Is udder ultrasonography a diagnostic tool for subclinical mastitis in sheep?

- Somatic cell count was significantly correlated with the ultrasonographic dimensions of mammary lymph node.
- Critical thresholds for ultrasonographic dimensions of mammary lymph node may be helpful in diagnosis.
- The ultrasonography is a rapid field test for supporting diagnosis of subclinical mastitis.

This study was designed to assess the application of ultrasonography for diagnosis of subclinical mastitis in sheep. A total of 56 udder halves and their corresponding superficial inguinal lymph nodes were examined ultrasonographically using a 7.5 MHz linear transducer. Milk samples were collected for bacteriological examination, somatic cell count (SCC) and the California mastitis test (CMT). Based on the bacterial culture growth, animals were classified into 4 groups as no growth (control, n = 4), coagulase-negative staphylococci (CNS, n = 14), Staphylococcus aureus (Staph. aureus, n = 4), and mixed CNS and Staph. aureus (n = 6). Blood analysis results showed non-significant difference (P > 0.05). The echogenicity of the infected udders' parenchyma was homogenous and hypoechoic. The gland cisterns were anechoic, while the teat canal appeared as a longitudinal echogenic line. The superficial inguinal lymph nodes were easily identified and appeared as oval-shaped structure with thin echogenic capsule. All nodes had an anechoic or hypoechoic parenchyma with a linear echogenic structure at the center. The ultrasonographic length, depth and area of the superficial inguinal lymph nodes were significantly increased in the udder infected groups (P < 0.05). SCC values showed significant difference among different udder health groups (P < 0.05). SCC was correlated with the ultrasonographic length (r = 0.59, P < 0.01), depth (r = 0.64, P < 0.01) and area (r = 0.66, P < 0.01) of superficial inguinal lymph nodes. The critical threshold for ultrasonographic length, depth and area of superficial inguinal lymph node was 11.5 mm, 7.8 mm and 13.5 mm2, respectively. Furthermore, the critical value for SCC was ≥400 × 103 cells/mL. In conclusion, the ultrasonography is a fast and field test for supporting diagnosis of subclinical mastitis. In addition, ultrasonographic examination of udder especially superficial inguinal lymph nodes can greatly aid in the assessment of subclinical mastitis in ewes and should be applied alongside SCC and CMT.