HIV/AIDS News & Information

the end of August 1996, 1465 cases of cyclosporiasis were reported in North America (740 sporadic and 725 cases related to 55 clusters). Preliminary investigations sug­gested a possible common source for this outbreak. Although several berries including strawberries were initially suspect, Guatemalan raspberries were ultimately implicated. This con­clusion was based on the strength of the cumulative evidence gathered in many jurisdictions, thus highlighting the impor­tance of the coordinated multiagency response led by the CDC.

Several key factors were crucial to the initial identification and investigation of this outbreak. A family physician was re­sponsible for reporting the first cluster of cases of cyclosporia- sis in Ontario. Subsequently, the municipal public health unit notified provincial public health authorities when local inves­tigations raised concerns of a widespread and ongoing out­break. The additional event-associated clusters and sporadic cases in Ontario were recognized in part because Ontario labo­ratories and health units received specific directions to facili­tate diagnosis and enhance surveillance. The initial responses from the local primary and public health systems are testimony to their important role in surveillance and control of communi­cable diseases, including emerging infectious diseases.

The different components of the outbreak investigation in Canada (cohort cluster investigations, a case-control study of sporadic cases, and the traceback of berries to a common source) and the necessity of combining evidence from investi­gations in other jurisdictions, each have potential strengths and limitations. An important limitation of the Ontario case- control study was potential recall bias from media attention on strawberries. Patients with cyclosporiasis may have been more likely to recall eating strawberries than other produce, thereby increasing the likelihood of an observed association. In several American case-control studies that were performed earlier than the Ontario study, consumption of fresh raspber­ries was most strongly associated with illness. The poten­tial for recall bias was less likely in the cluster investigations compared with the case-control study because persons were interviewed earlier and specifically about food eaten at a sin­gle event. At these events, however, the berry items included mixtures of berries and only three of seven clusters had well documented food histories. Tracing food to a contaminated common source can potentially provide strong direct evidence of causal association. However, tracebacks of fresh produce in general, and berries in particular, are often difficult to perform. Only two of the six cluster-associated events (events #1 and #6) in Ontario at which raspberries were definitely or probably served had well documented traceback data.

Raspberry exports from Guatemala to Canada are routed via the United States and occur primarily in the spring and fall. From March 23 to July 7, 1996, Guatemalan raspberries represented approximately 10% of the 88,565 kg of raspberries imported into Canada (personal communication, Horticulture Section, Agriculture and Agri-Food Canada, 1996). Of the Gua­temalan raspberries imported during this period, 86% were im­ported from May 1 to June 22 when the cluster-associated events occurred. During this period, Guatemalan raspberries were not imported into British Columbia, Manitoba, Sas­katchewan or the Atlantic Provinces and no increase in cases of cyclosporiasis was identified in these provinces. From May 1 to June 22, 1996, 79% of the Guatemalan raspberries were imported into Quebec, where only one cluster and a few sporadic cases of cyclosporiasis were reported, whereas only 20% were imported into Ontario. Enhanced surveillance, avail­ability of laboratory diagnostic testing and uneven distribu­tion of contaminated raspberries could have contributed to the burden of disease identified in Ontario.
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