Spatial distribution on pasture of infective larvae of the gastro-intestinal nematode parasites of sheep.
ABSTRACT The horizontal distributions of infective larvae on pasture grazed by sheep have been investigated. Using Taylor's Power Law it was found that larvae had a more aggregated distribution in September than August, the Law index of aggregation being 1.97 and 1.89 for the 2 months, respectively. However, at each time the degree of aggregation remained fairly constant for a range of spacings between points from 5 to 30 m. These results suggest that Taylor's Power Law could be used as a basis for devising an efficient pasture sampling strategy. More data are required, however, to determine the extent to which aggregation of the larvae varies with time of the year.
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ABSTRACT: Spatio-temporal pattern of larvae and eggs of gastrointestinal nematodes in cattle pastures in Veracruz, Mexico. The spatial and temporal distribution of gastrointestinal nematodes of cattle has been little studied in Mexico. Previous studies have described periods of higher larval presence, vertical and horizontal migration in grasslands, and the frequency of adult nematodes; as well as the effect of pasture trichomes on the migration and survival of Haemonchus larvae. The aim of this study was to determine the time-space layout and spread of gastrointestinal nematode larvae on pasture, and to estimate the effect of ivermectin applied to cattle on the time-dependent abundance of their eggs in a ranch in Veracruz. To determine the spatio-temporal arrangement, monthly morning grass samples were obtained from 30 sampling points from July 2008 to June 2009. Third stage larvae (L 3) from each point were counted, and aggregation patterns were estimated through variance/mean and negative binomial K indices. Additionally, the number of eggs per gram in cattle feces was determined, from samples with (CI) and without ivermectin (SI), using standard techniques. A total of 20 276L 3 larvae were recovered in the pasture, of which an 80% corresponded to Haemonchus contortus. The highest nematode density with more than 5 000L 3 /kgDM was detected in October 2008, and the lowest in February and March 2009. The L 3 showed an aggregated spatial pattern of varying intensity throughout the year. The number of eggs in the stool was not reduced with the ivermectin application to cattle, which suggested a failure of control. However, the highest parasite loads were observed from July to November 2008. We concluded that the application of ivermectin was not effective to control nematodes eggs, and that L 3 populations fluctuated on pasture for ten months, providing an infection source to grazing animals afterwards. Rev. Biol. Trop. 61 (4): 1747-1758. Epub 2013 December 01.Revista de biologia tropical 12/2013; 61:1747-1758. · 0.55 Impact Factor
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ABSTRACT: It is well known that Taylor's power law describes spatial aggregation, but its ecological interpretation remains elusive. Here we used data on spatial organization of soil nematode communities in urban landscapes to test relationships between nematode life history characteristics and the power law aggregation indicator, b. Forty seven genera of soil inhabiting nematodes were identified from 360 samples taken from turfgrass lawns at three public schools located in each of three northeast Ohio cities in July and October 2007. The nematodes were classified according to their life-styles in three life history classifications: trophic group, colonizer-persistor (cp) class, and functional guild, a combination of trophic group and cp-class. Estimates of Taylor's b for 28 more common genera ranged from 1.21 to 2.34. Estimates of b for trophic group, cp-class and functional guild ranged from 1.41 to 2.10, 1.34 to 1.97 and 1.41 to 2.39, respectively. Segregating genera by their trophic group, cp-class and functional guild each accounted for as much inter-genus variation in Taylor's power law as fitting 28 separate regressions. The improvements of fit in parallel line analyses for the three life-style categories over 28 lines for individual genera were highly significant. Bacterial- and plant-feeding groups were more highly aggregated than omnivorous and predatory nematodes. Nematodes in cp-classes 1 and 2 tended to be more aggregated than those in higher cp-classes. The functional guilds were generally more highly aggregated than individual genera, suggesting a higher degree of aggregation at the functional guild level. We conclude that nematode genera vary in their spatial organizations, but membership in a cp-class and functional guild accounts for much of this variability. Thus, Taylor's power law aggregation indicator, b, is sensitive to nematode feeding habits and life strategy traits as they influence a population's pattern of spatial organization.Applied Soil Ecology 64:214–222. · 2.11 Impact Factor
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ABSTRACT: Parasitic helminths present one of the most pervasive challenges to grazing herbivores. Many macro-parasite transmission models focus on host physiological defence strategies, omitting more complex interactions between hosts and their environments. This work represents the first model that integrates both the behavioural and physiological elements of gastro-intestinal nematode transmission dynamics in a managed grazing system. A spatially explicit, individual-based, stochastic model is developed, that incorporates both the hosts' immunological responses to parasitism, and key grazing behaviours including faecal avoidance. The results demonstrate that grazing behaviour affects both the timing and intensity of parasite outbreaks, through generating spatial heterogeneity in parasite risk and nutritional resources, and changing the timing of exposure to the parasites' free-living stages. The influence of grazing behaviour varies with the host-parasite combination, dependent on the development times of different parasite species and variations in host immune response. Our outputs include the counterintuitive finding that under certain conditions perceived parasite avoidance behaviours (faecal avoidance) can increase parasite risk, for certain host-parasite combinations. Through incorporating the two-way interaction between infection dynamics and grazing behaviour, the potential benefits of parasite-induced anorexia are also demonstrated. Hosts with phenotypic plasticity in grazing behaviour, that make grazing decisions dependent on current parasite burden, can reduce infection with minimal loss of intake over the grazing season. This paper explores how both host behaviours and immunity influence macro-parasite transmission in a spatially and temporally heterogeneous environment. The magnitude and timing of parasite outbreaks is influenced by host immunity and behaviour, and the interactions between them; the incorporation of both regulatory processes is required to fully understand transmission dynamics. Understanding of both physiological and behavioural defence strategies will aid the development of novel approaches for control.PLoS ONE 01/2013; 8(11):e77996. · 3.53 Impact Factor