How do microorganisms typically gain entrance to the mammary gland in cases of infectious mastitis?

Microorganisms typically gain entrance to the mammary gland in cases of infectious mastitis primarily through the streak canal. The streak canal serves as the primary opening of the teat and is the most direct route for pathogens to enter the udder. It is a narrow tube that connects the outside environment to the interior of the mammary gland.

During milking or when the cow is exposed to contaminated environments, bacteria can easily invade through this canal. Factors such as inadequate milking practices, environmental stress, or injuries can further increase susceptibility, allowing pathogens like Staphylococcus aureus or E. coli to colonize the mammary tissue.

The other anatomical structures mentioned do not serve as primary entry points for infection. The teat cistern and milk duct are internal components of the mammary system but are not as directly exposed to the external environment as the streak canal. Veins are involved in systemic circulation and not directly related to the entry of microorganisms into the mammary gland. Thus, the streak canal is crucial in understanding how infectious mastitis can develop in dairy cows.