In the microenvironment concept of avian botulism epizootiology, it is hypothesized that invertebrate carcasses may serve both as a substrate for toxin production by Clostridium botulinum type C and as a vehicle for toxin transmission to water birds. We field-tested that hypothesis by attempting to induce botulism in wing-clipped mallard ducks (Anas platyrhynchos) on sewage oxidation ponds in Utah. The experimental ponds were inoculated with C. botulinum spores in June 1974. Aquatic insect populations were monitored throughout the summer. Rotenone was used in August to kill insects in two ponds (one served as control), thereby providing potential substrate for clostridial growth and toxin production. Botulism was not detected among the birds even though they routinely ingested invertebrate carcasses. Samples of dead invertebrates contained no botulinum toxin. We concluded that the microenvironment concept, as it now stands, cannot always be a sufficient explanation of how type C botulism epizootics are initiated in nature. Other microbes may inhibit the growth of clostridial cells or destroy botulinum toxin.
