In 1937, a group of epidemiologists researching yellow fever in the Ugandan West Nile basin isolated an unknown virus for the first time from a 37-year-old woman suffering from fever. In 1940, researchers released the results of their laboratory tests: the virus caused encephalitis in inoculated mice. They called it the “West Nile virus” (WNV). Later, virologists included it in the genus Flavovirus, an RNA virus.
The virus was later identified in Egypt (1942) and India (1953). Fortunately, he was not very aggressive. Infected people were generally asymptomatic, and one in five had mild flu- like symptoms: fever, headache, muscle aches, feeling tired, rash, and swollen lymph nodes. These could last from a few days to several weeks and would usually go away on their own.
Humans responded well to the infection: the most current serological investigations have concluded that about 60% of the Nile inhabitants have antibodies against the virus. However, the disease could be life-threatening if the virus entered the brain, as occurs in six out of every 1,000 infected people.
In these cases, as detected in laboratory mice, it could cause inflammation of the brain (encephalitis) or meningitis, inflammation of the meninges, the tissue that surrounds the brain and spinal cord.
In 1953, WNV was isolated in the blood of rock pigeons ( Columba livia ) and the crow Corvus corone sardonius . In 1957 it was recognized as the cause of an outbreak of severe human meningoencephalitis in elderly patients during an outbreak in Israel. In the early 1960s the disease was detected in horses in Egypt and France; Throughout the decade, a multitude of cases appeared in southern Europe, southeast Asia, and Australia. In 1999 it was detected for the first time in the United States.
Acute cases were soon related to people or animals that had been bitten by common or trumpeter mosquitoes (Culex pipiens ). Several viruses can cause encephalitis, including Epstein-Barr virus, varicella-zoster virus, Coxsackie virus, polio virus, rabies virus, Powassan virus, and various transmitted viruses. by mosquitoes such as West Nile, St. Louis encephalitis virus, and Lacrosse virus.
As in other cases in which there is a viral reservoir (remember the case of bats and COVID-19), in the case of West Nile virus, birds asymptomatically harbored the virus, which passed to humans through bites infected mosquitoes after biting birds.
In some cases, people can contract the virus through blood transfusions and organ transplants. However, the risk of contracting the virus in this way is very low. It is important to emphasize that you cannot contract this virus directly from another person or from a pet.
The cases of bites are more frequent with high temperatures and drought, probably because the lack of water decreases the populations of fish and other aquatic predators that feed on mosquito larvae. As a consequence, adult populations increase dramatically.
Most people who become seriously ill with West Nile virus recover , although it can take weeks or months. For your peace of mind, remember that at least eight out of ten human infections are asymptomatic. The other two develop mild symptoms such as myalgias, arthralgias, nausea, vomiting, and adenopathies, and only about one in a hundred suffer from serious complications such as encephalitis, meningitis, flaccid paralysis, and pancreatitis.
Apparently, patients admitted to Seville have developed encephalitis after suffering mosquito bites, which has motivated the Junta de Andalucía to activate the surveillance and alert protocol for diseases. The main suspect is West Nile virus, and the diagnosis has already been confirmed in several of the patients.
Last Monday the Board declared two foci of encephalitis caused by the West Nile virus in two horse farms in Gibraleón (Huelva) and Jerez de la Frontera (Cádiz). In each of them, at least one horse tested positive for this pathogen.
Bearing in mind that epizootic outbreaks in horses precede cases in humans, the establishment of an adequate prevention system is essential.
Spain, a country ripe for the West Nile virus
Several species of trumpeter mosquitoes of the genus Culex ( C. pipiens, C. molesus, C. perexiguus and C. theileri ) are widely distributed in our peninsula, although C. pipiens is the most common vector of the West Nile virus.
In Spain, the optimal temperature conditions for the presence of C. pipiens and the possible circulation of the virus that concerns us occur between the months of April and October in the warm areas of Andalusia.
Since 1996, the WHO has considered West Nile virus fever as an emerging disease in Europe. In Spain, since 2010 there have been outbreaks in equines and humans, which shows that the pathogen circulates in our country. In 2019, five outbreaks were declared in the provinces of Huelva and Seville.
Who is at risk of infection?
You are at higher risk of infection if you live where the virus has been discovered in humans, birds, horses, or mosquitoes, if you spend a lot of time outdoors during the warmer months, and if you don’t protect your skin with an insect repellent containing DEET.
However, even in areas where cases of the virus have been reported, it is highly unlikely that a person will become ill from a mosquito bite.
Can it be prevented or avoided?
There is no vaccine to prevent West Nile virus in humans and no specific antiviral drugs. Treatment is symptomatic and supportive. The best way to prevent infection is to make it harder for a mosquito to bite you.
To do this, if you live in risk areas, take some basic measures:
- Mosquitoes breed in standing water. Drain water from birdbaths, pools, weirs, flowerpots, wading pools, and other places where larvae can accumulate.
- Repair window and door screens.
- Mosquitoes are most active at dawn, dusk, and in the early afternoon.
- Try to stay home during those hours.
- If you go outside when mosquitoes are active, wear shoes and socks, long pants, and a loose-fitting, light-colored long-sleeved shirt. If you are going to be outdoors, use an insect repellent that contains 20-30% DEET.
- Cross your fingers and reassure yourself: this is a friendly virus, nothing to do with the person responsible for COVID-19.
Manuel Peinado Lorca, University Professor. Department of Life Sciences and Researcher at the Franklin Institute for North American Studies, University of Alcalá
This article was originally published on The Conversation. Read the original.
