Confined animal feeding operations as amplifiers of influenza

Abstract

Influenza pandemics occur when a novel influenza strain, often of animal origin, becomes transmissible between humans. Domestic animal species such as poultry or swine in confined animal feeding operations (CAFOs) could serve as local amplifiers for such a new strain of influenza. A mathematical model is used to examine the transmission dynamics of a new influenza virus among three sequentially linked populations: the CAFO species, the CAFO workers (the bridging population), and the rest of the local human population. Using parameters based on swine data, simulations showed that when CAFO workers comprised 15-45% of the community, human influenza cases increased by 42-86%. Successful vaccination of at least 50% of CAFO workers cancelled the amplification. A human influenza epidemic due to a new virus could be locally amplified by the presence of confined animal feeding operations in the community. Thus vaccination of CAFO workers would be an effective use of a pandemic vaccine.

FIG. 1

Transmission dynamics between the CAFO species, CAFO workers, and the rest of the local community. In each group, susceptibles (S) become infected (I) and then removed (R) after recovery.

FIG. 2

Epidemic prevalence curves for humans corresponding to different percentages of CAFO workers in the community.

FIG. 3

Epidemic curves showing the prevalences for the confined species, CAFO workers, and local population. Local communities consisting of 0%, 15%, 30%, and 45% CAFO workers are considered in each case.

FIG. 4

Percentage increases in the final size of the human influenza epidemic as a function of the percentage of CAFO workers in the community. The curves correspond to pre-epidemic successful vaccination of 0% to 70% of the CAFO workers. Local communities with 5%, 15%, 25%, 35%, and 45% of CAFO workers are considered.

FIG. 5

Percentage increases in the final size of the epidemic as a function of the percentage of CAFO workers in the community with $R_0^H=1.1$, 1.2, 1.3, 1.4, and 1.5. CAFO workers are 5%, 15%, 25%, 35%, and 45% of the local population.

FIG. 6

Percentage increases in the final size of the human epidemic as a function of the percentage of CAFO workers in the community. The contact rate β_ws is based on data in which 20-80% of CAFO workers were seropositive to specific strains of swine influenza.