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The ability of cytokines to act as natural immunotherapeutics to enhance the health and the disease resistance of animals is of particular interest to the intensive livestock industries. Antibiotics have been used for such purposes over a long period of time, however, there is growing concern that this practice will enhance the development of antibiotic resistance in a range of bacterial pathogens. In several species, interleukin 5 (IL-5) is known to enhance B cell activity and to increase the numbers of eosinophils in blood and tissues. In this report, IL-5 was delivered to pigs, either as a recombinant protein or via a DNA delivery vector and was shown to elevate eosinophils in blood over a sustained period. Interleukin 3, a potent haemopoietic factor, did not synergise with IL-5 when both cytokines were given together, but did prime the pigs for a stronger response to IL-5. These results demonstrate that IL-5 can readily be delivered to commercial pigs to elicit a significant biological effect.