Guidelines for Surveillance, Prevention and Control of
West Nile Virus
(13 September 2002)
Background |
Epidemiological Surveillance |
Case Definitions |
Prevention & Control |
Biosafety |
Reservoirs & Vectors
Due to the threatened spread of West Nile virus to the countries of the Region, the Program on Communicable Diseases has summarized below some general guidelines as PAHO's recommendations on the surveillance of this disease, based on the proceedings and discussion in the "West Nile Virus Surveillance Workshop" organized by PAHO at CAREC in Trinidad and Tobago from 29 April to 3 May 2002.
The guidelines’ objective is the timely detection of West Nile virus activity in reservoir populations and vectors in order to institute the appropriate measures in terms of enhanced surveillance, transmission and vector control, as well as mass communication.
In late summer 1999, the first human cases of encephalitis caused by the West Nile virus were documented in the United States. The discovery of virus-infected, overwintering mosquitoes during the winter of 1999–2000 led authorities to predicted renewed virus activity for the coming spring, and they launched 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 the end of 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.
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. (Annex 1)
This surveillance requires the collection of birds that have recently died (within less than 48 hours) and the shipment of their remains (preserved in plastic bags on ice) to the national reference laboratory.
2. Active Mosquito Surveillance
The purpose of mosquito surveillance is to identify potential vectors, monitor the population densities of these vectors in a given area, and detect West Nile virus or other arbovirus activity. In 1999, West Nile virus in the United States was found primarily in bird-feeding mosquitoes. In 2000, virus-infected, mammal feeding mosquitoes were also found infected.
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 the transmission of West Nile virus, such as zoos, game reserves, nesting or feeding grounds of migratory birds, etc. (Annex 2)
3. Enhanced Passive Veterinary Surveillance
As a backup system to detect the presence of West Nile virus and monitor the extent of its transmission outside the bird-mosquito cycle, enhanced passive surveillance (passive surveillance enhance by general alerts to veterinarians) for neurological disease in horses mainly and other mammals should be implemented.
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 to the national reference laboratory for antibody detection and/or virus isolation . It is also useful to send tissue from the brain and cervical medulla (in formalin) 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, 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.
- A suspected case is any person who presents clinical symptoms of fever and serious neurological manifestations (from aseptic meningitis to 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;
- antibody in convalescent-phase serum (screened by ELISA or hemoagglutination inhibition (HI) and confirmed by 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 the plaque reduction neutralization test (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 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.
- 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 USDA-APHIS has granted a provisional license for the use of a vaccine composed of killed virus for horses.
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 to refer to the WHO Biosafety Standards.
Reservoirs and Vectors (Annexes)
Annex 1: Birds and Animal Species from which West Nile Virus Has Been Isolated
Free-Ranging Native North American Species
| Bittern, Least Ixobrychus sinensis | Jay, Blue Cyanocitta cristata |
| Blackbird, Red-winged Agelaius phoeniceus | Kestrel, American Falco sparverius |
| Bluebird, Eastern Sialia sialis | Killdeer Charadrius vociferus |
| Cardinal, Northern Cardinalis cardinales | Kingfisher, Belted Ceryle alción |
| Catbird, Gray Dumetella carolinensis | Merlin Falco columbarius |
| Chickadee, Black-capped Poecile atricapillus | Mockingbird, Northern Mimus, polyglottos |
| Cormorant, Double-crested Phalacrocorax auritus | Titmouse, Tufted Chordeiles minor |
| Cowbird, Brown-headed Molothrus ater | Ovenbird Seiurus aurocapillus |
| Crow, American Corvus brachyrhynchos | Owl, Great Horned Bubo virginianus |
| Crow, Fish Corvus ossifragus | Phoebe, Eastern Sayornis phoebe |
| Dove, Mourning Zenaida macroura | Rail, Virginia Rallus limicola |
| Duck, Mallard Anas platyrhynchos | Raven, Common Corvus corax |
| Finch, House Carpodacus mexicanus | Robin, American Turdus migratorius |
| Flicker, Northern Colaptes auroatus | Sanderling Calidris alba |
| Goldfinch, American Carduelis tristis | Skimmer, Black Rynchops niger |
| Goose, Canada Branta canadensis | Sparrow, Song Melospiza melodia |
| Grackle, Common Quiscalus quiscula | Thrush, Hermit Catharus guttatus |
| Grouse, Ruffed Bonasa umbellus | Thrush, Wood Hylocichla mustelina |
| Gull, Great Black-backed Larus marinus | Turkey, Wild Meleagris gallopavo |
| Gull, Herring Larus argentatus | Turnstone, Urdí Arenaria interpres |
| Gull, Ring-billed Larus delawarensis | Veery Catharus fuscescens |
| Hawk, Broad-winged Buteo platypterus | Vulture, Black Coragyps atratus |
| Hawk, Cooper's Accipiter cooperii | Warbler, Blackpoll Dendroica striata |
| Hawk, Red-tailed Buteo jamaicensis | Warbler, Canada Wilsonia canadensis |
| Hawk, Sharp-shinned Accipter striatus | Warbler, Yellow-rumped Dendroica coronata |
| Heron, Great Blue Ardea herodias | Warbler, Black-throated Blue Dendroica caerulescens |
| Heron, Green Butorides virescens | Waxwing, Cedar Bombycilla cedrorum |
| Hummingbird, Ruby-throated Archilochus colubris |
Captive North American Bird Species
| Crane, Sandhill Grus canadensis | Magpie, Black-billed Pica pica |
| Eagle, Bald Haliaeetus leucocephalus | Night-Heron, Black-crowned Nycticorax nycticorax |
| Gull, Laughing Larus atricilla | Owl, Snowy Nyctea scandiaca |
Other Free-Ranging Bird Species
| Dove, Rock (pigeon) Columba livia | Starling, European Sturnus vulgaris |
| Pheasant, Ring-necked Phasianus colchicus | Swan, Mute Cygnus olor |
| Sparrow, House Passer domesticus |
Free-Ranging Mammal Species
| Bat, Big brown Eptesicus fuscus | Raccoon Procyon lotor |
| Bat, Little brown Myotis lucifugus | Skunk, Striped Mephitis mephitis |
| Chipmunk, Eastern Tamias striatus |
Pet and Other Domesticated Species
| Cat | Chicken |
| Dog | Turkey, domestic |
| Rabbit, domestic | Peacock |
| Horse | Goose, domestic |
| Parakeet | |
| Macaw, Ara spp | |
| Cockatoo | |
| Cockatie | |
| Finch |
Exotic Species Housed in Zoos
| Cormorant, Guanay | Pheasant, Himalayan Impeyan |
| Duck Bronze-winged | Tragopan Blythe's |
| Flamingo, Chilean |
Annex 2: Mosquito Species from which the West Nile Virus Has Been Isolated
| Aedes albopictus | Aedes cinereus |
| Aedes vexans | Anopheles barberi |
| Anopheles punctipennis | Anopheles quadrimaculatus |
| Coquillettidia perturbans | Culex pipiens |
| Culex restuans | Culex nigripalpus |
| Culex quinquefasciatus | Culex salinarius |
| Culiseta melanura | Ochlerotatus atlanticus |
| Ochlerotatus atropalpus | Ochlerotatus canadensis |
| Ochlerotatus cantator | Ochlerotatus japonicus |
| Ochlerotatus sollicitans | Ochlerotatus taeniorhynchus |
| Ochlerotatus tormentor | Ochlerotatus trivittatus |
| Orthopodomyia signifera | Psorophora columbiae |
| Psorophora ferox | Uranotaenia sapphirina |
For posting and distribution purposes, the above document is available in Word and PDF formats.
Fact Sheet on West Nile Virus | Related Sites: West Nile Virus