West Nile fever is a non-contagious viral disease that primarily affects birds, horses, and humans and is transmitted by mosquitoes. It is a disease of African origin but has spread throughout the world thanks to migratory birds, who are the main hosts of the virus, maintaining a mosquito-bird-mosquito cycle that sometimes includes horses or people. The disease causes nervous signs that can sometimes become very serious and even cause the death of the infected being. Due to this, good epidemiological surveillance must be carried out to prevent the disease, as well as vaccination of horses in risk areas.
If you are curious or have heard about this disease and want to learn more about it, keep reading this article on our site about West Nile Fever in horses, its symptoms and control.
What is West Nile fever?
West Nile fever is a non-contagious infectious disease of viral origin and transmitted by a mosquito, generally of the Culex or Aedes genus. Wild birds, especially from the Corvidae family (ravens, magpies), are the main reservoir of the virus for its transmission to other beings by mosquitoes, since they develop a strong viremia after the bite of an infected mosquito. The best habitat for the spread of the virus are wet areas such as river deltas, lakes or marshy areas where migratory birds and mosquitoes abound.
The virus naturally maintains a natural cycle mosquito-bird-mosquito, with mammals sometimes being infected by the bite of a mosquito carrying the virus after biting a bird with the virus in its blood. People and horses are especially sensitive, where it can give rise to neurological symptoms more or less severe because the virus reaches the central nervous system and the spinal cord by blood. Transplacental transmission, breastfeeding or through transplants have also been described in people, being only symptomatic in 20% of cases. In horses there is no contagion between individuals, but the presence of a mosquito vector of the virus between them is always necessary.
Although West Nile fever is not one of the most common diseases in horses, it is very important to carry out a correct veterinary control to prevent this and other pathologies.
Causes of West Nile Fever
The disease is caused by West Nile virus, which is an arbovirus (arthropod-borne virus) of the family Flaviviridae and of the genus Flavivirus. It belongs to the same genus as the Dengue, Zika, yellow fever, Japanese encephalitis or Saint Louis encephalitis viruses. It was first identified in 1937 in Uganda, in the West Nile district. The disease is mainly distributed in Africa, the Middle East, Asia, Europe and North America
This is a Notifiable disease to the World Organization for Animal He alth (OIE), as well as registered in the Sanitary Code for Terrestrial Animals of this same organization. The increased circulation of the West Nile virus is favored by the presence of floods, heavy rains, rising global temperatures, population growth, extensive poultry farming and intensive irrigation.
West Nile Fever Symptoms
After the mosquito bite, the symptoms may take between 3 and 15 days to appear On other occasions they will never appear, due to that most of the horses that become infected never develop the disease, so they will not show any clinical signs.
When the disease develops, it is estimated that one-third of horses infected die. The signs that a horse with Nile Fever may present are:
- Fever.
- Headache.
- Swollen lymph nodes.
- Anorexy.
- Lethargy.
- Depression.
- Difficulty swallowing.
- Vision disorders with stumbling when walking.
- Slow and short step.
- Head dropped, tilted or supported.
- Photophobia.
- Incoordination.
- Muscular weakness.
- Muscle tremor.
- Teeth grinding.
- Facial paralysis.
- Nervous tics.
- Circular movements.
- Inability to stand.
- Paralysis.
- Seizures.
- Eat.
- Death.
Around 80% of infections in people do not produce symptoms and if they do occur they are nonspecific, such as moderate fever, headaches, tiredness, nausea and/or vomiting, skin rashes and enlarged lymph nodes. In other people, the severe form of the disease can develop with complications such as encephalitis and meningitis with neurological signs, but the percentage is usually minimal.
Diagnosis of West Nile Fever in Horses
The diagnosis of Nile Fever must be made through a clinical, differential diagnosis and verified by taking samples and sending them to the reference laboratory to have a definitive diagnosis.
Clinical and differential diagnosis
If a horse begins with some of the neurological signs that we have mentioned, even if they are very subtle, this viral disease should be suspected, especially if we are in an area at risk of viral circulation or the horse does not has been vaccinated. That is why calling the equine veterinarian before any unusual behavior of our horse is essential to treat it as soon as possible and control possible outbreaks. West Nile fever should always be differentiated from other processes that can present similar signs in horses, specifically:
- Equine Rabies.
- Equine herpesvirus type 1.
- Alphavirus encephalomyelitis.
- Equine protozoal encephalomyelitis.
- Eastern and western equine encephalitis.
- Venezuelan equine encephalitis.
- Verminous encephalitis.
- Bacterial meningoencephalitis.
- Botulism.
- Poisoning.
- Hypocalcemia.
Laboratory diagnosis
The definitive diagnosis and its differentiation from other diseases will be given by the laboratory. Samples must be taken for testing to detect virus antibodies or antigens for disease diagnosis.
Tests to directly diagnose the virus, specifically antigens, are performed on samples of cerebrospinal fluid, brain, kidneys or heart from necropsy if the horse has died, being useful the polymerase chain reaction or RT-PCR, immunofluorescence or immunohistochemistry in the brain and spinal cord.
However, the tests that are usually used to diagnose this disease in live horses are serological, based on blood, serum or cerebrospinal fluid, where instead of the virus antibodies that the horse has produced against it will be detected. Specifically, these antibodies are immunoglobulins M or G (IgM or IgG). IgG increase later than IgM and when clinical signs have been present long enough, so only detection of IgM in serum is diagnostic. The serological tests available for West Nile Fever detection are:
- IgM capture ELISA (MAC-ELISA).
- IgG ELISA.
- Hemagglutination inhibition.
- Seroneutralization: it is used to confirm positive or confusing ELISA tests, since cross-reactions with other flaviviruses can occur with this test..
Definitive diagnosis of West Nile fever in all species is made by virus isolation, but is not commonly practiced because it requires a level 3 biosafety. It can be isolated in VERO (African green monkey liver cells) or RK-13 (rabbit kidney cells), as well as in cell lines or chicken embryos.
Treatment of West Nile Fever in Horses
The treatment of Nile Fever is based on treating the symptoms that occur, since there is no specific antiviral, of so that the support therapy will be as follows:
- Antipyretics, analgesics and anti-inflammatories to reduce fever, pain and internal inflammation.
- Hold if you can hold the pose.
- Fluid therapy if the horse cannot be adequately hydrated.
- Nutrition by tube if you have difficulty eating.
- Hospitalization with a safe place, padded walls, comfortable bed and head protector to prevent injury from blows and control neurological signs.
Most of horses that become infected recover by developing specific immunity. On some occasions, even if the horse overcomes the disease, sequelae may remain due to permanent damage to the nervous system.
Prevention and control of West Nile fever in horses
West Nile fever is a notifiable disease but is not subject to an eradication program as it is not contagious between horses, but it is necessary for a mosquito to mediate between them, so the sacrifice of infected horses is not mandatory, except for humanitarian reasons if they no longer have quality of life.
For good control of the disease it is essential to carry out an epidemiological surveillance of mosquitoes as vectors, birds as main hosts and horses or humans as accidental. The objectives of the program are to detect the presence of viral circulation, assessment of the risk of appearance and the implementation of specific measures. Wetland areas should be especially monitored and bird surveillance is carried out with their carcasses, since many of the infected die, or by sampling suspects; in mosquitoes, through their capture and identification and in horses through sentinel sampling or suspected cases.
As there is no specific treatment, vaccination and reduction of exposure to transmitting mosquitoes are key to reducing the risk of horses contracting the disease. The preventive mosquito control program is based on applying the following measures:
- Use of topical repellents in horses.
- Stable horses by avoiding outdoor activities during times of greatest exposure to mosquitoes.
- Fans, insecticides and mosquito traps.
- Eliminate mosquito breeding sites by cleaning and changing the drinking water daily.
- Turn off the lights in the stable where the horse is to avoid attracting mosquitoes.
- Put anti-mosquito curtains in the stables, as well as mosquito nets on the windows.
West Nile Fever Vaccine in Horses
In horses, unlike in people, there are vaccines that are used in areas of greatest risk or incidence of the virus. The great utility of vaccines is to reduce the number of horses with viremia, that is, that have the virus in their blood, and to reduce the severity of the disease by presenting immunity if they become infected.
Inactivated virus vaccines are used from the age of 6 months of the horse, administered intramuscularly and require two doses. The first is at six months of age, revaccinated at four or six weeks and then once a year.
