Variation In Infection Intensity Research Articles

Temperature and duration of exposure drive infection intensity with the amphibian pathogen Batrachochytrium dendrobatidis.

The intensity of a pathogen infection plays a key role in determining how the host responds to infection. Hosts with high infections are more likely to transmit infection to others, and are may be more likely to experience progression from infection to disease symptoms, to being physiologically compromised by disease. Understanding how and why hosts exhibit variation in infection intensity therefore plays a major part in developing and implementing measures aimed at controlling infection spread, its effects, and its chance of persisting and circulating within a population of hosts. To track the relative importance of a number of variables in determining the level of infection intensity, we ran field-surveys at two breeding sites over a 12 month period using marked larvae of the common midwife toad (Alyes obstetricans) and their levels of infection with the amphibian pathogen Batrachochytrium dendrobatidis (Bd). At each sampling occasion we measured the density of larvae, the temperature of the water in the 48 h prior to sampling, the period of time the sampled individual had been in the water body, the developmental (Gosner) stage and the intensity of Bd infection of the individual. Overall our data suggest that the temperature and the duration of time spent in the water play a major role in determining the intensity of Bd infection within an individual host. However, although the duration of time spent in the water was clearly associated with infection intensity, the relationship was negative: larvae that had spent less than 3–6 months in the water had significantly higher infection intensities than those that had spent over 12 months, although this infection intensity peaked between 9 and 12 months. This could be due to animals with heavier infections developing more quickly, suffering increased mortality or, more likely, losing their mouthparts (the only part of anuran larvae that can be infected with Bd). Overall, our results identify drivers of infection intensity, and potentially transmissibility and spread, and we attribute these differences to both host and pathogen biology.

Changes in parasite traits, rather than intensity, affect the dynamics of infection under external perturbation.

Understanding the mechanisms that generate complex host-parasite interactions, and how they contribute to variation between and within hosts, is important for predicting risk of infection and transmission, and for developing more effective interventions based on parasite properties. We used the T. retortaeformis (TR)-rabbit system and developed a state-space mathematical framework to capture the variation in intensity of infection and egg shedding in hosts infected weekly, then treated with an anthelminthic and subsequently re-challenged following the same infection regime. Experimental infections indicate that parasite intensity accumulates more slowly in the post-anthelminthic phase but reaches similar maximum numbers. By contrast, parasite EPG (eggs per gram of feces) shed from rabbits in the post-treatment phase is lower and less variable through time. Inference based on EPG alone suggests a decline in parasite intensity over time. Using a state-space model and incorporating all sources of cross-sectional and longitudinal data, we show that while parasite intensity remains relatively constant in both experimental phases, shedding of eggs into the environment is increasingly limited through changes in parasite growth. We suggest that host immunity directly modulates both the accumulation and the growth of the parasite, and indirectly affects transmission by limiting parasite length and thus fecundity. This study provides a better understanding of how within-host trophic interactions influence different components of a helminth population. It also suggests that heterogeneity in parasite traits should be addressed more carefully when examining and managing helminth infections in the absence of some critical data on parasite dynamics.

First insight into the heritable variation of the resistance to infection with the bacteria causing the withering syndrome disease in Haliotis rufescens abalone.

Withering syndrome disease has experienced worldwide spread in the last decade. This fatal disease for abalone is produced by a rickettsia-like organism (WS-RLO), the bacterium "Candidatus Xenohaliotis californiensis". To evaluate the potential of the red abalone (Haliotis rufescens) to improve its resistance to infection by WS-RLO, the additive genetic component in the variation of this trait was estimated. For this, the variation in infection intensity with WS-RLOs and WS-RLOv (phage-infected RLOs) was analyzed in 56 families of full-sibs maintained for three years in a host-parasite cohabitation aquaculture system. A WS-RLO prevalence of 65% was observed in the analysed population; and from the total WS-RLO inclusions 60% were hyperparasited with the phage (WS-RLOv). The decrease in the food ingestion rate was the sole negative effect associated with increasing WS-RLO intensity of infection, suggesting that the high level of WS-RLOv load may have diminished the symptoms of WS disease in the analyzed abalones. The estimated heritabilities were moderate to mid, but significant, varying from 0.21 to 0.23 and 0.36 for WS-RLO and WS-RLOv infections, respectively. This suggests that variation in resistance to infection with WS-RLO may respond to selection in the evaluated red abalone population. Estimated response to selection (G) for the level of infection by WS-RLO indicated that if the 10% of red abalone with the lowest infection level is selected as broodstock, a 90% reduction in the intensity of infection in the progeny can be expected, even with the lowest estimation of heritability (h2=0.21). This strong response would be also due to the large phenotypic variance of this trait. Strong positive correlations, both phenotypic and genotypic, were observed between infection intensities with WS-RLO and WS-RLOv, indicating that selection to increase resistance to one of the types of RLOs will affect resistance in the other in the same direction. This is the first study that demonstrates the existence of additive genetic variation for resistance to WS-RLO in abalone. Consequently, it is possible to increase the resistance to WS-RLO in H. rufescens by selective breeding, which can be an economically attractive and environmentally friendly manner to reduce mortalities and growth effects caused by WS in abalone farms.

The importance of wild fish in the epidemiology of Clonorchis sinensis in Vietnam.

Preliminary findings of a high prevalence of Clonorchis sinensis in wild-caught fish in a North Vietnam reservoir (Thac Ba reservoir, Yen Bai Province) prompted a longitudinal epidemiological study of fish infections. Monthly collections of fish from September 2014 to August 2015 were processed for recovery of metacercariae; 1219 fish, representing 22 species, were examined. Seven species were infected with C. sinensis metacercariae. Four species, Toxabramis houdemeri, Hemiculter leucisculus, Cultrichthys erythropterus, and Culter recurvirostris, had high prevalence (31.1 to 76.7%); metacercarial intensities ranged from 3.9 to 65.7 metacercariae/fish. A seasonal variation of C. sinensis prevalence was observed in T. houdemeri. Variation in intensity of infection occurred in C. erythropterus and H. leucisculus. Intensity and prevalence of C. sinensis in the most highly infected species, T. houdemeri, varied by fish size; prevalence was higher in fish weighing more than 3g, and intensity was higher in fish weighing more than 5g. The distribution of metacercariae in the body region of T. houdemeri was significantly higher in the caudal fin (14.7metacercariae/g), compared to the body and head regions (0.7 and 1.4metacercariae/g, respectively). Further epidemiological investigations on C. sinensis in this reservoir region should include assessing the relative risk of the different fish species for humans based on the latter's food preferences, and the prevalence of C. sinensis in the community. The snail intermediate host(s) in the reservoir should also be identified along with the ecological factors influencing its exposure to C. sinensis eggs and its subsequent transmission of cercariae to fish. Also needed are investigations on the relative importance of wild and domestic reservoir hosts as sources of egg contamination of the reservoir.

Determinants of prevalence and intensity of infection with malaria parasites in the Blue Tit

AbstractParasite species are usually specialists utilising specific host species, but parasite assemblages may differ substantially even between populations of the same species and show seasonal and annual fluctuations. Host characteristics such as individual age and sex may also affect parasite species composition and abundance. Here, we report the occurrence of malaria parasites in the Blue Tit (Cyanistes caeruleus) inhabiting Gotland (Sweden) across 5 years. Using PCR-based techniques, we found eight cytochrome b lineages belonging to genus Plasmodium (pTURUD1, pBT7, pSGS1, pSW2, pGRW11) and Haemoproteus (hPARUS1, hPHSIB1, hWW2) with the overall prevalence of 65 %. The Plasmodium infections predominated (prevalence of 49.5 %), whereas Haemoproteus infection rate was much lower (prevalence of 16.5 %). We showed significant differences in infection status between study years and age classes when all parasite lineages were analysed together or the two most common lineages were analysed separately. Overall, older birds showed higher prevalence. A significant interaction between year and genus effect significantly explained variation in infection intensity. This interaction stems from a significant yearly variation of the infection intensity with Plasmodium, while such an effect is not present for Haemoproteus infections. More importantly, the intensity of infection with Haemoproteus was significantly higher than with Plasmodium in 3 out of the 4 study years with data.

Variation in WSSV in Litopenaeus vannamei during the period of cul-ture in ponds above sea level

The commercial culture of shrimp has expanded rapidly in China in recent decades and is now one of the most important agricultural industries.However,shrimp production decreased significantly between 1992-1997 due to the outbreak of white spot syndrome(WSS).WSS is of serious concern to shrimp farming throughout the world as mortality can approach 100% within 3-10 d of infection with WSSV.There is currently no effective treatment for this disease.We measured the variation in infection intensity of WSSV carried by shrimp(Litopenaeus vannamei) among six ponds in Hongtai Farm in Shanwei of GuangDong province between July and November 2010.In addition,we measured several environmental indices,water quality indicators,and the population structure of planktonic algae.We used real-time PCR to quantify the number WSSV copies in each shrimp.All the shrimp larvae carried WSSV at concentrations ranging from 1.3×103 to 1.7×104 copies/g.Fur-thermore,the shrimp were infected with WSSV throughout the entire farming period.The mean number of WSSV copies was higher in the gill tissue(2.3×109 copies/g) than in the muscle tissue(3.2×108 copy/g),but the differ-ence was not significant(P0.05).The number of WSSV copies tended to increase in both tissues throughout the farming period,and was characterized by a number of peaks.During the early rearing period,the number of copies ranged from 1.3×103–3×107copies/g,whereas in the mid and latter stages of rearing the number of copies in-creased to 1.5×106-1.2×1011 copies/g.In some ponds,outbreaks of WSS forced an end to farming.Rapid changes in water temperature,algae population structure,and weather were also associated with changes in the number of WSSV copies in shrimp.Our results suggest that shrimp can be cultured even when infected with WSSV.The in-tensity of infection with WSSV varied temporally and was affected by culture time and environmental factors.We suggest that maintaining the population structure of planktonic algae,keeping water quality constant,and de-creasing stress can significantly reduce the likelihood of WSS outbreak.

Individual Variability of Nosema ceranae Infections in Apis mellifera Colonies.

Since 2006, beekeepers have reported increased losses of Apis mellifera colonies, and one factor that has been potentially implicated in these losses is the microsporidian Nosema ceranae. Since N. ceranae is a fairly recently discovered parasite, there is little knowledge of the variation in infection levels among individual workers within a colony. In this study we examined the levels of infection in individual bees from five colonies over three seasons using both spore counting and quantitative real-time PCR. The results show considerable intra-colony variation in infection intensity among individual workers with a higher percentage of low-level infections detected by PCR than by spore counting. Colonies generally had the highest percentage of infected bees in early summer (June) and the lowest levels in the fall (September). Nosema apis was detected in only 16/705 bees (2.3%) and always as a low-level co-infection with N. ceranae. The results also indicate that intra-colony variation in infection levels could influence the accuracy of Nosema diagnosis.

Estimating patterns and drivers of infection prevalence and intensity when detection is imperfect and sampling error occurs

Summary1. Epidemiological studies are crucial for understanding the distribution and dynamics of emerging infectious diseases. To accurately assess infection states in wild populations, researchers need to account for observational uncertainty. We focus on two sources of uncertainty when estimating epidemiological parameters: nondetection of infection in sampled individuals and sampling error when quantifying infection intensity for infected individuals.2. We developed new analytical methods to simultaneously estimate prevalence and the distribution of infection intensities based on repeated sampling of individuals in the wild. The methods are an extension of those used for occupancy estimation and address both sources of observation error. At the same time, we account for heterogeneity in detection probability that results from individual variation in infection intensity.3. We use two estimation approaches to account for detection. The first is to use the complete likelihood in a hierarchical Bayesian model and fit using Markov chain Monte Carlo sampling. The second is to estimate the detection relationship using a mark–recapture abundance estimator and use those results to calculate weighted estimates for prevalence and mean infection intensities.4. We use data from a field survey of Batrachochytrium dendrobatidis in Illinois amphibians to test these methods. We show that detection probability using quantitative PCR is strongly related to infection intensity, measured in zoospore equivalents. Sites in the study varied greatly in estimated prevalence and to a lesser extent in mean infection intensities of infected individuals. We did not find evidence of a relationship of snout‐vent‐length to infection intensity or prevalence. Naïve estimates of prevalence that do not account for detection were less than estimates for either of our methods, which yielded similar prevalence values for most sites.5. Uncertainty when assessing disease state is a characteristic of most diagnostic tests. The estimators presented here account for this uncertainty and thus can improve accuracy when assessing the relationship of ecological factors to prevalence and infection intensity.

Comparison of within hive sampling and seasonal activity of Nosema ceranae in honey bee colonies

Nosema ceranae is a microsporidian parasite of the European honey bee, Apis mellifera, that is found worldwide and in multiple Apis spp.; however, little is known about the effects of N. ceranae on A. mellifera. Previous studies using spore counts suggest that there is no longer a seasonal cycle for N. ceranae and that it is found year round with little variation in infection intensity among months. Our goal was to determine whether infection levels differ in bees collected from different areas of the hive and if there may be seasonal differences in N. ceranae infections. A multiplex species-specific real-time PCR assay was used for the detection and quantification of N. ceranae. Colonies were sampled monthly from September 2009–2010 by collecting workers from honey supers, the fringe of the brood nest, and the brood nest. We found that all bees sampled were infected with N. ceranae and that there was no significant difference in infection levels among the different groups of bees sampled ( P = 0.74). However, significant differences in colony infection levels were found at different times of the year ( P < 0.01) with the highest levels in April–June and lower levels in the fall and winter. While our study was only performed for one year, it sheds light on the fact that there may be a seasonality to N. ceranae infections. Being able to predict future N. ceranae infections can be used to better advise beekeepers on N. ceranae management.

Tracking a heterosis effect in the field: tadpole resistance to parasites in the water frog hybridogenetic complex

Depending on the extent of evolutionary divergence among parent taxa, hybrids may suffer from a breakdown of co-adapted genes or may conversely exhibit vigour due to the heterosis effect, which confers advantages to increased genetic diversity. That last mechanism could explain the success of hybrids when hybridization zones are large and long lasting, such as in the water frog hybridization complex. In this hybridogenetic system, hybrid individuals exhibit full heterozygosity that makes it possible to investigate in situ the impact of hybridization. We have compared parasite intensity between hybrid Rana esculenta and parental R. lessonae individuals at the tadpole stage in two populations inhabiting contrasted habitats. We estimated intensity of Gyrinicola sp. (Nematoda) in the gut, Echinostome metacercariae in the kidneys and Haplometra cylindracea in the body cavity (both species belong to Trematoda). Despite high sampling effort, no variation in parasite intensity was detected between taxa, except a possible higher tolerance to H. cylindracea in hybrid tadpoles. The low effect of hybridization suggests efficient gene co-adaptation between the two genomes that could result from hemiclonal selection. Variation in infection intensity among ponds could support the Red Queen hypothesis.

Causes of inter-individual variation in reproductive strategies of the parasitic nematode Graphidioides subterraneus

Several factors capable of affecting the amount of resources available to an individual parasite, such as the number of other parasites in the host or host quality, may cause variability in reproductive success among parasites. Variation in egg output and mean egg volume was investigated among adult females of the nematode Graphidioides subterraneus, parasitic in the herbivorous subterranean rodent Ctenomys talarum (Octodontidae). Female nematode body size correlated strongly with the number of eggs produced. However, neither host body mass nor the number of other nematodes per host had any influence on the number or volume of eggs produced by the parasites. There was also no evidence for a trade-off between the number of eggs produced and mean egg volume among female nematodes. All these results suggest that resource supply to individual worms is not limited by host size or by the number of conspecific parasites vying for the same resources, despite the 30-fold variation in intensity of infection and the twofold variation in host body mass observed in the present study. Instead, resource availability does not appear to constrain reproduction in G. subterraneus, with its host providing a stable, predictable environment.

Frequency and spatial distribution of environmental Campylobacter spp.

Humans are exposed to Campylobacter spp. in a range of sources via both food and environmental pathways. For this study, we explored the frequency and distribution of thermophilic Campylobacter spp. in a 10- by 10-km square rural area of Cheshire, United Kingdom. The area contains approximately 70, mainly dairy, farms and is used extensively for outdoor recreational activities. Campylobacter spp. were isolated from a range of environmental samples by use of a systematic sampling grid. Livestock (mainly cattle) and wildlife feces and environmental water and soil samples were cultured, and isolates were presumptively identified by standard techniques. These isolates were further characterized by PCR. Campylobacter jejuni was the most prevalent species in all animal samples, ranging from 11% in samples from nonavian wildlife to 36% in cattle feces, and was isolated from 15% of water samples. Campylobacter coli was commonly found in water (17%) and sheep (21%) samples, but rarely in other samples. Campylobacter lari was recovered from all sample types, with the exception of sheep feces, and was found in moderate numbers in birds (7%) and water (5%). Campylobacter hyointestinalis was only recovered from cattle (7%) and birds (1%). The spatial distribution and determinants of C. jejuni in cattle feces were examined by the use of model-based spatial statistics. The distribution was consistent with very localized within-farm or within-field transmission and showed little evidence of any larger-scale spatial dependence. We concluded that there is a potentially high risk of human exposure to Campylobacter spp., particularly C. jejuni, in the environment of our study area. The prevalence and likely risk posed by C. jejuni-positive cattle feces in the environment diminished as the fecal material aged. After we took into account the age of the fecal material, the absence or presence of rain, and the presence of bird feces, there was evidence of significant variation in the prevalence of C. jejuni-positive cattle feces between grazing fields but no evidence of spatial clustering beyond this resolution. The spatial pattern of C. jejuni is therefore consistent with that for an organism that is ubiquitous in areas contaminated with cattle feces, with a short-scale variation in infection intensity that cannot be explained solely by variations in the age of the fecal material. The observed pattern is not consistent with large-scale transmission attributable to watercourses, wildlife territories, or other geographical features that transcend field and farm boundaries.

The mud flat anemone-cockle association: mutualism in the intertidal zone?

The intertidal cockle Austrovenus stutchburyi exists in a symbiotic relationship with the mud flat anemone Anthopleura aureoradiata, the latter using the shell of buried cockles as the only available hard substrate for attachment. The cockles are also host to a detrimental larval trematode Curtuteria australis that invades the bivalves through the filtration current, and here we demonstrate that the anemones significantly depress the rate by which cockles accumulate parasites in the field. Along the tidal gradient, the relative parasite load of cockles was lowest where anemones were most abundant, and the area occupied by anemones per square meter sediment surface explained 30% of the spatial variation in infection intensity. At a smaller spatial scale, parasite loads were significantly lower (34%) in cockles from patches with than without anemones at the same tidal height. A field experiment manipulating the density of anemones showed that the rate of parasite accumulation in cockles decreased with increasing anemone density, and that the generally positive relationship between infection intensity and cockle size tended to disappear in the presence of anemones. The results suggest that the anemone-cockle symbiosis is a non-obligate mutualistic relationship in which the former is provided with a suitable substrate for attachment whereas the latter obtains protection against parasitic infections.

Emerging patterns of hookworm infection: influence of aging on the intensity of Necator infection in Hainan Province, People's Republic of China.

We examined risk factors associated with Necator americanus infection among persons aged > or =50 years in Hainan Province, People's Republic of China. Age and sex made the most important contributions to the variation in infection intensity (28%-30%), with age alone responsible for 27% of this variation. When stratified by 20-year age intervals, the influence of shared residence was 23% for persons aged > or =50 years and 27% for those aged <20 years, who had the highest and lowest levels of infection intensity, respectively. This points to shared residence as a means of capturing the complex relationship between aging and shared socioeconomic, environmental, and behavioral factors that influence transmission of Necator infection. None of the other 26 personal or 32 household risk factors were found to be significant. The importance of aging in Necator infection reveals an emerging public health problem among the elderly population of developing countries.

Mistletoe infection of trees located at fragmented forest edges in the cloud forests of Central Veracruz, Mexico

We investigated the Psittacanthus schiedeanus mistletoe infection of its preferred host tree species Liquidambar styraciflua in remnants of cloud forest in Central Veracruz, Mexico. We studied whether or not the location of host trees on forest fragment edges has a direct effect on mistletoe infection, and explored among host tree variation in intensity of infection. In four 1 ha plots, we marked 200 L. styraciflua and recorded height, diameter at breast height (DBH), distance to fragment edge, distance to conspecific tree, and number of mistletoe adult plants per tree, or canopy area (upper or medium-low). Overall, tree densities varied among plots, but not the number of infected or non-infected L. styraciflua trees. Fifty-nine percent (118/200) of the L. styraciflua trees were infected. They were similar in the number of P. schiedeanus plants per tree, but not in height, DBH, or distance to conspecifics. The most severely infected trees were tall, located on fragment edges, and located near an infected conspecific. Trees located on fragment edges had more mistletoe plants per tree and medium-low area of the tree. Also individual tree features as height and distance to conspecific affected, but not DBH. We suggest that individual differences among host trees play an important role in explaining local abundance and distribution of mistletoe plants. Inside forest fragments, tree density of Liquidambar, and edges lengths are important factors in determining the presence and intensity of P. schiedeanus infection.

Intensity of infection in AIDS-related intestinal microsporidiosis.

To quantify intensity of infection in AIDS-related microsporidiosis, 20 patients with known microsporidiosis submitted stools for quantitative spore counts after staining with a calcofluor white stain. Nine patients collected stools for 24 h, for assessment of daily spore excretion, stool-to-stool variation in spore excretion, and patient-to-patient variation in intensity of infection. The number of organisms seen in small bowel biopsy specimens from 7 patients was compared with quantitative fecal spore excretion. Fecal spore concentration in 20 patients ranged from 4.5x105 to 4.4x108 spores/mL of stool. There was a strong correlation between fecal spore excretion and duodenal biopsy spore counts (r=.82; P<.024). Microsporidium infections in AIDS patients can be quantified by counting spores in stool and by small bowel biopsy. Variations in intensity of infection from patient to patient are great and are similar to those in AIDS-related Cryptosporidium infection.

Puberty and Age-intensity Profiles in Schistosome Infections: Another Hypothesis

In an excellent focus article on puberty and age-related changes in susceptibility to schistosome infection, Fulford et al.[1xFulford, A.J.C. et al. Parasitol. Today. 1998; 14: 23–26Abstract | Full Text | Full Text PDF | PubMed | Scopus (91)See all References[1]provide a new explanation for the characteristic age-intensity profile in areas where schistosomiasis is endemic. An age-dependent host factor seems to override—at least during adolescence—the variation in intensity of infection expected both from the variability of exposure with changing age and from the dynamics of acquired immunity. Fulford and colleagues put forward the hypothesis that physiological changes in the production of sex hormones could be one explanation. The authors explore how sex hormones might exert a regulatory effect on the immune system, eventually leading to an increased resistance to infection. Other modes of action by which sex hormones, particularly in females, may influence parasites are only mentioned briefly, but not commented on.Pelvic vascularization alters when girls enter puberty, resulting in multiple anastomoses between the rectal and the vesical plexus, and between the plexus of external and internal genital organs[2xSee all References, 3xSee all References]. These adaptive changes are accentuated during pregnancy. As adult worms tend to migrate, the adaptive changes in pelvic vascularization add to the chance that parasites reach the adjacent vascular beds, thereby increasing the probability of a settlement in female genital organs. In fact, at least in Schistosoma haematobium infection, involvement of external and internal genital organs is extremely common. Of women infected with this parasite species 40–80% are considered to have manifestations in the lower reproductive tract[4xFeldmeier, H. et al. Trop. Geogr. Med. 1995; 47: 1–15See all References, 5xKjetland, E. et al. Acta Trop. 1996; 62: 239–255Crossref | PubMed | Scopus (65)See all References, 6xLeutscher, P. et al. Acta Trop. 1997; 66: 27–34Crossref | PubMed | Scopus (27)See all References]. How many women also have worms located in the internal genital organs is unknown; post-mortem studies in female schistosomiasis patients, however, show that the ovaries, the fallopian tubes and the uterus are affected in 10–80% of cases[7xEdington, G.M. et al. Trans. R. Soc. Trop. Med. Hyg. 1971; 20: 846–849See all References, 8xGelfand, M. and Ross, W. Trans. R. Soc. Trop. Med. Hyg. 1953; 47: 218–220PubMed | Scopus (17)See all References, 9xYoussef, A.F. et al. J. Obstet. Gynaecol. Br. Commonw. 1970; 77: 847–851Crossref | PubMedSee all References].A shift of adult worms to topographic sites from which eggs cannot reach the lumen of the bladder, and thus are not excreted, obviously has an important consequence on age-intensity profiles: even if reinfection rates remain unchanged, egg excretion in urine will diminish and thus create the impression that there is an age-associated decrease of intensity of infection with onset of puberty.Two lines of evidence support this assumption. First, according to the literature, female genital schistosomiasis (FGS) occurs mainly after girls have reached sexual maturity[4xFeldmeier, H. et al. Trop. Geogr. Med. 1995; 47: 1–15See all References[4], ie. at an age when intensity of infection—as measured by egg excretion—decreases. Recently, these clinical observations were supported by data from a community-based study in Tanzania; the existence of FGS as measured by the presence of eggs in cervical biopsies and the prevalence/intensity of egg excretion in urine were inversely correlated during adolescence (Poggensee et al., unpublished).Second, if with the beginning of puberty a topographic shift of adult worms, rather than a diminished worm burden, is responsible for a decrease of egg excretion in urine, then the simultaneous measurement of egg excretion and circulating antigens should provide an important clue. Agnew et al.[10xAgnew, A. et al. Am. J. Trop. Med. Hyg. 1996; 55: 338–343PubMedSee all References[10]examined S. haematobium- or S. mansoni-infected populations for egg excretion as well as circulating anodic antigen (CAA). Schistosoma haematobium egg output relative to serum CAA was significantly reduced with onset of puberty. Nothing like that was observed in patients infected with S. mansoni, a parasite less commonly affecting the genital tract[4xFeldmeier, H. et al. Trop. Geogr. Med. 1995; 47: 1–15See all References[4]. Moreover, the effect observed by Agnew et al.[10xAgnew, A. et al. Am. J. Trop. Med. Hyg. 1996; 55: 338–343PubMedSee all References[10]was highly significant in women, but not in men, whose genitals are less frequently affected by ectopic schistosomiasis.Taken together, these circumstantial facts favour the hypothesis that in S. haematobium infection in women yet another mechanism may explain the familar age-intensity profile with the typical decline of the curve after onset of puberty. Whether vascular changes induced by sex hormones during puberty and pregnancy and subsequent anatomical compartmentalization of adult worms also have a regulatory effect on protective immune mechanisms is another intriguing question. The dynamics of schistosomiasis are probably much more complex than we still know. Why should it not be different between women and men?

Distribution and prevalence of Anguillicola crassus in eels from the tidal Thames catchmentb

Data gathered between 1988 and 1992 document the spread of the parasitic nematode Anguillicola crassus among eels in the tidal Thames catchment. Eel samples revealed a parasite prevalence ranging between 12 and 32% with a variation in intensity of infection of between one and five nematodes per infected host. Differences in the salinity regime between sampling points may be linked to the range of levels of infection in eels because of the saline tolerance limits of parasite developmental stages. The euryhaline teleost, the smelt (Osmerus eperlanus) found throughout the tidal river has been shown by others to be able to transfer nematode larval stages experimentally to large eels. Smelt found in the tidal Thames thus could possibly act as a further intermediate host to the eel population. The results support the theories proposed by previous workers that the parasite originally entered the tidal Thames via the commercial trade in live eels.

DISEASE IMPACT AND LOCAL GENETIC DIVERSITY IN THE CLONAL PLANT PODOPHYLLUM PELTATUM.

Genotypic diversity is restricted within local colonies of mayapple (Podophyllum peltatum), due to extensive asexual reproduction. Transplant experiments were used to examine whether disease impact from a specialist fungal pathogen (Puccinia podophylli) was affected by the local frequency of host genotypes within colonies. In each of six large mayapple colonies, I measured infection intensity on 1) ramets replanted in their native colony (which were thus surrounded mostly by identical genotypes) and 2) transplants from two foreign colonies (surrounded by different genotypes). Disease incidence during the pathogen's first generation did not vary significantly between native (11% infected) and foreign host genotypes (6% infected). In the pathogen's second generation, significant variation in infection intensity occurred among ramets from different source populations. However, at five of the six transplant sites, mean infection intensity was higher on some nonnative plants (locally rare host genotypes) than on natives (locally common host genotypes). In this system, pathogen attack does not act in a frequency-dependent manner to promote local genetic diversity among hosts.

A Survey of Lungworm Infection in Bighorn Sheep of Montana

A survey of lungworm infection (Protostrongylus stilesi and P. rushi) in the Rocky Mountain bighorn sheep (Ovis canadensis) of Montana was conducted from 1958 to 1963. Ninety-one percent of 900 fecal samples collected from 10 bighorn herds in western Montana contained protostrongylid larvae. Ninety-three percent of 143 lungs examined were infected with P. stilesi and 40 percent with both P. stilesi and P. rushi. No lungs contained only P. rushi. Total lesion area of P. stilesi infections averaged 2,332 mm2 and varied from I to 12,462 mm2. Numbers of adult P. rushi averaged 10 per infected lung and varied from 1 to 162. Sheep appeared to become infected during their first year of life. There seemed to be some association between infections of P. stilesi and P. rushi in lungs and the numbers of first-stage protostrongylid larvae in the feces when samples were compared on a herd basis. Numbers of first-stage larvae in individual fecal droppings from the same samples, in samples collected daily from the same animal, and in samples collected monthly from a herd of bighorns varied considerably. Feces collected during the spring contained more larvae than samples collected at other times of the year. Yearly variation in intensity of infection appeared to be directly correlated to the amount of precipitation in the preceding year. There has been a general decline in the number of Rocky Mountain bighorn sheep since the nineteenth century. During this period there have been some dramatic dieoffs which have stimulated a number of studies (Buechner 1960). Various disease conditions have been noted in these studies, and recently a lungworm-pneumonia complex has been implicated as the most important. Hunter and Pillmore (1954: 118) suggest that many of the earlier losses attributed to hemorrhagic septicemia may actually have been due to this disease complex. The parenchymal lungworm (P. stilesi) and the broncheolar lungworm (P. rushi) have been commonly associated with the complex. Protostrongylus stilesi has been reported to be the more virulent of the two species (Honess 1942:23). Basic to an understanding of the impact of lungworm on bighorn sheep is information on the intensity and distribution of fections in natural populations. Reports of lungworms in bighorn sheep have been reviewed by Buechner (1960:94-104). The most extensive and detailed survey was conducted in Colorado (Hunter and Pillmore 1954:125-126, Pillmore 1961:8793). The present survey in Montana was initiated in 1958 and continued to 1963. Its purpose was to determine the frequency and degree of protostrongylid lungworm infection so that they could be used with other data for evaluation of bighorn sheep population dynamics and management. This study was supported in part by a grant from the National Science Foundation (NSF G-19422). The 1958-60 data are from a thesis submitted by the senior author in partial fulfillment of the requirements for the degree of Master of Science in Wildlife Technology at Montana State University, Missoula. The authors wish to thank the personnel of the Montana Fish and Game Department and the hunters who helped in collecting bighorn lungs and fecal samples. R. E. Pillmore, for'Present address: Department of Veterinary Microbiology, University of California, Davis. 2 Present address: Department of Biology, Western Washington State College, Bellingham.