grey, however, this situation results in the invading squirrel increasing 25X faster as the red dies out. In fact the grey hardly infects the grey at all, presumably because it has carried the virus in North America for innumerable generations and has had time to adapt. Although, he sampled populations in Northern Ireland had low infection rates, the effects would still be applied, and perhaps many infected reds had already died. Antibodies to the virus studied showed up healthily in the grey, but were absent in almost all healthy reds (97.5%).
The key to a recovery of the red squirrel population is obviously the path taken by the virus. Most likely was the direct grey squirrel reservoir route via faeces, urine or ectoparasites. The latter seemed most likely, while warm summer weather helped the virus to persist in urine. One puzzle in this long-term problem is the way in which red squirrels have been declining for a century, but the first virus in the species was only found in 1981.bAs far as parasites were concerned, many greys had ectoparasites (69%), while only 23% of the reds were infected with them. Fleas, mainly on greys, ticks, mainly on reds, and mites, only on greys, were the main parasites found on the 246 animals sampled. However, many of the greys' parasites had no SQPV (73% of the fleas) while the one red positive squirrel had parasites that carried the virus.
Male squirrels of both species had more antibodies to the SQPV, presumably because they put themselves about a bit. They may also be more vulnerable to infection than females. Urine is also involved in transmission, if the typical rodent patterns are followed. If we attempt to prevent the greys from contacting red squirrels regularly, that effort would be worthwhile in containing the spread of the disease. More research on how to do this, and also to enlarge the sample will help researchers trying to conserve red populations in Northumberland, Wales and one of the few large viable British breeding populations, in Scotland.
Dr Neil Reid and his colleagues are published by the open-access journal, PloS One.
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