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Diarrhea and respiratory problems are the two main diseases that cause calf mortality, both at birth and after birth. Diarrhea usually occurs in the first 30 days after birth, and respiratory problems in calves older than 30 days, although they can occur at any age. In the United States, respiratory diseases are responsible for 21.3% of deaths in preweaned dairy calves and 50.4% in weaned calves, according to 2023 reports. In addition to mortality, other negative consequences of respiratory diseases include: costs and labor spent on treatment, severe growth retardation, and decreased fertility.

Detection of Bovine Respiratory Disease (BRD)
Early detection of respiratory diseases may increase the likelihood of successful treatment and help prevent unnecessary and inappropriate use of antimicrobials (especially antibiotics). Standard reference methods for the detection of bovine respiratory disease (BRD) in dead animals include necropsy with histopathological, microbiological, molecular and biochemical tests.

Ultrasonography and chest radiography can be used to detect BRD in live cattle. All of these require the use of expensive equipment and trained personnel.

In view of the above, the major advantage of clinical grading systems is that they do not require expensive detection equipment or tests. The first BRD score was developed in 1977 as a means of classifying the severity of BRD in calves infected with bovine respiratory syncytial virus (BRSV).

In 2014, several researchers proposed different scoring systems for assessing BRD in weaned calves. None of these BRD scoring systems provided an accurate and reliable assessment of BRD. In 2019, researchers from the University of California, Berkeley’s College of Agriculture and Natural Resources proposed a more accurate scoring system for BRD in suckling and weaned calves. They called this system the California BRD Scoring System, or CA BRD. It was based on six clinical signs for suckling calves and five clinical signs for weaned calves.

Total Score
All scores for these parameters are added together to give a total score.

For suckling calves, a total score of 5 or higher indicates a high probability of BRD.
One of the five parameters assessed for weaned calves is the diurnal variation in rectal temperature. If the calf has no rectal temperature variation, a total score of 1 or higher indicates a high probability of BRD. If the calf has rectal temperature variation of 15 (Celsius) or more, a total score of 2 or higher indicates a high probability of BRD.