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

Background: Equine influenza virus (EIV) is considered enzootic in North America and experimental studies have documented human EIV infections. Study design: This 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. Results: There 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. Conclusions: While we cannot rule out confounding exposures, these data support the premise that occupational exposure to EIV may lead to human infection.