Experimental infection of Echinoparyphium recurvatum von Linstow (Digenea: Echinostomatidae) cercariae in the snail second intermediate host Lymnaea peregra Müller shows that metacercarial encystment takes place on the lining of the mantle cavity, pericardial cavity and kidney lumen, with the mantle cavity the most preferred site. All three sites are accessible via the body openings. The metacercariae appear to be more susceptible to encapsulation in the visceral mass than in the cavity of the mantle, pericardium and the lumen of the kidney.

Evidence for the existence of 2 first intermediate host-specific sibling species of the 45 collar-spined echinostome Echinoparyphium recurvatum is presented. Experimental studies on their life-cycles were carried out under controlled laboratory conditions. The two entities were found to be morphologically indistinguishable in all major respects, yet they exhibited distinct biological characteristics. One of the sibling species utilizes the freshwater lymnaeid pulmonate snail Lymnaea peregra as first intermediate host, and the adults occur in the anterior small intestine of the wildfowl experimental definitive host Anas platyrhynchos. The other utilizes the freshwater mesogastropod prosobranch snail Valvata piscinalis as first intermediate host, and the adults occur in the posterior small intestine and rectum of A. platyrhynchos. The existence of the two sibling species in sympatry may be explained by the fact that the life-cycles of the two forms represent two distinct cycles of transmission serving to reduce excessive competition between them.

Transmission of cercariae of the echinostome digenean Echinoparyphium recurvatum was observed in experimental populations of the second intermediate host snail Lymnaea peregra. Populations of 100 snails were distributed among plastic mesh cages in an infection arena to produce different patterns of host dispersion. Cercarial transmission was investigated in host populations showing three separate patterns of host dispersion; Ideal Regular, Random and Contagious (‘Clumped’). Levels of cercarial transmission in populations showing each type of dispersion pattern were found to be significantly different. The highest rate of cercarial transmission occurred in the contagiously dispersed host population, and the lowest in the randomly dispersed population. Analysis of results from the randomly dispersed population also showed that both the mean percentage prevalence of infection, and also the mean number of cysts recovered per snail, increased significantly with the number of snails per cage. The positive relationship between the mean number of cysts per snail and the number of snails per cage was found to be best described by an exponential function. Chemosensory location of L. peregra snail hosts by E. recurvatum cercariae may provide an explanation for the patterns of cercarial transmission observed.

Survival and infectivity characteristics are described for cercariae of the echinostome Echinoparyphium recurvatum at 10, 15, 20, 25 and 30°C. Cercarial survival was markedly age- and temperature-dependent, maximum survival time being reduced from 68 h at 10°C to 12 h at 30°C, 50% survival times falling from 47·9 h at 10°C to 8·4 h at 30°C. The mean instantaneous per capita death rate of cercariae increased exponentially with temperature. Cercarial infectivity was also temperature dependent, the main effect of temperature being upon the rate at which infectivity diminished with increasing age of cercariae. Infectivity increased from zero at 10°C to a maximum at 25°C after which it declined to a low level at 30°C. Overall transmission efficiency of cercariae at each experimental temperature was calculated as the ratio of the mean instantaneous per capita death rate (μ) to the mean instantaneous per capita rate of infection (β). Transmission efficiency of cercariae was found to be maximal at 20°C and at a relatively high level between 10 and 25°C. Transmission efficiency was zero at 10°C and at a very low level at 30°C. These results suggest that the cercariae of E. recurvatum show transmission optima at water temperatures likely to be encountered in natural habitats in Britain and Europe.