One of these factors was discussed in Dr. Stephen LeBlanc’s webinar, “Supporting healthy immune function in transition dairy cows.” The webinar was presented on May 28, 2020 as part of the 2020 Animal Nutrition Conference of Canada, facilitated by the Animal Nutrition Association of Canada.
The state of the immune system in transition dairy cows
As dairy cows enter the transition period near and after calving, they experience many changes in both management and metabolism. Although this period can result in a decrease in immune competence, it is not true that all immune function is compromised in dairy cows during this time. Transition dairy cows also face lots of oxidative stress, meaning their ability to detoxify harmful free radicals is reduced. Transition dairy cows tend to face higher competition for feed and lying space, which may weaken their immune system further.
A dairy cow’s active immune system is made up of several elements and is divided into two main components: innate and adaptive. The innate system is composed of various elements including physical barriers, phagocytes such as neutrophils and macrophages and natural killer (NK) cells. In addition to this, the innate system is the rapid response force but generally is not directly affected by vaccines. The adaptive system includes humoral components such as antibodies and cell-mediated components such as cell lysis. The innate system and its main component, neutrophils, formed the basis of LeBlanc’s discussion.
The role and function of neutrophils
Neutrophils are white blood cells that activate in the presence of infection. Neutrophils are attracted to sites of infection or inflammation by chemical signals. Once the neutrophils are activated, they develop a “stickiness” and begin to migrate to the site of the infection or damaged tissue. In order to enter the infection site, neutrophils will change their shape. As the neutrophils leave the bloodstream and enter the site of infection, they begin to attack and destroy invading bacteria. The neutrophils will perform phagocytosis, to ingest and digest foreign particles. They will also release more chemical-signaling proteins called cytokines to initiate specified immune cell responses. Neutrophils are of special importance to the health of the mammary gland and the uterus.
Other phagocytes, called macrophages, monitor for invading bacteria and release pro-inflammatory regulators when bacteria are detected.
Neutrophils can work to combat infection in several ways. As mentioned, neutrophils can perform phagocytosis to destroy bacteria, but they can also generate reactive “bleach-like” compounds to kill bacteria. Neutrophils can summon more neutrophils to the site of infection, or they can turn down inflammation when their job is done. They are capable of casting (or excreting) a net of their own DNA, which can work to trap bacteria.
Neutrophils become compromised during the transition period. This compromise in function may be due to a number of factors. For example, one study found that mastectomized, pregnant cows saw a decrease in neutrophil function before calving, even though milk production was not being initiated. However, the same study found that after calving, lactating cows saw a continued decrease in neutrophil function, while the mastectomized cows saw a quick recovery in neutrophil function. Other factors that affect neutrophils include social stressors, antioxidant supply, glucose availability, calcium availability and exogenous immune modulators.
Antioxidant requirements
Antioxidants are an essential part of the diet of a dairy cow, as they impact the function of the immune system. When antioxidants such as zinc, manganese, copper, selenium and vitamins A and E are lacking, the function of the immune system is compromised. This results in fewer available enzymes for cell function and protection. Antioxidants help contain oxygen-free radicals that are generated as part of a neutrophil’s function.
Vitamin E and selenium are very important in transition dairy cows. If adequate amounts of these antioxidants are not present, it could result in neutrophils providing a short-lived response with little success compared with the desired sustained response and numerous bacteria killings. Additionally, if cows do not meet their dietary vitamin E requirement of approximately 1,000 international units (IU) per day, they become more likely to suffer from mastitis. Injections can work to address vitamin E deficiencies; however, their effect only lasts for about one week, so it is not a long-term solution.
Immune responses
Dairy cows in transition are often at risk of uterine disease due to the impairment of innate immune function that occurs during this period. Uterine disease has many risk factors including changes in the uterine microbiome, dry matter intake (DMI), metabolic health, social stressors and hormonal changes, as well as hypocalcemia caused by lower levels of calcium in the blood. Uterine disease triggers an inflammatory response, which can have effects on fertility for months after the visible disease has passed.
At calving and soon after, non-esterified fatty acids (NEFA) concentrations increase. Studies have found that elevated NEFA concentrations have a possible direct impact on the function and killing capacity of neutrophils. Similarly, increased levels of beta-hydroxybutyrate (BHB), a metabolic regulator, can impair neutrophil migration.
Immune responses consume a lot of energy when metabolic fuel is in short supply. LeBlanc described how in about 12 hours, a moderate immune response can “cost” approximately 2.2 pounds of glucose. He noted this is the energetic equivalent of about 33 pounds of milk.
The role of calcium
Lower blood calcium levels cause a decrease in phagocytosis, as well as oxidative bursts in neutrophils, which may lead to metritis. Although lower calcium levels do impair the function of neutrophils, administering it at calving has not been found to reverse this impairment. Dietary prevention of hypocalcemia is the preferred approach to this issue.
Transition dairy cows are prone to weakened immune systems as a result of reduced inputs but increased demands for immune responses. LeBlanc’s webinar explained how being mindful of this and ensuring proper nutrition, including adequate sustained intake of antioxidants, can be useful in reducing the risks this period can pose. 
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Emma Ohirko
- Editor
- Progressive Dairy
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