The Alimentary System

There are four major categories of diarrheagenic Escherichia coli: enterotoxigenic (a major cause of travelers' diarrhea and infant diarrhea in less-developed countries), enteroinvasive (a cause of dysentery), enteropathogenic (an important cause of infant diarrhea), and enterohemorrhagic (a cause of hemorrhagic colitis and hemolytic uremic syndrome). Besides manifesting distinct clinical patterns, these categories of E. coli differ in their epidemiology and pathogenesis and in their O:H serotypes. Common features (albeit distinct for each category) include plasmid-encoded virulence properties, characteristic interactions with intestinal mucosa, and elaboration of various types of enterotoxins or cytotoxins. A less-well-defined fifth category of diarrheagenic E. coli is that of enteroadherent E. coli, so far identifiable only by their pattern of adherence to Hep-2 cells in tissue culture.

Three strains of enteropathogenic Escherichia coli (EPEC), originally isolated from humans and previously shown to cause diarrhea in human volunteers by unknown mechanisms, and one rabbit EPEC strain were shown to attach intimately to and efface microvilli and cytoplasm from intestinal epithelial cells in both the pig and rabbit intestine. The attaching and effacing activities of these EPEC were demonstrable by light microscopic examination of routine histological sections and by transmission electron microscopy. It was suggested that intact colostrum-deprived newborn pigs and ligated intestinal loops in pigs and rabbits may be useful systems to detect EPEC that have attaching and effacing activities and for studying the pathogenesis of such infections. The lesions (attachment and effacement) produced by EPEC in these systems were multifocal, with considerable animal-to-animal variation in response to the same strain of EPEC. The EPEC strains also varied in the frequency and extent of lesion production. For example, three human EPEC strains usually caused extensive lesions in rabbit intestinal loops, whereas two other human EPEC strains usually did not produce lesions in this system.

Histologic sections and case histories from 23 dogs with proven fatal toxoplasmosis-like illness at the Angell Memorial Animal Hospital were reviewed. Toxoplasma gondii was identified in 13 dogs. A newly identified parasite, Neospora caninum, structurally distinct from T gondii, was found in 10 dogs. The newly discovered organism, belonging to a new genus and new species, formed meronts in many tissues of the dogs, especially the brain and spinal cord. Neospora caninum was located directly in the host cell cytoplasm without a parasitophorous vacuole; it divided by endodyogeny, contained more than 11 rhoptries, and did not react with the anti-T gondii serum in the immunoperoxidase test. Meningoencephalomyelitis and myositis were the main lesions associated with N caninum. Ulcerative and fistulous dermatitis was the major lesion in 1 dog.