Study finds leukotoxins are main BRD concern

Cattle Market & Farm Reports, Editorials
Dec 20, 2007
by WLJ
November 26, 2007

A new study conducted under feedlot conditions demonstrates that, despite improvements made in the quality of commercial vaccines against the bacterium Mannheimia haemolytica, those that don’t include a component to specifically stimulate protection against leukotoxin risk failure.

Leukotoxin is a natural byproduct produced by infection with M. haemolytica. Believed to be only one of the pathogenic factors M. haemolytica produces—but the most important—leukotoxin destroys the calf’s white blood cells, preventing them from fighting the infection and the damaging inflammatory process in the lung. Adequate protection against the effects of leukotoxin is so important, for instance, that mutant M. haemolytica which do not naturally produce the toxin have been shown to cause little or no disease.

The study, conducted by the independent Agri Research Center in Canyon, TX, randomly vaccinated 20 newly weaned and acclimated 352-pound-average heifers and steers against M. haemolytica, the most common bacterial cause of bovine respiratory disease (BRD), using one of two commercial vaccines. One contained a leukotoxoid to stimulate protection against leukotoxin (PULMO-GUARD PHM-1, from Boehringer Ingelheim Vetmedica) and one did not (Once PMH SQ, from Intervet). An additional 10 calves remained unvaccinated as controls.

Following challenge by direct introduction of M. haemolytica into the respiratory tract 28 days following vaccination, the Agri Research Center researchers found:

• Calves vaccinated with the leukotoxoid-containing vaccine had average temperatures significantly lower than either those vaccinated with the non-leukotoxoid vaccine or those not vaccinated at all.

• Calves vaccinated with the leukotoxoid-containing vaccine had significantly lower lung damage scores than either the non-leukotoxoid vaccinated or the control calves.
The study showed no statistically significant difference in scores indicating clinical disease, which included appetite, respiration rate, and eye and nasal discharge.

“Producers and veterinarians who were in practice 20 years ago may remember the problems associated with many of the whole-cell Mannheimia bacterins that didn’t include protection against leukotoxins,” observes Dudley Smith, DVM, professional services veterinarian for Boehringer Ingelheim Vetmedica. “And today’s manufacturers have made a lot of progress in improving vaccines compared to those first-generation products.