Experimental contact of bighorn sheep (Ovis canadensis) with horses and cattle, and comparison of neutrophil sensitivity to Pasteurella haemolytica cytotoxins.

This research paper investigates the vulnerability of neutrophils in various animal species—horses, cattle, and Rocky Mountain bighorn sheep—to cytotoxins produced by different strains of Pasteurella haemolytica bacteria. The results suggest that interactions between horses and bighorn sheep may not lead to harmful Pasteurella haemolytica-induced pneumonia, contradicting previous assumptions.

Research Context and Methodology

• The study was designed with the aim of understanding the susceptibility of neutrophils (a type of white blood cell) in different animal species to the cytotoxins produced by biotypes A and T of Pasteurella haemolytica, a bacteria that is often associated with respiratory diseases in animals.
• Neutrophils from horses, cattle, and Rocky Mountain bighorn sheep were exposed to different biotypes and serotypes of this bacteria. The susceptibility was determined by measuring cytotoxin-dependent lysis, where the neutrophils would be killed by the bacteria’s toxin.
• Additionally, in a comparative field study, bighorn sheep were pastured with horses and cattle separately to study the cross-species transmission of P. haemolytica.

Study Findings

• According to the in vitro study, all isolates of P. haemolytica were found noncytotoxic to horse neutrophils which means they posed no harm to horse neutrophils.
• P. haemolytica was cytotoxic for bighorn sheep neutrophils in 13 out of 18 cases and for cattle neutrophils in 4 out of 10 cases.
• In the field study where bighorn sheep were co-pastured first with horses and then with cattle, P. haemolytica biotype A was found in the cattle but not in the bighorn sheep or horses; instead, P. haemolytica biotype T was isolated from the bighorn sheep, which was noncytotoxic.
• One bighorn sheep died in each of the horse and cattle co-pasturing experiments, but P. haemolytica could not be isolated from the bighorn sheep that died in the horse contact experiment.
• Moreover, a noncytotoxic P. haemolytica biotype A strain was isolated at the necropsy from the bighorn sheep that died in contact with cattle.

Conclusions

• The research concludes that the association between bighorn sheep and horses may not likely lead to a harmful pneumonia condition due to infection by P. haemolytica, contrary to earlier beliefs. This finding may have crucial implications for managing cross-species contacts in areas where these species co-exist.