Serological evidence of equine influenza infections among persons with horse exposure, Iowa

Abstract

BackgroundEquine influenza virus (EIV) is considered enzootic in North America and experimental studies have documented human EIV infections. Study designThis cross-sectional study examined 94 horse-exposed and 34 non-exposed controls for serological evidence of EIV infection. Sera were evaluated for antibodies against three EIV and two human H3N2 viruses using microneutralization (MN), neuraminidase inhibition (NI), enzyme-linked lectin (ELLA), and hemagglutination inhibition (HI) serological assays. Risk factor analyses were conducted using logistic regression and proportional odds modeling. ResultsThere was evidence of previous infection by MN assay against A/equine/Ohio/2003(H3N8) but not the other 2 EIVs. Eleven (11.7%, maximum titer 1:320) horse-exposed and 2 (5.9%, maximum titer 1:160) control subjects had MN titers ≥1:80. Among the horse-exposed, 18 (19.1%) were positive by NI assay and 8 (8.5%) had elevated ELLA titers ≥1:10. Logistic regression modeling among horse-exposed revealed that having an elevated MN or ELLA titer (≤1:10) was associated with having a positive NI titer (OR=4.9; 95% CI=1.3–18.7, and OR=53.2; 95% CI=5.9–478.5, respectively). Upon proportional odds modeling, having worked as an equine veterinarian (OR=14.0; 95% CI=2.6–75.9), having a history of smoking (OR=3.1; 95% CI=1.2–7.7), and receipt of seasonal influenza vaccine between 2000 and 2005 (OR=2.3; 95% CI=1.1–5.0) were important independent risk factors for elevations in MN assay. ConclusionsWhile we cannot rule out confounding exposures, these data support the premise that occupational exposure to EIV may lead to human infection.