Asymptomatic carriage of gastrointestinal zoonoses is more common in people whose profession involves them working directly with domesticated animals. Subclinical infections (defined as an infection in which symptoms are either asymptomatic or sufficiently mild to escape diagnosis) are important within a community as unknowing (asymptomatic) carriers of pathogens do not change their behaviour to prevent the spread of disease; therefore the public health significance of asymptomatic human excretion of zoonoses should not be underestimated. However, optimal strategies for managing diseases where asymptomatic carriage instigates further infection remain unresolved, and the impact on disease management is unclear. In this review we consider the environmental pathways associated with prolonged antigenic exposure and critically assess the significance of asymptomatic carriage in disease outbreaks Although screening high-risk groups for occupationally acquired diseases would be logistically problematical, there may be an economic case for identifying and treating asymptomatic carriage if the costs of screening and treatment are less than the costs of identifying and treating those individuals infected by asymptomatic hosts.

Acquired immunity influences the severity of parasitic disease, but modelling the effects of acquired immunity in helminth infections has proved challenging. This may be due to a lack of suitable immunological data, or to the perceived complexity of modelling the immune response. We have developed a model of T. circumcincta infection in domestic sheep that incorporates the effects of acquired immunity on parasite establishment and fecundity. A large data set from commercially managed populations of Scottish Blackface sheep was used, which included relationships between IgA activity and worm length, and between worm length and fecundity. Use was also made of a recently published meta-analysis of parasite establishment rates. This realistic but simple model of nematode infection emulates observed patterns of faecal egg counts. The end-of-season faecal egg counts are remarkably robust to perturbations in the majority of the parameters, possibly because of priming of the immune system early in the season, reducing parasite establishment and growth and, therefore, faecal egg counts. Lowering the amount of early infection leads to higher end-of-season egg counts. The periparturient rise in egg counts in ewes appears to have an important role in supplying infection for the priming of the immune response. This feedback in the immune priming suggests that nematode infections may be difficult to eliminate.

We consider two phenomena, related to the host age-intensity profiles of parasitic infections, which have been suggested to be indicative of acquired immunity: (i) a lower age of peak intensity among more intensely infected hosts; and (ii) a decline with age in the dispersion of the distribution of parasites between hosts. We demonstrate that these phenomena occur among Kenyan schoolchildren infected with Schistosoma mansoni, although the magnitude of both is small. We also examine the mathematical models underlying these predictions and conclude that both phenomena are possible in the absence of acquired immunity or, indeed, in the absence of any density-dependent effect. In our opinion, insufficient attention has been focused upon mathematical models, describing the null hypothesis, i.e. density-independent models. In particular, we regard the usual assumptions made for the two stochastic components of these models, describing the heterogeneity between hosts and the probabilistic nature of infection and death of parasites, as too rigid and unrealistic. We demonstrate that deviation from these assumptions undermines the qualitative distinctions between models which describe acquired immunity or density dependence and those which are density-independent.

Host-parasite interactions are often seen as an arms race, with parasites attempting to overcome host resistance to infection. Herbivory is a common route of transmission of parasites that represents the most pervasive challenge to mammalian growth and reproduction. The present paper reviews the foraging skills of mammalian herbivores in relation to their ability to exploit plant properties to combat parasites. The starting point is that foraging behaviour may ameliorate the impact of parasitism in three ways; hosts could: (1) avoid foraging in areas contaminated with parasites; (2) select diets which increase their resistance to parasites; (3) select for foods containing anti-parasitic properties (self-medication). Details are given of the pre-requisite skills needed by herbivores if they are to combat parasitism via behaviour, i.e. herbivores are able to: (a) determine their parasitic state and alter their behaviour in relation to that state (behaviours 1, 2 and 3); (b) determine the environmental distribution of parasites (behaviour 1); (c) distinguish plant species or plant parts that increase their resistance to parasites (behaviour 2) or have anti-parasitic properties (behaviour 3). Mammalian herbivores cannot detect the presence of the parasites themselves and must rely on cues such as faeces. Despite the use of these cues contacting parasites may be inevitable and so mechanisms to combat parasitism are necessary. Mammalian herbivores have the foraging skills needed to exploit the heterogeneous distributions of nutrients and parasites in complex foraging environments in order to avoid, and increase their resistance to, parasites. Current evidence for the use of plant secondary metabolites (PSM) by herbivores for self-medication purposes remains equivocal. PSM have both positive (anti-parasitic) and negative (toxic) effects on herbivores. Here details are given of an experimental approach using tri-trophic (plant-herbivore-parasite) interactions that could be used to demonstrate self-medication in animals. There is strong evidence suggesting that herbivore hosts have developed the foraging skills needed to take advantage of plant properties to combat parasites and thus use behaviour as a weapon in the host-parasite arms race.

The effect of acquired immunity on the duration of Plasmodium falciparum infections is unclear, although this is an important term in models of malaria transmission. It is problematical to determine the duration of infections because of the difficulty of distinguishing persisting infections from new ones, and because parasite densities are often transiently below the limit of detection. We recently developed a dynamic model for infection incidence, clearance and detection of multiple genotype P. falciparum infections and fitted it to a panel dataset from a longitudinal study in Northern Ghana. We now extend this model to allow for seasonal and age variation in infection rates and also age dependence in clearance and in detectability of infections. These models indicate that there is seasonal variation in the infection rate, and age dependence in detectability. The best fitting models had no age dependence in infection or clearance rates, suggesting that acquired immunity mainly affects detectability.

We investigated population differences in immunological adaptation of three-spined sticklebacks (Gasterosteus aculeatus) to one of their most abundant macroparasites, the eye fluke Diplostomum pseudospathaceum. We compared infection success in lab-bred fish of 2 populations in northern Germany, from a lake, where eye flukes are prevalent, and a river, where these parasites do not occur. In order to discriminate between protection through innate and acquired immunity, we exposed fish either only once or repeatedly. Lake fish were significantly less susceptible than river sticklebacks already after a single exposure, indicating that in sympatric hosts innate immunity plays the major role in the defence against this helminth infection. In both habitat types, previous exposures only marginally decreased infection rates within 12 weeks. Lake fish showed higher immunocompentence by means of respiratory burst activity and spleen size, regardless of the infection status. Furthermore, they were in a better energy status than river fish, as indicated by a higher hepatosomatic index and haematocrit value. Interestingly, F1 hybrid fish of both populations ranged between the pure habitat types in parasite susceptibility as well as in immunological and condition parameters. Our results suggest that sticklebacks from lakes are better adapted to cope with higher parasite abundance in this habitat.

To investigate the prevalence of a flea-borne protozoan (Trypanosoma (Herpetosoma) microti) in its field vole (Microtus agrestis) host, we monitored over a 2-year period a range of intrinsic and extrinsic parameters pertaining to host demographics, infection status and vector (flea) prevalence. Generalized Linear Mixed Modelling was used to analyse patterns of both flea and trypanosome occurrence. Overall, males of all sizes and ages were more likely to be infested with fleas than their female counterparts. Flea prevalence also showed direct density dependence during the winter, but patterns of density dependence varied amongst body mass (age) classes during the summer. Trypanosome prevalence did not vary between the sexes but was positively related to past flea prevalence with a lag of 3 months, with the highest levels occurring during the autumn season. A convex age-prevalence distribution was observed, suggesting that individuals develop a degree of immunity to trypanosome infection with age and exposure. An interaction between age and whether the individual was new or recaptured suggested that infected animals are less likely to become territory holders than their uninfected counterparts.

The role of acquired immunity in lymphatic filariasis is uncertain. Assuming that immunity against new infections develops gradually with accumulated experience of infection, models predict a decline in prevalence after teenage or early adulthood. A strong indication for acquired immunity was found in longitudinal data from Pondicherry, India, where Mf prevalence was highest around the age of 20 and declined thereafter. We reviewed published studies from India and Subsaharan Africa to investigate whether their age-prevalence patterns support the models with acquired immunity. By comparing prevalence levels in 2 adult age groups we tested whether prevalence declined at older age. For India, comparison of age groups 20–39 and 40+ revealed a significant decline in only 6 out of 53 sites, whereas a significant increase occurred more often (10 sites). Comparison of older age groups provided no indication that a decline would start at a later age. Results from Africa were even more striking, with many more significant increases than declines, irrespective of the age groups compared. The occurrence of a decline was not related to the overall Mf prevalence and seems to be a chance finding. We conclude that there is no evidence of a general age-prevalence pattern that would correspond to the acquired immunity models. The Pondicherry study is an exceptional situation that may have guided us in the wrong direction.

The reproductive kinetics of Protopolystoma xenopodis primary and secondary infections in Xenopus laevis were monitored in a 3-year study. Thirty-five naïve, lab-raised, full-sib X. laevis from 1 spawning were each exposed to 30 P. xenopodis eggs. The course of infections at 20 °C was monitored by screening isolated hosts for parasite egg production. Ninety-four percent of toads supported the development of gravid parasites. Infections became patent 9–19 weeks p.i., lasted 3–30 months and produced estimated totals of 1–7152 eggs/host. Variation in primary infection characters was discontinuous: a subgrouping of hosts (16%) was characterized by extended infection duration and low reproductive rate. In order to test the effect of long-term infection history on a subsequent challenge, each host was re-exposed to P. xenopodis infective stages (30 eggs/host) 6 months after the loss of its original infection. Establishment of patent infection was significantly lower (15%), and pre-patent period (12–28 weeks) longer, than in primary infections of the same hosts, and than in concurrently exposed naïve controls (contemporary full-sibs of the primary/secondary infection group, maintained in parallel; n = 28). There was no relationship between primary infection characteristics and secondary infection outcome. Overall reproductive output per initial infective stage for the primary exposure exceeded that for the secondary exposure by a ratio of 15[ratio ]1. Results suggest that primary infection with P. xenopodis can elicit strong, long-term protective immunity against re-infection in X. laevis.

Strong resistance to reinfection developed in pigs as a result of trickle inoculations with Trichuris suis (250 infective eggs twice weekly for 4 weeks). At challenge (4000 eggs) 8 weeks after the last trickle inoculation, we observed a 96% reduction in the mean number of established challenge worms in immunized (TRI/CHA) compared with non-immunized controls (CON/CHA) (74 v. 1753; P = 0·004). Moreover, the total faecal egg excretion was significantly lower in immunized pigs compared with controls (P = 0·0003). The trickle inoculations depressed pig weight gains over the 8-week course of immunization (P = 0·042). It was observed that a small number of pigs developed a weaker level of resistance, reflecting the importance of genetic determinants. The relevance of these data to the population biology of porcine and human whipworm infections is discussed.

In order to investigate the phenomenon of age-related immunity to visceral leishmaniasis, a 1 year prospective survey was carried out on 5671 people in a Leishmania infantum focus in north-west Iran. The average incidence rate of infection since 1985 was 2·8%/year with all ages equally at risk. One in 13 infections in children led to visceral leishmaniasis (VL), and this ratio decreased significantly with age. Seroprevalence also dropped rapidly with age, suggesting that the same process may affect both clinical outcome and the humoral immune responses. Cell-mediated immunity was associated with a reduction in the seroconversion rate and an increase in the serorecovery rate. Even amongst people with no detectable cell-mediated immunity to Leishmania, the seroconversion rate decreased and the serorecovery rate increased with age. All current VL patients had a negative leishmanin skin test response. Hence, adults may develop protection against L. infantum through 2 processes, 1 dependent and 1 independent of acquired cell-mediated immunity.

Risk-benefit considerations have stimulated efforts to develop technology to overcome the limitations of procedures that place excessive reliance on the use of acaricides for the control of ticks affecting cattle. One of the alternatives to acaricides is the selection and use of breeds of cattle with a natural propensity for developing acquired resistance to ticks. The literature on aspects of the modes of expression and mechanisms of resistance of cattle to ticks, is reviewed along with information on the successful efforts in Australia to develop productive Bos indicus ï Bos taurus breeds with the ability to develop high levels of resistance. The use of cattle resistant to ticks is discussed in relation to other factors that should be considered in the design of a programme for tick control.