Summer heat is often measured in pounds of lost milk, but researchers are increasingly asking a different question: What does heat stress do to the dairy cow’s immune system?
A recent review published in Frontiers in Animal Science argues heat stress should be viewed as more than a production challenge. Heat stress can begin when the temperature-humidity index exceeds 72, long before conditions reach what is commonly considered to be extreme heat. Research has shown milk production declines 1% for every unit increase above that level.
While reduced feed intake, lower milk yield and reproductive losses remain important concerns, heat stress also disrupts immune function, alters cellular processes and may leave cows more vulnerable to disease.
That perspective could help explain a familiar pattern on many dairy farms. As temperatures climb, increases in common health challenges occur. It is estimated that these challenges, along with reduced productivity, cost the U.S. livestock industry between $1.7 and $2.4 billion annually. The issues related to this loss may be linked by a common underlying factor: impaired immune resilience.
When the Immune System Feels the Heat
Heat stress affects far more than a cow’s ability to dissipate body heat. It triggers physiological changes throughout the body, including changes in how immune cells respond to threats.
Heat stress has been associated with:
- Changes in circulating immune cell populations
- Reduced neutrophil function
- Impaired phagocytosis
- Decreased oxidative burst activity
- Altered lymphocyte responses
These processes are critical components of the cow’s first line of defense against invading pathogens.
Heat stress can also affect lymphocyte activity, potentially reducing the effectiveness of adaptive immune responses. The result is not simply a weaker immune system. Instead, heat stress appears to disrupt the body’s ability to mount an appropriate response. In some cases, immune activity may be suppressed. In others, inflammatory responses may become excessive or poorly regulated. Researchers increasingly describe this as immune dysregulation rather than simple immune suppression.
Those changes can occur before clinical disease becomes apparent, creating conditions that allow health problems to develop when cows encounter additional stressors or pathogens.
Why More Cows Get Sick During Hot Weather
The relationship between heat stress and disease has been recognized, but the biological mechanisms behind that relationship have remained less clear.
There is mounting evidence immune dysfunction contributes to increased disease susceptibility during periods of heat stress. Conditions such as mastitis, metritis, retained placenta and lameness frequently become more common during hot weather.
For example, reduced neutrophil activity may impair the cow’s ability to identify and eliminate bacterial pathogens. At the same time, altered inflammatory signaling can affect tissue repair and recovery. Together, these changes create opportunities for infections to establish and persist.
Heat stress may also compound challenges during the transition period, when cows already face significant metabolic and immune demands. A cow entering lactation with compromised immune function may be less capable of responding to infectious or inflammatory challenges, increasing the likelihood of clinical disease.
This means some summer health problems may be symptoms of a broader physiological response rather than isolated disease events.
The Cellular Cost of Heat Stress
Understanding why disease risk increases requires looking beyond clinical signs and into the cellular changes triggered by heat stress. Many of the effects described in the review begin at the cellular level.
One major consequence of heat stress is increased oxidative stress. Elevated temperatures can increase the production of reactive oxygen species while overwhelming the body’s antioxidant defenses. Excessive oxidative stress damages cells and tissues, contributing to inflammation and impaired function.
Heat stress also affects mitochondria, the organelles responsible for producing cellular energy. When mitochondrial function declines, cells have less energy available to support normal processes. This can affect immune cells, mammary tissue and reproductive tissues alike.
Heat stress also affects inflammatory pathways involved in immune signaling and pathogen recognition. Disruptions in these pathways can alter how the body responds to infection, potentially contributing to both inadequate immune responses and excessive inflammation.
Another emerging area of interest is gut barrier function. Heat stress can redirect blood flow away from the gastrointestinal tract as the body attempts to dissipate heat. Reduced blood flow may damage intestinal tissues and increase gut permeability, allowing inflammatory compounds and microbial products to enter circulation.
Taken together, these cellular changes help explain why heat stress can affect so many aspects of health simultaneously.
Supporting Immune Resilience During Hot Weather
There is no nutritional supplement or management tool that can fully eliminate the effects of heat stress. Environmental cooling remains the foundation of any mitigation strategy.
Several key areas to focus on during periods of elevated heat stress include:
- Maintain effective cooling systems, including fans, sprinklers and shade structures.
- Pay particular attention to cooling dry cows, which can influence future health and performance.
- Monitor disease trends closely during heat events.
- Evaluate transition-cow protocols to identify additional sources of stress.
- Consider nutritional strategies that support antioxidant defenses and immune function, including trace minerals, vitamins, betaine and rumen-protected amino acids when appropriate.
Nutritional interventions should complement, not replace, efforts to reduce heat load.
Look Beyond Production Losses
Heat stress has traditionally been viewed through the lens of milk production and reproduction. Those outcomes remain important, but they represent only part of the picture.
By altering immune function, increasing oxidative stress, disrupting cellular energy production and affecting inflammatory responses, heat stress can influence a cow’s ability to resist disease long before clinical signs appear. Milk loss may be the most visible consequence of heat stress, but the damage often begins much deeper, within the immune and cellular systems responsible for keeping cows healthy.
