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Etiologic investigations of infectious diarrhea were long limited to bacteria and protozoa. The advent of electron microscopy and molecular biology showed that diarrhea could also be caused by viruses, both in humans and in other animals. In 1969, electron microcopy was used to show, for the first time, the responsibility of a virus in a case of calf diarrhea. This "reo-like virus "was subsequently identified as a rotavirus, and was shown only four years later to be responsible for severe diarrhea in young children. Noroviruses, and particularly the human virus Norwalk, were subsequently discovered, followed by coronavirus, sapovirus, pestivirus, astrovirus, enteric adenoviruses, torovirus, and picobirnavirus. Some of viruses found in animals, and particularly rotaviruses, can also infect humans. Rotaviruses have been identified in numerous animal species and are generally host-specific, but zoonotic transmission has been suggested by cross-infection (especially in experimental models), by genetic studies showing a close relationship between certain human and animal rotaviruses, and by the discovery of new animal genotypes during epidemiological surveillance of human rotaviroses. Some animal strains of norovirus, sapovirus, picobirnavirus and astrovirus are genetically related to human strains, but their human transmission has not been demonstrated.

In order to ascertain the importance of Campylobacter spp., C.difficile, C.perfringens and Salmonella as agents of bacterial gastroenteritis in dogs, two groups of animals were studied prospectively. The first group consisted of 77 puppies in 14 litters, with fecal cultures performed weekly for 10 weeks, starting at birth. The second group consisted of a kennel population with every dog cultured at entry, and at two-month intervals thereafter. Incidence of Campylobacter spp. was 32 and 31 per 100 dog-month of observation for healthy pups and healthy adult dogs respectively, 46 and 0 for C.difficile, 51 and 36 for C.perfringens and 6.5 and 1.3 for Salmonella. The incidence of Campylobacter spp. in pups peaked at 8 weeks of age. This incidence (43 per 100 dog-months) was higher in pups reared together with older dogs than in pups reared without contact to other dogs (0 per 100 dog-months). Toxigenic strains of C.difficile were found in 61.5% of the healthy neonate dogs. None of the cases of non-watery and non-inflammatory diarrhea we observed was associated with any of the pathogens studied. Furthermore newly acquired colonization with Campylobacter spp. or Salmonella was never associated with episodes of diarrhea. No conclusions could be drawn about the role of bacterial pathogens for causation of watery or inflammatory diarrhea which were not observed in our study.