Hepatic Ultrasound Dimensional Characterization of Dairy Cattle - Relationship with Reproductive Status

Background: Ultrasound examination is an essential and accessible tool to evaluate the liver of dairy cows. Detailed ultrasound evaluation of the liver provides information on the size, position, morphology of the parenchyma, among other characteristics inherent to the organ. Morphological variations in hepatic ultrasound may be related to reproductive status, aiding in the differential diagnosis for liver alterations caused by diseases. However, the relationship between liver ultrasound measures and reproductive status is still unknown.  The objective of this study was to evaluate the liver dimensions of lactating dairy cows and their associations with reproductive status. Materials, Methods & Results: This work utilized lactating cows and aimed to find any possible relationship between reproduction and liver measurable characteristics. No previous preparation of the cows was necessary, only the application of liquid alcohol at 70% during ultrasound scanning. The ultrasound evaluation of liver morphology was done at the right antimer, at the cranial abdominal region, between 12th and 5th intercostal espace (ICE).  Liver measures evaluated: distances of spinous processes from dorsal vertebrae to liver apice (M1); distances of spinous processes from dorsal vertebrae to the furthest ventral limit of liver (M2); measures resulting from differences between 2 anterior vertebrae (M3), distances between lateral liver edge and portal vein (M4) and distances between lateral liver edge and vena cava (M5).  In addition, we studied the diameter measures of caudal vena cava and portal vein. The organ showed regular contours and parenchyma with fine homogeneous echotexture of habitual echogenicity, regardless of reproductive status. The caudal vena cava presented triangular shape (shape related to the inlay in the hepatic sulcus) or slightly oval (shape that is not considered normal, correlated with thrombosis, heart failure or compression caused by the chest), with hypoechogenic wall, located at the hepatic edge. In addition, in animals with BCS greater than 3.25, it was more difficult to locate and evaluate the vena cava. The portal vein presented a characteristic rounded shape with branching, hyperechogenic wall and was easier to be localized and evaluated, in relation to the vena cava, even in cows with BCS greater than 3.25. Liver measures were: M1: 35.09 ± 5.92 cm, M2: 66.14 ± 10.71 cm, M3: 31.05 ± 7.96 cm, M4: 95.08 ± 12.66 mm, M5: 128.21 ± 13.64 mm and the diameter of portal vein and caudal portal vein, in mm, were respectively: 40.34 ± 5.79 and 37.83 ± 6.93. Pregnant cows showed M1 < 36 cm, M2 < 66 cm, M3 < 31 cm, M4 < 96 mm and M5 ≥ 128 mm, and empty cows M1 ≥ 36 cm, M2 ≥ 66 cm, M3 ≥ 31 cm, M4 ≥ 96 mm and M5 < 128 mm.   Discussion: The distance of the liver thickness, in the position of the portal vein of pregnant cows, presented a lower value than that of empty cows. The use of reference values in different reproductive stages is recommended for comparison with animals affected by diseases. Liver dimensions and reproductive status of dairy cows showed a significant correlation and may be used in the future for prediction and aid in reproductive diagnosis, as well as for differential diagnoses of liver dysfunctions in these animals. There is enough scientific evidence in the present work to subsidize the presence of a correlation between the reproductive status and ultrasound measures evaluated in this research.  Keywords: liver, ultrasound, cattle, portal vein, vena cava.