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Should we regard Hodgkin's disease as infectious?

Environmental factors are known to be important in the aetiology of Hodgkin's disease, and recent interest has centred on the possible occurrence of case-to-case transmission, which might suggest an infectious cause. An extended "epidemic" of cases at a high school in New York State was described by Vianna, Greenwald, and Davies' in 1971. Hodgkin's disease developed in four students who had attended one school at the same time. Subsequent inquiry brought to light 31 additional cases who had had direct contact with the original four or contact through a single intermediary.2 The "epidemic" extended over a period of two decades, suggesting a long incubation period. Transmission seemed to occur particularly among young people and from young to the old. A similar type of outbreak with five cases occurring over a period of 11 years in a small Ohio township containing 405 homes and a population of around 1300 was reported by Klinger and Minton.3 The difficulty in interpreting such events has, however, been emphasized by Smith, Pike, and Kinlen4: if the study in a particular area is initiated because a small cluster has already occurred then the population has been artificially selected. The usual tests of statistical significance cannot, therefore, be applied. As an extension of their previous observations Vianna and Polan5 investigated the hypothesis that in schools where an index case of Hodgkin's disease had occurred there would be a subsequent excess of additional cases among students or staff. In five out of eight such secondary schools (with index cases in 1960-64) additional cases were diagnosed during the years 1965-69. In matched control schools there were no cases during the same period, and the difference in incidence was statistically significant. Subsequent correspondence in the columns of the New England Journal of Medicine has shown questionable support for this finding. Milham6 reported a twoto-threefold higher incidence in schoolteachers in the State of Washington. These observations have, however, been criticized on the grounds that the proportional mortality method of analysis fails to take into account that schoolteachers have a lower mortality from other causes and that they belong to higher social classes in which there is a known higher incidence of Hodgkin's disease.7 8 Recalculation of Milham's data by Hoover8 suggested that the transmission between teachers and students was not a major epidemiological feature of Hodgkin's disease. Vianna et al.9 have more recently asked whether there is an increased mortality from Hodgkin's disease among doctors.