Increased teat wall thickness in response to machine milking

摘要

Transiently increased teat wall thickness in responseto machine milking has been documented by variousmethods, including ultrasound. However, correlativeultrasonography and histology to detect the origin ofthis phenomenon is lacking. The first goal of the presentstudy was to evaluate and compare milking-relatedchanges of the teat tissue in 2 breeds of dairy cows (11Simmental and 3 Holstein) using B-mode ultrasonography.Additionally, the observed changes were comparedwith ultrasonographic findings in a Holstein cow withperiparturient udder edema. Finally, corresponding histologicalsections of the Simmental teats were analyzedand compared with those from a lactating nonmilkedAngus cow. We hypothesized that the mechanical loadof both stretching by the vacuum during phases ofopen teat cup liner and compression by the closed linerduring machine milking results in a transient congestionof blood vessels in the teat wall. The barrel of 1front teat of each cow was scanned immediately beforeand after machine milking (system vacuum: 42 kPa;pulsation rate: 60 cycles/min; pulsation ratio: 65:35).Shortly after milking (33 ± 6 min), the Simmentalswere slaughtered, and their scanned teat was immediatelyremoved and processed for investigation by lightmicroscopy. Ultrasonography after milking revealedanechoic tubular structures mainly in the inner half ofthe teat wall. Histological examination revealed thesestructures to be thick-walled veins. The left front andhind teats of the nonmilked lactating cow, collectedand prepared identically to those from the Simmentalcows, showed the same histological features. Ultrasonographicmeasurements showed that the diameter ofthese veins significantly increased after milking comparedwith matching images before milking. This effectwas most pronounced in the Holstein cows. Similarly,these veins were very prominent in the periparturientcow. However, neither the milked cows, including theperiparturient cow, nor the lactating nonmilked cowprovided any evidence of edematous extravasation onultrasonography or histology. These findings corroboratedour hypothesis that the increase in size of thickwalledveins in the teat tissue is the main reason forthe thickening of the teat walls in response to machinemilking.