Mastitis - is basically an inflammation of the mammary gland i.e. the udder and teats . It can affect any herd, from the most organised, to the least, at any time; all herds are therefore potentially susceptible
Clinical Mastitis -- visible signs of mastitis which include:
• Mild signs flakes or clots in the milk, may have slight swelling of infected quarter.
• Severe signs secretion abnormal, hot, swollen quarter or udder; cow may have a fever, rapid pulse, loss of appetite, dehydration and depression; death may occur.
Subclinical Mastitis -- no visible signs of the disease:
• Somatic cell count (SCC) of the milk will be elevated.
• Bacteriological culturing of milk will detect bacteria in the milk.
• Causes the greatest financial loss to dairy farmers through lowered milk production.
• For every clinical case of mastitis, there will be 15 to 40 sub-clinical cases.
Somatic Cell Count (SCC) -- the number of leukocytes or white blood cells per milliliter of milk.
• Normal milk will have less than 200,000 cells per milliliter.
• An elevated SCC is an indication of inflammation in the udder.
• Bulk tank SCC gives an indication of the level of sub-clinical mastitis and the loss of milk production in a herd due to mastitis.
Mastitis usually occurs in response to:
• Intramammary infection (IMI) - primarily bacterial infection, but also mycoplasmal, mycotic (fungal), or algal infections. Most mastitis is caused by bacterial infection of the mammary gland.
• Mechanical trauma, which predisposes the gland to IMI
• Thermal trauma, which predisposes the gland to IMI
• Chemical insult, which predisposes the gland to IMI
Factors Affecting Susceptibility to Mastitis
This section deals with :
• Determinants of Mastitis
• Timing of Infection and Stage of Lactation
• Nutrition and Mastitis
Determinants of Mastitis
Whether or not IMI occurs depends on the interaction of host, agent, and environmental factors.
HOST FACTORS include: the presence/absence of natural resistance to mastitis, the state of defense mechanisms (are they present and are they functional), the stage of lactation, and whether there are stress factors.
AGENT FACTORS include: the number of organisms in the gland, the pathogenicity of the organisms (they must penetrate the gland, then adhere to the tissue, and then reproduce), the presence of other virulence factors, and the state of the host defenses for resistance to infection.
ENVIRONMENTAL FACTORS include: the design and function of the milking machine, the milking environment, the milking hygiene practices, the type of housing and bedding, and the weather.
Timing of Infection and Stage of Lactation
Most environmental IMI occur during the periods of active involution after drying off, the period just prior to calving, and during early lactation, however, new IMI can occur at any time.
Active involution - At the beginning of the dry period the cow is no longer milked. The mammary gland undergoes a period of active involution Several situations which occur within the first two weeks after a cow is dried off predispose the mammary gland to mastitis:
• The gland continues to secrete milk with maximum fluid accumulation occurring 2 to 3 days after milk removal is stopped. Pressure in the gland can cause the streak canal to widen and the teat sphincter to dilate, allowing bacteria into the gland.
• Bacteria inside the gland are no longer regularly removed from the gland by the milking process.
• Teat dipping ceases.
• Phagocytes are involved in removal of secretory cell products (fat, casein), and are not particularly efficient at removing bacteria in the gland.
• Increasing amounts of immunoglobulins and lactoferrin (decreasing citrate : LF ratio) in the gland improve defense, but cannot override the problems noted above.
• Dry cow treatment of all quarters with antibiotics at the time of drying off reduces streptococcal and staphylococcal (but not coliform) IMI during active involution.
• Reducing the period of active involution by infusing colchicine (disrupts milk secretion mechanisms) decreases IMI during the active involution phase.
Peripartum Period - Several defense mechanisms are compromised during the period just prior to and after parturition (coinciding with colostrum formation) which predispose the gland to mastitis.
• Fluid volume in the gland increases resulting in increasing intramammary pressure and dilation of the streak canal and sometimes leakage of colostrum.
• Citrate concentration rises and lactoferrin is low.
• Phagocytic cells are not efficient at engulfing and killing bacteria in colostrum contained in the gland at this time.
• High immunoglobulin concentrations in the gland at this time are not effective in preventing new IMI. IgG1, the major immunoglobulin isotype in cow colostrum, is not normally an effective opsonin in the mammary gland.
• Antibiotic concentration from dry cow antibiotic therapy is too low to combat infection.
• Teat dipping during this period is not particularly effective in mastitis prevention.
Early lactation - Cows in early lactation are metabolically stressed. This stress can compromise the cow's disease resistance and result in clinical outbreaks of subclinical infections (eg. IMI acquired during the dry period or at calving time). In addition, mastitis is sometimes associated with high concentrate feeding which accompanies early lactation.
Nutrition and Mastitis
Deficiencies of the following vitamins and minerals have been shown to be related to increased incidence of clinical or sub-clinical mastitis, increased severity of infection, or elevated somatic cell counts:
o Selenium - deficient in soils of the Midwest and northeast. Protects mammary tissue from oxidative damage and augments phagocytic function.
o Vitamin E. Protects mammary tissue from oxidative damage and augments phagocytic function.
o Less well documented in their relationship to mastitis are: B-Carotene (high in fresh forage, low in stored feed), Vitamin A, Zinc, Copper, Cobalt, and some others.
Detection and Diagnosis of Mastitis
The ideal means of dealing with mastitis is to prevent it from happening. However, even under the best prevention and control programs, mastitis will occur. Remember that mastitis is an inflammation of the mammary gland. Detection of mastitis is generally based upon some manifestation of the inflammation (changes in the udder or in the milk). However, treatment of mastitis works best if there is some information on the particular bacterium causing the problem. So, detection of the inflammation and diagnosis of the specific cause of the inflammation are important.
Detection of the Inflammation : Detection of the inflammation is based upon the response of the animal to the infection. From the sections in Mastitis Lesson A you learned that several significant changes occur in the tissue and in the milk in response to infection. These include infiltration of leukocytes (referred to as somatic cells) and increased vascular permeability, resulting in alterations in the chemistry of the milk resulting from hydrolysis of milk proteins by hydrolytic enzymes and oxidative substances released from phagocytes, alterations in milk pH and ionic solutes, and ingestion of milk components by phagocytes.
Approaches to Detection of Mastitis
Visualization and palpation of the udder : In clinical mastitis, the udder may turn hard, red, and hot to the touch. Palpation of the udder may be painful to the cow. These symptoms arise from the changes in vascularity and blood flow of the gland when inflammed.
Detection of Somatic Cells : Several methods for detection of mastitis are available for detecting somatic cells in milk, including the California Mastitis Test (CMT; a cow-side test), the Wisconsin Mastitis Test (WMT; on-farm test), Microscopic Somatic Cell Count (usually requires a laboratory), and Electronic Somatic Cell Counting (requires sophisticated equipment). The CMT and WMT detect formation of a gel when DNA in somatic cells react with a detergent. The reaction occurs on a paddle (CMT) and is graded subjectively (neg, trace, 1,2,3), or in a tube (WMT) and is measured in millimeters. CMT or WMT results can be used as rough estimates of the number of somatic cells in milk and to identify subclinically infected mammary glands.
Somatic cell counts can be run on milk from the bulk tank (this indicates herd mastitis status) or from individual cows (detects a specific cow with possible mastitis). Somatic cell counts of less than 100,000 to 200,000 cells/ml of milk are not indicative of mastitis. As cell counts increase so does the chance that a bacterial infection is present. A bulk tank somatic cell count exceeding 750,000 cells/ml will result in loss of Grade A milk status (the producer cannot continue selling the milk until the problem is fixed).
N-acetyl-ß-D-glucosaminidase (NAGase) : NAGase is a lysosomal enzyme which increases in milk when mastitis is present. Some NAGase comes from the leukocytes infiltrating the tissue, but most of it comes from the epithelial cells in response to the presence of the leukocytes. So, the activity of NAGase in milk is highly correlated with the concentration of leukocytes (the somatic cell count). Assays for NAGase are enzyme assays and often use fluorescent substrates. The amount of fluorescence generated in these assays is correlated with the somatic cell count and with the degree of inflammation.
Electrical Conductivity : Electrical conductivity of milk increases during mastitis due to increases in Na+ and Cl- and decreases in K+ and lactose. Changes in conductivity can be detected by hand-held or in-milkline instrumentation.
Detection of Antibodies in Milk : An ELISA test has been developed to detect antibodies to Staphylococcus aureus in milk. Antibodies to S. aureus may be present in milk long after an infection is gone, or if it is a new infection the antibodies may be slow in increasing in concentration, these may result in false positives and false negatives in individual cases. Consequently, this method has limitations on detecting S. aureus as the cause of acute mastitis, but may be useful in situations of chronic mastitis and whole herd screening.
Identification of the Pathogen Causing Mastitis
Collecting sterile milk samples for bacteriological culture is the only way to definitively diagnose infectious mastitis and identify the causative organism. Proper technique in collecting the sterile milk sample is essential! Separate samples usually are collected from each quarter. Remove dirt and water from the teats and udder; wash and dry hands; strip out the foremilk; disinfect the teat ends with alcohol and/or teat dip (do the teats on the side away from you first); remove the cap from the sterile sample tube taking care to prevent contamination inside the tube; collect sample with tube at 45 degree angle to teat (collect the near teats first); replace the cap on the tube and dip the teats.
Proper sample handling : Refrigerate or keep the sterile milk samples on ice (if plating will occur in less than 24 hours) or freeze the samples (if storing for greater than 24 hours).
Plating the sample : Plating the samples requires special bacteriological medium, conditions of a laboratory and personnel with good technique in microbiology. Agitate the milk sample to mix. Plate 0.01 ml milk per each quarter of Blood agar plate using a loop or pipette. For suspected coliforms, plate 0.1 ml milk for each half of a Blood agar plate or plate on MacConkey agar. Incubate at 37 C for 48 hours.
Pathogens vs Contaminants : How do you tell if growth on the plate is a pathogen or a contaminant? These criteria assume you are using good technique in collecting the sterile sample and in plating the samples. Greater than or equal to 5 identical colonies from 0.01 ml milk (pure culture) = significant. Greater than or equal to 5 identical colonies from 0.01 ml milk (mixed culture) = significant, but questionable. Less than 5 identical colonies from 0.01 ml milk = a contaminant.
Intrepretation : Streptococcus agalactiae and Staphylococcus aureus are usually significant (representing a causative agent) in any number. Bacillus is usually a contaminant when present in any number.
Antibiotic Sensitivity Tests : are often used to determine what antibiotics may affect the bacterium isolated from the udder. However, these take time and additional microbiological techniques. These tests use either disc diffusion or minimal inhibitory concentration assays. The in vitro results may not necessarily reflect the in vivo sensitivity of the bacterium.
Control of Mastitis
Mastitis Control
Awareness of the economic losses associated with mastitis is resulting in a desire for mastitis control programs. Control programs are focused on detection of mastitis (by the above methods), identification of the causative agent(s) and prevention of transmission by removing the source of the agent (milk contaminated fomites, bedding, persistently infected cows, etc.). Knowledge of mammary anatomy and physiology, mammary defense mechanisms, microbial habitats, microbial virulence factors, milking machine function, and antibiotics/germicides is important in achieving effective mastitis control.
