1. In primary infections of chicks with 300 or 600 H. gallinae infective eggs, no clinical signs were apparent. Nodules were absent in all the experimental chicks as well as controls. Microscopically, few juveniles were observed in the caecal mucosa and the usual changes were mild congestion, few haemorrhages, and desquamation of superficial lining epithelial cells. Sections through a few thickened areas revealed a breached muscularis mucosa through which proliferated mucosal tissue had just invaginated into submucosa. The liver showed no changes.
2. With primary infection of chicks with 300 or 600 infective H. gallinae eggs, and their subsequent infection after 25 days with 1,000 infective eggs, there were no clinical signs after either primary or secondary infections. Nodules of varying sizes were present on the caecal wall of infected birds only, their size and number varying with age of infection. Histopathologically, the types of nodules observed were, one associated with juveniles and the other with their absence; and nodules were present both in the invaginated mucosa and in the submucosa. It was confirmed that these caecal nodules were due to irritation by H. gallinae juveniles and their metabbolites, and that these nodules were more a reaction of already sensitised caeca to subsequent infections, a fact hitherto unreported. The liver showed nothing abnormal, and there was not much difference between the haematological values of experimental and control chicks.
3. The actual process of formation of caecal nodules is demonstrated for the first time. The feasible explanation seems to be that on irritation of the already sensitised caeca by H. gallinae juveniles and their metabolites, hyperplasia of lymphoid tissue and epithelial cells results, which leads to a breach of the muscularis mucosa through which proliferated tissue of the mucosa invaginates. Later the muscularis mucosa constricts off and closes the nodule, the mucosa over it regenerates and becomes continuous. The present observations also indicate that the sterile nodular lesions usually found in natural infections may be the result of frequent infections, thus sensitising the chicks to subsequent infections.