West Nile fever is a zoonotic disease (an animal disease affecting humans). Disease is caused by West Nile virus (WNV), which is a flavivirus related to the viruses that cause St. Louis encephalitis, Japanese encephalitis, and yellow fever. West Nile virus is mainly transmitted to people through the bites of infected mosquitoes. WNV is maintained in nature by transmission between birds and mosquitoes; as birds are the natural hosts of the virus. Humans, horses and other mammals can be infected.
West Nile fever can cause severe neurological illness and death in people, however about 80% of people who are infected show no symptoms.
West Nile virus was first isolated in the West Nile district of Uganda in 1937. An epidemic of West Nile fever was reported in humans in Israel in 1951. In 1999 WNV was imported in New York and produced a large outbreak that spread throughout the continental United States of America (USA) during 1999-2010. Large outbreaks also occurred in Greece, Israel, Romania and Russia. The virus is now widely reported from Canada to Venezuela. Outbreak sites are on major birds’ migratory routes. WNV is commonly found in Africa, Europe, the Middle East, North America and West Asia.
WNV is highly prevalent in India. Infection usually presents as a mild, non-fatal dengue like illness in humans. Febrile illness and encephalitis cases in epidemic form were observed in Udaipur district of Rajasthan, Buldhana, Marathwada and Khandesh districts of Maharashtra. Human sera collected from Tamil Nadu, Karnataka, Andhra Pradesh, Maharashtra, Gujarat, Madhya Pradesh, Orissa and Rajasthan showed presence of WNV neutralizing antibodies. Serologically confirmed cases of WNV infections were reported from Vellore and Kolar districts during 1977, 1978 and 1981.
Presence of WNV was documented in north- eastern region of India during the year 2006 from four districts (Japanese encephalitis (JE) endemic areas) of Assam; in which11.6% of serum samples of AES (acute encephalitis syndrome) cases were found positive for IgM against WNV (these samples were negative for IgM against JE virus).
During an outbreak of AES in Kerala, in May 2011, presence of WNV was confirmed in clinical specimens. Since then, WNV encephalitis cases have regularly been reported in Kerala.
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The incubation period (the period between exposure to an infection and the appearance of the first symptoms) of WN fever is usually 3 to 14 days.
20% of people who become infected with WNV will develop West Nile fever whereas about 80% of infected people are asymptomatic (show no symptoms).
Various symptoms of WN fever include- Fever, headache, tiredness, and body aches, nausea, vomiting, sometimes skin rash (on the trunk of the body) and swollen lymph glands.
Severe disease such as West Nile encephalitis or meningitis (inflammation of brain and surrounding tissues) show symptoms of headache, high fever, neck stiffness, stupor, disorientation, tremors, coma, convulsions, muscle weakness and paralysis.
Serious illness can occur in people of any age; however people over the age of 50 and some immune-compromised persons (such as person who has received organ transplant) are at greater risk of serious illness.
It takes several weeks or months to recover from serious illness. Some of the neurologic effects may be permanent resulting in long-term sequelae or death.
About 10 % of patients with neurologic complications may die.
West Nile fever is caused by West Nile virus (WNV), belongs to the Japanese encephalitis virus (JEV) antigenic complex under family flaviviridae.
Transmission of disease- WNV is maintained in nature in a cycle involving transmission between birds and mosquitoes. Humans, horses and other mammals can be infected.
Birds acts both as carriers and amplifying host for WNV. Some bird species especially the crow family (corvidae) are more susceptible to the virus than others. Migratory birds are important in WNV transmission.
Ornithophilic (feeds on birds) mosquitoes belonging mainly to Culex species act as vectors for transmission of infection from infected birds to vertebrate hosts such as Cx.modestus (france),Cx.Vishnui complex(India and Pakistan), Cx univittatus complex(South Africa), Cx. pipiens pipiens(Romania, USA).
When Mosquitoes feed on an infected bird they become infected with the virus. The mosquitoes act as carriers (vectors) spreading the virus from an infected bird to other birds and to other animals. WNV is maintained in mosquito populations by transferring the infection through adults (mosquitoes) to eggs (vertical transmission).
Infection of other animals (e.g. horses, and also humans) is incidental to the cycle in birds since most mammals do not develop enough viruses in the bloodstream to spread the disease. They are “dead-end” hosts.
The virus may also be transmitted through contact with other infected animals, their blood, or other tissues.
A small proportion of human infections may be transmitted from organ transplant, blood transfusions and breast milk of infected person.
Various diagnostic tests are-
1. IgM antibody capture enzyme-linked immunosorbent assay (ELISA): IgM can be detected from cerebrospinal fluid (CSF) and serum specimens received from WNV infected patients at the time of their clinical presentation. Serum IgM antibody may persist for more than a year.
2. IgG antibody sero-conversion (or significant increase in antibody titers) in two serial specimen collected at a one week interval by enzyme-linked immunosorbent assay (ELISA);
3. Viral detection by reverse transcription polymerase chain reaction (RT-PCR) assay, and
4. Virus isolation by cell culture.
Specific antiviral treatment for West Nile virus infection is not available. In mild infection pain relievers can be used to reduce fever.
For patients with neuro-invasive West Nile virus infection treatment is supportive involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections. No vaccine is available for humans.
West Nile encephalitis or meningitis or West Nile poliomyelitis: 1 in 150 persons infected with the West Nile virus will develop a more severe form of disease.
About 10 % of patients with neurologic complications may die.
Reducing the risk of infection in people-
Vaccine is not available for human use hence raising the awareness among population about the risk factors and educating people about measures they can take to reduce exposure to the virus are some ways to reduce infection in human population.
Transmission of disease from mosquitoes to humans can be prevented by encouraging the use of mosquito nets, personal insect repellent, by wearing light coloured clothing (long-sleeved shirts and trousers) and by avoiding outdoor activity at peak biting times. In residential areas mosquito breeding sites should be destroyed with the help of community participation.
Persons should wear gloves and other protective clothing while handling sick animals or their tissues, and during slaughtering and culling procedures, for prevention of animal to human transmission.
Blood and organ donation restrictions and laboratory testing should be considered at the time of the outbreak in the affected areas after assessing the local/regional epidemiological situation.
Vector control –
Integrated mosquito surveillance and control programmes in areas where the virus occurs are the measure for prevention of human WNV infections. Integrated control measures include source reduction (with community participation), water management, chemicals, and biological control methods.
Occurrence of infection in human cases is usually preceded by outbreaks in animals therefore active animal health surveillance system to detect new cases in birds and horses is essential as an early warning signal for veterinary and human public health authorities. In the Americas it is important to reporting dead birds to local authorities. Vaccines are available for horses.