The epidemiology of gastrointestinal nematodes in summer rainfall and winter rainfall and even the uniform rainfall area has been fairly well described.
This paper is a belated report on trial work done many years ago to try to gain some understanding of the epidemiology of gastrointestinal nematodes in sheep grazing pastures irrigated through the summer in the Riverina.
Merino ram lambs born in May/June were run on white clover dominated pastures which were irrigated with 100mm of water every 10 days through the summer (November-March). The stocking rate was 20/hectare but they were rotated over 3 paddocks with weekly moves. This left the pasture in each paddock very short when the sheep were removed at the end of each rotation.
The ram lambs were removed from the paddocks in May 1987 because of heavy parasite burdens although the tracer lambs continued to be grazed.
In December 1987 the paddocks were fully stocked again with ram lambs born in May/June 1987.
In both years the lambs were drenched during the summer (December and March).
The anthelmintic used for all treatments was levamisole which had been 100% effective in a faecal egg count reduction trial.
Tracer lambs were run with the ram lambs during the periods indicated in Table 1, initially for 2 weeks but later for 4 weeks because of concerns that they might take a longer time to begin grazing.
The tracers were wether lambs which had been running on extensive rangeland and so assumed to have little prior exposure to nematodes. They were drenched prior to being run with the ram lamb mob. At the end of their grazing period they were removed and placed in a woolshed with a slatted floor for 4 weeks prior to being slaughtered and their gastrointestinal tracts being removed for total worm counts.
The faecal egg counts and total worm counts for the tracer lambs are shown in Table 1 below.
Other species of nematode were not detected.
For the purpose of analysis the larval pickup through the tracer period was assumed to be consistent and the daily pickup was calculated.
Chart 1 shows the mean daily larval pickup for each month that tracers were grazing in either year.
Chart 2 shows the mean daily larval pickup of Teladorsagia plotted against the mean daily temperature of the month.
Chart 3 shows the mean daily larval pickup of Nematodirus plotted against mean daily temperature.
The absence of the genera often seen in summer rainfall areas (Haemonchus and Trichostrongylus) is consistent with the author's experience. This is despite irrigation equivalent to 300mm of rainfall per month during the warmer months.
There is an obvious temperature effect on the pickup of Teladorsagia larvae in this trial. There appears to be two threshold mean daily temperatures: 20°C above which there is hardly any activity and 12.5°C below which the already high pickup becomes extreme. Whether there is a temperature below which larval pickup falls again cannot be tested in the environment.
The cause of this temperature effect is not clear, possibilities are:
Whether species better adapted to warm conditions, such as Haemonchus contortus, display similar behaviour warrants further investigation.
The absence of a clear temperature response in Nematodirus larval pickup has several possible explanations.
Nematodirus spp do not produce free-living larvae but rather the larvae develop to the infectious stage within the egg. This makes the larvae much more resistant to dessication. It also means that the larvae do not become active and exhaust their energy reserves under conditions of adequate moisture with warm temperatures.
Infective Nematodirus larvae are not motile and so do not move up plants away from the ground. This makes it necessary for animals to be grazing close to the ground to acquire infection. The rapid growth of the pasture limited the opportunities for infection with Nematodirus even though the pasture was usually quite short at the end of each rotation period. The tracers also picked up heavy loads of Nematodirus in November/December 1987 on paddocks which had not been grazed for months and where the pasture was thick and tall. This is an apparent contradicition with no ready explanation.
This project indicated that the parasite risk in irrigated summer pasture under the grazing regime used was in the autumn rather than the summer. The solution to this risk was to move the young sheep to annual irrigated pasture or to dryland pasture in the autumn.