Two populations of Apodemus sylvaticus, one in deciduous woodland and another in coniferous forest in Co. Down, Northern Ireland, were trapped at monthly intervals over 33 months. The dynamics, reproduction and age structure of the mouse populations differed between years with over a 40 fold range of population size. Nine species of helminths were recovered from the alimentary tract and associated organs of the mice. The prevalence and mean worm burden of the cestodes, Catenotaenia lobata, Hymenolepis hibernia and Taenia taeniaeformis varied erratically. The trematodes, Corrigia vitta and to a lesser degree Brachylaima recurva, had seasonal changes in abundance. The nematodes Capillaria murissylvatici and Nematospiroides dubius had strongly seasonal cycles of abundance associated with the onset of host reproduction. Trichuris muris was the rarest helminth. Despite protracted periods of absence this species maintained a seasonal increase in mean worm burden in late winter. The nematode, Syphacia stroma, had a weakly seasonal pattern of dynamics. The bases of these cyclical and non-cyclical dynamics of the helminths of A. sylvaticus are discussed.

Experiments were carried out to explore the survival of 14-day adult H. polygyrus following transplantation to mice of four strains, immunized by various protocols. Adult worm establishment and survival was unimpaired in CFLP mice which were totally refractory to larval challenge. Transplanted adult worms were also successful in NIH mice immunized by the 9-day abbreviated infection regime. However, NIH mice exposed to irradiated larvae or subjected to the divided primary infection, expelled transplanted adults. The 9-day abbreviated infection was further examined in SJL and (C57 Bl10 X NIH) F1 mice which expel adult worms during a primary infection and although this regime was unsuccessful in causing NIH mice to reject adult worms, expulsion of adult worms was accelerated in SJL and F1 mice. The survival of adult H. polygyrus was discussed in the context of stage-specific immunity and the delicate balance between the immunogenic stimuli from developing larvae, the immunomodulatory activities of adult stages and the host's genetically determined capacity to respond to these opposing signals.

Responses to parasite antigens were studied in three strains of mice, BALB/c, CBA and NIH, during the initial phases of a primary infection with the intestinal nematode Heligmosomoides polygyrus. Changes in the rate of in vivo cell division were analysed in mesenteric lymph nodes and spleens during the phases of larval maturation and adult establishment, and related to changes in organ size and cellularity. The nature of the proliferating cell populations was also investigated by flow cytometry, carried out on cell suspensions prepared at the time when larval development was complete. The variation in the ability of the strains of mice to become resistant to a challenge infection was manifest as only slight differences in their initial responses to infection. All three strains showed an increase in 125I-iododeoxyuridine incorporation in their mesenteric lymph nodes and spleen, and an increase in B cell frequency over that of T cells in the draining lymph nodes. Although lymph node weight in NIH mice continued to rise over a 4 week period, the majority of responses measured were short lived, peaking 10 to 14 days after infection. The low responder status of CBA mice was thus reflected in a transient and relatively small enlargement of lymphoid tissues, but their early proliferative responses to antigen were similar in scale to those of responder strains.

The intestinal nematode Heligmosomoides bakeri has undergone 2 name changes during the last 4 decades. Originally, the name conferred on the organism in the early 20th century was Nematospiroides dubius, but this was dropped in favour of Heligmosomoides polygyrus, and then more recently H. bakeri, to distinguish it from a closely related parasite commonly found in wood mice in Europe. H. bakeri typically causes long-lasting infections in mice and in this respect it has been an invaluable laboratory model of chronic intestinal nematode infections. Resistance to H. bakeri is a dominant trait and is controlled by genes both within and outside the MHC. More recently, a significant QTL has been identified on chromosome 1, although the identity of the underlying genes is not yet known. Other QTL for resistance traits and for the accompanying immune responses were also defined, indicating that resistance to H. bakeri is a highly polygenic phenomenon. Hence marker-assisted breeding programmes aiming to improve resistance to GI nematodes in breeds of domestic livestock will need to be highly selective, focussing on genes that confer the greatest proportion of overall genetic resistance, whilst leaving livestock well-equipped genetically to cope with other types of pathogens and preserving important production traits.

The infectivity of wild and laboratory strains of Heligmosomoides polygyrus (Nematospiroides dubius) in laboratory mice and in three species of wild British rodent was compared. Wild strains, of the subspecies H. p. polygyrus, were isolated from wild caught Apodemus sylvaticus. Only very low-level infections of the wild strains became established in laboratory mice. Similar worm burdens of the laboratory strain became established in laboratory mice and A. sylvaticus, although infections in A. sylvaticus were more short lived. Cortisone treatment of hosts increased the establishment and survival of the heterologous worm strain to that of the homologous strain. In contrast, neither strain of parasite established in Clethrionomys glareolus or Microtus agrestis, and cortisone treatment of C. glareolus did not increase establishment. Infection of laboratory mice with the wild-strain parasite induced significant immunity to a challenge infection with the laboratory strain.