Ecological Aspects of Helminth Transmission in Domesticated Animals.

SYNOPSIS. Some of the accumulated knowledge on the complex problems involved in the transmission of helminths of domestic animals in the USA is summarized. The lollowing conditions are usually necessary for the development outside the final hosts of infective stages of these helminths: moderate temperatures, adequate moisture, oxygen, and protection from freezing, desiccation and direct sunlight. Once the infective stages of helminths are attained, either free-living or parasitic in intermediate hosts, those of most species are more resistant to adverse conditions than pre-infective stages. Because temperature and moisture influence the development and survival of helminth infective stages, variable climatic and weather conditions drrectly or indirectly affect helminth transmission. Climate and weather maintain or alter the microclimates of the microhabitats in which helminth eggs and larvae normally live until infection of the final hosts is accomplished. We are only beginning to learn how long-term climatic trends and short-term weather influence helminth transmission. Recent attempts, however, to relate variations in climate and weather to degree and kind of helminth transmission by use of bioclimatographs, the interrelationship of evapotranspiration and precipitation, estimates of potential transmission periods, etc., have increased our understanding of helminth life cycles and helminth diseases of domestic animals, and have aided in developing increasingly effective methods of helminth control.