Cystic echinococcosis in the Berber of the Mid Atlas mountains, Morocco: new insights into the natural history of the disease in humans.

Precise knowledge of the natural history of cystic echinococcosis (CE) in humans remains somewhat arcane. The aim of the present study was to determine whether aspects of the natural history of established human infection with Echinococcus granulosus could be investigated by using a cross-sectional approach, in a community where CE was endemic. A mass ultrasound-screening survey, coupled with a questionnaire to record all previous surgical histories related to CE, was carried out amongst the primarily transhumant Berber people of the Mid Atlas mountains in Morocco. During two periods of intensive screening, of 10 days in May 2000 and 11 days in May 2001, 11,612 people, representing > 98% of the local population, were checked. One hundred and twenty six (1.1%) of the subjects--75 (59.5%) of the 6864 females investigated and 51 (40.5%) of the 4748 males--were found ultrasound-positive for CE. Overall, 14.1% of the CE cases detected were children aged 1-15 years (who made up 44% of the study population). Most (77.4%) of the ultrasound-positive subjects investigated were also found seropositive for CE. The frequency of a past history of surgery for CE increased with subject age (P = 0.024), 125 (1.1%) of the subjects being recorded as having had such surgery. The frequency of surgery for pulmonary CE was relatively constant in all age-groups, indicating that infection can occur at any time. The frequency of abdominal CE increased with age (R2 = 0.8102). Assuming that the incidence of infection remains fairly constant over time and that the longer a person spends in this endemic area the more likely it is that he or she will develop CE, then a progression from active through transitional to inactive CE should occur. The cysts observed by ultrasound in the present study were categorized, as type 1, 2, 3, 4 or 5, according to the new, standardized, ultrasound classification of CE developed by the World Health Organization (WHO). There was a clear exponential decline in the frequency of the various cyst types, from type 1 (the most frequent) to type 5 (the rarest). This decline validates the assumptions made, about the natural history of established CE, by those who developed the WHO's classification. The classification should therefore be invaluable to surgeons and clinicians, when they have to consider treatment options for patients with the various types of CE, and to policy makers trying to establish the economic costs of treating CE in endemic settings.