Seroepidemiology of leptospirosis in dogs from rural and slum communities of Los Rios Region, Chile.

BACKGROUND: Leptospirosis is a zoonotic disease of global importance and often neglected as a public health problem due to lack of awareness, under-diagnosis and under-reporting. Animals serve as a source of transmission through the shedding of Leptospira in their urine. Because of their proximity to humans, dogs may play a role in human infections. In order to assess and mitigate leptospirosis in dogs and the risk of transmission to humans it is important to understand the epidemiology of leptospirosis under natural conditions. This study aimed to characterize leptospirosis in owned dogs from three distinct community types. Blood, dog and household data were collected from 265 dogs in 190 households from 12 communities representing farms, rural villages, and urban slums in the Los Rios region, Chile. Serologic profiles with a 20-serovar microagglutination test panel were obtained. Binomial and multinomial logistic regression models were used to evaluate the associations between spatial, ecological, socio-economic variables and overall seropositivity as well as seropositivity to serogroup Canicola. RESULTS: Results from 247 dogs with no history of vaccination were used. Overall seroprevalence was 25.1% (62/247) with significant differences by community type: 10.9% (9/82) in dogs from farms, 22.3% (21/94) from rural villages, and 45.1% (32/71) from urban slums (p <0.001). This trend by community type was also observed for dogs with evidence of seropositivity to the Canicola serogroup. Factors associated with seropositive dogs included dog density and precipitation two-weeks prior to sampling. Presence of Leptospira positive puddles collected from the peri-domestic household environment was also associated with increased seropositivity. CONCLUSIONS: Results suggest that leptospirosis is actively maintained in the dog population in this study region with notably distinct patterns by community type. Dog populations from rural villages, and urban slums in particular, showed evidence of high levels of transmission probably as a result of the combined effects of dog living conditions as well as community-level ecological and environmental factors.

Agricultural landscape and spatial distribution of Toxoplasma gondii in rural environment: an agent-based model.

BACKGROUND: Predicting the spatial distribution of pathogens with an environmental stage is challenging because of the difficulty to detect them in environmental samples. Among these pathogens, the parasite Toxoplasma gondii is the causative agent of the zoonosis toxoplasmosis, which is responsible for public health issues. Oocysts of T. gondii are excreted by infected cats in the environment, where they may survive and remain infectious for intermediate hosts, specifically rodents, during months to years. The landscape structure that determines the density and distribution of cats may thus impact the spatial distribution of T. gondii. In this study, we investigated the influences of rural settings on the spatial distribution of oocysts in the soil. METHOD: We developed a spatially explicit agent based model to study how landscape structures impact on the spatial distribution of T. gondii prevalence in its rodent intermediate host as well as contamination in the environment. The rural landscape was characterized by the location of farm buildings, which provide shelters and resources for the cats. Specifically, we considered two configurations of farm buildings, i.e. inside and outside a village. Simulations of the first setting, with farm buildings inside the village, were validated using data from previous field studies. Then, simulation results of the two settings were compared to investigate the influences of the farm locations. RESULTS: Model predictions showed a steeper relationship between distance to the nearest farm and infection levels when farm buildings, and thus cats, were concentrated in the same area than when the farms were spread over the area. The relationship between distance to the village center and level of environmental contamination also differed between settings with a potential increased risk for inhabitants when farms are located inside the village. Maps of the risk of soil contaminated with oocysts were also derived from the model. CONCLUSION: The agent-based model provides a useful tool to assess the risk of contamination by T. gondii oocysts at a local scale and determine the most at risk areas. Moreover it provides a basis to investigate the spatial dynamics of pathogens with an environmental stage.

When should a trophically and vertically transmitted parasite manipulate its intermediate host? The case of Toxoplasma gondii.

Parasites with complex life cycles are expected to manipulate the behaviour of their intermediate hosts (IHs), which increase their predation rate and facilitate the transmission to definitive hosts (DHs). This ability, however, is a double-edged sword when the parasite can also be transmitted vertically in the IH. In this situation, as the manipulation of the IH behaviour increases the IH death rate, it conflicts with vertical transmission, which requires healthy and reproducing IHs. The protozoan Toxoplasma gondii, a widespread pathogen, combines both trophic and vertical transmission strategies. Is parasite manipulation of host behaviour still adaptive in this situation? We model the evolution of the IH manipulation by T. gondii to study the conflict between these two routes of transmission under different epidemiological situations. Model outputs show that manipulation is particularly advantageous for virulent strains and in epidemic situations, and that different levels of manipulation may evolve depending on the sex of the IH and the transmission routes considered. These results may help to understand the variability of strain characteristics encountered for T. gondii and may extend to other trophically transmitted parasites.

Quantitative estimation of the viability of Toxoplasma gondii oocysts in soil.

Toxoplasma gondii oocysts spread in the environment are an important source of toxoplasmosis for humans and animal species. Although the life expectancy of oocysts has been studied through the infectivity of inoculated soil samples, the survival dynamics of oocysts in the environment are poorly documented. The aim of this study was to quantify oocyst viability in soil over time under two rain conditions. Oocysts were placed in 54 sentinel chambers containing soil and 18 sealed water tubes, all settled in two containers filled with soil. Containers were watered to simulate rain levels of arid and wet climates and kept at stable temperature for 21.5 months. At nine sampling dates during this period, we sampled six chambers and two water tubes. Three methods were used to measure oocyst viability: microscopic counting, quantitative PCR (qPCR), and mouse inoculation. In parallel, oocysts were kept refrigerated during the same period to analyze their detectability over time. Microscopic counting, qPCR, and mouse inoculation all showed decreasing values over time and highly significant differences between the decreases under dry and damp conditions. The proportion of oocysts surviving after 100 days was estimated to be 7.4% (95% confidence interval [95% CI] = 5.1, 10.8) under dry conditions and 43.7% (5% CI = 35.6, 53.5) under damp conditions. The detectability of oocysts by qPCR over time decreased by 0.5 cycle threshold per 100 days. Finally, a strong correlation between qPCR results and the dose infecting 50% of mice was found; thus, qPCR results may be used as an estimate of the infectivity of soil samples.

Persistence of Bacillus thuringiensis israelensis (Bti) in the environment induces resistance to multiple Bti toxins in mosquitoes.

BACKGROUND: The simultaneous production of six different toxins by Bacillus thuringiensis israelensis (Bti) is thought to delay the evolution of resistance in treated mosquito populations. Recent studies have shown that Bti can persist and proliferate in the environment, thereby imposing continuous selective pressure on mosquito populations, raising concerns about the long-term effectiveness of this bioinsecticide. In order to evaluate the effect of Bti persistence on the evolution of resistance, the authors selected a laboratory Aedes aegypti L. strain with field-collected leaf litter containing Bti toxins. RESULTS: It is shown that resistance to each individual Bti toxin (up to 30-fold) can be obtained after only a few generations of selection. However, the resistance to commercial Bti and to environmental Bti remains low (twofold and 3.4-fold respectively) in the selected strain. Furthermore, some selected individuals exhibited resistance to Cry4B but not to Cry4A, suggesting that two distinct resistance mechanisms are involved in the resistance to these two toxins. CONCLUSION: Considering that resistance to Cry toxins might act as a first step to resistance to a complete Bti toxin mixture, the present results highlight the importance of testing each toxin individually in order accurately to monitor Bti toxin resistance evolution in field populations.

Development of a sensitive method for Toxoplasma gondii oocyst extraction in soil.

Toxoplasmosis is a world-wide infection caused by Toxoplasma gondii. Oocysts disseminated in the environment by infected cats provide a major source of infection for humans and intermediate hosts. The level of soil contamination and the dynamics of this contamination are mostly unknown due to the lack of sensitivity of detection method. Our aim was to improve the detection of T. gondii oocysts in soil samples by comparing three extraction protocols (A, B and C) on unsporulated and sporulated oocysts of different strains and ages, and by testing the effect of sporulation and soil characteristics on oocyst recovery using the most efficient method. The oocyst recovery obtained using protocol C, in which the flotation solution was placed under the sample solution after the dispersion step, was at least ten-fold higher than protocols A and B, in which the sample was just filtered before flotation. The efficiency of protocol C, tested on five artificial soil matrices and four natural soils inoculated with oocysts, was lowest in soils with high proportions of sand. We recommend the protocol C for field investigations, and we advise that results should be interpreted with caution, considering the effect of soil characteristics, especially sand content, on oocyst recovery.