Mucosa, host

Hookworm infection is caused by Ancylostoma duodenale or Necator americanus. N. americanus is found in the southeastern United States. Infective larvae enter the host in contaminated food or water or penetrate the skin and migrate to the small intestine. The adult worm attaches to gastrointestinal mucosa and causes injury by lytic destruction of the tissue. Over a period of time, the adult worm can cause anemia and hypopro-teinemia in the host. 

The protozoan Entamoeba histolytica causes amebiasis, an infection that is endemic in parts of the United States. The parasite can be present in the host as either an encysted or a trophozoite form. Initial ingestion of the cyst may result either in no symptoms or in severe amebic dysentery characterized by the frequent passage of bloodstained stools. The latter symptom occurs after invasion of the intestinal mucosa by the actively motile and phagocytic trophozoite form of the protozoan. 

Taeniid cestodes have a prey-predator life cycle involving two mammalian hosts. The definitive host is a carnivore or omnivore, which harbours the adult tapeworm parasite in the small intestine. Mature infective eggs are released with the faeces and, when these are ingested by a suitable species of intermediate host, the oncosphere contained within the egg is liberated through the influence of intestinal secretions, particularly bile. The activated parasite penetrates the mucosa of the small intestine... 

In heavily infected fish, the cestodes were so closely packed that individuals were (remarkably ) reported as being partially or totally fused together These zones of fusion gave strong histochemical reactions for -SH groups and appeared to be areas of intense metabolic activity. This suggested (750) that, in heavy infections, parasites which fail to find room for attachment on the host mucosa may derive their nutriment from another individual worm. This unusual observation clearly requires confirmation. 

HOST-PARASITE SPATIAL RELATIONSHIP The surface topography of intestinal and tissue sites is usually elaborate and difficult to reproduce in vitro, as are the spatial relationships between the adult worms and the mucosa. These relationships have been shown to be especially important in some species (e.g. Schistocephalus, Ligula, Echinococcus), for, if they are not reproduced within reasonable limits in vitro, insemination may not take place (See Fig. 10.2). 

Befus, A. D. Bienenstock, J. (1982). Factors involved in symbiosis and host resistance at the mucosa-parasite interface. Progress in Allergy, 31 76-177. 

Once Salmonella invade the intestinal barrier, the bacteria encounter host immune cells such as neutrophils, lymphocytes and macrophages, which engulf the bacteria (Fields et al., 1986 McCormick et al., 1993) (Fig. 5.1). Salmonella serotypes capable of causing gastroenteritis are killed by host macrophages, and effectively cleared by the immune system. Invasion of M cells and intestinal epithelial cells induces a pro-inflammatory response in the intestinal mucosa (Fig. 5.1a). A combination of inflammatory cytokine production, neutrophil infiltration and fluid and electrolyte secretion by the epithelium results in the diarrhea associated with gastrointestinal enteritis (Fig. 5.1a). 

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