Lineage 2 west nile virus as cause of fatal neurologic disease in horses, South Africa

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 South 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 South Africa.

Figure 1

Histopathologic section of 2 lumbar spine gray matter dendrites that stained immunohistochemically positive with flavivirus antiserum on postmortem tissue of horse HS101/08. Magnification ×1,000.

Figure 2

Maximum-likelihood comparison of the partial NS5 gene of West Nile virus (WNV) strains identified in horses in South Africa in 2008 with representative sequences of other WNV lineages. Bootstrap statistics are shown on the branches; only values >70% are included. Scale bar indicates 0.07 nt changes. Japanese encephalitis virus (JEV) was used as an outgroup. Black diamonds, WNV strains identified in horses in South Africa in the present study; white diamonds, WNV strains isolated from humans in South Africa in previous years. WNV strains and accession numbers and origin: HS123_08, FJ464376, South Africa; HS125_08, FJ464377, South Africa; HS101_08, FJ464378, South Africa; SAE126_08, FJ464379, South Africa; SAE134_08, FJ464380, South Africa; SA381_00, EF429199, South Africa; SA93_01, EF429198, South Africa; SPU116_89, EF429197, South Africa; goshawk_Hungary_04, DQ116961, Hungary; B956 polyprotein gene-1937, AY532665, Uganda; B956 117B3, M12294, Uganda; ArD76104, DQ318019, Senegal; H442, EF429200, South Africa; ArB3573_82, DQ318020, Central African Republic; Sarafend, AY688948, Uganda; Madagascar AnMg798, DQ176636, Madagascar; PTRoxo, AM404308, Portugal; Egypt101, AF260968, Egypt; EthAn4766, AY603654, Ethiopia; Kunjin MRM61C, D00256, Australia; WNV Italy 1998 equine, AF404757, Italy; WNV Ast02–2-25, DQ374653, Russia; LEIV-Vlg00–27924, AY278442, Russia; LEIV-Krnd88–190, AY277251, Russia; goose_Hungary_03, DQ118127, Hungary; NY385_99, DQ211652 NY, USA; NY99-eqhs, AF260967 NY, USA; NY99-flamingo382–99, AF196835 NY, USA; TYP9376 NY385_99, AY848697 NY, USA; WNV TX 2002 02, DQ164206 TX, USA; TM171_03, AY371271, Mexico; Mex03, AY660002, Mexico; TX 2002-HC, DQ176637 TX, USA; WNV804994, DQ256376, India; Rabensburg 97–103, AY765264, Czech Republic; JEV JaOAr5982, M18370, Japan.

Figure 3

Maximum-likelihood analysis of the E-protein region of a West Nile virus isolate, HS101_08 (black diamond), recovered from horses in 2008 compared with isolates obtained from humans and animals from South Africa and other regions of the world. Nucleotide differences between lineage 2 strains included in the alignment are shown in the summarized alignment below the tree, indicating only unique nucleotides. Vertical numbers above the alignment indicate the position of each variable site on the gene fragment. Scale bar indicates nucleotide substitutions per site.