The effect of combined rotavirus and Escherichia coli infections in rabbits.

In rabbits, experimentally induced rotavirus infection results in soft feces only; thus it is unlikely that it is the sole cause of the severe, often fatal diarrhea of weanling rabbits with which it is associated. To determine whether rotavirus acts synergistically with another pathogen, New Zealand White rabbits (10 to 38 weeks old) were inoculated with rotavirus (L:ALA:84) and/or Escherichia coli 015:H-(RDEC-1) via orogastric tube. A single dose of high-titer (10(6) fluorescent focus-forming units) rotavirus was used, whereas E. coli was administered in various doses (10(2) to 10(9) CFU) to determine the titer of E. coli that induced only mild diarrhea but, when combined with rotavirus, resulted in diarrheal disease. Doses of E. coli > 10(6) CFU resulted in infection in almost all rabbits 10 to 16 weeks old, as detected by fecal shedding, regardless of whether rotavirus was inoculated simultaneously. However, inoculation of > 10(6) CFU of E. coli, in conjunction with rotavirus, resulted in increased morbidity and mortality due to diarrheal disease compared with E. coli alone. Inoculation of rabbits 28 to 38 weeks old with similar doses of rotavirus and E. coli caused infection but failed to induce diarrhea, indicating that older rabbits were more resistant to the pathogenic effects of these two agents. A synergistic effect between rotavirus and E. coli occurred, causing more severe diarrheal disease in weanling rabbits than that resulting from either pathogen alone.