A rich body of work has reported levels of infection with Toxocara species in definitive hosts, and the frequency of eggs in the environment, in many different regions and situations. These have greatly increased our understanding of the relationship between egg excretion from companion and wild animals and the risk of human infection by inadvertent ingestion of eggs from soil and other environmental reservoirs. Nevertheless, it is difficult to compare studies directly because of vagaries in sampling and laboratory methods, a preponderance of prevalence rather than abundance data, and a lack of studies that systematically sample different sympatric definitive host populations. Such comparisons could be instructive, for example to determine the relative contributions of different definitive host populations and categories to environmental contamination in specified areas, and hence guide priorities for control. In this article we use estimates of host density and infection levels in the city of Bristol, UK, as a case study to evaluate the relative contribution of sympatric cats, dogs and foxes to overall environmental contamination with eggs. Results suggest that dogs, especially those less than 12 weeks of age, dominate total egg output, but that this is modified by degree of access to public areas and removal of faeces, such that foxes could take over as the primary source of eggs. Results and conclusions are likely to differ among specific locations. The general aim is to show how an improved quantitative framework for epidemiological studies of Toxocara spp. egg contamination can help to advance understanding and the effectiveness of control strategies in future.

Infestation by parasitic Psoroptes mites (Acari: Psoroptidae) is an important cause of economic loss and welfare problems in livestock in many areas of the world. At least five species within this genus have been recognized, based on the host infested, the infestation site and differences in length of the opisthosomal setae of adult male mites. Here the integrity of these species is considered by subjecting populations of mites from a range of host species and geographical locations to simultaneous morphological and molecular genetic analyses. Morphological analysis showed that there were significant differences in shape and size between mite populations from different hosts, and that length of the outer opisthosomal setae in males and the homologous seta in females were the most important distinguishing character in adults. However, considerable variation in outer opisthosomal seta length was evident within and between populations of mites, and differences were not clearly related to host-species or geographical origin and did not support the accepted species differences. Molecular characterization using sequence data from the mitochondrial second internal transcribed spacer (ITS-2) region and microsatellite markers found little or no consistent host-related variation between the mite population samples. The results suggest that there is no case for considering the Psoroptes mites from the different hosts examined as separate species and that the morphological variation observed therefore may represent phenotypic adaptation to the local microenvironment on particular species of host.

The astigmatid mite, Psoroptes ovis (Hering) (Acari: Psoroptidae), is an obligate, non-burrowing ectoparasite of vertebrates, of particular economic importance in domestic sheep flocks where it causes clinical psoroptic mange. To help understand the behaviour which facilitates transmission via the environment, the responses of P. ovis derived from rabbits (syn. Psoroptes cuniculi) to temperature and light were examined in the laboratory. On a vertical surface of uniform temperature, the presence and direction of illumination had a significant effect on the distance and direction moved by the mites. In darkness or with illumination from both above and below, the mites moved relatively little, but this movement was upwards. In contrast, with illumination from above only, mites moved downwards. When the direction of the illumination was reversed so that it came from below only, the mites moved upwards. On a vertical surface with a temperature gradient, in darkness or with illumination from both above and below, the mites moved up or down towards the area of highest temperature, depending on whether this was above or below, respectively. However, the movement of the mites in response to the temperature gradient was strongly displaced up or down by the presence of unidirectional illumination from above or below, respectively. The results indicate that the movement of these mites is strongly directed towards areas of high temperature but away from higher light intensity. These behaviours might be expected to maintain the position of the mites on a host animal and help them locate the skin surface of a new host when displaced into the environment.