Abstract
The effects of different types of exposure upon the host-parasite relationship of O. columbianum in young lambs is considered. Most histotropic larvae leave the mucosa of the small intestine between the 5th and 9th days following initial infections but are retained longer if there has been prior infection.
The main lesion of the disease is associated with vast accumulations of eosinophils around histotropic 4th stage larvae, or in tunnels left by the larvae as they move through the various tissues of the intestine. Necrotic masses of eosinophils eventually become encapsulated to form the characteristic nodules.
Nodular lesions are much more pronounced in the small intestines if there has been prior exposure. Host response in the large intestine is similar regardless of prior infection. Likewise, histotropic larvae enter and remain for weeks or months in the wall of the large intestine regardless of prior exposure.