Heliciculture is an excellent alternative to obtain edible snails but its viability is seriously threatened by pathogens. A parasitological survey was conducted in 3 mixed system-based heliciculture farms in Galicia (NW Spain), with the species Tetrahymena rostrata, Tetrahymena limacis, Tetratrichomonas limacis, Cryptobia helicogenae, Brachylaima aspersae (metacercariae and sporocysts), Alloionema appendiculatum, Nemhelix bakeri, and Riccardoella limacum being commonly found infecting Helix aspersa aspersa (petit-gris) snails. With the exception of C. helicogenae, N. bakeri, and B. aspersae sporocysts, all species were also detected in Helix aspersa maxima (gros-gris) snails, although generally with lower parameters. Most monoxenous infections, and consequently multiple parasitism, exhibited a rising trend during the first 2 months of intensive mating, with tendencies being slowed down or even reversed during the third month as a result of accumulated mortality and a sampling-derived reduction in host density. No parasites were vertically transmitted and infections were initially acquired from invading gastropod and micromammal reservoirs during fattening. Finally, artificial hibernation reduced significantly the prevalence of most species. These results confirm the importance of parasites in heliciculture and emphasize the need to prevent the entry of wild reservoirs into the farms and to rapidly remove the carcasses of dead snails from the reproduction units and fattening pens.

Trypanosoma cruzi (Tc), the causative agent of Chagas disease, is a diverse species with 2 primary genotypes, TcI and TcII, with TcII further subdivided into 5 subtypes (IIa–e). This study evaluated infection dynamics of 4 genetically and geographically diverse T. cruzi strains in 2 South American reservoirs, degus (Octodon degus) and grey short-tailed opossums (Monodelphis domestica). Based on prior suggestions of a genotype-host association, we hypothesized that degus (placental) would more readily become infected with TcII strains while short-tailed opossums (marsupial) would be a more competent reservoir for a TcI strain. Individuals (n=3) of each species were intraperitoneally inoculated with T. cruzi trypomastigotes of TcIIa [North America (NA)-raccoon (Procyon lotor) origin], TcI [NA-Virginia opossum (Didelphis virginiana)], TcIIb [South America (SA)-human], TcIIe (SA-Triatoma infestans), or both TcI and TcIIa. Parasitaemias in experimentally infected degus peaked earlier (7–14 days post-inoculation (p.i.)) compared with short-tailed opossums (21–84 days p.i.). Additionally, peak parasitaemias were higher in degus; however, the duration of detectable parasitaemias for all strains, except TcIIa, was greater in short-tailed opossums. Infections established in both host species with all genotypes, except for TcIIa, which did not establish a detectable infection in short-tailed opossums. These results indicate that both South American reservoirs support infections with these isolates from North and South America; however, infection dynamics differed with host and parasite strain.

Thermal effects on the dynamics of infection with metacercariae of Diplostomum baeri and D. spathaceum were monitored between May and September, in a year-class of perch Perca fluviatilis, in an artificial lake receiving warm water discharges from a nuclear power station and in an unheated reference site, for a two year period. In the heated area the prevalence of infection of the retinal form, D. baeri, was always 100%, whereas in the unheated site there was an increase from 93% in May to 100% in June 1986. The relative density of D. baeri was found to increase gradually during the first summer in both areas, although the accumulation rate of metacercariae was significantly increased in the heated area. In August of the first year the relative density of D. baeri peaked in the heated area, whereas it continued to increase in the unheated control. However, in September of the following year, the relative densities were at the same level in both thermal regimes. A concomitant decline in the degree of overdispersion of metacercariae within the host population was observed in the heated area as the population density of metacercariae decreased, whereas the index of dispersion remained at the same level throughout the study in the unheated area. However, it was not possible to sample perch in the unheated area between October 1986 and May 1987 and changes in the parasite population could have occurred during this period. The prevalence and relative density of D. spathaceum, the lens form, was on the contrary low, especially in the unheated site where it was recorded only occasionally. The infection of D. baeri exhibited a convex pattern in both thermal regimes, although the peak infection was noticed earlier in the heated area. Regulation of the parasite infrapopulation may have been achieved by the combined effects of a decreased transmission rate of cercariae with increasing age of the host, the mortality of metacercariae as a natural termination of the life span and to selective predation of heavily infected hosts. However, regardless of an increased accumulation rate of metacercariae in the heated area, the relative densities of D. baeri became equal in both thermal regimes at the end of the study. The mortality rate of metacercariae in the heated area was therefore presumably increased as compared with the unheated reference site.

Gnathostomiasis by Gnathostoma hispidum has occurred mainly in southern and western areas of Japan since 1980. In all cases, patients had eaten live loaches, freshwater fish imported from southeast Asia. Chief symptoms were creeping linear skin eruptions and eosinophilia, both of which disappeared in most patients within three months after the onset, and never recurred thereafter. In order to study how gnathostome larvae may survive and behave in humans, rats were infected with gnathostome larvae. Loaches were purchased in city markets in Fukuoka. Gnathostome larvae (G. hispidum) were recovered from the fish and fed to rats. They were killed at intervals of one and six months and one, two and three years postinoculation. Encysted larvae were found in the muscles of all rats and recovered larvae were all viable. A migrating larva was observed in muscle tissue from one rat that was killed three years postinoculation. In histological sections, walls around encysted larva were much thinner in a rat killed three years postinoculation than in a rat killed one month postinoculation.

The abundance of six species of trematodes: Aphanurus stossichi, Bacciger israelensis, Diphterostomum israelense, Plagioporus idoneus, Lepocreadium album and L. pegorchis, parasitic in the digestive tract of marine teleostei (Sparidae) collected near Jounieh (east Mediterranean), was analysed as a function of the host-size. In two parasite/host systems, infections were observed from the lowest size classes of the sample, with a clear tendency to an increase of abundance in older fish. In four others, parasites appear only above a rather high threshold class, young individuals never being infected. In the last three parasite/host systems, host invasion may occur early or late, but infection decreases above a well defined size class, old fishes rarely or never being infected. A given trematode species. when parasitizing several host species, shows similar abundance/host size relationships, e.g. P. idoneus in Diplodus vulgaris and Oblada melanura. When more than one species of trematode infects a single host species, curves can be markedly distinct; for instance, L. pegorchis was collected from Pagellus erythrinus below 15 cm. whereas D. israelense parasitized the same fish approximately above the same size. There is no evidence that such a replacement of one trematode by another in the course of host growth is a result of interspecific competition.

The kinetics of Breinlia booliati infection in 3 inbred rat strains (Lewis, Wistar and Sprague Dawley) were investigated. One group of rats was infected as neonates (less than 24 hours of age) with third-stage larvae of B. booliati and the other group was infected as juveniles (4 weeks of age). The results showed that infection in the neonates were significantly different from the infection in the juveniles. The 60 rats infected as neonates, when necropsied between 8 to 10 months postinfection, yielded adult worms. The 2 neonatal infection groups of Lewis and Wistar strains showed highest susceptibility to the infections. The mean prepatent period was 85 days. Ninety to 95% of the infected rats were patent with microfilaraemia and a large percentage (33 to 47%) of them had high microfilaraemia counts exceeding 3000 mff/20 mm3 of blood and larger sizes (mean 157·11 mm for female adult worms and 61·88 mm for male adult worms. The adult worms were distributed equally in both the pleural (57%) and peritoneal cavity (43%). In most aspects, the neonatal infection group of the Sprague-Dawley strain was intermediate in susceptibility between the 2 neonatal infection groups of the Lewis and Wistar strains and the 3 juvenile infection groups. In contrast to neonatal infection groups, the 3 juvenile infection groups exhibited low infection rates (37%, 58% and 47% for the Lewis, Wistar and Sprague Dawley strains respectively), longer prepatent periods (mean 101 days), lower recovery rates (2 to 4%), lower adult worm loads (mean 0·4 to 0·8 female worms, and 0·2 to 0·8 male worms per rat), and smaller sizes (mean 141·24 mm for female adult worms and 53·75 mm for male adult worms). Forty-four to 57% of these infected rats harboured either single male or single female adult worms in the body cavity. Most (92%) of the adult worms recovered from the juvenile infection groups resided in the pleural cavity and the remaining 8% were recovered from the peritoneal cavity. Microfilaraemia could be detected in only 3/20 Lewis rats, 5/20 Wistar rats and 5/20 Sprague Dawley rats. The mean peak microfilaraemia of the 3 pooled juvenile infection groups was 632 mff/20 mm3 of blood, ranging from 7 mff/20 mm3 to 1856 mmf/20 mm3. Our results indicate that the susceptibility to B. booliati infection in white rats is both genetic and age-associated. The responses of the 2 distinct infection groups to B. booliati infections are discussed.

The infection dynamics of Cyclops vicinus by the larvae of Bothriocephalus claviceps were studied experimentally. The mean number of procercoids/copepod rose to a plateau of approximately 14 as the density of eggs increased, whilst the number of copepods surviving decreased correspondingly. Copepods exposed to eggs of B. claviceps survived less well than controls, and those exposed to a higher density of eggs survived less well than those to a lower density. The mean number of parasites/copepod and the mortality of copepods were found to increase with the increasing time of exposure to eggs even when egg density was constant. These two parameters were also found to differ between gravid female copepods and unsexed young copepods. Young copepods suffered a higher parasite burden and thus a higher mortality, indicating that there are age or developmental-stage related differences in susceptibility of copepods to infection.

Myxobolus cerebralis, the myxozoan parasite responsible for whirling disease in salmonid fishes, has a complex life-cycle involving an invertebrate host and 2 spore stages. Water flow rate is an environmental variable thought to affect the establishment and propagation of M. cerebralis; however, experimental data that separates flow effects from those of other variables are scarce. To compare how this parameter affected parasite infection dynamics and the invertebrate and vertebrate hosts, dead, infected fish were introduced into a naïve habitat with susceptible hosts under 2 experimental flow regimes: slow (0·02 cm/s) and fast (2·0 cm/s). Throughout the 1-year study, uninfected fry were held in both systems, the outflows were screened weekly for spores and the annelid populations were monitored. We found clear differences in prevalence of infection in the worms, prevalence and severity of infection in the fish, and host survival. Both flows provided environments in which M. cerebralis could complete its life-cycle; however, both the parasite and its invertebrate host proliferated to a greater extent in the slow flow environment over the 1-year study period. This finding is of significance for aquatic systems where the flow rate can be manipulated, and should be incorporated into risk analysis assessments.

Pathozone behaviour of soil-borne plant pathogenic fungi is characterized by curves for the change in probability of infection with distance of inoculum from a host. We identify three major components that give rise to the pathozone profile for infection efficiency: the germinability of inoculum, given that it occurs in the pathozone (P1), the growth of the resulting fungal colony outwards to make contact with the host (P2), and the infectivity of mycelium once contact is made (P3). The probability of infection (P) is then given by the product, P1 × P2 × P3. Using Raphanus sativus and two contrasting types of inoculum of Rhizoctonia solani as a model experimental system, we measured each of the three components by quantifying germinability, colony architecture (to determine the chance of contact by one or more hyphae) and the change in susceptibility over time as a measure of infectivity as mycelium arrives from different distances away. The germinability of inoculum is uniform across the pathozone whereas the probability of contact declines with distance, and the net effect of host susceptibility and infectivity increases. The characteristics of these components result in pathozone profiles that vary in shape. Infection efficiency can decline in an exponential or sigmoidal fashion with increasing distance from the host or follow a curve that rises close to the host and then falls asymptotically to zero with increasing distance. Colony architecture was summarized by the distribution of the furthest extent of hyphal growth amongst colonies growing out from single inoculum units (described, because of its asymmetrical pattern, by a gamma distribution) and by the radial density of hyphae on concentric circles at different distances from the centre of the colony (described by a negative binomial distribution with parameters changing with distance).

The mechanistic model for the components of pathozone behaviour is tested by comparing the magnitude and shape of pathozone profiles predicted by the model with independent estimates for P obtained by placing inoculum at fixed distances from the host, and measuring the proportion of successful infections. The shape depends on the relative magnitudes and change with distance of the three components, P1, P2 and P3, which differed between the two types of inoculum. The profiles could be reproduced accurately when the probability of contact was based on the distribution and density of hyphae reaching the host rather than on the furthest extent of hyphal growth amongst colonies. The model is used to analyse the effects of replication and stochastic variability in the components of pathozone behaviour on the comparison of treatments for disease control.

This paper presents and tests a method to scale up from the dynamics of infection and disease on single plants in order to predict the behaviour of epidemics in populations of plants, in the presence of a biological control agent. Specifically, we quantify the effect of the antagonistic fungus, Trichoderma viride, Pers ex. Gray, on the pathozone dynamics of the damping-off fungus, Rhizoctonia solani Kühn on radish. The results from these individual-based experiments are used to predict the progress of an epidemic and the results are compared with experimental epidemics in microcosms. The addition of T. viride close to a germinating radish plant reduced the extent of the pathozone influence from 22·6 to 13·8 mm. Trichoderma viride also inhibited the evolution of infection efficiency of R. solani. The evolution of infection efficiency over time is described by a simple non-linear model for the probability of infection with distance, in which certain of the parameters vary with time. By combining this with a model based on conditional probabilities for the location and infectivity of randomly dispersed propagules within the pathozone, we were able to scale up and predict the disease progress of R. solani in a population of radish plants, and the extent of control effected by T. viride. Disease progress rose progressively to a maximum of 42% diseased plants in the unprotected crop compared with a sigmoidal approach to 13% in the protected crop. We show that these properties are consistent with a monomolecular function for primary infection with a temporally-varying rate parameter. We also show that the central region of the pathozone, where the joint probabilities of occurrence of a randomly located propagule and its ability to infect are maximal, has a large influence on the sensitivity of epidemics to pathozone dynamics. This is an important example of interpreting population behaviour of an epidemic from the infection and disease of single plants.

Infection experiments were conducted to assess the proportion of Steinernema feltiae (Site 76 strain) Filipjev infective juveniles which penetrated into the test host Galleria mellonella L. over an 8-week period. Using a combined ANOVA and infection model approach, the analyses showed that the proportion of infective juveniles which penetrated into the test hosts changed significantly over time. This change was found to be consistent with a fluctuation in the size of a non-infectious population structure within the infective juvenile pool. These fluctuations in the magnitude of the infectious structure would dynamically alter the number of juveniles available for infection in hosts and so impose the observed change in the proportion of juveniles penetrating into hosts, over the 8-week time-course. The empirical and ecological implications of such a dynamically limited pattern of infection and possible future research into the mechanisms responsible for the non-infectious population structure are discussed.

Pathozone behaviour of soil-borne plant pathogenic fungi is characterized by curves for the change in probability of infection with distance of inoculum from a host. We identify three major components that give rise to the pathozone profile for infection efficiency: the germinability of inoculum, given that it occurs in the pathozone (P1), the growth of the resulting fungal colony outwards to make contact with the host (P2), and the infectivity of mycelium once contact is made (P3). The probability of infection (P) is then given by the product, P1 × P2 × P3. Using Raphanus sativus and two contrasting types of inoculum of Rhizoctonia solani as a model experimental system, we measured each of the three components by quantifying germinability, colony architecture (to determine the chance of contact by one or more hyphae) and the change in susceptibility over time as a measure of infectivity as mycelium arrives from different distances away. The germinability of inoculum is uniform across the pathozone whereas the probability of contact declines with distance, and the net effect of host susceptibility and infectivity increases. The characteristics of these components result in pathozone profiles that vary in shape. Infection efficiency can decline in an exponential or sigmoidal fashion with increasing distance from the host or follow a curve that rises close to the host and then falls asymptotically to zero with increasing distance. Colony architecture was summarized by the distribution of the furthest extent of hyphal growth amongst colonies growing out from single inoculum units (described, because of its asymmetrical pattern, by a gamma distribution) and by the radial density of hyphae on concentric circles at different distances from the centre of the colony (described by a negative binomial distribution with parameters changing with distance).

The mechanistic model for the components of pathozone behaviour is tested by comparing the magnitude and shape of pathozone profiles predicted by the model with independent estimates for P obtained by placing inoculum at fixed distances from the host, and measuring the proportion of successful infections. The shape depends on the relative magnitudes and change with distance of the three components, P1, P2 and P3, which differed between the two types of inoculum. The profiles could be reproduced accurately when the probability of contact was based on the distribution and density of hyphae reaching the host rather than on the furthest extent of hyphal growth amongst colonies. The model is used to analyse the effects of replication and stochastic variability in the components of pathozone behaviour on the comparison of treatments for disease control.