The Importance of Soil as a Reservoir of Gastrointestinal Nematode Larvae of Sheep

Abstract

Several studies have shown that a significant proportion of infective L3 of 'Trichostrongylus colubriformis' (Tc) and 'Haemonchus contortus' (Hc) are found in soil (Rees 1950; Silangwa and Todd 1964; Rose and Small 1985; Stromberg 1997). We aimed to determine whether such larvae migrate on to pasture and thus form a true reservoir. A 2 x 3 x 2 x 6 factorial experiment with two replicates was conducted over a five-week period using 120 pots growing Italian rye grass ('Lolium multiflorum') in climate chambers. Pots contained either previously autoclaved or non-autoclaved soil and were inoculated with a solution containing 10,000 L3, of 50:50 Tc and Hc. Larvae were deposited on the soil surface or 0.5cm and 2.5cm below it at 10 sites in a volume of 0.1 ml per site using a 23-gauge needle. Total recovery of Tc and Hc L3 from pasture, the top 5cm of soil and the soil below 5cm was determined weekly. L3 migrated effectively from all soil depths onto pasture. There was no significant effect of L3 inoculation depth on larval recovery (P = 0.9). Significantly higher numbers of L3 were recovered from normal than from autoclaved soil (10.8% and 4.1% respectively, P ≤ 0.0001). The highest recoveries overall were from the top 5cm of the soil, and significantly higher numbers of Tc were recovered than Hc (13.5% and 1.4% respectively, P ≥ 0.0001). More L3 were recovered from the entire soil strata than those recovered from pasture over the duration of the experiment, which supports previous findings that suggest that the L3 utilise a sit-and-wait migration strategy, utilising a soil phase(Rose and Small 1985; Lettini and Sukhdeo 2006; Amaradasa, Lane et al. 2010).