David Renaud

Department of Population Medicine, University of Guelph, Guelph, ON

AHL Newsletter 2021;25(4):10.

It seems that an age-old question is: what bugs/pathogens are responsible for causing bovine respiratory disease (BRD) in dairy calves, especially in the fall?  To provide an answer about the pathogen profile causing BRD on farms that are having a respiratory disease outbreak or a challenge with chronically high levels of respiratory disease in their calves, deep nasopharyngeal swabs can be a practical on-farm tool.  These swabs will sample the respiratory and associated epithelium of the nasopharynx.  The collection of a meaningful sample can be done in a few minutes.  Contamination can be an issue when using these swabs which can compromise the clinical interpretation; however, using a guarded swab, such as a guarded mare uterine swab, will help minimize nasal contamination. 

To evaluate the pathogen profile responsible for causing BRD on several Ontario dairy farms, a descriptive study was undertaken on farms that were having challenges with respiratory disease.

During 2021, several different clinics and farms participated in this descriptive study.  In total, 68 animals from 14 different farms were sampled using deep nasopharyngeal swabs.  These farms mostly housed calves indoors in groups, and were either experiencing a respiratory disease outbreak at the time of sampling, or had a chronic-active situation.  Calves that were sampled were either sick or poor-doing, and had not been previously treated with antimicrobials or vaccinated within the preceding 2 weeks.  The age of the calves varied from 2 weeks to 6 months of age; however, the majority were under 6 weeks of age. 

The samples were submitted to the Animal Health Laboratory where they were cultured or underwent PCR testing for BHV-1, PI3, BRSV, bovine coronavirus BVDV and Mycoplasma bovis.

The bacterial results highlight the high levels of Pasteurella multocida isolated and positive    Mycoplasma bovis PCR tests from the calves (Fig. 1). 

Figure 1. Results of bacterial panel from 68 dairy calves on 14 dairy farms in Ontario that were experiencing a respiratory disease challenge.

The viral results demonstrate that overall, a small number of calves had a viral infection, and of these, a moderate number were infected with bovine coronavirus (Fig 2.).

Figure 2.  Results of viral PCR panel from 68 dairy calves on 14 dairy farms in Ontario that were experiencing a respiratory disease challenge.

On a herd level basis, most farms had a mixed population of pathogens isolated, including: Pasteurella multocida (10/14), bovine coronavirus (10/14), Mannheimia hemolytica (8/14), and Mycoplasma bovis (8/14).  With respect to the traditional viral pathogens, very few farms were positive for BRSV (1/14), PI3 (1/14), BHV-1 (0/14), or BVDV (0/14).

There are some interesting findings from the samples collected, but also some lessons learned.  We found that deep nasopharyngeal swabs can be useful to determine the pathogens present in a herd, especially when an appropriate calf necropsy is not available.  In addition, we found that the swabs are easy to use and only minor bleeding in the nostril was observed in some of the calves.

To request a comprehensive bovine respiratory disease panel at AHL, submit 2 deep nasopharyngeal swabs (normally guarded uterine swab) for each animal and order test code brsppnl.  

Note: separate samples are needed for bacteriology (gel media) and PCR (VTM or dry swab).  The swabs cannot be transferred into alternate media upon arrival at the lab.  More information on the AHL comprehensive bovine respiratory disease panel is available in the September 2021 AHL Newsletter: https://www.uoguelph.ca/ahl/comprehensive-bovine-respiratory-disease-panel [1].

Funding for this descriptive study was provided by Merck Animal Health.