Single- and five-worm infections of Echinostoma revolutum (Trematoda) in the golden hamster

Low genetic diversity in wide-spread Eurasian liver fluke Opisthorchis felineus suggests special demographic history of this trematode species

Opisthorchis felineus or Siberian liver fluke is a trematode parasite (Opisthorchiidae) that infects the hepato-biliary system of humans and other mammals. Despite its public health significance, this wide-spread Eurasian species is one of the most poorly studied human liver flukes and nothing is known about its population genetic structure and demographic history. In this paper, we attempt to fill this gap for the first time and to explore the genetic diversity in O. felineus populations from Eastern Europe (Ukraine, European part of Russia), Northern Asia (Siberia) and Central Asia (Northern Kazakhstan). Analysis of marker DNA fragments from O. felineus mitochondrial cytochrome c oxidase subunit 1 and 3 (cox1, cox3) and nuclear rDNA internal transcribed spacer 1 (ITS1) sequences revealed that genetic diversity is very low across the large geographic range of this species. Microevolutionary processes in populations of trematodes may well be influenced by their peculiar biology. Nevertheless, we suggest that lack of population genetics structure observed in O. felineus can be primarily explained by the Pleistocene glacial events and subsequent sudden population growth from a very limited group of founders. Rapid range expansion of O. felineus through Asian and European territories after severe bottleneck points to a high dispersal potential of this trematode species.

Endemic echinostome infections of candidate hosts

Wild Lymnaea tomentosa snails, recovered from Lake Wanaka, New Zealand, were established in the laboratory. Wild snails, naturally infected with echinostomes, provided metacercariae for infection of laboratory maintained snails. Metacercarial cysts from wild and laboratory snails were then used to attempt infection of definitive host candidates. Laboratory snails provided convenient packaging of known numbers of cysts. Metacercariae excysted in the small intestines of ducklings to mature in 6 days. Worms were expelled as they became gravid. Attempts to establish infections in experimental hosts other than ducklings were not successful. No worms were recovered from mice, white rats, guinea pigs, hamsters or immunosuppressed white rats.

Insemination and population density in Entobdella soleae, a monogenean skin parasite of the common sole, Solea solea

Immature specimens of the monogenean skin parasite Entobdella soleae, experimentally isolated from other individuals, fail to self-inseminate when maturity is reached and self-insemination has not been recognized during many hours spent observing living parasites. E. soleae lives on the lower surface of its flatfish host, the common sole (Solea solea), in relatively small numbers, most commonly 1, 2 or 3 adults per fish, but in spite of the low density and extensive habitat, freshly collected wild parasites were found to be inseminated. If it is assumed that E. soleae is unable to self-inseminate in the wild, then it seems likely that single isolated parasites on wild soles have lost their partners as a result of death or transfer to another host and that parasites have an efficient way of locating a mating partner. Mathematical modelling indicates that random locomotion with searching movements of the body would lead to mating contacts between 2 individuals on a small sole within the reproductive life-time of the parasites, but is unlikely to maximize reproductive output because it cannot ensure that mating will occur soon after sexual maturity is achieved. It is suggested that pheromonal attraction may ensure early meetings and that the thin stagnant layer of sea water between the lower surface of the relatively inactive sole and the sea bottom is an ideal environment for the operation of such a system and may account for the preference of adult E. soleae for the host's lower surface.

Echinostoma trivolvis: mating behavior of adults raised in hamsters

Echinostoma trivolvis adults grown in golden hamsters were treated in vitro with [3H]-tyrosine to label sperm and transplanted to uninfected hamsters alone or with four to six unlabeled adults. After 4-5 days, worms were recovered, processed for autoradiography, and observed for silver grains over their seminal receptacles. Of 11 worms transplanted singly, 6 (55%) self-inseminated. In multiple-worm situations, 7 of 12 (58%) labeled worms self-inseminated, and in the process the 12 labeled worms inseminated only 5 of 35 (14%) possible unlabeled worms. E. trivolvis adults self-inseminated when isolated and both self- and cross-inseminated in groups, suggesting an unrestricted mating pattern. These results were compared with the mating patterns of other digenetic trematodes.

Single and multiple worm infections of Echinostoma caproni (Trematoda) in the golden hamster

Six of 10 hamsters fed a single metacercarial cyst of Echinostoma caproni (single-worm infections) and 13 of 19 hamsters fed either 2 or 5 cysts (multiple-worm infections) were infected with adult echinostomes at necropsy 22 days post-infection. Considerable histopathological changes to the small intestine occurred in hamsters carrying single-worm infections. There were no differences in either mean length, width or wet weight of echinostomes in single- versus multiple-worm infections. The mean number of eggs/worm from single-worm infections (525) was significantly greater than that from multiple-worm infections (288). The average percentage of fully developed miracidia/worm from single worms (94%) was similar to that from worms in multiple infections (92-95%). Single worms of E. caproni were capable of self-fertilization and production of viable eggs. Miracidia derived from single worms were as capable of infecting laboratory-reared Biomphalaria glabrata and producing patent rediae as were those from multiple infections.

Single- and five-worm infections of Echinostoma caproni (Trematoda) in the ICR mouse

Twenty-nine (64%) of 44 ICR mice fed a single metacercarial cyst of Echinostoma caproni and all of 23 mice each fed five cysts were infected with ovigerous worms at necropsy 2-4 weeks post-infection. Each host fed five cysts had two to five worms at necropsy, and all worms were either paired or clustered. Distribution of worms in the small intestine was similar in single- and five-worm infections and all worms were located 17-20 cm anterior to the ileo-cecal valve. Both single and multiple worms produced eggs with fully-developed miracidia. The number of eggs per uterus in 2-week-old multiple worms was almost twice that of single worms. The body area of 3- and 4-week-old multiple worms was significantly greater than that of single worms of the same age.

Echinostoma population regulation in experimental rodent definitive hosts

Echinostoma population regulation in the experimental rodent host is governed by the capacity of the latter to express an effective regulatory response. Parasite establishment, survival and fecundity are affected by host-related factors such as species, strain and age and by parasite-related factors such as species, age and the burden of infection. The genetic heterogeneity in the regulatory response to infection is marked. The most intensively studied host/echinostome combinations comprise E. caproni and E. trivolvis in the mouse host, for which a range of interesting host-parasite relationships has been demonstrated, including concomitant immunity with rapid expulsion of superimposed infections, a long-lasting resistance to secondary infection, a negatively dose-dependent pattern of expulsion of primary infections, a positively dose-dependent reproductive potential, an infective-dose independency of primary worm establishment, and a range of heterologous antagonistic and synergistic interactions in concurrent infections with related and unrelated parasite species. The Echinostoma/rodent model is highly suitable for studying aspects of parasite population regulation in intestinal trematode infections.