The UK bird population would appear to be safe from attack by West Nile virus (WNV), according to new research, because the vast majority of birds have already been exposed. A new study carried out by researchers from the Centre for Ecology and Hydrology (CEH), examined more than 350 wild-caught and farm birds of 30 different species, from blackbirds and carrion crows to song thrushes and robins. Most of the birds had blood samples taken before being released, while brain tissue samples were also taken from crows and magpies caught and destroyed as part of pest control programmes.
The samples were tested for antibodies against up to 3 WNV types: NY, isolated in New York State, USA, in 1999; DAK, isolated in Central African Republic in 1967; and Is, isolated in Israel in the early 1980s. The results were startling, showing that 84 (59.6 percent) of the 141 samples tested for WNV-Is, 233 (66 percent) of the 353 samples tested for WNV-NY and a staggering 157 (91.3 percent) of the 172 samples tested for WNV-DAK had antibodies against the virus and had, therefore, been exposed to WNV at some time. Tests were also carried out to try and find active WNV in the samples taken – without success – although researchers were confident that it would be found if enough birds were tested.
While it will take some time for the full repercussions of the work to be explored, one initial conclusion is that the UK bird population is not at risk from WNV by virtue of its herd immunity, built up over many years. While no work has been done to verify this, it is also likely that all captive bird populations with access to open-air flights will have been challenged by WNV at some stage by virtue of the virus's transmission methods. Whether or not imported birds from the Americas could prove vulnerable if exposed to WNV from the UK population is yet to be seen, but the current high numbers of bird deaths in the USA and Canada show that there is no in-built immunity to WNV on that continent, leaving American stock open to challenge from the virus.
The apparent herd immunity against WNV found in UK birds is hardly surprising, considering the annual migration patterns of the many species that overwinter in Africa. This is backed up by the fact that antibodies to the Central African WNV-DAK strain proved the most prevalent in the tests carried out by CEH. With migratory birds arriving back in the UK each spring, the transfer of the disease to the native wild – and captive – bird population was just a matter of the presence of the appropriate methods of transmission, in this case mosquitoes and, to a lesser extent, ticks and mites.
It is impossible to tell when this process began, but it is clear that migration has been going on for many thousands of years, and it's therefore possible that WNV was in the UK bird population well before any of the virus strains were isolated in the laboratory. Dr Ernie Gould of CEH also suggests it's likely that some antibodies to WNV are passed from mother to chick: "The egg is a rich source of antibodies, but the levels of antibody we have recorded in adult birds mean this is not likely to be the only source of antibodies in the chicks. I would go as far as to say that the initial antibodies received from the egg are sufficient to allow the young birds to fight off the virus when they are exposed to it. This could be the reason why bird deaths from WNV have never been seen as a problem in the UK."
While Dr Gould and his team were unable to isolate any active virus in any of the samples tested, he is positive it is there. Tests on 69 juvenile spring-born birds caught and sampled during the summer of 2002 showed that 35 of these had been exposed to the virus and had developed antibodies. In each of these cases the source of infection had to be in the UK.
"We have to presume that the virus is circulating harmlessly in the bird population at a sub-clinical (showing no symptoms) level and is building herd immunity," Dr Gould said, "while we have not physically isolated the live virus, we have found RNA from the virus which suggests it is present.
To draw another analogy with the human population, measles virus is known to persist in the jejunum (part of the small intestine) of people who have immunity to the virus. If you know where to look you will find it. Purists say that until the virus has actually been isolated, it cannot be considered to be in a population, but I am convinced that if we do enough sampling, it will be found."
One of the most worrying points about WNV is that it can prove fatal to humans. Indeed it was deemed responsible for the deaths of about 270 people in the USA during 2002. To date there have been no human cases of WNV found in the UK. The Public Health Laboratories recently studied brain samples from 150 unexplained brain encephalopathies and found no evidence of the virus. Dr Gould said, however, that would be surprised if there has been no cross-infection of WNV to humans in the UK. "Research in the Volgograd region of Russia has shown that ticks can be important in the transmission of WNV. While it's likely that this is one method of infection at work in the UK, the spread rate is too high for ticks to be responsible for the level of antibodies we have seen. The primary route of infection has to be the mosquito."
So, if UK mosquitoes are carrying the virus (something Dr Gould hopes to start studying shortly), why haven't we seen cases in the human and animal population? "Possibly because we've not looked hard enough," Dr Gould says. In July, the Chief Medical Officer – who had seen an early draft of the paper produced by Dr Gould and his team – suggested that medical professionals should step up their efforts to test for WNV in the human population.
Contact:
Graeme Kirk
Acting Deputy Editor
Cage and Aviary Birds
E-mail: Graeme_Kirk@ipcmedia.com
ProMedMail
3 August 2003
Original web page at ProMedMail
