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How summer temperatures affect immune system function in dairy cattle

Gabrielle Dumas and Annie Pelletier for Progressive Dairyman Published on 30 April 2019

Summer is upon us. It is time to think about and be prepared for the stress that will live in our dairy herds in the coming months.

Let’s face it, the summer of 2018 brought its share of particularly high temperatures, and the impacts on productivity were significant. Moreover, July 2018 was the hottest month observed by the Quebec government in nearly 100 years in the south of the province.

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Heat stress in dairy cattle

When we talk about heat stress, the term temperature-humidity index (THI) is regularly mentioned. THI is calculated using the temperature, in degrees Fahrenheit, as well as the relative humidity (HR, percentage) in the following equation:

THI = T - [0.55 - (0.55 × HR / 100)] × (T - 58)

Dairy cows experience heat stress when the THI reaches 68. This stress is reflected among other things by a rectal temperature exceeding 32.9ºC and a respiratory rate of more than 60 respirations per minute. At this stage, the impacts are multiple and can first be seen in the state of health of the animals.

When health is compromised, the energy of the animal is not devoted to milk production. Thus, performance declines to result in a decrease in dry matter intake (DMI), a decrease in milk production, increase in metabolic disorders, decrease in the percentage of fat and protein, a decrease in reproductive performance, as well as the weakening of the immune functions.

What about heat stress in Canada?

Being located farther north, we tend to think our cows are not subject to heat stress. However, studies show Canadian dairy cows are also subject to a decline in milk yield caused by heat and humidity. As demonstrated by the study conducted at Laval University, cows subjected to several consecutive days of heat stress had lower milk, fat and protein yields than cows subjected to normal temperature and humidity conditions. The cows most affected by heat stress were those in their third lactation or greater, with declines of 4.1 and 2.9 percent in fat and protein. Not counting the longer-term impact on reproduction and overall health, thermal stress causes a decrease in direct income for the producer.

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Effect on immunity

To understand how heat stress impacts immunity, let’s start by defining normal immune function. In times of stress, the functions of the immune system fluctuate. When the number of pathogens in the environment is not too high, combined with a compromised immune system, the animal manages to effectively handle the challenge, and the negative impacts are limited.

As shown in Figure 1, when the load of pathogens in the environment and stress is higher, and both at the same time, the animal will likely be more susceptible to diseases.

Immunosuppression immune dysfunction

Age, levels of production, genetics, nutrition, transport, toxins, calving, THI, etc., are examples of stresses and factors that affect immunity.

In times of stress, it is first possible to observe an increase in blood cortisol. Cortisol, a steroid hormone secreted by the adrenal gland, is important in many situations such as calving. The objective is not to inhibit this increase of cortisol. However, this rise in cortisol causes a decrease in the expression of a protein called L-Selectin in the blood.

The neutrophils are considered to be the dairy cow’s first line of defense against pathogenic invaders, and L-Selectin is a marker of innate immunity. Decreased L-Selectin expression prevents neutrophils from functioning normally and penetrating into the invading tissues.

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This situation has dramatic consequences for the immediate response of the immune system and the clinical outcome of exposure to an infectious organism. Thus, in times of stress such as heat and extreme humidity, the defenses of the immune system are less able to play their role as they should.

The left side of Figure 2 shows a picture of a blood vessel and immune cells in a normal situation.

Well-protected tissue

Yellow beads, around neutrophils, represent L-Selectin. However, on the right side, there is a less optimal situation where the neutrophils cannot cross through the blood vessel to reach the site of infection because the expression of L-Selectin is compromised by the increase of cortisol.

The weakening of immune function is therefore associated with the inability of neutrophils to migrate to tissues attacked by pathogens. These immune cells are less effective at destroying invading organisms, including phagocytosis, making the dairy cow highly susceptible to infectious diseases.

Recent research suggests acute exposure to high THI damages cells in the digestive tract of the cow, allowing pathogens and endotoxins to enter the bloodstream. Following the introduction of these pathogens, there is activation of a strong immune response that requires a large amount of glucose, which would otherwise be used for milk production.

Cows exposed to high THI during the dry period also respond poorly to vaccination during this period. Thus, the secretion of antibodies is weaker, and the innate immune functions (phagocytosis of the neutrophils and oxidative burst after calving) are altered. If the innate immune functions are negatively affected, it is now shown this stress will have an impact on the health of the calf to be born as well as on its future development.

How to prepare for heat stress

Strategies to improve neutrophil function during stress may help improve immune function and promote disease resistance. Preparing the immune system of the dairy cow to mobilize large numbers of neutrophils could therefore be an important advantage against infectious organisms, such as those associated with mastitis.

In order to achieve this, some modifications in the barn must be done, such as the addition of fans and sprinklers. Other important factors to consider are to feed the cows during cooler periods of the day, clean feeding areas frequently and serve good-quality forages.

In the diet, increasing energy density, maintaining adequate dietary fibre intake, increasing potassium intake and positioning certain feed additives to maximize immune system functions are also good ideas. Thermal stress has significant repercussions several days after the heat. In the U.S., the effects of heat stress are estimated to cost the dairy industry between $897 million and $1.5 billion a year – it is therefore very relevant to pay attention and make sure the immune system is optimized during this time of year.  end mark

Written in collaboration with Annie Pelletier, agr., from Phibro Animal Health.

References omitted but are available upon request. Click here to email an editor.

Gabrielle L. Dumas
  • Gabrielle L. Dumas

  • Technical Representative
  • DCL Animal Health and Nutrition
  • Email Gabrielle L. Dumas

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