Association between clinical respiratory signs, lung lesions detected by thoracic ultrasonography and growth performance in pre-weaned dairy calves

Abstract

Background: Bovine respiratory disease (BRD) is the main cause of mortality among 1-to-5 month old calves in Ireland, accounting for approximately one-third of deaths. Despite widespread use of clinical respiratory signs for diagnosing BRD, lung lesions are detected, using thoracic ultrasonography (TUS) or following post-mortem, in calves showing no clinical signs. This highlights the limitation of clinical respiratory signs as a method of detecting sub-clinical BRD. Using 53 purchased artificially-reared male dairy calves, the objectives of this study were to: (i) characterise the BRD incidence detected by clinical respiratory signs and/or TUS, (ii) investigate the association between clinical respiratory signs and lung lesions detected by TUS, and (iii) assess the effect of BRD on pre-weaning growth.

Results: Clinical BRD (based on Wisconsin clinical respiratory score and/or rectal temperature > 39.6 ºC) was detected in 43 % and sonographic changes (lung lesions) were detected in 64 % of calves from purchase (23 (SD; 6.2) days of age) until weaning, 53 days post-arrival. Calves with clinical BRD were treated. Sixty-one per cent calves affected with clinical BRD had lung lesions 10.5 days (median) before detection of clinical signs. Moderate correlations (r_sp 0.70; P < 0.05) were found between cough and severe lung lesions on arrival day, and between rectal temperature > 39.6 ºC and lung lesions ≥ 2 cm² on day 7 (r_sp 0.40; P < 0.05) post-arrival. Mean average daily live weight gain (ADG) of calves from purchase to weaning was 0.75 (SD; 0.10) kg; calves with or without clinical BRD did not differ in ADG (P > 0.05), whereas ADG of those with severe lung lesions (lung lobe completely consolidated or pulmonary emphysema) was 0.12 kg/d less (P < 0.05) than calves without lung lesions.

Conclusions: Thoracic ultrasonography detected lung consolidation in calves that did not show signs of respiratory disease. The presence of severe lung lesions was associated with reduced pre-weaning growth. These findings emphasise the importance of using TUS in addition to clinical respiratory scoring of calves for an early and accurate detection of clinical and sub-clinical BRD.

Keywords: Bovine respiratory disease; Calf; Clinical respiratory scoring; Lung consolidation; Pneumonia; Thoracic ultrasonography.

Fig. 1

Cumulative percentage of calves diagnosed with clinical BRD or lung lesions during 30 days after arrival. ¹Clinical bovine respiratory disease. ²Thoracic ultrasonography score. ³Calves with cBRD and/or TUSS > 1.

Fig. 2

Experimental design. Timeline representing the experimental design from arrival of animals to the Research Centre facilities (d 0) until weaning (d 52 (SD; 10.4) post-arrival). Research veterinarian performed TUS and CRS on days 0, 7 and 14. Moreover, an extra TUS was performed in 33 out of 53 calves between day 14 and 30. Farm technician checked for visual clinical signs twice daily during all study period. Calves with CRS ≥ 5 and/or RT > 39.6 ºC were treated and classified as cBRD+. Animals not receiving treatment during all study period were classified as cBRD-. BRD, bovine respiratory disease; cBRD, clinical BRD; CRS, clinical respiratory score; RT, rectal temperature; TUS, thoracic ultrasonography; TUSS, thoracic ultrasonography score.

Fig. 3

Ultrasonograms of the classifications by thoracic ultrasound score categories (8 MHz). A bright hyperechogenic line (pleura) of demarcation between the intercostal muscles and the lung tissue is observed in all ultrasonograms. The ultrasonograms of normal lung surface are classified as TUSS1, where reverberation artefacts of the pleura are observed. The presence of comet tail artefacts (arrow head) is likewise included in this category. Ultrasonograms classified in the category TUSS2 and TUSS3 include presence of lung lesions (marked with stars) with < 2 cm² and ≥ 2 cm², respectively. In the category TUSS4, ultrasonograms of pulmonary emphysema (to the left) or complete consolidated lung lobe (to the right) are included. The heart is delimited by circular shape dashed line and lung lobe consolidation by triangular shape dashed line. Squares delimited with green lines in the ultrasonograms images represent 1 cm² each. ICM, intercostal muscles; P, pleura; TUSS, thoracic ultrasonography score.