West Nile Virus (WNV) is a flavivirus causing West Nile fever, primarily spread by mosquitoes. It was first identified in the West Nile district of Uganda in 1937. Since its arrival in North America in 1999, its transmission patterns have become relevant for public health. The virus circulates in a cycle involving mosquitoes and birds.
Mosquitoes as Primary Carriers
Mosquitoes, particularly those belonging to the Culex genus, are the main carriers of West Nile Virus. These mosquitoes acquire the virus when they feed on infected birds. Once a mosquito ingests the virus, it undergoes a period of replication within its body, known as the extrinsic incubation period.
During this extrinsic incubation period, the virus multiplies and travels from the mosquito’s gut to its salivary glands. This process takes about 8 to 12 days, varying with environmental temperatures; higher temperatures can shorten the period. Only after this incubation can the mosquito transmit the virus to other animals, including humans, through subsequent bites.
The Role of Birds
Birds play a central role as reservoir and amplifying hosts for West Nile Virus. Over 320 bird species can become infected with WNV, and some, like crows and jays, are particularly susceptible. These birds can develop a high concentration of the virus in their blood, a condition known as viremia, making them efficient sources of infection for mosquitoes.
This circulation between mosquitoes and birds maintains WNV in the environment. Migratory birds can also contribute to the geographic spread of the virus to new areas. While many birds can be infected, the level and duration of viremia vary by species, influencing their individual contribution to the transmission cycle.
Transmission to Humans and Other Mammals
Humans, horses, and other mammals can contract West Nile Virus through the bite of an infected mosquito. However, these hosts are considered “dead-end hosts” for the virus. This means that while they can become infected and show symptoms, the level of virus in their bloodstream does not reach a concentration high enough to infect another mosquito that bites them.
Humans and most other mammals do not contribute to the virus’s spread within the mosquito-bird cycle. In humans, about 80% of infections are asymptomatic, while others may develop West Nile fever with symptoms like fever, headache, and body aches. Horses can also develop neurological symptoms, including incoordination and muscle tremors, though many infected horses show no signs of illness.
Other Routes of Transmission
While mosquito bites are the most common way WNV is transmitted, there are other, less frequent routes. These include transmission through blood transfusions and organ transplantation. Blood donations are now screened for WNV, reducing this risk, though a possibility remains, especially from asymptomatic donors.
The virus can also be transmitted from a pregnant mother to her child during pregnancy or delivery, or through breastfeeding. Accidental exposure to WNV has also occurred in laboratory settings. These alternative transmission methods account for a very small proportion of overall WNV infections.