Comment on "Parasites as a viability cost of sexual selection in natural populations of mammals"

Gut protozoa of wild rodents - a meta-analysis

Protozoa are well-known inhabitants of the mammalian gut and so of the gut microbiome. While there has been extensive study of a number of species of gut protozoa in laboratory animals, particularly rodents, the biology of the gut protozoa of wild rodents is much less well-known. Here we have systematically searched the published literature to describe the gut protozoa of wild rodents, in total finding records of 44 genera of protozoa infecting 228 rodent host species. We then undertook meta-analyses that estimated the overall prevalence of gut protozoa in wild rodents to be 24%, with significant variation in prevalence among some host species. We investigated how host traits may affect protozoa prevalence, finding that for some host lifestyles some protozoa differed in their prevalence. This synthesis of existing data on wild rodent gut protozoa provides a better understanding of the biology of these common gut inhabitants and suggests directions for their future study.

Patterns of adult tick parasitization of coexisting European (Erinaceus europaeus) and Algerian (Atelerix algirus) hedgehog populations in eastern Iberia

The availability of data regarding population abundance and dynamics of ticks in wild vertebrate populations is crucial to understand the host-tick relationship and to assess the risk for domestic animals and humans from possible zoonotic diseases. In this study we analyse several host-intrinsic and environmental factors affecting the tick population of two sympatric hedgehog species (the European and the Algerian hedgehog) in two differently anthropized habitats in eastern Spain. We captured 215 hedgehogs and sampled 356 adult ticks of four different species (Rhipicephalus sanguineus, R. turanicus, R. bursa and R. pusillus). We analysed by General Linear Mixed Models how the hosts' species, sex, capture month and site and their interactions affected tick presence and tick load. Further, we carried out Spearman correlation tests to analyse how environmental temperature and precipitation affect tick presence and load on hedgehogs. We found that, in general, male hedgehogs were more infested than females. However, the effects of the different factors depended on the tick species, especially related to their endophilic or exophilic character. High values of general tick infestation can be explained by the coincidence of tick activity peaks and higher male host activity levels, especially within areas with higher habitat diversity and species richness. We also discuss how the potential immunosuppressive effect of testosterone could be affecting our results. Our results show that in highly anthropized environments hedgehogs potentially act as important mixing vessels for tick-borne zoonotic pathogens and that monitoring ticks in periurban wildlife is important to assess potential health risks for pets and humans.

How do biases in sex ratio and disease characteristics affect the spread of sexually transmitted infections?

The epidemiology of sexually transmitted infections (STIs) is inherently linked to host mating dynamics. Studies across many taxa show that adult sex ratio, a major determinant of host mating dynamics, is often skewed - sometimes strongly - toward males or females. However, few predictions exist for the effects of skewed sex ratio on STI epidemiology, and none when coupled with sex biased disease characteristics. Here we use mathematical modelling to examine how interactions between sex ratio and disease characteristics affect STI prevalence in males and females. Notably, we find that while overall disease prevalence peaks at equal sex ratios, prevalence per sex peaks at skewed sex ratios. Furthermore, disease characteristics, sex-biased or not, drive predictable differences in male and female STI prevalence as sex ratio varies, with higher transmission and lower virulence generally increasing differences between the sexes for a given sex ratio. Our work reveals new insights into how STI prevalence in males and females depends on a complex interaction between host population sex ratio and disease characteristics.

Human toxocariasis, a silent helminthic disease revealed in Savannakhet, Lao PDR

Toxocariasis is a zoonotic helminthiasis caused by the migrating larvae of Toxocara canis and T. cati, common roundworms of dogs and cats. Our previous study in Savannakhet Province of Lao PDR showed an infection rate of 44.1% of Toxocara spp. in dogs. Thus, we investigate if this previous high prevalence in the definitive hosts influenced the occurrence of human toxocariasis. For that we used a 38 kDa recombinant protein derived from T. canis larvae excretion secretion products in ELISA. Human serum samples were collected in the Lahanam area of Savannakhet province. The population attending the study in Lahanam village were aged from 7 to 59 years old (y/o) 65.9% (54/82) were male and 34.1% (28/82) were female. The total percentage of seropositivity to Toxocara sp. was 30.4% (25/82). Males were more likely to test positive for toxocariasis with a risk ratio of 2.70 (CI95 0.87–4.93). No significant differences between ages were seen. However, it was possible to observe an increase of optical density (OD) values in ELISA according to age. The awareness of the health system on the high prevalence of seropositivity to Toxocara sp. in Savannakhet can prevent irreversible consequences as permanent vision loss and seizures caused by this silent chronic disease revealed in the Lahanam area.

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The first survey for human toxocariasis was carried out in Savannakhet, Lao-PDR.

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Seropositivity was higher in males than females.

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Despite no reported clinical cases of toxocariasis 30.4% of seropositivity was found.

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People are at risk of exposure to Toxocara sp. larvae in Savannakhet, Lao-PDR.

Sex bias in infectious disease epidemiology: patterns and processes

BACKGROUND

Infectious disease incidence is often male-biased. Two main hypotheses have been proposed to explain this observation. The physiological hypothesis (PH) emphasizes differences in sex hormones and genetic architecture, while the behavioral hypothesis (BH) stresses gender-related differences in exposure. Surprisingly, the population-level predictions of these hypotheses are yet to be thoroughly tested in humans.

METHODS AND FINDINGS

For ten major pathogens, we tested PH and BH predictions about incidence and exposure-prevalence patterns. Compulsory-notification records (Brazil, 2006-2009) were used to estimate age-stratified ♂:♀ incidence rate ratios for the general population and across selected sociological contrasts. Exposure-prevalence odds ratios were derived from 82 published surveys. We estimated summary effect-size measures using random-effects models; our analyses encompass ∼0.5 million cases of disease or exposure. We found that, after puberty, disease incidence is male-biased in cutaneous and visceral leishmaniasis, schistosomiasis, pulmonary tuberculosis, leptospirosis, meningococcal meningitis, and hepatitis A. Severe dengue is female-biased, and no clear pattern is evident for typhoid fever. In leprosy, milder tuberculoid forms are female-biased, whereas more severe lepromatous forms are male-biased. For most diseases, male bias emerges also during infancy, when behavior is unbiased but sex steroid levels transiently rise. Behavioral factors likely modulate male-female differences in some diseases (the leishmaniases, tuberculosis, leptospirosis, or schistosomiasis) and age classes; however, average exposure-prevalence is significantly sex-biased only for Schistosoma and Leptospira.

CONCLUSIONS

Our results closely match some key PH predictions and contradict some crucial BH predictions, suggesting that gender-specific behavior plays an overall secondary role in generating sex bias. Physiological differences, including the crosstalk between sex hormones and immune effectors, thus emerge as the main candidate drivers of gender differences in infectious disease susceptibility.

The costs of risky male behaviour: sex differences in seasonal survival in a small sexually monomorphic primate

Male excess mortality is widespread among mammals and frequently interpreted as a cost of sexually selected traits that enhance male reproductive success. Sex differences in the propensity to engage in risky behaviours are often invoked to explain the sex gap in survival. Here, we aim to isolate and quantify the survival consequences of two potentially risky male behavioural strategies in a small sexually monomorphic primate, the grey mouse lemur Microcebus murinus: (i) most females hibernate during a large part of the austral winter, whereas most males remain active and (ii) during the brief annual mating season males roam widely in search of receptive females. Using a 10-year capture-mark-recapture dataset from a population of M. murinus in Kirindy Forest, western Madagascar, we statistically modelled sex-specific seasonal survival probabilities. Surprisingly, we did not find any evidence for direct survival benefits of hibernation-winter survival did not differ between males and females. By contrast, during the breeding season males survived less well than females (sex gap: 16%). Consistent with the 'risky male behaviour' hypothesis, the period for lowered male survival was restricted to the short mating season. Thus, sex differences in survival in a promiscuous mammal can be substantial even in the absence of sexual dimorphism.

Gender, immunity and the regulation of longevity

For humans and many other animals, gender is a fact of life. Most individuals are born either male or female and their sex will have an enormous influence on their behaviour, physiology and life history. In this review, I consider the effect gender has on lifespan. In particular, I discuss the role played by behaviour, immunity and oxidative damage in determining sex-dependent differences in longevity. I consider existing explanations for the effect of gender on lifespan and how these explanations fit together. Finally, I expand on the recent suggestion of a key role for the insulin/IGF-1 signalling pathway in regulating sex-dependent differences in lifespan and I highlight a number of areas for future investigation.

Steroid hormone related male biased parasitism in chamois, Rupicapra rupicapra rupicapra

Parasites are linked with their host in a trophic interaction with implications for both hosts and parasites. Interaction stretches from the host's immune response to the structuring of communities and the evolution of biodiversity. As in many species sex determines life history strategy, response to parasites may be sex-specific. Males of vertebrate species tend to exhibit higher rates of parasites than females. Sex-associated hormones may influence immunocompetence and are hypothesised to lead to this bias. In a field study, we tested the prediction of male biased parasitism (MBP) in free ranging chamois (Rupicapra rupicapra rupicapra), which are infested intensely by gastrointestinal and lung helminths. We further investigated sex differences in faecal androgen (testosterone and epiandrosterone), cortisol and oestrogen metabolites using enzyme immunoassays (EIA) to evaluate the impact of these hormones on sex dependent parasite susceptibility. Non-invasive methods were used and the study was conducted throughout a year to detect seasonal patterns. Hormone levels and parasite counts varied significantly throughout the year. Male chamois had a higher output of gastrointestinal eggs and lungworm larvae when compared to females. The hypothesis of MBP originating in sex related hormone levels was confirmed for the elevated output of lungworm larvae, but not for the gastrointestinal nematodes. The faecal output of lungworm larvae was significantly correlated with androgen and cortisol metabolite levels. Our study shows that sex differences in steroid levels play an important role to explain MBP, although they alone cannot fully explain the phenomenon.