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Variation in the intensity and duration of infections is often driven by variation in the network and strength of host immune responses. While many of the immune mechanisms and components are known for parasitic helminths, how these relationships change from single to multiple infections and impact helminth dynamics remains largely unclear. Here, we used laboratory data from a rabbit-helminth system and developed a within-host model of infection to investigate different scenarios of immune regulation in rabbits infected with one or two helminth species. Model selection suggests that the immunological pathways activated against Trichostrongylus retortaeformis and Graphidium strigosum are similar. However, differences in the strength of these immune signals lead to the contrasting dynamics of infections, where the first parasite is rapidly cleared and the latter persists with high intensities. In addition to the reactions identified in single infections, rabbits with both helminths also activate new pathways that asymmetrically affect the dynamics of the two species. These new signals alter the intensities but not the general trend of the infections. The type of interactions described can be expected in many other host-helminth systems. Our immune framework is flexible enough to capture different mechanisms and their complexity, and provides essential insights to the understanding of multi-helminth infections., Author summary Hosts infected with parasites have developed complex immune strategies to regulate infection severity, however, these strategies are not always successful to confer long term protection. Here, we examine the immune reactions of hosts infected with one or two gastrointestinal parasites, using a rabbit-helminth system and a modeling approach to laboratory experiments. We found that similar immune interactions operate for the two parasites, however, changes in the strength of the relationships lead to contrasting dynamics of infections, where one parasite is quickly removed while the other shows no evidence of control. For hosts infected with both parasites we also found the activation of new immune reactions that asymmetrically affect the dynamics of the two parasites. While we observed changes in parasite intensity the general trends are conserved. Understanding how the immune reaction modulates host-parasite interactions can help to explain the often large variation in the host response to infections in natural systems.

During December 2007 and November–April 2008/2009, 128 gastrointestinal tracts and 1680 faecal samples of hares from different parts of the Czech Republic were examined. Evaluation of the faeces samples indicated that the samples could have been infected with any one of the following parasites: Eimeria spp., Trichostrongylus retortaeformis, Trichuris leporis, Graphidium strigosum, Strongyloides sp., Protostrongylus spp. and Anoplocephalidae tape-worms. Trichostrongylus retortaeformis was the most common nematode (75.8 %). 39.8 % of the animals were infected with Trichuris leporis. Only four hares were in-fected with Graphidium strigosum. Eimeria spp. was the most commonly occurring parasite (90.5 %). The follow-ing species were found: Eimeria coquelinae, E. cabareti, E. leporis, E. babatica, E. europea, E. pierrecouderti, E. macrosculpta, E. orbiculata and E. stefanski. Eimeria coquelinae was the most prevalent species (68.4 %). Eimeria cabareti and E. leporis were the next most frequently found species (64.5 % and 57.8 %, respectively).

Italian hare Lepus corsicanus De Winton, 1898, is a true species living in simpatry with Lepus europaeus in mainland Italy and with Oryctolagus cuniculus in Sicily, where L. europaeus is absent. Up to date, nobody has studied the helminths of this endemic Italian Mammal. Therefore, in order to describe Italian hare gastro-intestinal helminths, gastro-intestinal tract of 29 Italian hares, coming from mainland Italy (#14) and from Sicily (#15) were collected between 1997 and 2009. Twentyfive hares were positive for at least one parasite (86 %). Six parasite species were isolated, 2 cestodes (Cittotaenia pectinata, prevalence 3 %) and Paranoplocephala sp., 3 %) and 4 nematodes (Trichostrongylus retortaeformis, 86 %, Graphidium strigosum, 14 %, Trichuris sp., 10 % and Teladorsagia circumcincta, 7 %). Both Teladorsagia circumcincta circumcincta and Teladorsagia circumcincta trifurcata morphotypes were identified. Comparison with available data regarding L. europaeus and O. cuniculus in Italy is provided. Being not T. circumcincta and Paranoplocephala sp. typical parasites of leporids, a description of the observed specimens is given.

Cytokines play a key role in maintaining communication between organs and in so doing modulate the interaction between concurrent infections. The extent of these effects depends on the properties of the organ infected and the intensity and type of infections. To determine systemic bystander effects among organs, IFN-γ, IL-4 and IL-10 gene expression was quantified at 7 days post-challenge in directly infected and uninfected organs during single and co-infections with the respiratory bacterium Bordetella bronchiseptica and the gastrointestinal helminths Graphidium strigosum and Trichostrongylus retortaeformis. Results showed that cytokine expression in a specific organ was influenced by the type of infection occurring in another organ, and this bystander effect was more apparent in some organs than others. Within the same organ the relative cytokine expression was consistent across infections, although some cytokines were more affected by bystander effects than others. For the infected gastrointestinal tract, a stronger cytokine response was observed in the tissue that harbored the majority of helminths (i.e. duodenum and fundus). Overall, co-infections altered the intensity but to a lesser extent the relative cytokine profile against the focal infection, indicating clear bystander effects and low organ compartmentalization. However, organs appear to actively modulate cytokine expression to avoid potential immuno-pathological consequences.

Given the health and economic burden associated with the widespread occurrence of co-infections in humans and agricultural animals, understanding how coinfections contribute to host heterogeneity to infection and transmission is critical if we are to assess risk of infection based on host characteristics. Here, we examine whether host heterogeneity to infection leads to similar heterogeneity in transmission in a population of rabbits single and co-infected with two helminths and monitored monthly for eight years. Compared to single infections, co-infected rabbits carried higher Trichostrongylus retortaeformis intensities, shorter worms with fewer eggs in utero, and shed similar numbers of parasite eggs. In contrast, the same co-infected rabbits harbored fewer Graphidium strigosum with longer bodies and more eggs in utero, and shed more eggs of this helminth. A positive density-dependent relationship between fecundity and intensity was found for T. retortaeformis but not G. strigosum in co-infected rabbits. Juvenile rabbits contributed to most of the infection and shedding of T. retortaeformis, while adult hosts were more important for G. strigosum dynamics of infection and transmission, and this pattern was consistent in single and co-infected individuals. This host-parasite system suggests that we cannot predict the pattern of parasite shedding during co-infections based on intensity of infection alone. We suggest that a mismatching between susceptibility and infectiousness should be expected in helminth coinfections and should not be overlooked.

Between 1992 and 1996, 95 rabbits from the immediate locality of Malham Tarn, North Yorkshire, UK were examined for the presence of helminth parasites. All the examinations took place in late September or October. Three species of nematodes, Graphidium strigosum, Passalurus ambiguusand Trichostrongylus retortaeformis and two species of cestodes, Taenia pisiformis and Cittotaenia pectinata were identified. There were no associations between helminth species richness and year of sampling, host weight or sex. A logistic model was fitted to the prevalence data from these helminths as was an over-dispersed Poisson model to the worm burden data. Graphidium strigosum was the most frequently identified species with an average prevalence of 78%. The mean prevalence and intensity of Graphidiuminfection were significantly effected by sampling year. The lower than normal rainfall recorded at the Tarn during the years 1995 and 1996 may have be one reason for this pattern. The worm burden of G. strigosum was significantly positively associated with rabbit body weight. The intensity of infection with P. ambiguus was significantly higher in female rabbits. There was a significant non-linear relationship between P. ambiguus worm burden and rabbit weight (P = 0.002) with worm burdens being highest in the 1000 g to 1499 g weight cohort. Trichostrongylus retortaeformis was only identified in 1994 and male rabbits harboured significantly higher worm burdens than females (48 vs. 7, P = 0.022). Over the five years, the average Taenia pisiformis prevalence was 31% and there was a significant positive association between worm burden and rabbit weight (P = 0.001). Cittotaenia pectinata had a prevalence of 37% over the whole study period with no interactions between prevalence or intensity and body weight, year of sampling or rabbit sex. All five helminths showed an overdispersed distribution with k values less than 1.

THE widespread and common occurrence of endoparasites in free-living Australian wild rabbits, Oryctolagus cuniculus (L.), prompted an investigation into any effect they might have on the severity of infectious myxomatosis in partially resistant animals, or in cases due to attenuated strains of the virus. Although the results so far obtained are based on a small number of rabbits and are insufficient to answer this complicated problem fully, results collected simultaneously on the behaviour of parasites, especially Graphidium strigosum, in virus-infected hosts are interesting and worth recording.

THE curious food pellets described by Morot and mentioned in Dr. Madsen's letter must be familiar to all laboratory workers who have occasion to open the rabbit's stomach. They have frequently been seen in the parasitological department of this Laboratory during the course of work carried out on the stomach worm Graphidium strigosum, and we have wondered how it could be possible for the mucous membrane of a large hollow organ to form such perfectly rounded pellets.

ABSTRACr: In rabbits, Oryctolagus cuniculus, confined to a 0.63-acre enclosure a previously reported field observation was confirmed that, during the breeding season, the magnitude of infections with two nematodes, Trichostrongylus retortaeformis and Graphidium strigosum, is much greater in female rabbits than in males. At the same time in ovariectomized females the numbers of both nematodes were similar to those seen in males. During the breeding period salpingectomized females carried worm burdens less than those in the breeding females and greater than in the males and the ovariectomized females. These results support the hypothesis that the reproductive hormones of the host influence the host-parasite relationships of T. retortaeformis and G. strigosum.

Replicated rabbit populations were maintained in experimental enclosures at each of three densities (approximately 12.5, 25, and 50 rabbits per ha); in addition, populations were maintained at a density of about 50 per ha but in enclosures of three different sizes (about 0.1, 0.2, and 0.4 ha). Altogether 230 rabbits commenced the experiment. Towards the end of the breeding season all the surviving rabbits (or their replacements) were killed and counts made of the numbers of G. strigosum and T. retortaeformis. Statistical tests were done relating the numbers of these parasites to the treatment to which their hosts had been exposed and to a range of factors that had been measured either during or at the termination of the experiment. Numbers of G. strigosum varied in relation to the treatment to which their hosts were exposed, in particular to the length of time that the rabbits had been under the conditions of the experiment and to factors directly related to the treatment, especially rabbit density and the amount of social contact with other rabbits. These extrinsic factors had a much less marked effect on the numbers of T. vetortaeformis in the rabbits. Male rabbits of low social status carried significantly more T. uel'ortaeformis than did their more dominant counterparts. It is suggested that this was the result of social factors affecting the susceptibility of these subordinate animals to infection with this parasite.