Researchers calculate that UK cattle and sheep are at risk from bluetongue
(press release 25 July 2007)
Research just published in the Royal Society’s journal Interface predicts that if midges carrying bluetongue virus (BTV) were to arrive in the UK, then it would be likely to spread amongst ruminants such as cattle and sheep. This prediction is all the more apposite given the reports last week of fresh cases of bluetongue near the coast of Belgium, and in Germany.
This conclusion was the result of collaboration between the Institute for Animal
Health and the Universities of Cambridge and Liverpool. The researchers
developed a new mathematical model for the transmission of BTV and applied it
using existing data on the speed of growth of BTV in Culicoides biting
midges, the survival rate of the midges, the ratio of midges to ruminants in an
area, the rate at which midges bite cattle and sheep, and the likelihood that
being bitten would result in infection. Published data applicable to
conditions
in the UK were used whenever possible.
Temperature is a critical factor in the spread of BTV as it affects several aspects of the transmission process: the time it takes for the virus to develop in the midges until it is ready to be transmitted onwards; the rate at which the midges bite ruminants for a blood meal; and how long the midges live.
The analysis suggests that the risk of spreading BTV is greatest when the temperature is between 15 and 25°C. At lower temperatures the virus does not grow in the midges sufficiently for it to be transmitted. At higher temperatures the midges might die before the virus has grown to sufficient levels to be transmitted. However, local temperature variation and other environmental conditions might well enable the midges to survive sufficiently even when the peak temperature in a given day does rise above 30°C.
The research was supported by the Biotechnology and Biological Sciences Research Council (BBSRC) and the Department of the Environment, Food and Rural Affairs (Defra).
Assessing the risk of bluetongue to UK livestock: uncertainty and sensitivity analyses of a temperature-dependent model for the basic reproduction number
Simon Gubbins1, Simon Carpenter1, Matthew Baylis2, James L.N. Wood3 & Philip S. Mellor1
Published in Interface, a journal of the Royal Society, on 17th July 2007.
(available online at http://www.journals.royalsoc.ac.uk/content/fr145782r5223174/fulltext.pdf).
1Institute for Animal Health, 2Faculty of Veterinary Clinical Science, University of Liverpool, 3Cambridge Infectious Diseases Consortium, Department of Veterinary Medicine, University of Cambridge.
Bluetongue in northern Europe
Bluetongue was unknown in northern Europe until last summer, when bluetongue virus serotype 8 was introduced by means unknown. The new cases of bluetongue reported in Belgium (17 July 2007) and Germany (20 July 2007) demonstrate that infectious midges are actively spreading the virus in those countries. (BT_PressRel_23jul07.htm)
The successful spread of this serotype to domestic animals in Belgium, France, Germany, Luxembourg and the Netherlands is believed to have been helped by higher than normal temperatures (John_Gloster_3apr07.htm). The virus is spread by biting Culicoides midges, in which the virus replicates. Warm temperatures increase the rate at which the virus grows in midges, and increase the rate at which the midges bite their victims for a blood meal. It is during this process that the virus is transmitted from midge to ruminant and vice versa. A few years ago other serotypes of bluetongue virus had spread to southern Europe from more southerly latitudes, and became established. Those serotypes have not spread to northern Europe.
Bluetongue virus causes disease in sheep, cattle, and other ruminants
Although all ruminant species can be infected by the bluetongue virus, clinical signs of the disease are usually restricted to domesticated breeds of sheep. Other animals such as goats and cattle rarely show any symptoms. Bluetongue virus may therefore spread into new areas without being noticed. However, a relatively high number of cattle have been clinically affected during the current outbreak in Northern Europe.
In sheep, bluetongue is characterised by a fever that may last for several days. The virus mainly affects small blood vessels and this can lead to reddening and swelling of the lips, mouth, nasal linings and eyelids. Swelling of the tongue can lead to a restriction of the blood supply, leading to a blue colouration, hence the name of the disease. Animals may have quickened breathing. Nasal discharges, excess salivation and frothing are common. Lameness may occur. Animals can lose condition rapidly, including muscle degeneration. BT_clinical.pdf
There is no treatment for bluetongue. Prevention may be possible by vaccination (but only if a vaccine of the appropriate serotype is available) and by controlling midge populations (with insecticides or, where practical, by control of breeding sites), but neither is totally successful, and by applying animal movement controls.