Population genetics seeks to understand the genetic relationships within and between populations of a species and the processes that generate these patterns. Little is known about the population genetics of parasitic nematodes. This is a notable gap in our knowledge since understanding the population genetic patterns and processes of parasitic nematodes has profound implications for our ability to fully understand this important group of pathogens. For example, it is only possible to begin to understand how a parasite population will respond to an imposed selection pressure (such as an anthelmintic drug, a vaccine, or resistant hosts) when the population genetic structure and patterns of gene flow of that population is known. Equally, the epidemiology of many nematode parasites is well known empirically and theoretically, yet this epidemiological information is of limited use without a good understanding of the genetic structure of those populations (Anderson & May, 1992).

The relatively small literature on mitochondrial DNA (mtDNA) diversity in nematode species is summarized here. Nematodes show a wide range of overall genetic diversities and population genetic structures. Species-wide levels of diversity correlate strongly with the breeding system and other life cycle features that control effective population size. Obligate outcrossers that parasitize mobile vertebrate hosts are the most diverse, species having hermaphroditic stages are less so, and species having asexual reproductive stages appear even less diverse. Nevertheless, these conclusions are preliminary because there exist so few data on DNA diversity in nematodes. What is needed are more comparative studies using similar sampling designs and the same DNA markers, including nuclear loci and further work with mtDNA

Polymorphic molecular markers are being identified to characterize the genomes of parasitic nematodes. The aim is to construct a map with markers evenly spread over the six chromosomes. With such a map, regions can be identified that are under selection pressure when attempts are being made to eradicate worms, be it by drugs, vaccines or genetic resistance in the sheep. Several types of markers have been developed, microsatellites, transposon-associated markers, amplified fragment length polymorphism (AFLP) and expressed sequence tag (EST) markers. Linkage groups can be constructed using several generic crosses between inbred and drug resistant strains. EST markers will be especially important for comparative mapping with the genome of Caenorhabditis elegans, and therefore localization of the linkage group on a chromosome. It will then be possible to identify functional genes close to markers that have changed allele frequencies under selection pressure and identify the mechanisms of resistance to parasite control.

We review how constraints on the mating probability of female worms by segregation within individual host guts form a critical element in the parasite population structure of nematodes. We consider the effects of these constraints on the population genetics of nematodes under various assumptions regarding worm competition as reflected in the relationship between the abundance of a certain genotype within the gut and the probability of reproductive success. The consequences for the emergence of resistance to drugs and immunotherapy and implications for host-parasite coevolution are discussed. We also review evidence for genetic heterogeneity in parasite populations as a necessary prerequisite for the applicability of mating probability models designed to assess the population genetics of nematodes.

Most studies of helminth communities in the European eel Anguilla anguilla have been undertaken in the British Isles, and there are very few analyses of community composition and structure from continental Europe. To fill this gap and test the hypothesis that helminth communities in freshwater eels in the British Isles are not typical of those of continental Europe, helminth communities of eels in the River Tiber below Rome were analysed by season using data collected in 1980 and new data from 1996. The intestinal helminth communities in the Tiber eels were species poor and characterized by low diversity. Most eels harboured one or no parasite species and communities were heavily dominated by the acanthocephalan Acanthocephalus clavula. Intestinal helminth infra-community richness and diversity did not differ between seasons within a year or between the same seasons in 1980 and 1996, although some changes in composition were apparent. Intestinal infracommunities from Tiber eels were very similar in characteristics to those analysed from the British Isles, and their temporal changes also showed close similarities to those reported from rivers in the UK. It seems likely therefore that conclusions derived from British studies can be applied to helminth communities of eels on the continent.

Of the 13 species of helminths that parasitize stream fishes in Hawai'i, seven species are considered to be native to the archipelago and the remaining six species to be introduced by man. Sources of colonization for native species are piscivorous birds for three species, and marine fishes for four species. Non-native helminths have been brought to Hawai'i in association with the importation of parasitized exotic species of poeciliids introduced into streams for mosquito control and as aquarium releases. Many of these introduced parasites have broad host specificity and now infect the five species of native gobioid stream fishes. Exotic parasites, including a roundworm Camallanus cotti, a tapeworm Bothriocephalus acheilognathi and a leech Myzobdella lugubris, are more widely distributed among Hawaiian streams than are native species.

The length and width of 1297 Fasciola hepatica eggs shed in cattle hosts, 337 in sheep and 199 in nutria, were measured from several parts of France. The data were compared with those obtained from other studies in Spain, France (where rats were also investigated), Germany and the Netherlands. One way analysis of variance and discriminant analysis were used to assess differences between host origins. The distribution of length and width of eggs were analysed using skewness and kurtosis Fisher coefficients. The eggs recovered from sheep, cattle, rodents and lagomorphs were different in size: the eggs found in rodents (length L × width W in μm: 8592) and lagomorphs (L × W in μm: 9100) were smaller than those found in sheep and cattle (L×W in μm: 10,000). These morphological differences in F. hepatica eggs were host-induced in rats (L×W in μm: 9709 in cattle to 8949 in rats) and rabbits (L×W in μm: 9709 in cattle to 8432 in rabbits). These differences in size of eggs might correspond to their being less able to develop into miracidia in less frequent hosts such as rodents and rabbits.

Lymnaea tomentosa, the intermediate host of a schistosome which causes schistosome dermatitis in Lake Wanaka, was found to depths of 16 m. The snail recruits in January, lives for up to 21 months, is associated with all water plant species and is found in areas devoid of macroscopic plants. It is host to several trematode species. Avian schistosome infections appear in the spring when echinostome prevalence is low, and a decrease in schistosome prevalence may be correlated with an increase in echinostome prevalence during the summer. A multiple-kind lottery model analysis of parasite species richness implies that interspecific interactions may be occurring in the host snail during the months of December and January.

Miracidia of Echinostoma caproni were exposed to solutions varying in salinity, pH, and temperature in 1 ml concavity slides. Half-lives of the miracidial populations were determined and longevity curves constructed to find maximum life spans of the miracidia in the different conditions. Control miracidia in aquarium water at pH 7.2 and 22°C. had a half-life of 3.6 h and a maximum life span of 9 h. Miracidia of E. caproni were not very tolerant of saline solutions from 0.1% to 0.4%, the latter being lethal within an hour. A bimodal effect was found with exposure to aquarium water of varying pH, with a peak at pH 5 in acid solutions and pH 9 in alkaline solutions. Miracidia tolerated pH ranges from 3 to 11 exhibiting half-lives of 2.4 h or greater in these solutions. At lower than ambient temperatures, E. caproni miracidia lived longer, the greatest being a half-life of 5.0 h and a maximum life span of 15h at 5°C. At warmer temperatures, the half-life was reduced until lethality was reached at 40°C. ANCOVA analysis of log transformed longevity curves supported the observation that in pH trials many miracidia survived initially with a major die-off after 3.3 h. The opposite trend of early die-off and gradual mortality of the survivors was supported in the temperature trials.

The distribution of two acanthocephalan species (Pomphorhynchus laevis and Acanthocephalus anguillae) in the chub (Leuciscus cephalus) was studied in foriver reaches characterized by different levels of pollution: the River Ticino ne Abbiategrasso (unpolluted), the Naviglio Grande Canal, in Milano (slightly polluted), the River Lambro near Merone village (polluted) and the Riv Lambro near Monza (severely polluted).

Pomphorhynchus laevis was restricted to the unpolluted and the slight polluted sites, while the intensity of A. anguillae increased proportionally water pollution. These differences were partially explained by the variation in abundance of their intermediate hosts (Echinogammarus stammeri for P. lavis and Asellus aquaticus for A. anguillae). Data on the occurrence of P. laevis at A. anguillae showed a significant negative binomial frequency distributic suggesting their tendency to be aggregated within the host populations L. cephalus.

Echinococcus granulosus is widely distributed throughout Iran, where a variety of animals act as intermediate hosts. In this study, the development and morphological characteristics of E. granulosus of sheep, cattle and camel origin were compared and, overall, the characteristics of larval and adult isolates from sheep and cattle, although similar, showed significant differences from those of the camel isolates, especially with respect to the size of rostellar hooks. Differences in the fertility rates of hydatid cyst in sheep, cattle and camel were recorded as 88%, 19% and 70% respectively.

In dogs experimentally infected with the three ‘strains’ of E. granulosus, the rate of development was more rapid in the camel than in the sheep and cattle ‘strain’. In addition, adult E. granulosus derived from the camel demonstrate differences in worm length, segmentation and size, shape, number and arrangement of the genitalia when compared with worms derived from sheep and cattle. The epidemiological significance of these findings is discussed in relation to human hydatid infections in Iran.

A single oral dose of an aqueous suspension containing 11,350,000 chlamydospores of a Mexican isolate of Duddingtonia flagrans (FTHO-8) given to sheep, resulted in a maximum reduction of 88% (range 86.7–90.4%) of the population of Haemonchus contortus infective larvae in the faeces. The effect of this treatment continued for 4–5 days after administration of the suspension. The possible use of this treatment as a method of control of ovine haemonchosis is discussed.

Using DBA/2J mice, tissue homogenates of larval Echinococcus multilocularis were injected into the mesenteric veins to generate the liver infection. Mice were treated with either albendazole or mebendazole for prolonged periods to examine the morphological changes of the metacestode. Albendazole induced disorganization of both laminated and germinal layers and suppressed the maturation of vesicles. Amorphous but loosely laminated PAS-positive material was observed inside the damaged vesicles, although new vesicles slightly developed inside or outside of the damaged ones. Active proliferation of vesicles occurred after treatment with albendazole was terminated. Hydatid cysts were more severely damaged in mice treated with mebendazole and new vesicles did not develop around the damaged ones. Also, hydatid cysts reappeared after treatment with mebendazole was terminated. These results indicate that these drugs do not eliminate larval E. multilocularis in the long-term, but mebendazole has a higher suppressive effect on multivesiculation than albendazole.

High performance thin layer chromatography (HPTLC) was used to determine phospholipids and sphingolipids in the intestinal mucosa of ICR mice infected with Echinostoma caproni for two weeks. The major phospholipids detected in both infected and non-infected mucosa were phosphatidylcholine (PC) and phosphatidylethanolamine (PE). HPTLC-densitometric analysis showed that there was a significant decrease in the weight of both PC and PE in the intestinal mucosa of infected mice compared to that of the uninfected controls. Cerebrosides and sulphatides, but not sphingomyelin, were identified in the intestinal mucosa of both infected and uninfected hosts. There was an apparent increase in the cerebroside content of the mucosa of infected versus control mice. The pathobiochemical changes seen in the polar lipid content of infected hosts probably reflect the feeding and behavioural activities of E. caproni in the mouse intestine.

The specific gravity of the eggs of the liver fluke Opisthorchis viverrini was determined using a sucrose gradient centrifugation and found to range from 1.2713 to 1.3043. The peak egg count was located at the sucrose fraction with a specific gravity of 1.2814. An attempt to float eggs in saturated sodium nitrate solution, sp.gr. 1.4, failed. Examination of human stool specimens for O. viverrini eggs by simple flotation in saturated sodium nitrate solution and the formol-ether sedimentation technique revealed that the flotation technique was not as efficient as the sedimentation technique. It was suggested that the flotation techniques were inappropriate for the detection of O. viverrini eggs in faeces or contaminated soil.