West Nile virus (WNV)

West Nile virus (WNV) is a mosquito-borne flavivirus related to St. Louis encephalitis virus (SLEV), Murray Valley encephalitis virus, Kunjin virus and Japanese B encephalitis virus. The virus has been isolated from at least 43 species of mosquitos arond the world, including Culex spp., Aedes spp. and Anopheles spp. West Nile virus was first isolated from a woman in Uganda in 1937. Since that time, it has been recognized elsewhere in Africa, as well as in India, Pakistan, Asia, the Middle East and Europe. Most recently, WNV appeared in the United States in 1999. This is the first time that WNV has been recognized in the Western Hemisphere.

• On August 23, 1999, an astute physician in Queens, NY recognized that it was unusual to have 2 cases of febrile encephalitis at the same time within a very small area of one neighborhood (Whitehurst). Reporting to the NYC Health Department led them to uncover additional cases from other area physicians. This outbreak in 1999 ultimately involved 62 human cases, 7 deaths (primarily in elderly persons with underlying illnesses) and seroconversion in 2.6% of the ~46,000 local Queen's population.
  • These infections were initially attributed to SLEV. However, the death of crows in the area (ultimately thousands died in the northeastern U.S.), exotic birds at the Bronx Zoo (Chilean flamingos, cormorants, a bald eagle and a snowy owl) and horses on nearby Long Island (all of which are species not typically made ill by SLEV) led to a search for a different agent, which was ultimately shown to be WNV.

How was WNV introduced to the United States and from where?

• Genetic and phylogenetic data suggest that the strain of WNV in the United States is mostly closely related to a WNV strain from the Middle East, specifically a WNV isolated from a goose in Israel in 1998. West Nile virus may have been introduced to the United States by several methods:
  • introduction of an infected bird -- Given the requirements for quarantine of birds brought into the United States, this would have had to occur via illegal smuggling or trans-Atlantic migration.
  • introduction of infected mosquitos -- This could occur via the transportaion of mosquitos on airliners or on-board trans-Atlantic ships.
  • travel by a person who was incubating the virus?

West Nile virus infection in people:

• The presentation of WNV infection in people can range from asymptomatic infection to fatal encephalitis and may include:
  • fever
  • maculopapular rash
  • muscle weakness
  • flaccid paralysis
  • seizures
  • nausea and vomiting
  • lymphopenia (This was a prominent finding in one group of patients in the 1999 outbreak in NYC.)
    • The viremia phase is reported to be relatively short in people (6 days).

West Nile virus infection in horses:

• In the equine epizootic in 1999, 22 horses were affected, with 13 deaths.
• Clinical signs of WNV infection in horses include an acute onset of ataxia, hypermetria and weakness (often parparesis), in some cases ascending to tetraparesis, complete recumbency and death. Fever (possibly biphasic) occurs in some, but not in all cases.
  • It should be noted that not all horses present with the full set of clinical signs, e.g. 2 horses initially presented in 1999 with unilateral forelimb lameness and 1 horse presented with an apparent unilateral radial nerve paralysis.
• Other clinical signs in some animals include muscle fasiculations, anisocoria, slow pupillary light responses, lacrimation, and seizures.
• 6 of 8 horses in one study exhibited hypersensitivity to touch and sound.
• Although a substantial percentage of horses infected with WNV in the U.S. have succumbed (~17-39%), many of the surviving horses have returned to training and do not exhibit residual neurologic signs.
• The primary differential diagnoses in horses include equine herpesvirus myelitis, equine encephalitis virus (EEEV, WEEV, VEEV), rabies or equine protozoal myelitis (EPM). In fact, the initial working diagnosis for many of the equine cases reported in 1999 was EPM.
• During the summer of 2001, a killed-virus vaccine was conditionally licensed for use in horses in the U.S.

WNV activity in the U.S. since 1999:

• WNV has proven that it can overwinter successfully in the U.S. and it has spread to include a progressively wider geographic region in 2000-2001.
  • In 2000, there were 21 humans cases with 2 deaths, 60 equine cases with 23 deaths (from MA, CT, RI, DE, NY, NJ and PA) and 4,323 wild bird cases (primarily crows and related species such as ravens and jays).
  • In 2001, the virus was detected from horses, people and/or birds throughout the eastern U.S. (with the exceptions of NH, WV and SC) and as far west as WI, IA, MO, AR and LA. There were 45 human cases (5 deaths), 347 equine cases (59 deaths) and 5991 birds cases, about 3/4 of which were, again, in crows.
  • In 2002, WNV infections have already (as of February) been detected in Florida.
• Public health officials now consider that WNV will henceforth be a permanent, enzootic/endemic virus in the U.S. It has been suggested that geographic expansion of the WNV range will also continue, perhaps reaching as far west as the Rocky Mountains during 2002.