Evidence that a hydatid cyst is seldom "as old as the patient".

Human hydatid disease caused by the strain of Echinococcus granulosus endemic in Australia and New Zeland has been regarded as a very lonstanding condition, and most cysts diagnosed in adults were believed to have resulted from infection in childhood. A significant drop in the number of new cases among people and 25 and over (P less than 0.05) in two hydatid-control programmes makes it necessary to revise this view. In both Tasmania (P greater than 0.50) and New Zealand (0.50 greater than P greater than 0.10) the incidence was halved without significantly altering the age-distribution. These findings indicate that adults are relatively susceptible and that the latent period between infection and diagnosis in many cases is only a few years.