West Nile Fever: the disease

West Nile Fever or encephalitis is a zoonotic disease caused by an arbovirus, with a growing importance in recent years due to its great expansion worldwide. One of the possible reasons for this expansion is that the virus is capable of infecting a wide range of hosts. The main hosts of the virus and its natural reservoir are numerous species of wild birds. Occasionally, the virus can infect domestic poultry (geese, graylag geese, pigeons and chickens). The virus also infects amphibians, reptiles, and mammals, being especially pathogenic for horses and people. Transmission between all these animals is primarily through the bite of mosquitoes of the genus Culex, although it has also being related with other genera, such as Aedes.

The virus is maintained in nature infecting mosquitoes and wild birds, which do not develop the disease, despite the high levels of viremia they acquire. It is what is known as a enzootic, rural or sylvatic cycle. Under certain conditions, if there are a large number of infected mosquitoes, they may bite other hosts, including horses and people, causing disease. These are incidental or dead-end hosts, because they do not transmit the disease, since their viremia is not high enough as to be able to infect another new mosquito. How does the virus produce disease? Once the infected mosquito bites the host, the virus replicates in the Langerhans cells and dendritic cells of the skin, and in the regional lymph nodes.

Then, the virus spreads to different organs through a transient viremia with a low level of virus. Days later, neutralizing antibodies develop and eliminate the virus. In a small percentage of cases, the virus may cross the blood-brain barrier and infect neurons in the brain and in the spinal cord, producing encephalitis and encephalomyelitis that may be severe and can cause death. Most horses infected with the virus are asymptomatic or present mild and transient clinical signs from which they recover. However, around 10% of them develop severe neurological disease.

The neurological signs include: ataxia and incoordination, weakness, and paralysis of the limbs, facial and tongue paresis with difficulty in swallowing, grinding of teeth, convulsions and marching in circles. When clinical signs appear, 20 to 50% of the animals can die. In people, the majority of cases are asymptomatic. Sometimes there may be fever and mild symptoms, and only in about 1% of the cases, encephalitis. In some birds that become ill, such as geese, there is incoordination, and paralysis of legs and wings. 00:03:10.150 –> 00:03:12.150 Crows are especially sensitive

and often die without any previous clinical signs. The clinical signs in horses can be mistaken with other diseases that cause encephalitis, so it is necessary to confirm the diagnosis with laboratory tests. Usually the virus is detected by RT-PCR in blood or in the cerebrospinal fluid, or in the brain and the spinal cord in dead animals. The presence of specific antibodies in serum is detected by ELISA. The diagnosis in birds is performed in a similar way, and it is very important for the control of the disease, as we will see in another step. Why is West Nile fever so important?

It’s important because the increase in the geographic distribution of the virus has paralleled an increased incidence of the disease in horses, people and in some types of birds. From its initial isolation in the 1930s in the district of West Nile in Uganda, the virus has spread through different African countries, the Middle East, Asia, the Mediterranean, and Eastern Europe. In 1999, the virus was detected for the first time in America, in New York City, causing the death of horses, people and birds, especially crows. From here it has extended in less than 10 years to all of Mainland America, and later, to Australia. Since 2010, the incidence of cases in people and horses in the South and East of Europe has increased.

The factors that may have contributed to the wide spread of the virus could be: migratory birds, which can spread the virus over long distances; the increase of intercontinental travel and trade of exotic birds; the climate change, since the increase of temperatures favours the shift of the mosquitoes to more northern latitudes, and the replication of the virus in the vector, increasing, thus, the possibility of transmission to the hosts; the emergence of new, more virulent viral strains; and lastly, the adaptation of the virus to new vectors and hosts. In conclusion, West Nile Fever is considered one of the emerging arboviral diseases of growing importance in animal and in human and animal health, as it is a zoonosis.

For this reason, it is a notifiable disease. We will see the measures for its prevention and control in another step.