Post by juju on Mar 24, 2006 2:13:22 GMT -5
In the fall of 1999, the identification of West Nile virus (WNV) in dead birds found near and at the Bronx Zoo in New York City came as a huge surprise to investigators. The virus responsible for the disease had never before been seen in the United States. In a few, short years the virus rapidly spread across the United States, causing illness in over 15,000 horses in 2002. At this time, West Nile virus disease in horses has been identified in all 48 continental states. Understanding the disease can help owners take steps to protect their horses from WNV.
West Nile Virus is a mosquito-borne virus that causes encephalitis in the animals it infects. People, birds, and horses appear to be the most common victims, although there have been reported cases of the disease in other animals. Birds are the primary host of the disease. This means that the virus lives in birds and is picked up by mosquitoes. It can then be spread back and forth between birds and biting mosquitoes. It appears that birds are the only primary host and the only animals that can make infective virus for the mosquitoes to transmit. Unfortunately, mosquitoes that are carrying the virus can bite other animals and spread the disease to them. These other animals, including horses, are dead-end hosts. They cannot and do not spread the virus to other horses or other animals. So a horse that is suffering from West Nile virus disease is not a threat to neighboring horses or people.
The virus causes encephalitis, which is inflammation of the brain. The virus multiplies in the blood, then crosses into the brain, and causes disease signs. The signs of the illness can vary, but include any and all signs of central nervous system disease. Early signs may be loss of appetite and depression. Signs of the brain involvement can include fever, weakness, paralysis of the hind legs, incoordination, inability to swallow, and inability to see, along with twitching of the face and other muscles, head pressing, wandering, convulsions, circling, and coma. Some horses will go down and rapidly die. The list of signs is similar to any other disease that causes brain inflammation, so West Nile virus must be differentiated from rabies, equine protozoal myeloencephalitis, head trauma, brain abscess, and lead poisoning. In addition, other viral diseases carried by mosquitoes, such as Eastern, Western, and Venezuelan equine encephalitis, cause the same set of problems for horses.
There is no way that anyone can make a definitive diagnosis of West Nile virus simply by looking at a sick horse. Any horse with signs of central nervous system disease should be seen by the veterinarian. If the veterinarian believes that the horse is suffering from WNV, special blood tests will be done to positively identify West Nile virus disease. Because treatment of any encephalitis depends on the cause of the problem, a definitive diagnosis must be made to ensure proper therapy.
Horses vary in their response to the bite of a virus-carrying mosquito. It is believed the virus incubates between 5 and 15 days before disease signs are noticeable. Many horses that are infected with the virus show no signs of illness. In those horses that the virus multiplies and enters the brain, illness will result. Horses with the illness are given supportive care. In addition, there has been some positive response to the use of West Nile virus antibody. The product is given at the early onset of signs and may help reduce severity of the disease. The earlier the antibody is given, the more likely that it will be effective. Aside from this, there is no specific therapy to treat the signs of illness or kill the virus; once it replicates and invades the central nervous system, the disease must run its course. Some animals are sick for only a day or two, some are sick for weeks. Most recover, some die. It has been estimated that approximately 1/3 of infected, ill horses have died. Those with fewer signs and a shorter course of illness have a better prognosis than those with severe, protracted illnesses.
It is important to repeat that WNV is not spread horse to horse. It has to be spread by a virus-carrying mosquito that picks up the virus from an infected bird. Even though the virus is detectable in an infected horse's blood for a few days, a horse is not contagious to another horse. If a sick horse carries the virus and is bitten by a mosquito, the mosquito will not carry the disease to another horse. This makes control of the disease simpler for horse owners.
There are logical steps that horse owners can take to control WNV and to reduce the chances of it infecting their horses. First, correct vaccination is needed. The vaccines against other mosquito-borne viral encephalitis illnesses are not protective against West Nile virus. The horse must be vaccinated against West Nile virus using one of the two WNV vaccines presently on the market. Both vaccines recommend that the initial vaccination be two doses, given a few weeks apart. It is critical to read the label, as the directions vary between the two products. According to the label directions of both products, the vaccine is then boostered with one dose annually. Some veterinarians choose to booster the vaccines twice per year. The initial vaccine series and yearly boosters should be completed 2-3 weeks before mosquito season. If a second booster is given, it is often given towards the end of the season.
There is confusion regarding changing brands of vaccine in horses that have previously been vaccinated. It is okay to use a different brand of vaccine after the initial series is completed. When giving the initial series of two vaccinations, the same brand should be used. After the initial series, one can switch brands. One does not need to start over, but can simply continue the single boosters as recommended by your veterinarian.
In addition to vaccination, several steps can help to reduce horses' exposure to disease-carrying mosquitoes. The most important step is to eliminate water sources that are used for breeding by mosquitoes. This includes water troughs, water buckets, old tires, kiddy pools, clogged gutters, bird baths, wheel barrows and any other source of standing water. Even a pool cover can hold enough water for mosquitoes to breed in. Any water that stands stagnant for more than 4 days is a potential breeding site. Turn over, clean out, or throw away items that can possible hold water. In addition, drain or landscape low lying areas that hold stagnant water. Finally, clean water buckets, troughs, and tubs often to eliminate mosquito eggs.
Next, take steps to keep mosquitoes off the horses. Stall the horses at dusk and dawn in enclosures that are mosquito-resistant. Make sure there are no mosquitoes living in the mosquito-resistant enclosure before closing the doors. It may be wise to fog the stable with an insecticide at dusk to reduce the mosquito population. Screened stalls can be very useful. Fans will keep air moving, which helps keep the mosquitoes from landing on the horses. In addition, topical insecticides that repel and kill mosquitoes can be of help. Products that contain ingredients such as permethrins are often recommended. Always follow label directions and do not over-treat the horses if mosquitoes are still present. Topical insecticides are only one part of a multi-step plan needed to reduce mosquitoes and should be used with adequate environmental control.
In addition, be alert for signs of problems. Check stable areas and pastures for dead birds. Report any citings to the local health department and avoid handling dead animals with bare hands (use gloves or a shovel). If any horses show signs of neurological problems, call the veterinarian immediately. It is crucial to eliminate other diseases, such as rabies, and if a diagnosis of West Nile virus is made, to start therapy immediately.
The use of vaccines and mosquito control has dramatically reduced the incidence of horses becoming ill from West Nile virus. Actually, the decline has been remarkable. Reported, positively identified cases in horses fell to just over 5,000 in 2003, approximately 1,400 in 2004, and less than this in 2005. This proves that horse owners do not need to overreact, but do need to protect their animals. Horses can be protected by making sure they are properly vaccinated and by making sure that the mosquito population is kept to a minimum. These common sense approaches to horse care will help to keep horses safe from West Nile virus.
Sincerely,
Dr. Jane Leon
West Nile Virus is a mosquito-borne virus that causes encephalitis in the animals it infects. People, birds, and horses appear to be the most common victims, although there have been reported cases of the disease in other animals. Birds are the primary host of the disease. This means that the virus lives in birds and is picked up by mosquitoes. It can then be spread back and forth between birds and biting mosquitoes. It appears that birds are the only primary host and the only animals that can make infective virus for the mosquitoes to transmit. Unfortunately, mosquitoes that are carrying the virus can bite other animals and spread the disease to them. These other animals, including horses, are dead-end hosts. They cannot and do not spread the virus to other horses or other animals. So a horse that is suffering from West Nile virus disease is not a threat to neighboring horses or people.
The virus causes encephalitis, which is inflammation of the brain. The virus multiplies in the blood, then crosses into the brain, and causes disease signs. The signs of the illness can vary, but include any and all signs of central nervous system disease. Early signs may be loss of appetite and depression. Signs of the brain involvement can include fever, weakness, paralysis of the hind legs, incoordination, inability to swallow, and inability to see, along with twitching of the face and other muscles, head pressing, wandering, convulsions, circling, and coma. Some horses will go down and rapidly die. The list of signs is similar to any other disease that causes brain inflammation, so West Nile virus must be differentiated from rabies, equine protozoal myeloencephalitis, head trauma, brain abscess, and lead poisoning. In addition, other viral diseases carried by mosquitoes, such as Eastern, Western, and Venezuelan equine encephalitis, cause the same set of problems for horses.
There is no way that anyone can make a definitive diagnosis of West Nile virus simply by looking at a sick horse. Any horse with signs of central nervous system disease should be seen by the veterinarian. If the veterinarian believes that the horse is suffering from WNV, special blood tests will be done to positively identify West Nile virus disease. Because treatment of any encephalitis depends on the cause of the problem, a definitive diagnosis must be made to ensure proper therapy.
Horses vary in their response to the bite of a virus-carrying mosquito. It is believed the virus incubates between 5 and 15 days before disease signs are noticeable. Many horses that are infected with the virus show no signs of illness. In those horses that the virus multiplies and enters the brain, illness will result. Horses with the illness are given supportive care. In addition, there has been some positive response to the use of West Nile virus antibody. The product is given at the early onset of signs and may help reduce severity of the disease. The earlier the antibody is given, the more likely that it will be effective. Aside from this, there is no specific therapy to treat the signs of illness or kill the virus; once it replicates and invades the central nervous system, the disease must run its course. Some animals are sick for only a day or two, some are sick for weeks. Most recover, some die. It has been estimated that approximately 1/3 of infected, ill horses have died. Those with fewer signs and a shorter course of illness have a better prognosis than those with severe, protracted illnesses.
It is important to repeat that WNV is not spread horse to horse. It has to be spread by a virus-carrying mosquito that picks up the virus from an infected bird. Even though the virus is detectable in an infected horse's blood for a few days, a horse is not contagious to another horse. If a sick horse carries the virus and is bitten by a mosquito, the mosquito will not carry the disease to another horse. This makes control of the disease simpler for horse owners.
There are logical steps that horse owners can take to control WNV and to reduce the chances of it infecting their horses. First, correct vaccination is needed. The vaccines against other mosquito-borne viral encephalitis illnesses are not protective against West Nile virus. The horse must be vaccinated against West Nile virus using one of the two WNV vaccines presently on the market. Both vaccines recommend that the initial vaccination be two doses, given a few weeks apart. It is critical to read the label, as the directions vary between the two products. According to the label directions of both products, the vaccine is then boostered with one dose annually. Some veterinarians choose to booster the vaccines twice per year. The initial vaccine series and yearly boosters should be completed 2-3 weeks before mosquito season. If a second booster is given, it is often given towards the end of the season.
There is confusion regarding changing brands of vaccine in horses that have previously been vaccinated. It is okay to use a different brand of vaccine after the initial series is completed. When giving the initial series of two vaccinations, the same brand should be used. After the initial series, one can switch brands. One does not need to start over, but can simply continue the single boosters as recommended by your veterinarian.
In addition to vaccination, several steps can help to reduce horses' exposure to disease-carrying mosquitoes. The most important step is to eliminate water sources that are used for breeding by mosquitoes. This includes water troughs, water buckets, old tires, kiddy pools, clogged gutters, bird baths, wheel barrows and any other source of standing water. Even a pool cover can hold enough water for mosquitoes to breed in. Any water that stands stagnant for more than 4 days is a potential breeding site. Turn over, clean out, or throw away items that can possible hold water. In addition, drain or landscape low lying areas that hold stagnant water. Finally, clean water buckets, troughs, and tubs often to eliminate mosquito eggs.
Next, take steps to keep mosquitoes off the horses. Stall the horses at dusk and dawn in enclosures that are mosquito-resistant. Make sure there are no mosquitoes living in the mosquito-resistant enclosure before closing the doors. It may be wise to fog the stable with an insecticide at dusk to reduce the mosquito population. Screened stalls can be very useful. Fans will keep air moving, which helps keep the mosquitoes from landing on the horses. In addition, topical insecticides that repel and kill mosquitoes can be of help. Products that contain ingredients such as permethrins are often recommended. Always follow label directions and do not over-treat the horses if mosquitoes are still present. Topical insecticides are only one part of a multi-step plan needed to reduce mosquitoes and should be used with adequate environmental control.
In addition, be alert for signs of problems. Check stable areas and pastures for dead birds. Report any citings to the local health department and avoid handling dead animals with bare hands (use gloves or a shovel). If any horses show signs of neurological problems, call the veterinarian immediately. It is crucial to eliminate other diseases, such as rabies, and if a diagnosis of West Nile virus is made, to start therapy immediately.
The use of vaccines and mosquito control has dramatically reduced the incidence of horses becoming ill from West Nile virus. Actually, the decline has been remarkable. Reported, positively identified cases in horses fell to just over 5,000 in 2003, approximately 1,400 in 2004, and less than this in 2005. This proves that horse owners do not need to overreact, but do need to protect their animals. Horses can be protected by making sure they are properly vaccinated and by making sure that the mosquito population is kept to a minimum. These common sense approaches to horse care will help to keep horses safe from West Nile virus.
Sincerely,
Dr. Jane Leon
