THE TRANSITION PERIOD (part 1): Immune challenges and disease vulnerability
Defining the Transition Period
The transition period spans approximately six weeks, from two weeks before calving to four weeks post-calving. This critical phase represents the time when cows are most vulnerable to disease, including mastitis, as they navigate through significant physiological, metabolic, and immune challenges.
During this period, cows experience a range of stressors:
- Physical stress of parturition
- Metabolic and hormonal shifts supporting gestation to lactation
- Dietary changes to support milk production
- Environmental and social stress (e.g. pen movements)
Stressors are critical as they play a direct role in modulating immune function, which is key to disease susceptibility during this period.
Immune System Changes During the Transition Period
These changes are primarily driven by the cow’s hormonal and metabolic adaptation to lactation, and they can have profound effects on immune cell populations and their function:
- Immunotolerance during pregnancy
During pregnancy, cows maintain a state of immunotolerance to protect the developing fetus and to prevent its rejection as foreign tissue.
- Pro-inflammatory shift near calving
Shortly before calving, a pro-inflammatory shift occurs, driven by both fetal signals and changes in maternal hormone levels. This shift, which can last from a few days to several weeks, prepares the cow for the demands of lactation but also creates an environment where the immune system is more susceptible to dysfunction and infection.
Impact on Disease Incidence and Immune Function
The immunological changes during the transition period contribute to an increased risk of disease, including mastitis.
- Neutrophil shifts and impaired function
The hallmark is an increase in circulating neutrophils. This surge is a response to both hormonal changes and fetal signals, leading to the release of a wave of neutrophils from the bone marrow. Unfortunately, a significant proportion of these neutrophils are immature, which impairs their ability to effectively combat pathogens.
Cows in the transition period may have a weakened defense against mastitis-causing pathogens, as immature neutrophils have reduced pathogen-killing capabilities.
- Relocation of immune cells
The role of immune cells during the transition period includes critical tasks such as tissue remodeling during calving and post-calving uterine involution. As neutrophils migrate to the reproductive tissues, there is a reduction in their blood levels, which compromises the cow’s systemic immune response.
Migration of neutrophils is necessary for effective uterine healing and pathogen clearance, but it can ‘distract’ immune cells from addressing mastitis infections.
- Lymphocyte function and variability
Changes in lymphocyte function also occur. Lymphocyte responses, particularly the balance between Th1 and Th2 immune responses, can vary widely between cows, leading to substantial individual variability in immune function. This variability has implications for developing vaccination strategies, particularly those targeting the dry period to optimize immune response before calving. The effectiveness of these strategies can depend on the specific immunological status of each cow.
Article written by Shannon Walsh, DVM
Technical Services Veterinarian (Ruminants)
REFERENCE
Chase, C. C. L., Parreño, V., Bradford, S., Sordillo, L. M., Prenafeta, A., Piepers, S., Saif, L. J., Strain, S., Timsit, E., Woolums, M. E., Thiry, E., Newcomer, B. W., Aldridge, B. M., Nodar, P., & Casademunt, C. (2022). Bovine immunity: Making immunology and vaccinology come alive (DL: GI 1666-2021). HIPRA, S. A.
