A Case-Control Approach to Examine Diseases for Evidence of Contagion, Including Diseases with Long Latent Periods

A number of situations have recently been described in which Hodgkin's lymphoma appears to have shown epidemiological characteristics of a contagious disease, with a long and variable latent period between the transmission of the disease and the onset of symptoms (Vianna et al. [1971; 1972]). The statistical significance of these reported outbreaks has been impossible to assess. In this paper we first suggest a case-control approach to evaluate the statistical significance of (A) the total effective contact between all possible pairs of patients, and (B) the number of patients involved in effective contact with at least one other patient: this method requires matched controls to be selected for each patient. We then generalize this case-control approach to the situation where groups of patients are matched with groups of controls. Finally we show how a previously published extension of Knox's space-time clustering method (Pike and Smith [1968]) might also be used in studies of such diseases in which a long latent period is postulated: this generalization of the Knox method does not require the collection of information from control persons. The case-control method is likely to be the more sensitive approach.