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Understanding why cows get sick during the transition period

Michael A. Ballou and Matthew D. Sellers Published on 10 October 2013

Dairy cows are most susceptible to infectious diseases, such as mastitis and metritis, during early lactation. All animals on a dairy are being exposed to micro-organisms every day; however, most of them do not become clinically diseased.

The immune system defends the cow from constant attack by recognizing the micro-organisms as foreign and safely eliminating them from the body.

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It is called an “immune system” because there are many components that work together in synchrony to protect the cow from disease. The immune system is balanced and working well most of a cow’s life, keeping her healthy.

During the transition period, however, the immune system of many cows becomes dysfunctional, which increases the risk for infectious diseases.

We often hear people say that the immune systems of transition cows are suppressed, which is partly true, but this is likely an overgeneralization. Some components of the immune system are not suppressed at all, and in fact, some immune responses are actually elevated during the transition period.

To demonstrate our point, many cows are vaccinated with a J5 or similar vaccine during the dry period and again at calving to prevent the overactivation of an immune response during early lactation that results in inflammation and the classic signs of clinical disease.

Therefore, we prefer to say the immune system of a transition cow can become dysfunctional rather than suppressed.

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First, we need to understand what this dysfunctional immune system looks like. As we mentioned previously, the immune system is composed of a lot of responses by many different types of white blood cells.

We want to simplify our discussion and only consider two types of white blood cells – macrophages and neutrophils.

Macrophages play an important role in recognizing micro-organisms as foreign and signaling to the rest of the body that there are potentially dangerous intruders.

The signal from the macrophages causes inflammation. The intensity of the inflammation should be proportional to the threat because if the response is too strong it causes a lot of tissue damage, reduces milk production and can kill the cow.

The analogy we like to use is: You do not want to send a full-blown military response to deal with a shoplifter that a mall security guard could easily handle. This is an example of a disproportionate response that would likely cause a lot of unnecessary damage and casualties.

Data suggest that cows in early lactation have elevated, rather than suppressed, inflammatory responses. Remember that the J5 vaccine is preventing this inflammation signal, so through vaccination we are actually trying to reduce this immune response during early lactation because it may be too strong.

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The other white blood cell we want to discuss is the neutrophil, which was consistently shown over the last four decades of research to be suppressed during the transition period.

Neutrophils can be thought of as law enforcement; they are ready to fight and respond to signals indicating there is a problem.

Early lactating cows that have decreased neutrophil responses are more likely to have severe mastitis because they cannot control the growth of the invasive micro-organisms as well as cows with more functional neutrophils.

The suppressed neutrophil response, when coupled with more aggressive inflammation, is an example of a dysfunctional immune system.

There are many variables that contribute to this dysfunctional immune system. They can be broadly classified as either related to stress or the metabolic demands of lactation (We recognize that the onset of lactation could also be classified as stress).

Using mastectomized cows, the USDA-ARS group in Ames, Iowa, reported that the metabolic demands of the mammary gland contributed more to the dysfunctional immune system than the stressors associated with parturition.

Animals, including humans and cows, stress about things beyond their control. Any change can be perceived as a stressor to a cow, and parturition is inundated with changes.

Chronic stress suppresses neutrophil functions, and although some of the stressors of parturition are unavoidable, an attempt should be made to limit any additional stressors during that critical period.

Transition cows should not be overcrowded; if a dairy producer is going to invest in additional pen space, make sure the close-up and fresh cows are not overstocked.

The environment of the cow should also be pleasant, no matter how bad the weather is outside. Therefore, cows should be cooled when it is hot, dry when it is rainy, and drafts should be prevented when it is cold.

Cows should be offered a palatable, well-balanced ration daily, and the feedbunk should never be slick. The pen should be clean, which will not only reduce the stress on the cow but also decrease exposure to the micro-organisms that cause infectious disease.

There is not a clear verdict on how pen movements influence the stress response, the immune system and transition success.

Some data support that an “all-in all-out” approach improves the transition; however, a recent study conducted at the University of Minnesota indicated that weekly re-grouping of cows during the transition period did not influence neutrophil responses or health when compared to an “all-in all-out” approach.

We need a better understanding of how management strategies and environment can reduce the stress of transition cows, but a good rule of thumb is to ask yourself, “If I was in that cow’s place, would I be comfortable?”

The shift from non-lactating to lactating is abrupt and a major physiological change. The change in the metabolic demands of the mammary gland during the transition period affects the immune system at many levels.

The mammary gland requires a lot of nutrients and energy, and in many cases, the cow does not consume enough nutrients and energy to meet these demands, which may directly or indirectly cause immune dysfunction.

Negative energy and protein balances during early lactation play a part in immune dysfunction; however, a clear picture of their role is unknown.

A study at the University of Illinois reported that there were no effects of inducing negative energy balance in mid-lactation on neutrophil function or the response to an intramammary Streptococcus uberis challenge.

Decreased availability of calcium in the blood and body also contributes to immune system dysfunction. Recent data from the University of Florida indicated that cows with subclinical hypocalcemia, less than 8.6 mg per dL total serum calcium on the day of parturition, had reduced neutrophil function and a greater incidence of metritis.

Most dairymen, nutrition consultants or veterinarians would say they do not have a hypocalcemia problem because they do not have many clinical milk fevers. However, data indicate that as many as 25 to 50 percent of cows in a herd can have subclinical hypocalcemia.

Therefore, it is important that cows in the close-up are receiving a low DCAD diet and high-risk cows are receiving a bottle of calcium IV at parturition.

Data from Michigan State indicates that a reduced antioxidant status in early lactation is causative to increased inflammation; therefore, balancing the ration for appropriate antioxidants including vitamin E, selenium, copper, zinc and manganese, among others, is important.

The NRC committee in 2001 recognized this and increased their recommendations for vitamin E in the diets of transition cows because it was reported that supplementing vitamin E at those concentrations decreased the incidence of mastitis.

All cells in the body need energy, protein, minerals and vitamins to function properly, and white blood cells are no different.

It may be possible to decrease the severity of metabolic stress during transition by gradually transitioning cows to a decreased energy diet prior to calving rather than having the change occur abruptly following calving.

Homeostatic or homeorhetic adaptations may allow the support of both the demands of lactation and immune system. Are we able to manage cows in a way that maintains the proper function of the immune system during the metabolic demands of lactation?

This will be an interesting area of research in the next few years that may lead to new and unique management strategies.

Cows in a herd deal with the transition period with varying success. Many factors, some management-related, some cow-related and some completely uncontrollable, affect any individual cow’s transition success.

A recent study from our laboratory indicates there is a moderate amount of variation between herds in white-blood-cell responses, but there is a much larger amount of variation within the herd.

The next great hurdle in identifying and preventing immune dysfunction and subsequent subclinical and clinical disease is determining what makes cows within the herd respond differently (i.e., why don’t all the cows get sick or all the cows stay healthy?).

Once the primary reasons for these differences are identified, we can develop management strategies to improve the immune responses of those cows that would be at risk for a dysfunctional immune system during the transition period.

Ultimately, transition success depends on a multitude of factors. While some of these factors are out of reach of the dairy producer and their veterinarian, several are easy enough to monitor or correct.

Limit metabolic and physiologic stress to the cows and maintain cow comfort.

To make great strides in understanding the roots and causes of immune dysfunction during the transition period, we must first understand why transition success is so variable in the first place. PD

Sellers is a Ph.D. candidate in animal science and biostatistics. Ballou is an associate professor of nutrition and immunology. Both are with Texas Tech University.

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