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PubMed1990

Structure, stability and species interactions in helminth communities of wood mice, Apodemus sylvaticus.

The helminths of the alimentary tract of wood mice, Apodemus sylvaticus, were studied at two sites over a 33-month period. Nine helminth species were recovered regularly. All but one was absent for at least 1 month. Monthly samples from these helminth communities were more similar to samples taken in the following month than to samples taken at progressively greater intervals up to a year. Helminth communities at both sites, however, had cyclical elements. Comparison of mean similarity indices for helminth faunas from 33 monthly samples and those from data sets generated by four null models suggests that observed values did not differ from a model where relative abundance of each species was determined randomly with specific maxima of abundance and species absences based on observed data. Loss of helminth species may decrease measurements of community stability based on relative abundance while persistence of abundant species increases stability. Helminth faunas in samples of A. sylvaticus from six localities taken at the same time of year at 5-year intervals indicated that some changed radically while others remained virtually unchanged. The considerable variation in helminth communities from different localities was not related to proximity or gross habitat characteristics. Nematospiroides dubius, Corrigia vitta and Capillaria murissylvatici were important in discriminating between the parasite faunas at different sites. Abundance of Syphacia stroma varied considerably between spatial surveys reflecting differences in host population dynamics in the 2 years. The present report and data from elsewhere in Ireland suggest that species composition of the helminths associated with A. sylvaticus may be stable over a wider geographical scale. There were neither strong nor consistent positive or negative interactions between pairs of helminth species. It is concluded that the stability characteristics of this parasite community, in terms of species composition and relative abundance, are the product of the population biology of independent parasite species rather than interspecific interactions. Variation in the role of competition in parasite communities is discussed.

Animals↗
PubMed1990

The role of helminths in the biological control of mammals.

Biological control of invertebrates has been successful while that of vertebrates has been, with the exception of myxomatosis in rabbits, unsuccessful; reasons for this are discussed. Demographic studies of small mammals suggest that population regulation occurs by several different mechanisms, more than one of which may be acting at the same time. Coevolution is an important phenomenon in host-parasite associations, nevertheless parasites may limit host population abundance. The basis of the regulatory effect on the host population is that parasite-induced host mortality or reduction in fecundity is density-dependent. Increasing evidence of the density-dependent effects of helminths on host survival and reproduction is forthcoming from laboratory studies but has not been confirmed in the field. The theory that a helminth parasite may regulate mammal population abundance has been verified recently in the laboratory. A multidisciplinary research programme aimed at understanding the mechanisms responsible for formation of house mouse (Mus domesticus) plagues and seeking strategies to reduce mouse numbers is discussed. One aspect of the work involves investigation of the potential of the nematode, Capillaria hepatica, as a biological agent in the control of wild mice in the cereal-growing regions of Australia. Biological control of mammals is viewed within the context of integrated pest management. A helminth species which reduces host survival or fecundity at an increasing rate as host abundance increases has a role in host population regulation. There is potential to capitalize on that role and apply the helminth as a biological agent in the control of mammals which have attained pest status.

Animals↗
PubMed1992

Effect of genetic type, lactation and management on helminth infection of ewes in an intensive grazing system on irrigated pasture.

A survey of helminth infection was conducted in a flock of 290 ewes distributed into Romanov (R), Merinos d'Arles (M) and Romanov x Merinos (R x M) genetic types, grazing irrigated pasture in the south of France. Faecal egg and larval counts were done seven times per year from 1981 to 1984 on homogeneous groups of ewes and then individually once to four times every autumn from 1985 to 1988. Helminth fauna was diverse and more abundant during autumn. High levels of strongyle infection occurred in the ewes that remained on the same irrigated pastures during summer. Moving to Alpian pastures during the summer lowered autumnal infection. Significant differences between genotypes in intensity of infection were observed in the order R > R x M > M for strongyles (Teladorsagia circumcincta and/or Trichostrongylus vitrinus, Chabertia ovina and/or Oesophagostomum venulosum, Nematodirus spp.), Moniezia spp. and Dictyocaulus filaria. The hierarchy was reversed for Fasciola hepatica infection and not consistent from one year to another for protostrongylid infections. The effect of lactation intensity on the postparturient rise was studied by equilibrating number of ewes according to reproductive status (zero, one or two lambs in lactation). Merino ewes with two lambs in lactation, as well as primiparous Romanov ewes, had significantly higher strongyle infections than the others. The repeatabilities of the larval and egg counts between the four trial years were 0.24, 0.23 and 0.16, respectively, for protostrongyles, Nematodirus and strongyles, with higher intra-annual values for protostrongyles and inconsistently significant results for strongyles due to the presence of several species.

Animals↗
PubMed1992

Determinants of host-specificity in parasites of freshwater fishes.

Factors responsible for interspecific variability in host-specificity were investigated within 15 genera (including 176 species) of metazoan parasites found in Canadian freshwater fish. For each species in a genus, the parasite's number of known hosts was determined from published host-parasite records. The effects of the total number and mean size of potential hosts (i.e. all fish species belonging to the family or families that include a parasite's known hosts) on number of hosts of congeneric species were evaluated using multiple regressions. Since parasite species that have been recorded often tend to have greater numbers of known hosts than do seldom-recorded parasites, it was necessary to control for the confounding effect of study intensity. In all parasite genera, whether from highly specific taxa such as monogeneans or from less host-specific ones, there was a positive relationship between the number of potential hosts and the number of known hosts. However, no consistent relationships were observed between the mean size of potential hosts and number of known hosts. These results suggest that the availability of suitable host species may have been a key factor limiting the colonization of new hosts by fish parasites.

Animals↗
PubMed1993

Comparative histopathology of Longicollum (Acanthocephala: Pomphorhynchidae) infection in the alimentary tract and spleen of Acanthopagrus australis (Pisces: Sparidae).

The pomphorhynchid acanthocephalan Longicollum alemniscus infected the rectum of 16 and the intestine of five of 628 Acanthopagrus australis from Moreton Bay, Queensland. The neck and proboscis penetrated the entire gut wall and were surrounded by a layer of compact, rounded fibroblasts inside a layer of elongated fibroblasts and scattered connective tissue fibres. These layers extended across the entire gut wall and into the capsule that enclosed the proboscis outside of the gut. A similar parasite occurred extraintestinally in three fish; the proboscis of one penetrated into the spleen. The spleen was shrunken, with the proboscis and anterior part of the neck enclosed in a capsule of proliferated connective tissue and cellular elements similar to that in the response to infection of the gut wall.

Acanthocephala↗
PubMed1995

The epidemiology of helminth infections of growing sheep in Argentina's western pampas.

Seasonal population trends of helminth parasites of growing lambs were investigated over 4 years. Successive worm-free lambs were grazed together with untreated lambs for 20-30 days and then slaughtered for helminth counts 2 weeks after their removal from pasture. Likewise untreated lambs from the same flock were slaughtered from 2 to 15 months of age for worm counts in the same way as tracer lambs. The predominant parasites were Haemonchus, Nematodirus and Trichostrongylus. Haemonchus contortus was found to be of major importance and the flock acquired massive worm burdens from summer to mid-autumn. Minimum burdens were seen from winter to early spring and maximum L4 stages were found from mid-autumn to early winter. Nematodirus burdens increased from December with a peak in late summer and then decreased to low values. Maximum larval availability was in autumn to early winter. Trichostrongylus (mainly T. colubriformis) populations increased in autumn and peaked in June-July, while the highest larval availability was in autumn. The minor genera recovered were Ostertagia, Cooperia, Trichuris, Oesophagostomum and Moniezia. Dictyocaulus, Chabertia and Teladorsagia were noted occasionally and liver flukes were not detected. No important and pathogenic numbers of L4 stages were seen and all predominant species were able to survive over summer or winter in pasture.

Animals↗