Pathophysiology of single and challenge infections of Haemonchus contortus in Merino sheep: Studies on red cell kinetics and the “self-cure” phenomenon

The alterations in red cell kinetics and iron metabolism occuring in sheep infected with 10,000 Haemonchus contortus larvae and similarly reinfected 7–8 weeks later were monitored by radio-isotopic methods and the results obtained correlated with the clinical course of anaemia and development of the parasite populations. It is suggested that there are essentially three stages in the development of anaemia in infected sheep. The first, occurring during the initial 3 weeks, and characterized by a progressive fall in PCV, negative or low faecal egg count, and normal serum iron level, is attributable both to the blood-sucking activities of developing larvae and young adults and to a latency in the response of the host's erythropoietic system. The second stage, which may not, at least initially, involve further reduction in PCV is nonetheless accompanied by continuous abomasal haemorrhage and marked stimulation of erythropoiesis achieved at the expense of the animal's iron stores. The final stage, characterized by a dramatic reduction in PCV and serum iron concentration, signals exhaustion of the host's synthetic machinery due to frank deficiencies of iron and possibly available protein. The nature and timing of the underlying changes in worm activity following re-infection add further support to the concept that experimental ‘self-cure’ is immunological in origin.