Synergistic effects of concurrent challenge with bovine respiratory syncytial virus and 3-methylindole in calves.
OBJECTIVE
To evaluate the potential synergy between bovine respiratory syncytial virus (BRSV) and 3-methylindole (3MI) in inducing respiratory disease in cattle.


METHODS
20 mixed-breed beef calves.


METHODS
A 2 X 2 factorial design was used, with random assignment to the following 4 treatment groups: unchallenged control, BRSV challenge exposure (5 X 10(4) TCID50 by aerosolization and 5.5 X 10(5) TCID50 by intratracheal inoculation), 3MI challenge exposure (0.1 g/kg of body weight, PO), and combined BRSV-3MI challenge exposure. Clinical examinations were performed daily. Serum 3MI concentrations, WBC counts, PCV, total plasma protein, and fibrinogen concentrations were determined throughout the experiment. Surviving cattle were euthanatized 7 days after challenge exposure. Pulmonary lesions were evaluated at postmortem examination.


RESULTS
Clinical respiratory disease was more acute and severe in cattle in the BRSV-3MI challenge-exposure group than in cattle in the other groups. All 5 cattle in this group and 3 of 5 cattle treated with 3MI alone died or were euthanatized prior to termination of the experiment. Mean lung displacement volume was greatest in the BRSV-3MI challenge-exposure group. Gross and histologic examination revealed that pulmonary lesions were also most severe for cattle in this group.


CONCLUSIONS
Feedlot cattle are commonly infected with BRSV, and 3MI is produced by microflora in the rumen of all cattle. Our results suggest that there is a synergy between BRSV and 3MI. Thus, controlling combined exposure may be important in preventing respiratory disease in feedlot cattle.
D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000473:pathology / Q000503:physiopathology*; D005163:Factor Analysis, Statistical; D008168:Lung / Q000187:drug effects / Q000473:pathology / Q000503:physiopathology*; D009929:Organ Size / Q000187:drug effects; D015656:Respiratory Mechanics / Q000187:drug effects; D018357:Respiratory Syncytial Virus Infections / Q000473:pathology / Q000503:physiopathology / Q000662:veterinary*; D018112:Respiratory Syncytial Virus, Bovine; D012862:Skatole / Q000097:blood / Q000493:pharmacokinetics / Q000633:toxicity*