Very recently, I read an editorial written in the Australian Veterinary Journal for Jan/Feb, 2003 by the eminent Australian veterinary researcher, Dr Michael Studdert of the School of Veterinary Science, University of Melbourne, Parkville, Victoria. His subject was the ongoing outbreak of infection caused by West Nile Virus (WNV) in the USA and Canada, and its implications for Australia. Dr Studdert points out the fact that a strain of WNV called Kunjin virus exists in Australia now. Kunjin virus is not considered to cause a significant infection in humans, although this virus has been isolated from two fatal cases of the disease in horses in Australia. Dr Studdert's point in mentioning Kunjin virus is that, because it is a WNV agent that cycles through mosquitoes and birds, in Australia there are mosquitoes and species of birds that could allow the strain of West Nile Virus now plaguing North America to establish itself if it ever entered Australia.

West Nile Virus was originally isolated in Uganda in 1937 and has since spread through the eastern hemisphere, and in 1999, it surfaced in the eastern USA. Since then it has moved into many eastern and western states in the USA as well as into the eastern and middle provinces of Canada. As I write this, it is early July, and we have just learned that two magpies found dead in my home province, Alberta, are positive for WNV disease. The source of this virus to North America isn't known, but it seems that the North American virus is identical to the virus isolated from a dead goose in Israel in 1998.

With the steady movement of West Nile Virus through eastern Canada and now into the western Canada, and spurred by Dr Studdert's article, I thought it might be an idea to discuss the disease as it relates to pigeons. No doubt fanciers and the general public alike have concerns about domestic and feral pigeons when WNV surfaces each summer, simply because they are birds. Now some facts. The natural course of this virus is a through a mosquito-bird cycle, and only very occasionally do humans and horses intervene in this cycle.

West Nile Virus is carried by mosquitoes. When an infected mosquito feeds on a bird, virus is transmitted to the bird which then becomes infected. Within the bird, the virus multiplies, and within days, viral particles appear in the blood, in a phase called 'viraemia' (vir = virus; aemia = in the bloodstream). When another mosquito feeds on this infected bird, it too becomes infected with WNV, and the mosquito-bird cycle continues. In many species of birds studied so far, the infected bird appears healthy throughout the whole process.

The presence of virus in the blood of an infected bird is a brief phase. However, during this time, the immune system of the infected bird kicks into high gear. The result is that the bird develops solid immunity to WNV and clears the infection from its system. Later, if this bird is fed upon again by an infected mosquito, no viraemia develops --because it is now solidly immune -- and the bird does not transmit the virus to mosquitoes. However, birds such as crows, magpies, blue jays, etc. are notable exceptions, and the disease moves beyond viraemia, the birds actually get the disease, and die. However, before death intervenes, these infected birds can transmit the virus to mosquitoes that feed on them. Some of the signs in infected susceptible species of birds include circling, convulsions, staggering, abnormal head posture, weakness, inability to stand, etc..

The virus remains in the infected mosquitoes, and can be transmitted through its eggs and the larval stages of its life cycle. In this way, the mosquito becomes a reservoir for the virus, and is most likely the way WNV overwinters.

If an infected mosquito feeds on an animal such as a human or a horse, very occasionally, the individual may develop signs of illness which, in most humans is an influenza-like condition. Many other infected humans simply do not develop symptoms of the disease. Very occasionally, the disease does affect the brain of humans, producing inflammation called encephalitis, and in some aged humans, death. Many humans infected with the virus develop solid immunity and recover very well. Over 80% of infected humans do not develop symptoms of the disease. Humans and other mammals like horses are what is known as "dead end" hosts, because the number of viral particles that circulate in their bloodstream is so low that they cannot infect mosquitoes. As health officials in Canada mentioned several times in television reports during the summer of 2002, the risks to humans from WNV are very small indeed compared with the much, much greater risks associated with the effects of influenza in the elderly. Even so, there have been deaths associated with some cases of WNV in humans.

Now what is the role of pigeons in this disease? Because it is a bird, what part does the pigeon play in the spread of WNV? Overall, the news is good. Laboratory experiments with pigeons that were exposed to WNV by mosquito bites have shown that, yes indeed, these birds became infected, virus appeared in their bloodstream and reached a maximum level after the second or third day, and was eliminated from the system by the fifth day after infection. Protective antibodies were produced by the immune system and were detected by the seventh day after infection. Of importance to us as pigeon fanciers was the fact throughout this whole study, these infected pigeons remained completely healthy, with no indication of disease. These infected pigeons also did not pass the virus from pigeon to pigeon.

This experiment also studied the levels of viraemia, that is, how many viral particles were present in the blood of these pigeons. The maximum level in any of these experimental pigeons was 50,000 viral particles per cc of blood (a cc is approximately 1/30th of an ounce), but the average number of viral particles per bird was actually closer to 20,000 per cc. These figures sound high, but what do they really mean? Generally, levels of 10,000 to 100,000 viral particles per cc of blood in birds are believed to be "slightly" infectious to mosquitoes. This means that fewer than 10 mosquitoes in 1000 that bite these birds, would become infected. So the good news is that, while pigeons can and do become infected, it is important to be aware that during their highest level of viraemia, pigeons are remarkably poor transmitters of WNV. Thus, more good news is that pigeons may be the same as humans and horses, in the sense that they too may well be "dead end" hosts, incapable of spreading the virus, simply because of the very low levels of virus that circulate in their bloodstream.

By contrast, the common house sparrow (English sparrow) develops a very high level of viremia greater than 100,000,000 viral particles per cc of blood, so they are highly infectious to mosquitoes, and therefore are of major importance in the spread of WNV. In North America, it seems that there are now more than 110 avian species known to be infected with WNV. Among domestic birds and animals, geese and horses seem to be the only species at great risk of developing serious illness if they are bitten by an infected mosquito.

Now, what are a few of the protective things we might do as pigeon fanciers? Firstly, in dealing with neighbors, city councillors, etc., in fact anyone who might be concerned about pigeons, inform them of the facts presented here, particularly those facts regarding pigeons and their non-role in the transmission of WNV. This is an important point, since an alarmed public can get a very wrong idea about the role of pigeons in the spread of WNV -- simply because they are birds. Secondly, as a personal opinion, in the face of an outbreak of WNV disease, I do not believe we should be encouraging birds such as crows, magpies, blue jays and house sparrows to nest in our neighbourhoods, or to gather at our bird feeders. Why encourage these birds to become next summer's primary agents in the spread of WNV? Thirdly, eliminate pools of standing water in bird baths, wading pools, eaves troughs, etc., as these can be good breeding sites for mosquitoes. Securely cover rain barrels and avoid filling small ornamental pools in the yard to prevent mosquitoes from using them as breeding sites. Fourthly, the use of insecticides such as the permethrin-based products on loft structures during mosquito season may be helpful in keeping their numbers down.

If you find any of the key transmitting species of birds dead, before picking up the bird, slip your hand and arm into a plastic bag, pick up the bird, then turn the bag inside out, tie it and get it to your nearest city or state health official as soon as possible. If there is any delay, keep the carcass chilled. Avoid getting the blood of birds on your hands.

I hope that this brief look at West Nile Virus and the non-role of pigeons in the spread of this disease may provide some measure of reassurance to concerned fancier Australian who fear that their pigeons may become targets of those who mistakenly believe that our pigeons are a source of this virus for susceptible species. They are not. I also hope very much that the strain of WNV now in North America stays well away from the shores of Australia!