The epidemiology of Echinococcus granulosus in the U.K. VIII. The structure of adult colonies of E. g. equinus Say in farm dogs in Wales

The status of Echinococcus granulosus equinus in farm dogs was examined in two valleys in South Powys, Wales. In one valley 76% of the dogs examined were infected, and there was at least one infected dog on every farm except one. In the second valley 49% of the dogs were infected. The structure of natural adult colonies of Echinococcus is described, and the population dynamics of the parasite in the U.K. is discussed. Every colony in the dogs, excepting the very smallest and the very youngest, consists of parasites at several stages of development. Most colonies comprise less than 20 worms. Colony density is greatest about the second or third eighth of the total length of the small intestine. The general characteristics of a colony (other than number) are reflected in the characteristics of the group of worms 'resident' in each successive eighth of the small intestine. Neither the age nor the sex of the host has any obvious effect on the incidence or the size of the colony, its development, or the ultimate infectivity of the colony. The parasites continue to grow throughout life, and growth may be largely independent of the worms' development. As a result, their length is very variable. The number of eggs in each gravid proglottid varies considerably, from 12-1626. All the evidence available and derived from the study of natural populations confirms that E. granulosus is an obligate self-fertilizing hermaphrodite, and that serial development of gravid proglottids does not occur.