—from Epidemiological Bulletin, Vol. 23 No. 4, Diciembre 2002—

Guidelines for Surveillance, Prevention and Control of West Nile Virus

Due to the threat of spread of West Nile virus to the countries of the Region, based on the proceedings from the “West Nile Virus Surveillance Workshop” organized by PAHO at the Caribbean Epidemiology Center (CAREC) in Trinidad and Tobago in 2002, PAHO’s general recommendations for the surveillance of the West Nile Virus are presented as summarized by its Communicable Diseases Program.

The guidelines’ objective is the timely detection of West Nile virus activity in reservoir and vector populations in order to put in place the appropriate measures in terms of enhanced surveillance, transmission and vector control, as well as mass communication.

Background
In late summer 1999, the first human cases of West Nile virus encephalitis were reported in the United States. The discovery of virus-infected, overwintering mosquitoes during the winter of 1999–2000 led authorities to predict renewed virus activity for the coming spring, and to launch early season vector control and disease surveillance in New York City and surrounding areas at the beginning of the summer of 2000. These surveillance efforts were aimed at detecting and documenting West Nile virus infections in birds, mosquitoes, and horses, as well as in sentinel animals that could predict the appearance of the disease in humans. By August 2002, West Nile virus activity had been detected in 41 states and the District of Columbia in the United States and four Canadian provinces. West Nile virus has been detected in birds (more than 78 species), mosquitoes (14 species), horses, certain other mammals, and humans.

Epidemiological Surveillance
The populations in which surveillance should be implemented in countries where West Nile virus circulation has not been detected, in order of priority, are birds, mosquitoes, horses, and finally, humans.

1. Active Bird Surveillance
Active bird surveillance is aimed at monitoring arbovirus activity in free-ranging and sentinel birds. Surveillance of dead crows in particular and other members of the family Corvidae may be an indicator of West Nile virus in a geographic area. However, in some areas, other wild bird species may be the first birds identified with West Nile virus infection (Table 1).

This surveillance requires the collection of birds that have recently died (within less than 48 hours) and the shipment of their remains (preserved on ice in plastic bags ) to the national reference laboratory.

2. Active Mosquito Surveillance
The purpose of mosquito surveillance is to identify potential vectors, monitor their population densities in a given area, and detect West Nile virus or other arbovirus activity. In 1999, the West Nile virus in the United States was found primarily in bird-feeding mosquitoes. In 2000, virus-infected, mammal-feeding mosquitoes were also found.

Entomological surveys should target mainly adult populations of Culex spp., followed by surveillance of Aedes spp. and other species in areas where probable or confirmed cases have been reported in birds, animals, or humans, and in areas with a high risk of West Nile virus transmission, such as zoos, game reserves, nesting or feeding grounds of migratory birds. (Table 2).

3. Enhanced Passive Veterinary Surveillance
As a support system to detect the presence of West Nile virus and monitor the level of its transmission outside the bird-mosquito cycle, enhanced passive surveillance for neurological disease (passive surveillance enhanced by general alerts to veterinarians) should be implemented mainly in horses and other mammals.

This requires the investigation of cases in horses with neurological symptoms consistent with encephalitis (such as listlessness, ataxia, lack of coordination, staggering, drooping lower lip, partial paralysis, or death) and the shipment of serum and brain samples from these animals to the national reference laboratory for antibody detection and/or virus isolation . It is also useful to send formalin-preserved tissue from the brain and cervical medulla for histopathology. In some countries, it will be necessary to make a differential rabies diagnosis.

4. Enhanced Passive Human Surveillance
Enhanced passive surveillance for human cases of viral encephalitis (passive surveillance by alerting the health services) can be implemented as a backup system for the detection of West Nile virus activity. If resources permit, surveillance of aseptic meningitis should also be implemented.

The objective of human surveillance is to detect serious cases of West Nile virus infection in order to offer treatment.

Case Definitions
A suspected case is any person who presents clinical symptoms of fever and serious neurological manifestations (from aseptic meningitis or encephalitis) of unknown etiology.

A probable case is defined as a suspected case with one or more of the following criteria:
— demonstration of serum IgM antibody against West Nile virus using ELISA immunoenzyme assay (ELISA);
— antibody in convalescent-phase serum screened by ELISA or hemoagglutination inhibition (HI) and confirmed by plaque reduction neutralization test (PRNT).

A confirmed case is a probable case with one or more of the following criteria:
— isolation of West Nile virus or demonstration of West Nile viral antigen or genomic sequences in tissue, serum, cerebrospinal fluid, or other body fluid;
— demonstration of seroconversion (a fourfold or greater increase in antibody titer) to West Nile virus in PRNT in serum or paired samples of cerebrospinal fluid (acute and convalescent);
— demonstration of IgM antibodies to the West Nile virus in a cerebrospinal fluid sample in the acute phase by MAC-ELISA.

Note: Detection of IgM specific antibodies to West Nile virus and/or IgG antibodies (by ELISA) in a single serum or cerebrospinal fluid sample should be confirmed by any of the other previous techniques.

Prevention and Control Measures
Prevention
At the present time, the most effective way to prevent transmission of West Nile virus and other arboviruses to humans and other animals, or to control an epidemic once transmission has begun, is to reduce human exposure to the mosquito vectors. To prevent domestic animal and human disease, public health services must have adequate vector control capabilities.

A critical component of any program for the prevention and control of vector-borne disease transmission is public education about these diseases, how they are transmitted, and how to prevent or reduce the risk of exposure. Public education efforts should make use of behavioral science and social marketing methods to communicate the information effectively to the target populations.

There are some basic precautions that individuals may undertake to limit their exposure to the virus in their homes:
— Put screens on windows and block any holes in the house where mosquitoes can enter.
— Wear long pants and long-sleeved shirts, particularly when remaining outdoors for extended periods, especially at times when mosquitoes are active.
— Minimize outdoor activities at dawn and dusk, when mosquitoes are most likely to bite.
— Use insect repellents with up to 35% of the active ingredient DEET for adults and up to 20% for children.

Herbal and ultrasonic repellents are ineffective against mosquitoes.

Control
The most effective and economical way of controlling mosquitoes is by larval source reduction. Experience shows that this is best done through programs that reduce breeding sites, monitor mosquito populations, and initiate control measures before the disease is transmitted to humans and domestic animals. Such programs can also be used as the first-line emergency response for mosquito control in the event that viral activity is detected in a particular area or the disease is reported in humans. Control of adult mosquito populations through aerial spraying of insecticides is usually kept as a last resort.

In addition to preventing exposure to the mosquitoes, in the United States the Animal Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA) has granted a provisional license for the use of a vaccine composed of killed virus for horses.

Biosafety
Universal precautions for animal necroscopy should be employed, for example: personal protection (use of protective clothing, gloves, facial and eye protectors), sanitary disposal of dead birds and animals or contaminated samples, and the disinfection of all devices after processing the samples.

For the storage and transport of specimens, please refer to the WHO Biosafety Standards at the following Internet site: http://www.who.int/emc-documentos/bioseguridad/whoemc973c.html

Source: Prepared by the Communicable Diseases Program of PAHO’s Division of Disease Prevention and Control (HCP/HCT). This information was adapted from the material used in the West Nile Virus Surveillance Workshop, held at CAREC in April/May 2002, with the support of the Centers for Disease Control and Prevention (CDC) of the United States, PAHO’s Communicable Diseases (HCP/HCT), Veterinary Public Health (HCP/HCV), and Essential Drugs and Technology (HSP/HSE) Programs.

Return to Index
Epidemiological Bulletin, Vol. 23 No. 4, Diciembre 2002