Data from slaughter plants (n = 3) and feedlots (n = 18) in eastern Washington were analyzed to characterize occurrence patterns of cysticercosis in Washington during 1984. Three concurrent peaks in cysticercosis rates (0.6/1,000 to 5/1,000 slaughtered cattle) were detected at 3 slaughter plants. Peaks were observed at 8 feedlots from December 1983 to March 1984, at 6 feedlots from April to July 1984, at 2 feedlots from August to October 1984, and at 3 feedlots from November 1984 to February 1985. Affected feedlots were not closely associated geographically and were feeding cattle from many, predominantly northwestern, origins. For 3 feedlots for which time in the feedlot was available for each slaughter shipment, an increase in cysticercosis rate with increasing time in the feedlot was noticed. Within these 3 feedlots, cases of cysticercosis were widely scattered spatially. The pattern of cysticercosis indicated human fecal contamination of a regionally available feed source. Of feedstuffs in use, potato waste, a byproduct of the processed potato industry, appeared to be the most likely source of Taenia saginata ova.

The goal of outbreak investigation is the identification of the risk factors over which management has control; these factors are known as key determinants. The risk factor approach to disease causality is much more functional than simplistic theories such as agent causality. Many outbreaks, even of infectious disease, do not have simple microbiologic explanations. In the planning of an outbreak investigation, the most critical step is the identification of management procedures that have been found, by scientific studies, to influence risk of the type of disease present. During the investigation, specific disease data, rather than the client's impressions, should be collected so that temporal and risk group analyses may be performed. Performance evaluation should be conducted to determine the breadth of management systems involved and the duration of involvement. Management evaluation must be specific, comprehensive, and targeted by scientific studies, risk group and temporal analyses, physical examination, and laboratory diagnosis. The entire investigation--the planning, the data gathering, the sample collection--must be concentrated on identifying the ways in which management should be changed to restore equilibrium to a herd experiencing excessive losses due to disease.