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Study underestimated morbidity due to specific pathogens

Paul R Hunter, Consultant medical microbiologist (Paul.Hunter3@virgin.net)

Public Health Laboratory, Countess of Chester Health Park, Chester CH2 1UL
Infectious Disease Epidemiology Unit, Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London WC1E 7HT
Leeds Public Health Laboratory, Leeds LS15 7TR
Centre for Applied Microbiology and Research, Porton Down, Salisbury, Wiltshire SP4 0JG
Southampton University, Southampton SO6 6YD
Scottish Centre for Infection and Environmental Health, Ruchill Hospital, Glasgow G20 9NB
Food Safety Authority of Ireland, Dublin 1, Republic of Ireland

EDITOR—Wheeler et al's paper estimates the burden of symptomatic infectious intestinal disease in the United Kingdom.1 One aspect of their findings was the high proportion (55%) of community episodes in which no pathogen was identified. I would suggest that this study severely underestimates the prevalence of certain intestinal pathogens. For example, the authors calculate the incidence of cryptosporidium infection in their community study to be only 0.81 (95% confidence interval 0.26 to 2.5) per 1000 person years.

Another approach to determining the incidence of an infectious disease is to determine the prevalence of antibodies in the community and then calculate the annual attack rate required to give that degree of antibody positivity. If an individual is assumed to remain antibody positive for life after an infection then this attack rate is given by a simple formula: cases/100 person years=% seropositive/average life expectancy.

Few studies …