Abstract:
Background: Influenza A viruses pose a significant risk to human health because of their wide host range and
ability to reassort into novel viruses that can cause serious disease and pandemics. Since transmission of these
viruses between humans and pigs can be associated with occupational and environmental exposures, we
investigated the association between occupational exposure to pigs, occurrence of acute respiratory illness (ARI),
and influenza A virus infection.
Methods: The study was conducted in Kiambu County, the county with the highest level of intensive small-scale
pig farming in Kenya. Up to 3 participants (> 2 years old) per household from pig-keeping and non-pig-keeping
households were randomly recruited and followed up in 2013 (Sept-Dec) and 2014 (Apr-Aug). Oropharyngeal (OP)
and nasopharyngeal (NP) swabs were collected from participants with ARI at the time of study visit. For the animal
study, nasal and oropharyngeal swabs, and serum samples were collected from pigs and poultry present in enrolled
households. The human and animal swab samples were tested for viral nucleic acid by RT-PCR and sera by ELISA
for antibodies. A Poisson generalized linear mixed-effects model was developed to assess the association between
pig exposure and occurrence of ARI.
Results: Of 1137 human participants enrolled, 625 (55%) completed follow-up visits including 172 (27.5%) pig
workers and 453 (72.5%) non-pig workers. Of 130 human NP/OP swabs tested, four (3.1%) were positive for
influenza A virus, one pig worker, and three among non-pig workers. Whereas none of the 4462 swabs collected
from pig and poultry tested positive for influenza A virus by RT-PCR, 265 of 4273 (6.2%) of the sera tested positive
for virus antibodies by ELISA, including 11.6% (230/1990) of the pigs and 1.5% (35/2,283) of poultry. The cumulative
incidence of ARI was 16.9% among pig workers and 26.9% among the non-pig workers. The adjusted risk ratio for
the association between being a pig worker and experiencing an episode of ARI was 0.56 (95% CI [0.33, 0.93]), after
adjusting for potential confounders.
Conclusions: Our findings demonstrate moderate seropositivity for influenza A virus among pigs, suggesting the
circulation of swine influenza virus and a potential for interspecies transmission.