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SUMMARYEnvironmental sex determination in the mermithid nematodeRomanomermisculicivorax is examined in the context of parasite reproductive success and population regulation. Experimental results show that the sex ratio of the nematode within its mosquito host (Culex quinquefasciatus) is dependent on parasite density. Sex ratios are biased to females at low parasite burdens and to males at high parasite burdens. Low temperature further enhances female-biased ratios. The net effect of density-dependent sex determination on parasite and host population growth is shown to be critically dependent on the frequency distribution of parasite numbers/host. Mermithid parasite distributions within natural host populations show low degrees of aggregation relative to other helminth species. The population regulation of the parasite is examined with respect to environmental sex determination and parasite-induced host mortalities by means of simple mathematical models of the dynamics of parasite transmission via its life-cycle. The significance of regulatory constraints on population growth are discussed in relation to the use of mermithids as biological control agents of insect pests or disease vectors.

In a study of faecal egg counts of Schistosoma mansoni from 359 people of all ages from a rural Kenyan community, a positive association was demonstrated between infection intensity in individuals before treatment and reinfection intensity in the same individuals 9 months after treatment in certain age groups of the sampled population. Consequences and possible causes of these observations are discussed in terms of the epidemiology and control of schistosomiasis.