Role of Pasteurella granulomatis and Dermatobia hominis in the etiology of lechiguana in cattle.

Attempts were made to reproduce bovine lechiguana, a disease associated with Dermatobia hominis and Pasteurella granulomatis infections. Suspensions of Pasteurella granulomatis were mixed with each of the following: saponin, oil adjuvant, ground Dermatobia hominis, or 5% mucin. Each preparation was inoculated into 6 cattle. Twelve more cattle, 6 of which received dexamethasone, were inoculated with bacterial suspension alone. Abscesses but no lechiguana was produced in all 36 cattle. After abscess regression, 12 cattle were reinoculated with a suspension of mouse-passed P. granulomatis. Only abscesses were produced. The intralymphatic inoculation of P. granulomatis in 6 cattle did not produce the disease. Eleven cattle infected naturally with D. hominis had lesions containing dead larvae. These lesions were inoculated with P. granulomatis. Nine cattle were experimentally infected with larvae of D. hominis that had been contaminated with the bacteria. No lechiguana lesions were produced in these 20 cattle. Six cattle with severe natural D. hominis infection were inoculated in the larval lesions with P. granulomatis. One developed lesions indistinguishable from those of natural lechiguana. The lesions regressed after treatment with chloramphenicol. D. hominis larvae and exudate from lesions caused by the fly were collected from 7 cattle on 3 farms and examined bacteriologically. P. granulomatis was isolated from the larvae and the exudate of a healthy calf from a farm where lechiguana had never been observed. These results suggest that P. granulomatis has a causal role in lechiguana, and that D. hominis may be a carrier of the bacterium. These observations suggest that lechiguana occurs when severe D. hominis lesions are infected with P. granulomatis. The apparent long incubation period, the negative results obtained in the other experiments, and also the infrequent occurrence of the natural disease suggest that lechiguana is a disease for which Koch's postulates are not easily fulfilled.