Analysis of the Sheep, Haemonchus Contortus Relationship (Project Management & Research Organisation)

Summary

Gastrointestinal nematodes cost the Australian sheep industry $369 million annually or 8.7% of its total value (Sackett, et al. 2006). Barbers pole worm (Haemonchus contortus) is a blood feeding nematode that parasitises the abomasum. It is one of the three most important sheep roundworms in Australia (along with Trichostrongylus and Ostertagia) and represents a serious constraint to sheep production in regions with summer dominant rainfall such as northern NSW, south-eastern Queensland and the coastal regions of south-west WA. Its high fecundity leads to rapid contamination of pastures with larvae and high levels of ingestion cause acute death. Chronic blood loss caused by lighter Haemonchus burdens can result in anaemia, anorexia, reduction in body weight and wool growth, depression and death. The costs of haemonchosis are related to reduced productivity, prophylaxis, treatment and sheep deaths. The continuing rise of anthelmintic resistance is a severe threat to the profitability of the Australian sheep industry.
To develop improved and/or alternative methods for controlling Haemonchus such as genetic selection of resistant sheep, a better understanding of the host parasite relationship is essential. This is a dynamic relationship and altering it in favour of the host could help ameliorate parasitism.The objectives of this project were;
To describe parasite population dynamics and the host response to infection.
To develop a method for serial sampling of worms and abomasal tissue from the same animal during an Haemonchus infection.
To identify parasitological and molecular events associated with a switch to inhibition of larval establishment.
To determine the morphological and molecular effects of host immunity on Haemonchus.
To examine the influence of diet and immunosuppression on sheep immunity to Haemonchus.

The first step in this project was to understand the changes in parasite levels during an infection. For this reason, the first experiment examined the establishment of Haemonchus populations and the immune response by comparing sheep given a bolus dose of larvae with sheep given trickle doses of larvae prior to the bolus dose.
Experiment 2A used staggered doses of ivermectin-resistant larvae as a marker to examine the timing of the immune response on the establishment of larvae. Permanent abomasal fistulas were implanted into 18 sheep and serial sampling of abomasal mucosa and Haemonchus was undertaken through these fistulas to enable cellular and molecular events to be observed in the same animal over time. The effect of sheep immunity on adult Haemonchus fecundity was examined in Experiment 2B.
Experiment 3 repeated important time points identified in Experiment 2A to allow larger amounts of post-mortem material to be collected. In the final trial, sheep immunity to Haemonchus was observed under corticosteroid-induced immunosuppression and a reduced protein diet (Experiment4).
These experiments successfully generated pathological symptoms of haemonchosis such as anaemia, and detected an immune response to the worms. Immunohistochemistry results showed general trends which are consistent with a Th2 (humoral) immune response. A Th2 response is associated with reduced nematode burdens in several species. In a parallel study in SheepGenomics, 41 microarrays have been performed using RNA from serial biopsies of abomasal mucosa in live sheep and abomasal mucosa collected at post-mortem. These will help to identify genes involved in immunity to Haemonchus which will assist in breeding sheep which are resistant to infection.
The use of image analysis software to measure the area and perimeter of worms collected at post mortem has been highly effective. The most statistically significant reductions were measured from worms in trials where larval challenge continued closest to the slaughter date, suggesting that the presence of worms or recent worm exposure is required to maintain the immune response.
A unique finding was a correlation between adult worm size and egg content. This leads to the conclusion that the immune regulation of control of egg production is manifest through the regulation of worm size and presumably growth which will help us to identify a mechanism. Differential gene expression in Haemonchus has been demonstrated via differential display PCR using worms collected from immune and nonimmune sheep. Twenty-five differentially expressed bands have been identified to date. Cloning and sequencing of such bands may lead to the identification of proteins which can be targeted for vaccination or chemical control.