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Lineage 2 West Nile Virus as Cause of Fatal Neurologic Disease in Horses, South Africa

Year: 2009

Type: article

Abstract: Serologic evidence suggests that West Nile virus (WNV) is widely distributed in horses in southern Africa.However, because few neurologic cases have been reported, endemic lineage 2 strains were postulated to be nonpathogenic in horses.Recent evidence suggests that highly neuroinvasive lineage 2 strains exist in humans and mice.To determine whether neurologic cases are being missed in southern Africa, we tested 80 serum or brain specimens from horses with unexplained fever (n = 48) and/or neurologic signs (n = 32) for WNV.From March 2007 through June 2008, using reverse transcription-PCR (RT-PCR) and immunoglobulin (Ig) M ELISA, we found WNV RNA or IgM in 7/32 horses with acute neurologic disease; 5 horses died or were euthanized.In 5/7 horses, no other pathogen was detected.DNA sequencing for all 5 RT-PCR-positive cases showed the virus belonged to lineage 2. WNV lineage 2 may cause neurologic disease in horses in southern Africa.W est Nile virus (WNV), a mosquito-born flavivirus of the family Flaviviridae, is widely distributed throughout Africa, the Middle East, Asia, parts of Europe, Australia, North and South America, and the Caribbean.The WNV transmission cycle involves birds as vertebrate hosts and ornithophilic mosquitoes as maintenance vectors (1).Isolates of WNV fall into 2 major genetic lineages: lineage 1 is found in North America, North Africa, Europe, and Australia; lineage 2 strains are endemic to southern Africa and Madagascar (2,3).Recently, additional lineages in central and eastern Europe (lineages 3 and 4) (4,5) and India (lineage 5) have been reported (6).Humans and horses are incidental hosts for WNV (7).Although most infections are benign, ≈20% of infected persons have fever, rash, arthralgia, and myalgia, and for ≈1% of these, severe disease, including meningoencephalitis, encephalitis, and polio-like flaccid paralysis, may develop.Rare cases result in hepatitis, myocarditis, pancreatitis (8), and death (1).Signs in horses are ataxia, weakness, recumbency, and muscle fasciculation (9-11).Seroepidemiologic studies suggest that asymptomatic infections frequently occur in horses (12,13), but neurologic infections result in a high case-fatality rate (30%-40%) (14).In 2002 the largest outbreak of WNV encephalomyelitis in horses was recorded in the United States; 15,257 cases were reported from 40 states (11).This outbreak was followed in 2003 by the largest outbreak in humans in the Northern Hemisphere (9,832 cases) (1).The number of cases among horses was greatly reduced after the introduction of an inactivated vaccine for animals (10,15,16).In the Karoo, a semidesert region in South Africa, in 1974, WNV caused one of the largest outbreaks ever recorded in humans, affecting tens of thousands of people.During this outbreak thousands of persons visited their local clinicians; however, no cases of neurologic disease were reported.In the 1980s, an epizootic involving WNV and Sindbis virus occurred in the Witwatersrand area of the Gauteng Province in South Africa; this epizootic Lineage 2 West Nile Virus as Cause of Fatal Neurologic Disease in Horses, South Africa

Source: Emerging infectious diseases

DOI: 10.3201/eid1506.081515