Seasonal pasture contamination and availability of nematodes for grazing sheep

Abstract

In December 1970 and in January, February, March, April, May and September 1971 separate plots of sown pasture, each 0.1 ha, were contaminated by grazing sheep infected with gastrointestinal nematodes. In succeeding months each plot was grazed by worm-free tracer lambs for 2 weeks; the lambs were then withdrawn and held for 2 weeks in pens before slaughter for total differential worm counts. Observations on each plot continued for 12 months; the numbers of worms found in the tracer lambs indicated the seasonal occurrence of nematode larvae on pasture. For Haemonchus contortus, larval availability from deposition was rapid in summer and slow in autumn, maximum inhibition at the fourth larval stage occurring in larvae picked up in the winter months. Ostertagia spp. presented a marked contrast, with curtailed development in summer and contamination in autumn producing high levels of infection on pasture in late winter and early spring when inhibition was at maximum levels. Of the other species studied, intestinal Trichostrongylus spp, showed a similar pattern of development to H. contortus in summer, but as with Ostertagia spp. autumn contamination could produce infection peaks in late winter and spring. Inhibition at the fourth larval stage was not a characteristic of intestinal Trichostrongylus spp. For T. axei autumn and winter conditions favoured development, and peak infestations occurred in spring and coincided with maximum inhibition. Nematodirus spp. developed mainly in summer and most inhibition occurred at this time. Spring (September) contamination with Nematodirus spp. did not result in detectable levels of infection. For all other species spring contamination was rapidly translated to pasture and the infection was comparatively short-lived. All species were capable of overwintering on pasture and with the possible exception of T. axei a persistence of infection of at least 12 months was demonstrated. For Ostertagia spp. the importance of late summer and autumn contamination in its epizootiology and control in a summer rainfall region has been confirmed.

Impact of gastrointestinal parasitic nematodes of sheep, and the role of advanced molecular tools for exploring epidemiology and drug resistance - an Australian perspective

“…In summer rainfall environments of Australia, infective L3 of Teladorsagia are most abundant on pastures grazed by spring-born lambs. Worm burdens follow closely the trend of larval availability, with peaks during summer, then declining during autumn to early winter [31]. …”

“…Infective larval populations on pastures peak during January and February decline from March to August and reach their lowest levels around September to November [31]. From the beginning of autumn, an increasing proportion of larvae arising from eggs deposited during the summer and autumn months accumulate as arrested larvae in sheep of all ages, to resume their activity in the following spring [10].…”

The integration of grazing management and summer treatments for the control of trichostrongylid infections in Merino weaners

“…It is known that in summers with above average rainfall, infective larvae are more available to sheep. 11,12 Therefore, contamination of pastures with worm eggs may be substantial in years with above average summer rainfall.…”

Grazing systems and worm control in sheep: a long-term case study involving three management systems with analysis of factors influencing faecal worm egg count

“…On the other hand, L 3 are comparatively resistant to the effects of cold, and survive frosts and cold temperatures in the Armidale region that inhibit hatching of eggs to L 3 (Southcott et al 1976;Bailey et al 2009a). Larval survival under these conditions is prolonged with Barger et al (1972) predicting L 3 population half-lives of 93 days at 12 C and 85% relative humidity (RH), and only 9 days at 28 C and 35% RH.…”