How does immune suppression affect the cows?

All dairy farmers know that in the first weeks after calving, cows are more vulnerable and at risk of disease. Few know why. During the critical time around calving, the cow’s immune system is put under strain, leading to a reduced resistance to infections..

 First, a few words about immunity, or the body’s resistance to infection.

[+] Immunity, the cow’s self-defence

Immunity is the natural whole-body system protecting the cow from infections. The immune system is essential for life as it protects against invaders like bacteria and viruses. Two separate but overlapping branches of the immune system coordinate the body's protection:

[+] First line of defense: Innate immune system

This is the critical first line of active defence against invading pathogens that cause infection.

The innate immune system has three major components:

· Physical barriers, such as intact skin, mucous membranes or the teat sphincter. We all know that wounds (such as damaged teats, sole ulcers or a bruised birth canal) leave the door open to infections.

· Inflammation is triggered when the body senses it is under attack. It is a normal and necessary response. Damaged cells send chemical signals to the body's defence mechanism. White blood cells called neutrophils respond quickly to these signals by migrating to the site of infections.

· Phagocytosis, whereby the neutrophils and other "defence cells" recognise, ingest and kill the invading harmful microbes - before disaster can occur. Neutrophils are a key component in the immune response.

A functioning innate immune system is essential to prevent new infections. The ability of neutrophils to respond to the first signs of a bacterial invasion is key to eliminating new infections.

[+] Targeted response: Acquired immunity

Acquired immunity results in a "memory" within the body, which "remembers" a previous infection. Unlike the innate immune system it is usually directed against a specific microbe.

 

Some of the cells that carry out the acquired immune response produce proteins called antibodies, which protect the body against future encounters with this microbe.

Dairy farmers routinely use this branch of the immune system.

· Vaccination makes the body produce protective antibodies against diseases caused by specific viruses or bacteria.

· Colostrum contains the cow's antibodies (also known as immunoglobulins), and is nature's way of helping to protect the new born calf against diseases until it can fight them off on it's on own.

[+] Why fresh cows are more vulnerable

During the vital 90 days, some components (neutrophils) of a cow's immune system drop below normal levels.

Several factors - which all undergo changes during the transition period - are known to contribute to immune suppression.2

· Cortisol, the "stress hormone", is elevated at calving. Additional stressors in the enviroment can increase cortisol levels even more. This has a negative effect on movement of neutrophils to fight of invasion by bacteria.

· Ketones and non-esterified fatty acids (NEFA). Elevated levels of ketones, which are commonly associated with a negative energy balance, were found to have a negative impact on the activity and migration of the white blood cells.

· Calcium is a critical component of immune cell activation and in migration of white blood cells to the site of infection. 

Mastitis

Research shows that 25% (multiparous) to 39% (1st lactation) of clinical mastitis cases occur within the 1st month post calving6. Immune suppression around the time of calving is one of the reasons why so many cows are affected during this period.

 

Retained placenta

Neutrophils play a role in placental detachment and decreased neutrophil function has been demonstrated in cows with retained placenta5.

 

Metritis is an inflammation of the uterus (womb). Metritis can occur in cows with immune suppression that are unable to mount a strong defence against invading bacteria after calving. 

[+] References

1. J.C. Detilleux et al., (1994), Study of immunological dysfunction in periparturient Holstein cattle selected for high and average milk production, Veterinary Immunology and Immunopathology, 44, 251-267.

2. Mulligan F.J. et al., (2008), Production diseases of the transition cow, The Veterinary Journal, 176, 3 -9.

3. Sordillo L.M., (2005), Factors affecting mammary gland immunity and mastitis susceptibility. Livestock Production Science, 98, 89 -99.

4. Hammon D.S. et al,(2006), Neutrophil function and energy status in Holstein cows with uterine health disorders, Veterinary Immunology and Immunopathology, 113, 21 -29.

5. Kimura K., Goff J.P., Kehrli M.E., (2002), Decreased neutrophil function as a cause of retained placenta in dairy cattle, Journal of Dairy Science, 85 (3), 544 -550.

6. Miltenburg J.D., (1996), Incidence of clinical mastitis in a random sample of dairy herds in the southern Netherlands, Veterinary Record, 139 (9), 204-7.

 

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