Life cycle and host-parasite relationships of Khawia iowensis Calentine and Ulmer, 1961 (Cestoidea: Caryophyllidea)

Gravid Khawia iowensis occur within the intestinal tract of the cyprinid fish (Cyprinus carpio). Proceroids occur within the coelom of the tubificid oligochaete (Aulodrilus pigueti). Naturally infected annelids were not found; in Iowa, fish infections are strictly seasonal with maximal prevalences and mean intensities occurring during August;Dero digitata, Stylaria lacustris, Ilyodrilus templetoni, Branchiura sowerbyi, Limnodrilus hoffmeisteri and A. pigueti were exposed to embryonated eggs of K. iowensis. Infections, acquired by egg ingestion, occurred only in A. pigueti. After hatching in the tubificid, oncospheres penetrate the gut wall and migrate anteriad within the host\u27s coelom where procercoid development occurs;A cercomere, bearing six embryonic hooks, is characteristic of the procercoid larva. Procercoids possessing a scolex typical of the adult worm are probably infective to fish by day 60. Progenetic development does not occur in K. iowensis procercoids;Experimental feedings of infected oligochaetes to carp did not definitely estabish how fish acquire infections. Evidence indicates that periodicity of adults in fish may result from natural senescence of gravid worms and from variations in feeding activities of carp during the year. Two other intestinal helminths of carp, Pomphoryhnchus bulbocolli and Capillaria sp., did not exhibit seasonal periodicity;K. iowensis, Capillaria sp. and P. bulbocolli occupy discrete regions of the carp gut. Khawia occurs primarily in the anterior half of the gut, P. bulbocolli mostly in the third quarter and Capillaria, in light infections, primarily in the fourth quarter of the gut. Evidence exists for intraspecific competition by Capillaria and for radial segregation between Capillaria and P. bulbocolli. For K. iowensis, prevalence and mean intensity increased with increase in carp length. Male and female carp had similar prevalences and mean intensities for each helminth species;K. iowensis is redescribed and its close morphological similarity to K. japonensis is noted. Pathology elicited by K. iowensis in fish and oligochaete hosts is examined. Procercoids may cause rupture and death of A. pigueti. K. iowensis appear to be relatively benign in wild carp