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Use of quick thoracic ultrasonography (qTUS) to guide and evaluate antimicrobial therapy in three natural outbreaks of Mycoplasma bovis pneumonia
Jourquin, S., Chantillon, L., Lowie...
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Objectives
Quick thoracic ultrasonography (qTUS) is an accessible on-farm method to reliable diagnose (subclinical) pneumonia in both dairy and beef calves, offering great potential for more rational antimicrobial use and better control of bovine respiratory disease. The objective of this study was to describe the application of qTUS to guide and evaluate antimicrobial therapy in three natural outbreaks of Mycoplasma bovis associated pneumonia in a single beef farm.
Materials and methods
A retrospective analysis was performed on ultrasonographical and diagnostic data collected from 3 different outbreaks of respiratory disease in the same closed commercial beef farm housing 1000 Belgian Blue beef cows. Calves at risk aged between 2 and 6 months and were housed in groups of 6 in fully separated pens. Pneumonia was defined as a lung consolidation ≥1cm while cure was defined by reaeration of these lesions to <1cm in depth. In each outbreak, qTUS was used as a tool to initiate, stop or generally evaluate antimicrobial therapy. In outbreak 1, ultrasound-guided full metaphylaxis was used, meaning that all animals at risk (n = 130) were metaphylactically treated (one injection), after which antimicrobial therapy was only prolonged in animals which still had lung consolidation. Antimicrobial treatment was stopped as soon as ultrasonographical reaeration of the lung occurred. Outbreak 2 (n = 94) was approached by individual treatment of animals with ultrasonographically confirmed pneumonia, without metaphylaxis. Finally, in the third outbreak (n = 61), an ultrasound-guided targeted metaphylaxis, only using group treatments in pens were 3 or more animals had consolidations ≥1cm, was applied. Again, as was done in outbreak 1, antimicrobial treatment was discontinued when reaeration appeared. In each outbreak, animals older than 3 months were treated with florfenicol while animals less than 3 months were treated with oxytetracycline, both administered as long acting formulation every 48 hours. Identification of primary pathogens was done by means of whole genome nanopore sequencing (WGS) on non-endoscopic bronchoalveolar lavage samples (nBAL). Strain typing of M. bovis and detection of possible genomic antimicrobial resistance was done by long-read nanopore sequencing.
Results
At the time of intervention 43.3% (55/130), 50% (47/94) and 37.7% (23/61) of the calves from outbreaks 1, 2 and 3 had a lung consolidation ≥1cm, respectively. In the first outbreak, where full metaphylaxis was applied, cure rate after 14 days was 98.2% (54/55). no new cases of pneumonia were detected, resulting in a total proportion of healthy calves of 99.2% (129/130) after the outbreak. In outbreak 2, over the 21- day follow up period 24 new cases of pneumonia were detected, resulting in a total of 75.5% (71/94) pneumonia within the herd. Cure rates were 61.7% (29/47) and 79.2% (19/24) for the initial and new pneumonia cases, respectively (total cure= 67.6 % (48/71). Finally, in outbreak 3, incidence of pneumonia started at 37.7% (23/61). After a 14 day follow up, the combination of targeted metaphylaxis and individual treatment resulted in a total cure rate of 78.9% (30/38). Of the initial cases, 78.2% (18/23) was cured. 15 new cases were detected in this outbreak, of which 80% (12/15) was cured at the end of observation. Cure rate was significantly lower in outbreaks 2 and 3, compared to 1 (P<0.01). In outbreak 1, 2 and 3, M. bovis was detected by WGS and strains were classified as belonging to Belgian M. bovis genomic clusters IV, I and I, respectively. In the third outbreak the coronavirus was also detected. Evaluated over the outbreak, in outbreaks 1, 2 and 3, a number of animal daily dosages (ADD) per animal of 3.0, 8.0 and 5.3 ADD/ animal were necessary to control the outbreak, respectively. For comparison a standard metaphylaxis with tulathromycin stands for 7 ADDs/animal.
Conclusion
The qTUS technique showed potential as a reliable and practically feasible method both for detection of pneumonia and therapy evaluation in larger herds. Large differences in cure rate between the outbreaks were observed which may be related to co-infection, timing of the initiation of antimicrobial therapy, strain difference or the use of metaphylaxis. Especially the latter is a point of interest given that both ultrasound-guided metaphylaxis (full or targeted) resulted in lower antimicrobial use compared to individual treatment.
Keywords: Bovine respiratory disease, Mycoplasma bovis, lung ultrasound, antimicriobial reduction.
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