Clinical signs of bovine respiratory disease (BRD) in young dairy heifers tell only part of the story, and more objective diagnosis could help evaluate treatment success. During the recent BRD Symposium in Denver, University of Wisconsin veterinarian Terri Ollivett, DVM, PHD, outlined how individual and herd-level factors can contribute to treatment failures and how using thoracic ultrasound to guide treatment protocols could improve measurement of treatment response and validate dosage regimens for antimicrobial drugs.
Effective treatment, Ollivett says, is a component of prevention and antimicrobial stewardship. However, she adds, many dairies do not measure treatment response, or typical measures based on resolution of clinical signs or records for retreatment rates and average treatments per calf. Using these measures, research suggests that 20 to 35% of treated calves need multiple treatments due to relapse of BRD signs. Instead of basing resolution of disease on clinical signs, she says, we should use resolution of lung lesions as the expected standard.
Ollivett has conducted several studies using thoracic ultrasound (TUS) to detect non-aerated or consolidated lung lesions associated with pneumonia. She has found a high correlation between the amount of consolidated lung identified on TUS and gross post-mortem exams. Regardless of the clinical picture, she says TUS lung lesions in dairy calves are associated with reduced pre-weaning ADG, increased mortality and less milk production during the first lactation.
Ollivett says that by using a systematic clinical scoring system, such as the Wisconsin Respiratory Score, along with TUS, practitioners can define three BRD subtypes:
- Upper respiratory tract infections
- Clinical pneumonia
- Subclinical pneumonia
“Although the distributions of BRD subtypes will vary from farm to farm, we have found that approximately 1/3 of new cases are subclinical,” she says.
Noting a high incidence of relapse and retreatment, Ollivett hypothesizes that incomplete bacterial killing sets the stage for bacterial replication and relapse or recurrence of consolidation once the antibiotic pressure has been removed. Poor treatment response, coupled with misleading clinical criteria for treatment success puts calves at risk for future clinical disease and prolonged periods of slow growth.
Implementing TUS-guided treatment protocols should improve calf-level response result in fewer relapses, decreased duration of disease, thereby improving calf welfare and decreasing cost of disease, while ensuring the treatments are effective at establishing a bacteriological cure within the lungs.
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