OCTOBER 2009 IN THE SHADOW OF EQUUS - WEST NILE VIRUS ARRIVES IN BC
By Margaret Evans
West Nile virus (WNV) has finally arrived in British Columbia. Since arriving in the New York area ten years ago, the virus spread rapidly across the continent yet failed to reach BC until this summer when one horse (possibly two) became infected in Osoyoos. Just a few weeks later another horse was diagnosed with WNV in Aldergrove. None of them has been out of the province, confirming that the virus is present in various areas in BC. In 2006, a horse that had travelled to Colorado was infected with the virus before it returned to the province.
“This is not a surprising development,” said Dr. Paul Kitching, chief veterinarian and branch director at the Animal Health Centre. “British Columbia has long anticipated the arrival of West Nile virus and now it’s in multiple regions of the province. What is important is preventing infection as best as possible, for both humans and animals. Of all large land mammals, horses are particularly susceptible. There are vaccines for horses and horse owners should talk to their vets about getting their animals immunized.”
The virus is a mosquito-borne illness which is transmitted by biting the host. The virus can cause swelling and inflammation of the brain and spinal cord in horses, birds, and humans, sometimes leading to permanent neurological damage. However, many horses bitten by an infected mosquito will not develop the clinical disease. According to information on Alberta Agriculture’s website regarding WNV, horses may develop an asymptomatic infection and eliminate the virus. They will be none the worse for it. However, those that do show clinical signs may display a range of symptoms.
According to Dr. John Twidale, Chair, Equine Committee, BC Veterinary Medical Association, signs of the disease in horses include a dull sleepy attitude and muscle tremors in the face, chest, and hind legs, progressing to staggering, falling, and being totally unable to rise. Once a horse goes down, it may never regain its feet and has to be euthanized. Even horses which remain standing may have residual nerve damage causing lack of coordination of the hind end which will affect their gait for life. Handlers of horses showing clinical signs of instability should be extremely cautious in case a horse stumbles and collapses on them. Once neurological symptoms appear there is no specific treatment to cure the disease, which is why many veterinarians are urging horse owners to vaccinate their animals.
WNV is commonly found throughout Africa, southwest Asia, the Middle East, the Mediterranean region of Europe, and Australia. It was first diagnosed in a woman in the West Nile district of Uganda in 1937. In 1957 there was an outbreak among elderly patients in Israel where it was found to cause severe cases of both meningitis and encephalitis. In the 1960s the virus was recorded in Egypt and France, infecting both horses and humans. Studies on the evolution of the virus, though, show that it first emerged in Africa about 1000 years ago.
The first North American outbreak was in the New York City area in 1999 and prior to that it had never been documented in the Western Hemisphere. It arrived in Canada in 2001 in a wild bird in Ontario. Just a year later it showed up in horses in Ontario, Quebec, Manitoba, and Saskatchewan and in 2003 it infected horses in Alberta.
Horses and humans are the end carriers of the virus which means that it cannot be spread from horse to horse, human to human, or animal to human. But while a horse cannot give the virus to another, the fact that it has become ill (even mildly so) is a head’s up that there are infected mosquitoes in the area that could be a risk to other horses and people.
“While there are West Nile virus vaccines for horses, there is no vaccine for humans,” cautioned Dr. Bonnie Henry, director of the Vector-Borne Disease Program at the British Columbia Centre for Disease Control, an agency of the Provincial Health Services Authority. “Common sense precautions against getting bitten by mosquitoes should be taken, especially when outdoors, and even as we head into fall. This includes wearing light-coloured, long-sleeved shirts and long pants – especially in the evenings and early mornings when mosquitoes are most active – and using mosquito repellents that are federally registered, such as those that contain DEET and lemon eucalyptus oil.”
Preventative maintenance should be practiced around the barn. Whether or not owners decide to vaccinate their horses, they should be diligent about draining standing water where mosquitoes can breed such as buckets, barrels, any outside container, bird baths, the inside of tires, clogged gutters where rainwater can collect, and puddles that don’t drain. Everything should be turned over, emptied, or backfilled. Keep a lookout for dead crows and other corvids such as ravens, magpies, or jays and report these to the local health authority. Repair broken screen doors or windows to keep mosquitoes out of the house or closed areas of the barn.
In addition to infecting people and horses (including mules and donkeys), the virus can infect dogs, cats, domestic rabbits, alpacas, and a variety of wildlife. But as much as the virus is a real concern, it is not a cause for panic. According to the Canadian Animal Health Institute, less than one percent of mosquitoes in a region affected by WNV actually carry the virus.
WNV will likely remain a permanent fixture in the environment as it assimilates into regional ecosystems. Therefore, if owners decide not to vaccinate their horses for the remainder of the mosquito season, they may want think seriously about planning for it when it comes to vaccinating their horses next year.