EXPERIMENTALLY INDUCED CORONAVIRUS INFECTIONS IN CALVES - VIRAL REPLICATION IN THE RESPIRATORY AND INTESTINAL TRACTS

Abstract

Eleven 3- to 50-day-old colostrum-deprived gnotobiotic calves and seven 25- to 63-day-old colostrum-deprived conventional calves were allotted into 3 groups. Each group was inoculated with a fecal isolate of bovine coronavirus via different routes: orally/intranasally OR/IN, No. 1 through 8, group 1 calves; OR, No. 9 through 13, group 2 calves; IN, No. 14 through 18, group 3 calves. Nasal swab specimens and fecal specimens were collected daily and were examined for coronavirus antigen by use of direct immunofluorescent staining (nasal epithelial cells) or by use of immune electron microscopy (fecal specimens). All but 4 calves (No. 11, 13, 17, and 18) were euthanatized on postinoculation days (PID) 3 to 7. Calves 11 and 17 became severely dehydrated and died at PID 5. Calves 13 and 18 were evaluated for nasal and fecal shedding of coronavirus through PID 14. Distribution of coronavirus antigen in the respiratory and intestinal tracts of the 14 euthanatized calves was evaluated by use of direct immunofluorescent staining. All calves developed profuse diarrhea by PID 2 to 4; however, calves did not develop clinical signs of respiratory tract disease before euthanasia or death. Inoculated calves shed coronavirus in their feces as detected by use of immune electron microscopy. Infected nasal epithelial cells were detected in all but 2 orally inoculated calves (No. 9 and 10). Route of inoculated influenced the sequence of initial detection of coronavirus antigen from fecal specimens or nasal swab specimens. Shedding of coronavirus in the feces and nasal passages was concurrent for all but 1 calf inoculated OR/IN, whereas detection of fecal shedding preceded nasal shedding in all but 1 OR inoculated calf (No. 12). The converse was true for most IN inoculated calves. Nasal and fecal shedding of coronavirus persisted through at least PID 6 in the 1 calf euthanatized at PID 7 (No. 8) and in the 2 calves examined through PID 14 (No. 13 and 18). Of the 14 euthanatized calves examined, all had coronavirus antigen in the intestine, with most infected cells in the ileum and colon. Villous atrophy was evident in the ileum of all but 2 calves (No. 9 and 14). Coronavirus-infected cells were detected in upper respiratory tract tissue, including the nasal turbinates or trachea of all but 2 OR inoculated calves (No. 9 and 10). Four of 18 calves (2 OR/IN inoculated, 1 OR inoculated, and 1 IN inoculated) had coronavirus antigen in their lung tissue: 2 of these 4 calves (No. 1 and 12) had focal interstitial emphysema in the lung tissue and had higher than normal rectal temperatures. Results of the present study indicated that oral or intranasal inoculation of calves with bovine coronavirus can induce respiratory tract and enteric infections in calves.