West Nile fever: symptoms and treatment

This emerging zoonosis, caused by just a mosquito bite, can affect your horse. Today we explain everything about West Nile fever, a disease with some recent cases in the south of the peninsula. 

What is West Nile fever?

West Nile fever (WNF) is a zoonotic viral disease transmitted mainly through the bite of infected mosquitoes. Although it primarily affects birds, horses can be one of the animals susceptible to the virus, and in some regions the disease has caused concern because of its impact on equine health.

Zoonoses in humans: what are they and how are they transmitted?

Horses can be infected with the West Nile fever virus (WNF) when bitten by mosquitoes that have previously bitten infected birds. There are occasional outbreaks in humans that follow a seasonal transmission pattern, with the first cases appearing in summer and continuing into autumn.

When we talk about zoonoses in humans, we mean infectious diseases that are transmitted from animals to people. The causative agents may be bacteria, viruses, parasites or even unconventional agents, and transmission occurs through direct contact with animals, or through contaminated food, water or the environment.

Types of zoonoses

• Direct zoonosis: Transmitted through direct contact with animals, via saliva, stings, bites or the air.
• Indirect zoonosis: Requires a vector or intermediary, such as mosquitoes, ticks or contaminated food.
• Viral zoonoses: Examples include rabies, Ebola, avian flu and yellow fever.
• Bacterial zoonoses: Such as salmonellosis, tuberculosis and bubonic plague.
• Fungal zoonoses: Ringworm, cryptococcosis or histoplasmosis.
• Parasitic zoonoses: Toxocariasis, leishmaniasis, mange and fleas

West Nile fever: what are its symptoms?

Most infections are asymptomatic (80%). When symptoms do appear, the most serious manifestation of the infection is encephalitis, which occurs in both humans and horses.

Symptoms in horses

It mainly affects the brain and peripheral nervous system. The disease can progress to seizures and an inability to remain standing. Approximately one third of infected animals die, while the rest recover. The main symptoms of West Nile fever are:

• Fever
• Loss of appetite
• Lethargy (fatigue or weakness)
• Lack of coordination (ataxia)
• Muscle tremors
• Depression
• In severe cases, paralysis or death

Symptoms in humans

Among humans, around 80% of West Nile infections are asymptomatic, although only between 20% and 40% develop the disease. Most people have no symptoms, although some may experience a moderate fever, headache and swollen glands. In older people, complications such as encephalitis or aseptic meningitis may occur. 

Diagnosis and prevention

Diagnosis is generally carried out through serological tests (such as ELISA) or PCR, which detect the presence of the virus or antibodies in the horse's blood, or through recognition of the symptoms.

Prevention involves the following measures:

• Vaccination: There are vaccines for West Nile fever that are effective in preventing infection in horses. These vaccines form part of the vaccination protocol in many countries, especially in areas where the virus is more common.
• Mosquito control: Reducing exposure to mosquitoes is key. This may include the use of mosquito nets, protective masks, repellents,spraysand removing mosquito breeding areas (such as stagnant water).

Is there treatment for West Nile fever?

There is no specific antiviral treatment for West Nile fever. Treatment is usually symptomatic, aimed at relieving symptoms and supporting the horse during recovery. In severe cases, horses may need intensive veterinary care, which could include fluid therapy, anti-inflammatory medication and support for breathing or movement.

Therefore, vaccination and preventive measures are essential in endemic areas. If you have horses or work in an environment where the risk of West Nile fever is high, it is essential to speak to a vet to make sure your horses are properly protected.