Comparative strategies and success of sympatric and allopatric Fasciola hepatica infecting Galba truncatula of different susceptibilities.

Allopatric infections of French Galba truncatula with an Argentinean isolate of Fasciola hepatica were carried out to determine the infectivity of foreign miracidia in three snail populations differing by their susceptibility to French miracidia (two highly and one poorly susceptible populations). Sympatric infections of G. truncatula with French miracidia were used as controls. Compared to sympatric infections of G. truncatula, snail survival at day 30 post-infection in allopatric groups was significantly lower in a highly susceptible population and significantly greater in the other two. Prevalence in snails infected with the allopatric isolate was significantly lower (16.4-34.5 % instead of 58.6-72.1 %), whereas their patent period was significantly longer (a mean of 69.9-85.9 days instead of 6.4-20.7 days). The mean number of metacercariae was also higher in allopatric groups (236.5-897.3 per cercariae-shedding snail instead of 70.7-222.1). Owing to longer patent periods, the Argentinean isolate of F. hepatica was less pathogenic for these snails. The lower prevalence of infection, the longer patent period and the higher number of metacercariae noted in allopatric groups might be the consequence of an adaptive mechanism used by this digenean introduced to the New World to infect new populations of unusual intermediate hosts.

Intermediate snail hosts of French Fasciola hepatica: Lymnaea neotropica and Lymnaea viatrix are better hosts than local Galba truncatula.

Allopatric and sympatric infections of Lymnaea neotropica and Lymnaea viatrix var. ventricosa with Argentinean and French isolates of Fasciola hepatica were carried out to determine the capacity of these snails to produce metacercariae and to verify if this capacity changed with snail generation. The same process was also made with a French population of Galba truncatula known to be highly susceptible to French isolates of the parasite. In each lymnaeid species separately considered, the survival rate at day 30 post-exposure and prevalence of F. hepatica infection in the group infected with Argentinean miracidia were significantly greater than those recorded in the corresponding French one. Compared to infected G. truncatula, both South American lymnaeids had longer patent periods and produced a higher number of metacercariae. The highest infections were noted with L. v. ventricosa. In the three snail species, metacercarial production was more important with the Argentinean isolate of miracidia than with the French one. If three successive generations of L. v. ventricosa are exposed to the same French isolate of miracidia, cercarial production significantly increased from parents to the F2 generation, while the other characteristics of infection only showed insignificant variations. L. neotropica and L. v. ventricosa are better intermediate hosts for French F. hepatica than local G. truncatula. The numerical increase of shed cercariae in the F1 and F2 generations of L. v. ventricosa demonstrates a rapid adaptation of this species to the French isolate of the parasite.

The steppe species of gastrointestinal nematodes of small ruminants, with a focus on Marshallagia: climate as a key determinant.

We intended to relate the geographic distribution of ruminant gastrointestinal nematodes in relation to steppe climate (and vegetation). Data are either from literature or from newly acquired/available results. Simple or more sophisticated meteorological indices were used to characterize the climate. Regression analyses were used to correlate climatic factors and presence of endoparasites from steppe areas. The distribution of one (Marshallagia) out of five endoparasite genera was concentrated mostly in steppic areas whereas other species were found also in other areas. In wild hosts the distribution of Marshallagia was much larger from Sptizberg to New World (northern territories in Canada or extreme south of America). In domestic small ruminants the presence of Marshallagia was identified more frequently and constantly in the area of original domestication and its early diffusion (from Northern Africa to Kashmir, Caucasia). The distribution of this parasite was correlated to low rainfalls which were not the case for all other endoparasites. After host switch (reindeer or south America camelids), it has expanded in other climatic areas, either colder or dryer.

Changes in gastro-intestinal helminth species diversity in lambs under mixed grazing on irrigated pastures in the tropics (French West Indies).

The development of gastro-intestinal helminth diversity was monitored in lambs grazing alone or grazing with heifers in the ratio one heifer to four lambs. Five successive cohorts of lambs were studied from January 1994 to May 1996. Each cohort of lambs grazed irrigated pastures of Pangola grass for 4 months (from weaning to 6 months of age). A total of 50 lambs was necropsied and their worms counted and identified at the end of each grazing period. Four heifers were also necropsied on one occasion. Special attention was dedicated to the identification of the most pathogenic worm, i.e. Haemonchus spp. Malate dehydrogenase polymorphism in H. contortus was studied in order to evaluate changes between cohorts and between grazing managements. The species diversity was estimated by Shannon diversity indices (main species or all species). It was higher in the mixed grazing group than in the lambs that grazed alone. Diversity increased in successive cohorts. This was due in part to the acquisition of Cooperia spp. of cattle origin. The increase in diversity in the mixed grazing lambs corresponded to the lower faecal egg excretion and better weight gains recorded previously in that group. There seemed to be no cross-transmission of H. similis found in heifers and H. contortus harboured by lambs. The latter species was not morphologically or genetically different in the lambs grazed alone or with heifers, indicating that the presence of cattle did not modify qualitatively the transmission of H. contortus.

Nematode genera diversity in cattle: similarity of between-sire progenies.

Breeding cattle for resistance to nematode infection is mostly based on egg excretion. This, however, does not allow for generic identification of the nematodes involved. Unless we know whether the selected resistance is directed against one or several particular genera, a strong bias could be introduced in the selection programs. In order to estimate the likelihood of this potential bias we investigated nematode genera diversity in the progeny of four sires in 1992 and seven sires in 1994. Three of the four Aberdeen Angus sires used in 1992 were related while the seven sires in 1994 were unrelated. Diversity was assessed using at least ten individual faecal cultures for each progeny group during each of the two sampling periods (beginning and end of grazing period, April and September). It was estimated by the relative proportion of each genera (Ostertagia, Cooperia, Haemonchus, Trichostrongylus and Oesophagostomum) or by either the Shannon (genera diversity) or Pielou (genera evenness) index. The Shannon index was repeatable when measured at 2-week intervals within the same progeny group on ten random faecal samples. No significant difference was recorded between sire genera diversity over the two sampling periods. This indicated that hosts have a limited effect on the nematode genera diversity as assessed by faecal cultures, and that the selection of resistant hosts could probably be achieved using faecal egg counts.

Morphological polymorphism in the nematode Teladorsagia circumcincta in relation to age of larvae, infection mode and lamb characteristics in experimental conditions.

Variations in the proportion of the less-frequent morph of Teladorsagia circumcincta, T. c. trifurcata, were studied under experimental conditions. Infections with older larvae (over 7 months of age at 4 degrees C) yield a lower proportion of T. c. trifurcata compared with those obtained from fresh larvae (15 days). This phenomenon could account for the reduction in the proportion of the morph trifurcata in two isolates that were acclimated to laboratory rearing for several generations. The infection mode (single or trickle infections did not significantly modify the proportions of both morphs. The morph trifurcata was found in higher proportions (> 5.5%) in more susceptible male and/or Romanov lambs, and in lower proportions (< 2.5%) in less susceptible female and/or Mérinos d'Arles lambs.