Mastitis: Its Diagnosis and Control

Introduction

Mastitis is a potentially fatal inflammation of the cow’s mammary gland, which is usually caused by bacteria entering the teat canal and moving into the udder tissue. Toxins released by mastitis bacteria damage milk-secreting tissue and ducts throughout the mammary gland, reducing milk yield and quality. Mastitis can occur at any stage of lactation, including the dry period, but is most likely in the first month after calving and in late lactation.

What Causes Mastitis in Cows

Mastitis occurs when large numbers of white blood cells (leukocytes) migrate into the mammary gland, usually in response to bacteria invading the teat canal through environmental contact or during the milking process.

Streptococcus  agalactiae  is the most  common  cause  of  subclinical mastitis and may rarely cause acute mastitis. The organism lives inside the cow’s udder and survives only for a short time outside the mammary gland. It spreads primarily during milking via the milking machine and contaminated hands and materials (cloths). The microorganism can be eradicated from the herd by appropriate treatment combined with good milking practices

Staphylococcus  aureus  lives inside  and  outside the  udder on  the teat skin.  The microorganism can cause both clinical and sub-clinical mastitis and spreads the same way as S. agalactiae.

Yeasts are also responsible for causing mastitis. Overuse of antibiotics and poor sanitation contribute to yeast mastitis. It can also be caused by an injury to the cow’s udder.

Factors Influencing Susceptibility to Mastitis

1. Type of bacteria: Influences the type of symptoms and disease. Some bacteria are more virulent than others in causing mastitis.
2. Physiological status of cow: Although infection can occurs at any time, most of the new infections take place during the first three weeks of the dry period and during the first month after parturition, suggesting that level of milk production is not directly related to mastitis. It is likely that intra-mammary pressure is a predisposing factor for  mastitis  during  these periods.
3. Age of the cow: The incidence of mastitis increases with age. It is possible for the udder of the first-calf heifer to be infected at parturition.
4. Level of milk production: Not directly related to incidence of mastitis. However, other factors, which affect milk, yield such as milking rates, pendulous udders may be related to mastitis incidence.
5. Inherited features of the cow: Length of the leg in proportion to the udder size and relative strength of the udder attachment are the examples of inherited factors that influence the occurrence of  mastitis.  Large, pendulous udders tend to exceed the capacity of the supporting ligaments, resulting in breakdown of the udder that will ultimately subject the udder to more physical injuries and thus increases the incidence of mastitis.
6. Milking machine: Improper use  of  milking  machine  (irregular fluctuation  of  vacuum  level,  over-milking,  and  incomplete  milking)  is related to tissue irritations and incidence of mastitis.
7. Environment: Mastitis often increases when cows are turned onto pastures. Housing as it relates to the degree of udder and teat injury are some contributing factors.

Types of Mastitis

There are several ways of classifying mastitis but a simple classification recognizes it into two major groups:

1. Contagious Mastitis: Caused by bacteria living on the skin of the teat and inside the udder. It can be transmitted  from  one cow  to another during milking.
2. Environmental mastitis: Is caused by the certain organisms such as Escherichia coli which do not normally live on the skin or in the udder but which enter the  teat  canal. When  the  cow  comes  in  contact  with contaminated  environment .

Contagious mastitis can be divided into three groups

1. Clinical Mastitis

Characterized by the presence of gross inflammation signs (swelling, heat, redness, pains).

• Peracute Mastitis

Is  usually  characterized  by  visibly  gross  inflammation,  disrupted functions  (reduction  in  milk  yield,  changes  in  milk  composition)  and systemic  signs  (fever,  depression,  shivering,  loss  of  appetite  and  loss  of weight).

• Acute Mastitis

Similar to peracute mastitis, but with lesser systemic signs (fever and mild depression).

• Sub-Acute Mastitis

In this type of  mastitis, the  mammary  gland  inflammation  signs  are minimal with no visible systemic signs.

2. Sub-Clinical Mastitis

Mastitis characterized by change in milk composition with no signs of gross inflammation or milk abnormalities. Changes in milk composition can be detected by special diagnostic tests

3. Chronic Mastitis

An inflammatory process that exists for months and may continue from one lactation to another. Chronic mastitis for  the most part exists  as sub-clinical but may exhibit periodical flare-ups, sub-acute or acute form, which last for a short period of time.

Environmental mastitis

Environmental organisms are found in bedding, soil, walkways, on pasture or any surface with which the cow or her manure comes in contact. Organic bedding materials such as wood shavings, straw or recycled dry manure solids are common sources.  Cattle that congregate in cool, shaded areas while on pasture may contaminate the ground with manure and other discharges, thus making this area a source of mastitis causing bacteria.

The prevention of mastitis can be achieved by:

1. Proper milking hygiene. Bacteria are transmitted to the udder from the contaminated hands of the milker.  Thus the milker’s hands should be washed thoroughly with disinfected soaps before milking while the clinically infected  cows  should  be  milked  last.  Teats should be cleaned and dried before milking.
2. Milking machine. Machines  for  milking  should  function  and operate properly. Vacuum level in the milking unit should be between 300 mm of mercury with little fluctuation. The vacuum regulator should be kept clean and checked regularly.
3. Dipping the teats  after  milking.  Teat  dipping  does  not  reduce existing infection. However, when suitable disinfectant is used to immerse or spray the teats the rates of new infection can be reduced by up to 50%.
4. Dry treatments. Incidence of mastitis during the dry period can be considerably reduced by effective use of antibiotic infusions in each quarter of the udder at the last milking of lactation. Dry cow therapy is the one of the best way to cure chronic and subclinical  mastitis that  are difficult  to treat successfully during lactation.
5. Culling of chronically infected cows. This is an effective method because  in most  herds only  6-8%  of all  cows account  for 40-50%  of all clinical mastitis.
6. Nutrition. Deficiencies of selenium and vitamin E in the diet have been associated with an increased rate of new mammary infections.

Diagnosis of Mastitis

Current  trends  in  diagnosis  of  mastitis  involves  following  routine diagnostic tests:

1. Physical examination of udder: It is emphasised to view the shape, size, consistency and contour of the udder properly. Detailed examination of the teat and teat orifices should be made to assess inflammation, hot painful swelling and loss of
2. Strip cup test: In individual  animals  and  in  herds,  strip  cup  or  strip  plate  test  is routinely  used  in milking  parlor for detection of  clinical mastitis. In  herd health  management  practice  operators  of  the  milking  machines  visually examine  the fore  milk for gross  abnormalities by  squirting few stripes  of milk on strip cup where the abnormalities are usually manifested in the form of blood, flakes, clots and wateriness suggestive of mastitis.
3. California mastitis test: It estimates the number of somatic cells present in milk. Procedure for conducting CMT test is by mixing the test reagent (CMT  reagent)  with  an  equal quantity  of  milk.  The  reagent reacts with DNA of the nuclei of the somatic cells in the milk to form a gel. Depending upon the amount of  gel formation. Formation of more gel indicates higher somatic cell count.
4. Wisconsin mastitis test: Is primarily a laboratory test which is generally conducted on bulk tank milk samples. In both WMT and CMT same type of reagent is used. WMT the test result reaction  are  measured  (mm).  The test is conducted by combining a measured quantity of milk with an equal amount of reagent. The milk and reagent are then mixed for 8 to 10 seconds. The mixture is drained  for  a period  of  18 seconds  and  returned to  an  upright position. After waiting for one minute, the amount of fluid remaining in the tube is measured. WMT scores are generally calculated in millimetres (mm) and used to predict the average number of somatic cells present in the milk.
5. Modified white side: Test Increased leukocytic count  of  milk  forms  the  basis  of  this  test.  In mastitis, test shows formation  of white  flakes,  while in  normal  milk; test result shows formation of milky opaque fluid.
6. pH Determination test: The normal pH of milk is 6.4 to 6.8 being isotonic with blood plasma. In mastitis occurring during late lactation and dry period, the concentration of lactose and  casein  in  the  milk  is  reduced  whereas  sodium  chloride  and sodium bicarbonate pass into the alveoli from plasma to maintain isotonicity. Therefore during such situations milk becomes alkaline with greater amount of chlorides.
7. Chloride test: This test detects presence of increased quantity of chlorides in mastitic milk. Normal milk contains about 0.07 per cent of chlorides. In mastitis there is decreased amount of lactose and increased amount of sodium chloride  to maintain the normal  milk osmotic  pressure hence during inflammation there is increase in the chloride content (> 0.14 per cent).
8. Electrical conductivity test. The conductivity of milk may be defined as the property of substances in solution which can ionize and therefore can conduct an electrical current. When the concentration of sodium chloride rises in milk, the conductivity rises proportionately. Therefore, measurement of  electrical  conductivity is used  as  a  simple  physical  method  to  diagnose  mastitis.  Electrical conductivity  of  milk  can  be determined  by  using  a  hand  held  (portable) electrical conductivity meter (milk checker or digital mastitis detector). The EC of milk is expressed in the unit of milk seimens/cm (Ms/cm).
9. Somatic cell count of milk. In direct microscopic somatic cell counts, milk smear is made on a clean glass slide in the area of 1 cm stained with 1% methylene blue and 60 fields are examined for  the  count.  The  average  numbers  of  cells  per  field  are multiplied by the  multiplication factor  of the  microscope and the  value so obtained is considered equal to the number of cells/ml of milk sample.
10. N-acetyl-β-D-glucosaminidase test (NAGASE) test. This test is based  on the  measurement of a cell  associated enzyme N-acetyl  B-D  Glucosaminidase  in the  milk.  A  highest level  of  the enzyme indicates high cell count. NAGASE test is a simple, effective and most reliable for diagnosis of sub clinical mastitis. The activity ranges  of NAGASE  for  normal milk  (<0.5  x  10 4  cells /ml)  and mastitis  milk  (1.5  x  104  cells/ml)  are  0.0053  and  0.034/  mole  /min/ml respectively.  Among the  all-indirect diagnostic  tests, NAGASE  activation estimation is found to have the highest accuracy.
11. Methylene blue reduction test (MBRT). Methylene blue reduction test measures chemical activities, especially the respiratory activities of the bacteria in the milk. The microorganisms in the milk first remove oxygen by respiration and when all oxygen and some other reducible components of the milk has been removed or reduced, there is change in colour of methylene blue to methylene white.
12. Milk Antitrypsin Assay (Maum Test): Milk antitrypsin activity is due to leakage of blood Protease inhibitor into milk and represents  increased permeability. This  was  increased  to  many  folds  and  the  trypsin inhibitor test appeared to be  superior over CMT  in diagnosing  sub-clinical mastitis.