Eric R. Morgan

Parasites are generally overdispersed among their hosts, with far-reaching implications for their population dynamics and control. The factors determining parasite overdispersion have long been debated. In particular, stochastic parasite acquisition and individual host variation in density-dependent regulation through acquired host immunity have been identified as key factors, but their relative roles and possible interactions have seen little empirical exploration in parasite populations. Here, Taylor’s power law is applied to test the hypothesis that periodic parasite removal destabilises the host-parasite relationship and increases variance in parasite burden around the mean. The slope of the power relationship was compared by analysis of covariance among 325 nematode populations in wild and domestic ruminants, exploiting that domestic ruminants are often routinely treated against parasite infections. In Haemonchus spp. and Trichostrongylus axei in domestic livestock, the slope increased with the frequency of anthelmintic treatment, supporting this hypothesis. In Nematodirus spp., against which acquired immunity is known to be strong, the slope was significantly greater in post-mortem worm burden data than in faecal egg counts, while this relationship did not hold for the less immunogenic genus Marshallagia. Considered together, these findings suggest that immunity acting through an exposure-dependent reduction in parasite fecundity stabilises variance in faecal egg counts, reducing overdispersion, and that periodic anthelmintic treatment interferes with this process and increases overdispersion. The results have implications for the diagnosis and control of parasitic infections in domestic animals, which are complicated by overdispersion, and for our understanding of parasite distribution in free-living wildlife. Parasite-host systems, in which treatment and immunity effectively mimic metapopulation processes of patch extinction and density dependence, could also yield general insights into the spatio-temporal stability of animal distributions.

Background Toxocarosis is a globally distributed zoonotic disease, but sources of infection are not well documented over large geographical scales. To determine levels of environmental contamination, soil from 142 parks and recreational areas across the UK and Ireland was assessed for the presence of Toxocara . Methods Toxocara ova (eggs) were isolated from soil samples by sieving and flotation and then enumerated. Individual eggs were isolated and imaged, and a subset was characterised by species‐specific PCR and Sanger sequencing. Results Characteristic Toxocara ‐type eggs were found in 86.6% of parks, with an average of 2.1 eggs per 50 g of topsoil. Representative eggs were confirmed as Toxocara canis by Sanger sequencing, with many eggs containing developed larvae, hence being viable and potentially infective. Positive samples were more common, and egg density was higher, in parks with greater perceived levels of dog fouling. Limitations Samples were collected at a single timepoint and with limited spatial mapping within parks. Further study is needed to discern spatiotemporal differences within parks and recreational areas. Conclusion Toxocara is widespread in soil in public parks, indicating a need for further efforts to reduce egg shedding from pet dogs. Standardised methods and large‐scale surveys are required to evaluate risk factors for egg presence and the impact of interventions.