Survival of chicken ascarid eggs exposed to different soil types and fungi

The eggs of intestinal ascarid parasites (Ascaridia galli and Heterakis spp.) of chickens can survive long-term in soil and this makes contaminated yards and pastures infective to chickens for years. The fungi Pochonia chlamydosporia Biotype 10 and Metarhizium brunneum KVL04-57 can kill ascarid eggs in agar assays but their efficacy against these eggs in soil is unknown. We therefore initially tested the ovicidal effect of the two fungi in laboratory soil assays. Unembryonated eggs were added to sterilised and non-sterilised soil with or without fungi, and egg recovery was examined before and after incubation (22°C, 30days). Egg recovery was substantially reduced by P. chlamydosporia and M. brunneum in sterilised soil. However, in non-sterilised soil only M. brunneum slightly reduced egg counts. Notably, egg recovery was reduced markedly in non-sterilised soil though no fungi were applied. To test if this is a general characteristic of natural soil, eggs were incubated in similar assays comprising 7 different soils (sterilised and non-sterilised). After incubation, egg recoveries were reduced substantially in all non-sterilised soils whereas there were only minor reductions in sterilised soils. We next isolated and examined if particular egg-degrading fungi occurred in the non-sterilised soil that had exhibited the most extreme ovicidal activity (99%). The fungal isolates belonged to two genera (Metarhizium and Acremonium) with reported egg-degrading properties but when tested in vitro in agar assay, none of the three tested isolates of native fungi degraded >34% eggs, suggesting that these fungi may not individually have caused the reduction in egg survival in the non-sterilised soil. In conclusion, a range of natural soils showed inherent properties related to biotic factors to degrade nematode eggs. The biocontrol efficacy of both P. chlamydosporia Biotype 10 and M. brunneum KVL04-57 was very good in sterilised soil but currently limited in non-sterilised soil, potentially due to competition with natural soil biota in fungal establishment. Further research may determine if efficacy may be increased by changing the formulation.