The spread of West Nile virus (WNV) across the mid-western United States and into Canada this year (2002) has garnered a great deal of media attention. For North America it is an emerging disease entity. As such the facts concerning the disease are often intermixed with rumors, old-wives tales and meaningless gossip which is often purported to be the truth when in reality the latter information is most likely false and without any factual foundation whatsoever. I think the best way to present this paper is in the form of a fact sheet.

TO BEGIN: WHAT IS THE WEST NILE VIRUS (WNV)?

The virus was first identified in Uganda, Africa, in 1937. Since that time it has been identified in outbreaks, which occurred in Europe, Asia, North Africa, West Africa and Israel. WNV first appeared in Long Island, New York, where it was isolated from dead crows. It did not take long before WNV cases were diagnosed in birds, horses and man in New York. In the New York City area that year, there were 62 human cases reported with 7 deaths.

The virus itself is an arbovirus, which means it is carried by mosquitoes.

In Europe some researchers have found that several species of ticks may also be carriers of the disease.

The Culex Tarsalis species of mosquito found in the western U.S. is believed by some scientists to be very efficient in spreading the WNV.

HOW IS THE WNV SPREAD AND TO WHAT SPECIES OF BIRDS AND ANIMALS?

The virus mainly infects mosquitoes and birds. The main reservoir for the WNV is birds of the crow family some of which are blue jays, gray jays, magpies, ravens and the common crow. Other species of birds may also be infected such as the English sparrow, starling, owls and members of the raptor family. Domestic geese and ducks are susceptible but the wild Canada goose is resistant. Some other domestic poultry species may also be infected along with the wild and domesticated pheasant.

As WNV spread across North America its progress was monitored by serotesting dead crows and blue jays, which are especially vulnerable to the virus. When infected, probably by infected mosquitoes, some of these birds develop a viremia and die. Therefore the spread of the disease can be monitored by testing the dead birds. In addition, health authorities trap and test mosquitoes in suspected areas. Positive identification of the virus in these insects will make the presence of the disease known. Thus the presence of WNV is known in an area long before cases appear in horses or humans.

WNV arrived here in Sioux County, Iowa, in the late spring and in our yard we have picked up 2 dead crows, 1 blue jay, 1 starling and a number of dead sparrows. Usually I do not find dead birds on our premises here in town.

A local zoo reported the deaths of a Sand Hill Crane and a Golden Eagle from the WNV, this past summer.

Humans, horses, mules, hinneys and members of the ass family are susceptible to the virus when bitten by an infected mosquito. They are called dead-end hosts as the virus does not spread from horse to horse or human to human except under extraordinary circumstances such as organ transplants or blood transfusions which may occur from the infected human or horse to its like species.

Cattle are not known to be infected nor are dogs and cats. One dog was experimentally infected however.

Three out of 50 black bears were found to be sero-positive to WNV when blood tested.

The WNV is spread by the bites of mosquitoes from bird to bird and to other animals such as the horse and human. Again, the virus does not spread from a horse to a human or human to human or human to horse or horse to horse.

The WNV cannot be spread by coming in contact with one infected bird. One should however use rubber or plastic gloves in handling a dead bird. When dressing wild birds such as pheasants, which have been shot, a person should use the same precautions.

HOW DID WNV ARRIVE IN NORTH AMERICA SPECIFICALLY IN LONG ISLAND, NEW YORK, IN 1999?

At this time there was an outbreak of the disease in Israel, which was especially severe in the domestic goose population. No one knows for sure but it is speculated the virus arrived here from that area via infected mosquitoes on board an airplane.

WHAT ARE THE SYMPTOMS OF A WNV INFECTION IN THE HORSE?

Horses and other members of the equine species, when infected with the WNV, will show symptoms of an encephalomyelitis or in layman's terms, a brain infection.

A rise in temperature may or may not be present. Only 24% of affected horses will show a temperature rise or fever.

The most common symptoms are depression, listlessness, stumbling and incoordination, especially of the rear legs. All of the limbs may be weak but often the hind feet appear to "drag." Sensory activity in these legs is almost always retained.

Facial muscle twitching may be seen as well as general muscle twitching. The lower lip may "hang" and the eyes may have a "glazed" appearance.

Generally speaking, in the early stages of the disease, the front limbs do not seem to be involved.

Some horses may show an inability to swallow. A few animals will constantly push against a fence, building or other object. This is called "head pressing."

Most horses in the early stages of the disease will continue to eat and drink.

These symptoms may progress and the horse will go down and die. The length of time from onset to death will be 5 to 7 days.

Occasionally an animal will be found dead. For some reason these animals are usually found near a body of water.

All ages of horses can contract the disease, however it is thought animals in poor physical condition are more likely to be affected. The very young and very old animals seem to be more susceptible to the virus possibly due to compromised immune systems.

The incubation period in the horse and human is the same, 3 to 15 days from exposure to the onset of symptoms. Ten days seems to be the average.

WHAT ARE THE SYMPTOMS OF WNV INFECTION IN THE HUMAN?

In the first place most people who are bit by an infected mosquito never contract the disease or develop any symptoms. If one does, the symptoms are usually very mild and resemble those seen and felt in a slight case of the flu. They may include a fever, body aches and pains, mild headache, swollen lymph glands, and a skin rash.

If the fever becomes high, the headache severe and the neck stiff and very sore; along with confusion, muscle tremors, weakness, and paralysis, a more severe infection is in progress and it can be fatal. As in the horse the elderly and those persons with stressed immune systems are more susceptible to infection and death from WNV.

WHAT OTHER DISEASES MAY BE CONFUSED WITH WNV?

As soon as a horse owner recognizes that his animal is sick a veterinarian should be consulted. Other diseases to be considered in the diagnosis are rabies, equine viral rhinopneumonitis (the nervous form), E.P.M. (equine protozoal myeloencephalitis), tetanus, sleeping sickness, and so-called moldy corn poisoning. Of these diseases I would most worry about the "dumb" or paralytic form of rabies being confused with the WNV infection.

In the U.S. most states require that all cases of WNV be reported to the proper state agency usually the state veterinarian or the Department of Agriculture. The veterinarian will draw a blood sample and submit it to a laboratory to be tested for the presence of WNV antibodies. Clinical signs of the disease plus a positive ELISA blood serum test is diagnostic of WNV infection.

In most states, and certainly here in Iowa, the attending veterinarian must fill out a form detailing certain aspects of the case and submit it to the proper authorities.

If the animal dies and rabies has not been ruled out, the brain and brain stem should be submitted to a lab for further testing for rabies and WNV.

IS THERE A TREATMENT OR TREATMENTS TO BE USED ON AN ANIMAL WITH WNV?

Since WNV infection of the brain and spinal cord is caused by a virus there is not an antibiotic or drug which will destroy the virus in these areas of the body. Therefore any treatment which is instituted is called symptomatic and supportive. This means that the symptoms are being treated as they appear in the animal. The proper treatment of these horses will dramatically reduce the fatality rate if instituted early.

It is reported that the fatality rate in the horse is about 30 to 40%. I have told my clients to check their stock twice a day. Look them over very well and if they notice anything resembling the symptoms of WNV infection, get on the ball, do not wait, and institute a treatment. To wait until the animal is recumbent and exhibiting convulsions is just too late. I truly believe if you can get them early you can save almost all of them.

What drugs are used? That is up to the veterinarian. The treatment might include fluid therapy, antibiotics (for secondary infection), anti-inflammatory drugs, diuretics and changes in the diet. Don't forget the use of a sting in certain cases. In order to save most of these horses it also requires excellent, diligent nursing care, along with some form of drug therapy.

I have had, just lately, about a half-dozen calls on the use of peroxide in horses with WNV. One gentleman injected a draft mare with 60cc of peroxide in the hip. The mare developed an abscess as big as a basketball in the area.

Another put a pint of peroxide into the vein of his horse. It died!

Where these people have heard that peroxide will cure a WNV horse is beyond my comprehension.

Here's another one–the use of DOMSO in these WNV horses. DOMSO is a drug that can be used, however you do not take the 99% pure drug and give it IV in its undiluted form.

One individual put a pint of pure DOMSO into the vein of his draft gelding. The horse died. Others have given a less amount and their patient died. Some gave less and their horses did not die but it caused problems in addition to the WNV.

So if you must treat your horse be sure what you do will help the cause and not hinder it.

IS THERE ANY WAY TO PREVENT WNV INFECTIONS IN THE EQUINE SPECIES?

Yes, there is. There are several things you can do. A vaccine is available through your veterinarian which when used correctly will prevent almost all of the cases of WNV in the equine. When using the vaccine you must understand the following. The first dose is given to an unvaccinated horse. This sensitizes the animal but does not cause any immunity to be produced. A second shot is given exactly 3 weeks from the first shot. This causes the horse to produce the immunity. However as immunity builds up it will take 2 or 3 more weeks from the date of the second shot for the horse to be fully immune.

It does not help nor is it advised to give the 2nd vaccination before the 3-week period after the first inoculation.

So you can figure this one out. From the time of the first and second shot, the horse is not immune for 5 weeks.

Once it has had the two vaccinations, which established its primary immunity, the animal can then be vaccinated once a year, with one shot, to maintain its immunity.

I blame many veterinarians for not instructing their clients on the proper use of the vaccine and to what happens in the horse regarding the immune process. Any person can see that even if the horse is being properly vaccinated with the vaccine for WNV it could come down with a natural infection within 5 weeks of the first shot. That is not the fault of the vaccine. The immune process in the horse is not as simple as turning on a water faucet. As a matter of fact almost all vaccines administered to the equine operate the very same way in the establishment of primary immunity.

Besides vaccination of horses and mules the only other preventative measure that can be taken is to try and reduce the population of mosquitoes and if possible keep them off the horse. This in part can be accomplished by draining all sources of standing, stagnant water. If this is not possible a mosquito larvacide can be placed in this water. Get rid of brush and weeds which harbor the mosquitoes. Use insect sprays and repellents on the premises and on the animals.

As for human WNV prevention, mosquito control is very important. Wear long sleeved shirts and keep the head covered. Wear long pants and apply insect repellents containing permitherin or 35% DEET to your clothing.

I was privileged on September 12, 2002, to participate in a telephone conference call to be updated on the West Nile Virus and West Nile Virus vaccine. Rob Daily, Director of Equine Business Group for Fort Dodge Animal Health, moderated the conference call. Participants included four doctors, all members of the Professional Services Department at Fort Dodge, who addressed the most current information about the spread of the disease and the WNV vaccine. This session lasted one hour followed by a half hour question and answer period where questions were taken from veterinarians online and answered by a staff member.

I was extremely pleased with the conference and I wish to thank the Fort Dodge personnel for providing this learning session for the veterinarian, which by the way cost the vet nothing but his or her time.

I will list some of the points I learned which are often reflected by questions from clients and others about the WNV.

This information is of September 12, 2002:

1. The movement of WNV in 3 years. Found first in Long Island, N.Y. is now found in 42 states, Canada, Mexico, and Columbia, South America.
2. Certain species of birds are the reservoir and the virus is carried by mosquitoes.
3. Humans and horses are "dead end" victims. They will not infect other animals or humans under most circumstances. The incubation period in both is 3-15 days.
4. Fort Dodge vaccine for WNV at this time is the only vaccine available for use in the equine. There is not a human vaccine available but one could be produced in 2 years.
5. Fort Dodge WNV vaccine is produced under a conditional license. This means it has met all of the purity tests and other tests of the FDA but they have not been able to challenge enough horses. This of course is ongoing.
6. The vaccine is a dead virus vaccine. It cannot cause the WNV disease.
7. The vaccine must be used in 2 doses given ideally 3 weeks apart. If the second dose is given later than 6 weeks from the first dose a 3rd dose should be given 3 weeks from the date the 2nd dose was administered. One month after the 3rd dose is given the animal is fully immune to the WNV.
8. An FDA challenge study revealed that trial horses vaccinated with WNV vaccine, 2 doses at 3 weeks apart were challenged in 12 months with the live virus. 81.8% of the horses developed the viremia but there were no clinical signs of the WNV disease. Preventable fraction in the blood was 94%, which was excellent.
9. One can vaccinate pregnant mares. To date there have been no reports of adverse reactions in either the mares or foals. Vaccination late in pregnancy will protect the neonate better.
10. No adverse reaction in the semen of vaccinated stallions has been noted.
11. A vaccinated horse will not infect others, as the vaccine is a killed virus vaccine.
12. The vaccine is safe for all breeds of horses (including miniatures), donkeys, mules and members of the ass family.
13. The vaccine has been used to vaccinate many species of birds and other animals in zoos and like establishments with no adverse reactions.
14. If the mare is vaccinated there is a passive transfer of immunity to the foal. Vaccinate these foals with the first dose given at 3 to 4 months of age.
15. If the mare is not vaccinated, vaccinate the foal at 6 to 8 weeks of age. Give 3 doses of the vaccine at 3 to 6 week intervals. This is the same for the eastern and western encephalitis vaccine.
16. A horse recovering from a WNV infection can be safely vaccinated. The animal should keep a natural immunity from the infection up to 6 months.
17. A horse is vaccinated and comes down with the infection. It was already incubating the disease. The vaccine will not make the sickness worse. In fact the presence of the vaccine may aid the case.
18. The presence of the vaccine in the above case will not interfere with the diagnosis by sero-typing the blood. In other words the test used to diagnose WNV infection in the horse will not be affected by the vaccination of the animal.
19. Adverse reactions to the vaccine are extremely low. Site reactions (swelling and etc.) number less than 2 reports per 100,000 ds. Allergic reactions - less than 2 per 100,000 ds. Systemic illness (fever-anorexia) less than 5 per 100,000 ds. Lack of efficacy - 20 reports for millions of doses used. Stress may be a factor here.
20. One can vaccinate after the herd is exposed.
21. Mosquitoes do not transmit the WNV from the infected horse or human because the virus load to the mosquito is too small.
22. Under the right environmental circumstances a mosquito population can regenerate in 1 week.
23. In testing random horses which have not been seen to be sick with WNV, positive blood tests have been obtained which suggests horses can get the disease and their immune systems can handle it to the extent symptoms do not appear.
24. 10% of recovered horses have some residual deficit. This often also occurred in recovered cases of eastern and western encephalitis.
25. 45 days after exposure to the WNV the blood test on these horses may no longer be positive.
26. There is a great concern of WNV infection in wild birds; whooping cranes, eagles and owls to mention a few. Birds in zoos have been vaccinated but immunity studies are still underway.
27. Using the same needle to vaccinate horses in an exposed herd will not cause the disease to be transferred from a sick or incubating horse to others. However this is a very poor practice.
28. Mortality rate of WNV infected horses: their figure is 30-50%.
29. One can administer Fort Dodge WNV vaccine with the other vaccines for other equine diseases when vaccinating horses.
30. One good thing about the WNV. It is stable and has not changed.

I believe the WNV will be with us for a long period of time and be a menace to the health of our equine and human population. The cold temperatures of fall and winter in some areas of the U.S. will for a time prevent the breeding and hatching of mosquitoes. However some of the adult population will winter in the warmth of homes, barns and other outbuildings. Cases of WNV could possibly appear during the cold weather months in both humans and horses.

To control this WNV disease I strongly believe in vaccination of as many horses as possible against WNV. Iowa has approximately 100,000 horses and from August 2001 until September of 2002, 97,000 doses of WNV have been sold to Iowa veterinarians. This means that close to 50,000 or half of our horses have been vaccinated.

I also believe every effort should be made to reduce or eliminate the mosquito population. To accomplish this end pesticides must be used and should be administered by trained personnel. The use of these chemicals should be deemed safe by the proper federal, state, and local officials. When these criteria have been met I do not know a single pesticide spraying program, which has caused serious harm to people or the environment. This will and should greatly reduce the number of equine and human cases occurring in this country during 2003.

Here is a list state by state of the number of reported equine cases which have occurred in the United States from January 1, to October 6, 2002.

The total equine cases to date are 9,036 in 36 states.

Alabama (49), Arkansas (73), Colorado (343), Delaware (1), Florida (182), Georgia (29), Illinois (431), Indiana (504), Iowa (714), Kansas (500), Kentucky (403), Louisiana (324), Maryland (8), Massachusetts (1), Michigan (145), Minnesota (725), Mississippi (217), Missouri (569), Montana (99), Nebraska (1,041), New Jersey (28), New Mexico (35), New York (17), North Dakota (562), Ohio (461), Oklahoma (128), Pennsylvania (40), South Carolina (2), South Dakota (656), Tennessee (96), Texas (500), Vermont (3), Virginia (17), West Virginia (3), Wisconsin (66), and Wyoming (64).

As of Monday, October 7, 2002, the Center for Disease Control reported there were 2,796 probable or confirmed cases in humans from 34 states with 146 fatalities. They are state by state as follows:

Illinois (654), Michigan (401), Ohio (313), Louisiana (293), Mississippi (168), Indiana (157), Missouri (138), Texas (101), Nebraska (80), New York (60), Kentucky (44), Tennessee (40), Alabama (39), Pennsylvania (37), Iowa (33), South Dakota (33), Minnesota (31), Wisconsin (28), Virginia (20), Georgia (19), Massachusetts (17), North Dakota (15), Maryland (14), Arkansas (11), Connecticut (11), Florida (10), New Jersey (7), the District of Columbia (6), Colorado (5), Oklahoma (4), North Carolina (2), West Virginia (2), California (1), Rhode Island (1), and South Carolina (1).

To date in 2002, 6,071 dead crows, and 4,514 other dead birds have been tested positive with WNV from 43 states and the District of Columbia.

The presence of WNV has been detected by sero conversions in 342 sentinel chicken flocks from Florida, Iowa, Nebraska, Pennsylvania and New York City.

In addition 4,010 WNV positive mosquito pools have been reported from 26 states.

In closing the following information has just become available to me. I quote, “It is currently believed that any type of bird or mammal may be susceptible to WNV infection, but very few species appear to develop clinical illness due to infection. WNV infection has been reported in several bat species, chipmunks, gray squirrels, striped skunks, a rabbit, dog, 3 cats, in addition a domestic sheep, a mountain goat, a 7 month old wolf pup, a llama and an alpaca. From what is known, WNV does not appear to pose a significant health risk for species other than birds, horses and humans. There is also no current evidence that any of these species are capable of serving as a reservoir for the virus." End of quote. There you have it as of October 9, 2002. I will keep you posted!