Bovine mastitis is one of the most common diseases of the dairy herd. It has a great impact on the health of the animals and especially on the economy of the farm and the dairy sector.

Basic physiology of the udder

To fully understand bovine mastitis disease, it is necessary to explain how cow udders work. Cows have four mammary glands (called quarters) that produce milk in large quantities. Each gland has a nipple through which the milk drains. The components for the formation of milk are obtained from blood. This blood passes through the mammary epithelial cells and is filtered there. Besides, these cells are responsible for adding substances such as casein, lactose, and fat. Once the milk has been formed, it is stored in the udder tanks.

The main defense mechanisms that cattle have developed to resist mastitis are listed below:

1. Physical: the udder nipples have ducts that, due to their tortuous anatomy, provide a physical barrier against infection. Also, after a few hours of milking, a keratin plug that prevents the entry of microorganisms is generated. When these physical defense mechanisms are affected by poor milking practices, handling, hygiene, or anatomic problems, the appearance of mastitis is favored.
2. Cellular: when a microorganism enters the nipple ducts, the cow’s immune system generates defense cells to counteract a possible infection. The majority of the cells that appear in this response are macrophages; a normal count of these cells in the milk is around <100,000 cells/milliliter. However, when the infection becomes more serious, that is, mastitis appears, a large number of other cells, called neutrophils, are generated with up to 1 million cells/milliliter in milk. This count of defense cells (leukocytes) is called the Somatic Cell Count (SCC) and it is an important parameter when evaluating milk. We will review it later.
3. Non-cellular: In addition to the cells that defend the integrity of the udder, other components appear to combat mastitis. Among them, the following stand out: Immunoglobulins, which are proteins that seek to eliminate microorganisms; lactoferrin, which prevents the multiplication of bacteria; the cytokines that modulate the entire cellular defense cascade.

Then, what exactly is mastitis? It is the inflammation of the mammary gland of cows, usually as a result of an infection. The cow has all the defense mechanisms explained above and use them to reduce the possibility of a mastitis.

Some authors (Smith, 2015) mention that the moment of birth of cows is a period of greater risk for the appearance of mastitis. This is explained because at the time of calving, the cow’s cortisol rises considerably and this hormone depresses the function of the immune system. This generates a lower cellular and humoral response in the udders against the entry of microorganisms.

Therefore, for those cows which are about to give birth it is a must to keep adequate housing measures considering good hygiene practices, adequate nutrition, welfare, and veterinary monitoring of the state of the udder and in general of the delivery.

Types of mastitis

Bovine mastitis is the inflammation of the mammary glands. Depending on the degree of inflammation, it is classified into two types: subclinical mastitis and clinical mastitis:

• Subclinical mastitis: In this mastitis, there are no obvious clinical signs or abnormal udder examination. However, there are changes in the amount of milk produced and in its composition.
• Clinical mastitis: In this, the affected quarter presents a decrease in milk production. Also, it has evidence of inflammation (pain, redness, swelling, warmth) on clinical examination. Even the cow may have systemic signs (fever, depression, low feed intake, diarrhea).

Diagnosis of bovine mastitis

Before addressing the diagnostic tests for mastitis, it is necessary to clarify that these can fluctuate depending on some factors. One of these factors is the time of lactation of the cow: in the peripartum period or at the end of the lactation, elevated levels of somatic cells can affect the reading of the test. On the other hand, other diseases (such as traumatic reticulum-peritonitis) can also influence these tests.

Therefore, for a correct diagnosis of mastitis, a holistic approach is essential.

• Somatic Cell Count (SCC)

The Somatic Cell Count (SCC) is a rapid and inexpensive test that allows evaluating the inflammatory status of the quarters of the cow’s udder. The quarter somatic-cell counts (QSCCs) can be performed in one or all four quarters at the same time.

Regarding the interpretation of the test, it is recognized that results slightly above 100,000 somatic cells/milliliter of milk indicate inflammation. When the count is excessively high, it may also indicate an infection. This is explained by a high presence of inflammatory cells in response to an infectious agent that invades the udder and ends up contaminating the milk.

To carry out a correct analysis of the SCC it is essential to establish the values ​​of the prevalence of mastitis on the same farm. This makes it possible to correlate the results obtained with the behavior of mastitis in the population, and thus identify sick and healthy animals.

• California Mastitis Test (CMT)

It is a quick and inexpensive test to assess possible inflammation of the mammary gland in cows.

The procedure consists of adding about 2 milliliters of milk from each quarter in the four spaces available on the paddle. Then, a reagent that comes with the kit is added and shaken gently and in circles for a few seconds.

The test reagent lyses leukocytes in milk. Their content is released and reacts with the components of the milk, increasing its viscosity. In cases where there is a high SCR, the viscosity can be so high that a gel is formed.

The test is subjectively analyzed, a visual inspection should be quickly performed (less than 20 seconds). The degree of viscosity of the milk generated in each quadrant of the paddle is assessed.

The results are interpreted as follows:

• Negative: there is no viscosity of the sample.
• Traces: there is a slight viscosity of the sample that disappears if the paddle is shaken a little more (in less than 20 seconds).
• Grade 1: there is viscosity in the sample, but it does not tend to form a gel; when the paddle is shaken for more than 20 seconds it will be diluted again.
• Grade 2: there are instantaneous sample viscosity and slight gel formation that covers the entire quadrant uniformly.
• Grade 3: there are complete gel formation and a rise in the center of the sample (such as a fried egg) that does not disappear when shaking the paddle.

The grades of the CMT test correlate with those of the RCS as indicated in this table:

What Bacteria Cause Bovine Mastitis?

Cow mastitis can be caused by a wide variety of bacteria. In general, when preventive measures have failed, and circumstances favor their appearance, bacteria colonize the mammary glands and cause mastitis.

The most common bacteria found to cause bovine mastitis are:

1. Streptococcus agalactie
2. Staphylococcus aureus
3. Coliforms (E.coli and less frequently Klebsiella spp.)

Other bacteria that can also cause bovine mastitis, but less frequently, are:

1. Mycoplasma
2. Corynebacterium bovis
3. Environmental Streptococcus.
4. Coagulase negative Staphylococcus
5. Fusobacterium pyogenes
6. Pseudomonas
7. Serratia
8. Nocardia
9. Mycobacterium

In general, the appearance of these bacteria is favored in hot environments with high humidity levels, poor hygiene conditions, and inadequate handling of the udder during milking.

Besides, it should be clarified that most subclinical mastitis has a developmental course of months or years. Also, not all clinical mastitis has systemic symptoms.

Some of these bacteria have developed resistance against antibiotics, so a specific group of antibiotics that is useful for each type of bacteria must be used. Without a doubt, drug treatment is not enough and this must be complemented with other prevention and treatment measures.

Therapeutic measures

The treatment of clinical mastitis depends on many factors that vary widely: the region or country, the breed of cow, the zootechnical management and the milking system. Therefore, here we mention some general aspects to take into account for the treatment of this disease in dairy herds.

1. Lactation period: should be taken into account since the antibiotic treatment may change if the cow is starting the productive cycle, if it is in the middle of the lactation, or if it is ending its production and is approaching the dry period.
2. Clinical history: it depends on the number of sick cows in the dairy farm, the number of births of each one and the number of affected quarters, since all this generates a lower response to antibiotic treatment for bovine mastitis.
3. Severity of the infection: the therapy established for mastitis depends on the bacterial agent involved and the amount of this population of pathogens present in the mammary glands. Furthermore, the intensity of the injury to the affected udder should be considered.
4. Choice of antibiotic: it is advisable to do a milk culture (it can be from only sick cows or the entire herd, if possible) and to do an antibiogram to select the best antibiotic against the specific microorganism that causes mastitis.
5. Complementary treatment: depending on the presentation, other therapeutic measures can be used to help improving the health of cows with mastitis. For example, the administration of fluids and electrolytes or the use of anti-inflammatory drugs.

Economic impact

Mastitis is probably the disease of the world’s dairy herds that generates the most economic costs due to its presentation and consequences. On one side, subclinical mastitis generates economic losses due to the low production of milk. On the other side, clinical mastitis, in addition to low milk production, demands an investment of resources focused on the therapeutic measures to be implemented. A large part of these losses is in the milk that must be excluded because it has drug residues (such as antibiotics) or a high load of cells and bacteria. Finally, cows that suffer from severe clinical mastitis and do not respond to treatment have to be slaughtered.

Bovine mastitis prevention measures

1. Milking process: all stages of this process should be reviewed, including disinfection of the nipple and the sealing of it. The entire vacuum system (in automatic milking) or personal hygiene (especially in manual milking) should also be check. Management after milking is critical, due to the high probability of mammary infection during the 2 hours after this process (especially if cows lie on or step on their teats).
2. Nutrition: There is evidence that indicates that making a contribution of micro and macronutrients in the diet of prepartum and postpartum cows has benefits in the prevention of mastitis. This is explained by a lower impact of negative energy balance (NEB, which is the production of ketone bodies) in individuals; This NEB favors the appearance of mastitis. Besides, ensuring the consumption of vitamins such as E or A and selenium provides an antioxidant effect that has shown to have benefits in preventing bovine mastitis.
3. Stress: peripartum is one of the most stressful moments for cows, which predisposes them to suffer from mastitis. Reducing stress at all possible times generates benefits at the immune level and strengthens the body’s response to bacterial invasion of the mammary glands. For example: offer shady places or use sprinklers to avoid heat stress.
4. Periodic evaluation: the farm must constantly evaluate the health status of the cows to quickly attend to the cows that may show signs of mastitis. In this disease, a quickly established treatment has good results.

Conclusions

Bovine mastitis is one of the most common diseases in dairy herds in the world and has a high impact on health and the economy of the farm. Producers, zootechnicians, and veterinarians must know the key aspects of this disease. Having clarity about the concepts related to the prevention of mastitis, its diagnosis, its treatment, and its economic impact, allows to face this disease and to avoid its frequent appearance in cow’s milk herds.

References:

• Calderón, A., & Rodríguez, V. C. (2008). Prevalencia de mastitis bovina y su etiología infecciosa en sistemas especializados en producción de leche en el altiplano cundiboyacense (Colombia). Revista Colombiana de Ciencias Pecuarias, 21(4), 582-589.
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