Acute Contagious Mastitis (Staphylococcus aureus, Streptococcus agalactiae, Mycoplasma bovis)

Acute contagious mastitis is a highly prevalent and economically devastating inflammation of the bovine mammary gland, primarily caused by contagious pathogens including Staphylococcus aureus, Streptococcus agalactiae, and Mycoplasma bovis. This condition is characterized by a sudden onset of clinical signs, ranging from mild milk abnormalities to severe systemic illness, and is a major concern in dairy herds worldwide. The disease is defined by its contagious nature, spreading from cow to cow primarily during the milking process, and its tendency to cause chronic, subclinical infections that persist within a herd. Acute contagious mastitis is a significant cause of reduced milk yield, increased somatic cell counts (SCC), premature culling, and treatment costs, making it a primary target for mastitis control programs. The condition is particularly relevant in high-producing dairy cows, especially during early lactation and the periparturient period, where immune function is compromised and metabolic stress is high. In beef cattle, contagious mastitis is less common but can occur, particularly in suckler herds where calf transmission may play a role. The economic impact includes direct losses from discarded milk, reduced milk quality premiums, increased veterinary costs, and indirect losses from decreased reproductive performance and increased culling rates. Effective management requires a comprehensive understanding of the epidemiology, pathogenesis, and control measures specific to these contagious pathogens.

The primary etiological agents of acute contagious mastitis are Staphylococcus aureus, Streptococcus agalactiae, and Mycoplasma bovis. Staphylococcus aureus is a Gram-positive, coagulase-positive coccus that produces a range of virulence factors, including surface adhesins (e.g., fibronectin-binding proteins, clumping factor), exotoxins (alpha-toxin, beta-toxin, leukocidins), and enzymes (coagulase, hyaluronidase, staphylokinase). These factors facilitate adherence to mammary epithelial cells, evasion of the host immune response, and tissue destruction. S. aureus can form microabscesses and scar tissue within the mammary gland, leading to chronic, refractory infections. Streptococcus agalactiae is a Gram-positive, beta-hemolytic coccus that is an obligate parasite of the bovine mammary gland. It produces enzymes such as hyaluronidase and streptokinase, which promote tissue invasion and inflammation. S. agalactiae is highly contagious and spreads rapidly through contaminated milking equipment and hands. Mycoplasma bovis is a small, cell-wall-deficient bacterium that is a major cause of mastitis, arthritis, and pneumonia in cattle. It adheres to mammary epithelial cells and can invade the gland, causing severe inflammation and necrosis. M. bovis is resistant to many antibiotics due to its lack of a cell wall, making treatment challenging. Other less common contagious pathogens include Corynebacterium bovis and Mycoplasma californicum, but S. aureus, S. agalactiae, and M. bovis are the most significant. The etiology also includes management factors that facilitate transmission, such as inadequate milking hygiene, poor teat disinfection, and lack of dry cow therapy.

Acute contagious mastitis is a global disease affecting dairy cattle of all breeds, but it is most prevalent in high-producing Holstein-Friesian cows. The disease is more common in intensive dairy systems with high stocking densities and year-round calving, where the risk of transmission is increased. Age and parity are significant risk factors, with older cows (third lactation and beyond) being more susceptible due to cumulative exposure and damage to the teat canal. The transition period and early lactation are critical times for new infections, as the immune system is suppressed and the udder is under metabolic stress. Seasonal variations are observed, with higher incidence in summer months in some regions, possibly due to increased environmental contamination and fly populations. Herd size is positively correlated with mastitis prevalence, as larger herds have more opportunities for pathogen spread. Morbidity rates can be high, with up to 30-50% of cows in an infected herd showing clinical or subclinical mastitis. Mortality is rare but can occur in severe cases of acute gangrenous mastitis caused by S. aureus, especially in first-calf heifers. The economic impact is substantial, with estimated losses of $200-400 per cow per year in affected herds, including reduced milk production, discarded milk, treatment costs, and increased culling. The prevalence of contagious mastitis has decreased in many developed countries due to successful control programs, but it remains a significant problem in herds with poor milking hygiene and inadequate biosecurity.

The pathophysiology of acute contagious mastitis involves a complex interplay between bacterial virulence factors and the host's inflammatory and immune responses. The primary route of infection is ascending through the teat canal, which is the first line of defense. The teat sphincter and keratin lining provide physical and chemical barriers, but these can be compromised by milking machine malfunction, trauma, or poor hygiene. Once bacteria enter the gland cistern, they adhere to the epithelial lining and multiply. Staphylococcus aureus produces alpha-toxin, which is a pore-forming cytolysin that damages epithelial cells and leukocytes, leading to tissue necrosis and abscess formation. The bacteria can also invade epithelial cells and survive intracellularly, evading the immune system and antibiotics. Streptococcus agalactiae colonizes the teat cistern and large ducts, causing inflammation and edema. It produces enzymes that break down tissue components, facilitating spread. Mycoplasma bovis adheres to the epithelial surface and can invade the gland, causing severe inflammation and necrosis. The host response involves the recruitment of neutrophils to the site of infection, which release reactive oxygen species and proteolytic enzymes, contributing to tissue damage. The inflammatory response leads to increased vascular permeability, resulting in edema, heat, and pain. In severe cases, systemic inflammatory response syndrome (SIRS) can occur, characterized by fever, tachycardia, and endotoxemia, leading to shock and death. The release of bacterial toxins and inflammatory mediators can also cause damage to other organs, such as the liver and kidneys. Chronic infections lead to fibrosis and atrophy of the mammary tissue, resulting in permanent loss of milk production.

Predisposing factors for acute contagious mastitis can be divided into intrinsic and extrinsic categories. Intrinsic factors include high milk yield, which increases the risk of teat canal damage and bacterial colonization; parity, with older cows having more exposure and damage; genetics, as some cows are more susceptible to mastitis; and immune suppression, particularly during the periparturient period when cortisol levels are elevated and neutrophil function is impaired. The transition period is a critical time, as cows experience negative energy balance, hypocalcemia, and oxidative stress, all of which compromise immune function. Extrinsic factors include poor milking hygiene, such as inadequate teat disinfection before and after milking, contaminated milking equipment, and improper milking procedures (e.g., overmilking, high vacuum pressure). Environmental factors such as dirty bedding, overcrowding, and poor ventilation increase the risk of exposure to pathogens. Lack of dry cow therapy and inadequate treatment of clinical cases can lead to chronic infections and increased transmission. Nutritional factors, such as vitamin E and selenium deficiency, can impair immune function and increase susceptibility. Management practices that stress cows, such as abrupt ration changes, overcrowding, and poor cow comfort, also predispose to mastitis.

Clinical signs of acute contagious mastitis vary depending on the causative agent and the severity of the infection. In peracute cases, especially with S. aureus, cows may exhibit severe systemic signs including fever (rectal temperature > 40°C), depression, anorexia, tachycardia, and tachypnea. The affected quarter is swollen, hot, painful, and edematous, with a hard consistency. Milk from the affected quarter is abnormal, appearing watery, with flakes, clots, or a serous or bloody appearance. In cases of gangrenous mastitis, the skin of the teat and udder may become cold, blue, and necrotic, with a characteristic foul odor. Streptococcus agalactiae infections often cause milder clinical signs, with mild to moderate swelling and milk abnormalities, but can progress to severe inflammation if untreated. Mycoplasma bovis mastitis is often acute and severe, with marked swelling, pain, and systemic signs, and can affect multiple quarters simultaneously. The milk may be watery with a sandy or granular appearance. Chronic infections may be subclinical, with no visible signs but elevated SCC and reduced milk production. Herd-level signs include an increase in bulk tank SCC, an increase in clinical mastitis cases, and a higher incidence of new infections. In beef cattle, clinical signs may be less obvious, but calves may fail to thrive due to poor milk quality.

Differential diagnoses for acute contagious mastitis include other forms of mastitis caused by environmental pathogens such as Escherichia coli, Klebsiella pneumoniae, and Streptococcus uberis. These are typically associated with environmental contamination and may present with similar clinical signs, but they are not contagious and require different control measures. Other differentials include teat injuries, udder edema, and abscesses. Teat injuries can cause swelling and milk abnormalities but are usually unilateral and associated with trauma. Udder edema is common in fresh cows and is characterized by pitting edema without systemic signs. Abscesses may be palpable as fluctuant masses. Additionally, conditions such as toxic mastitis due to coliforms can cause severe systemic signs and must be differentiated from contagious mastitis. Laboratory tests, including bacterial culture and milk SCC, are essential for definitive diagnosis. The presence of multiple affected quarters and a high bulk tank SCC may suggest contagious pathogens, while environmental pathogens often affect single quarters and are associated with poor bedding hygiene.

The diagnostic algorithm for acute contagious mastitis begins with a thorough herd history, including recent mastitis incidence, bulk tank SCC, and milking practices. A complete physical examination of the cow, including assessment of the udder, is performed. The California Mastitis Test (CMT) is a rapid, cow-side test that detects elevated SCC in milk, indicating inflammation. A positive CMT result warrants further investigation. Milk samples should be collected aseptically from affected quarters for bacterial culture and sensitivity testing. Samples should be taken before antibiotic treatment and transported to a laboratory promptly. Culture results can identify the causative agent and guide antimicrobial therapy. For Mycoplasma, special culture media and longer incubation times are required. Molecular techniques such as PCR can provide rapid and specific identification. Additional diagnostic tests include measurement of milk SCC using an electronic counter, which is more precise than CMT. Blood tests may be performed to assess systemic involvement, including complete blood count and serum biochemistry. Ultrasonography of the udder can be used to assess the extent of tissue damage and abscess formation. In cases of suspected Mycoplasma, PCR on milk samples is the preferred method. The diagnostic algorithm should also include evaluation of milking equipment and hygiene practices to identify potential sources of transmission.

Laboratory findings in acute contagious mastitis include elevated milk somatic cell count (SCC), typically > 200,000 cells/mL in infected quarters, and often > 1,000,000 cells/mL in clinical cases. The California Mastitis Test (CMT) is positive, with scores of 2 or 3. Bacterial culture of milk samples yields the causative organism, with S. aureus, S. agalactiae, or M. bovis identified. For S. aureus, culture on blood agar shows beta-hemolysis and coagulase positivity. S. agalactiae is beta-hemolytic and is CAMP-positive. M. bovis requires special media (e.g., Hayflick's medium) and grows slowly, taking 3-10 days. Blood tests may reveal leukocytosis with a left shift, indicating an inflammatory response, or leukopenia in severe cases. Serum fibrinogen levels may be elevated. In cases of systemic involvement, blood gas analysis may show metabolic acidosis. Milk composition is altered, with increased chloride and sodium, and decreased lactose and potassium. In Mycoplasma mastitis, the milk may have a characteristic sandy appearance. Additionally, PCR assays can detect bacterial DNA in milk samples, providing rapid and sensitive diagnosis.

Imaging modalities are not routinely used for the diagnosis of acute contagious mastitis, but they can be helpful in assessing the extent of tissue damage and complications. Ultrasonography of the udder can be performed using a 5-7.5 MHz linear probe. In acute mastitis, the affected quarter may show increased echogenicity of the parenchyma, edema, and abscess formation. Abscesses appear as hypoechoic or anechoic cavities with hyperechoic walls. Ultrasonography can also be used to guide aspiration of abscesses for culture. In cases of gangrenous mastitis, Doppler ultrasound may show reduced blood flow to the affected area. Radiography is not useful for mastitis diagnosis, but it may be used to evaluate underlying bone or joint involvement in cases of Mycoplasma arthritis. Endoscopy is not applicable. In research settings, advanced imaging such as MRI or CT may be used to study the pathophysiology of mastitis, but these are not practical in clinical practice.

Cytological examination of milk from affected quarters reveals a high number of neutrophils, often with degenerative changes, and the presence of bacteria. In chronic cases, macrophages and lymphocytes may be more prominent. Histopathological examination of mammary tissue from biopsy or necropsy shows acute inflammation with edema, neutrophil infiltration, and necrosis of epithelial cells. In S. aureus infections, microabscesses and fibrosis are common. S. agalactiae infections cause less tissue destruction but significant inflammation. M. bovis infections are characterized by severe necrotizing mastitis with extensive tissue damage and infiltration of mononuclear cells. Histopathology can also reveal the presence of bacterial colonies within the tissue. In cases of gangrenous mastitis, there is coagulative necrosis of the skin and underlying tissue. Histopathology is not routinely performed in clinical cases but is valuable for research and for confirming the extent of tissue damage.

Treatment of acute contagious mastitis should be initiated promptly to minimize tissue damage and prevent systemic complications. The approach includes supportive care, antimicrobial therapy, and anti-inflammatory treatment. For cows with severe systemic signs, intravenous fluids (e.g., isotonic crystalloids at 20-40 mL/kg) and non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine (1.1-2.2 mg/kg IV) or meloxicam (0.5 mg/kg SC) are indicated. Intramammary antibiotics are the mainstay of treatment for mastitis. For S. aureus and S. agalactiae, commonly used intramammary preparations include ceftiofur, penicillin, and amoxicillin. However, S. aureus infections are often refractory to treatment due to intracellular survival and abscess formation. Systemic antibiotics may be used in addition to intramammary therapy, especially for S. aureus and Mycoplasma. For Mycoplasma, effective antibiotics include oxytetracycline (10 mg/kg IV or SC), tylosin (10 mg/kg IM), or enrofloxacin (7.5-10 mg/kg SC). However, Mycoplasma is often resistant to many antibiotics, and treatment may be unsuccessful. Supportive care includes frequent milking of the affected quarter to remove toxins and bacteria, and the use of oxytocin (20-40 IU IV or IM) to aid milk let-down. In cases of gangrenous mastitis, surgical amputation of the affected teat or quarter may be necessary. The prognosis for recovery is guarded, especially for S. aureus and Mycoplasma infections. Treatment should be based on culture and sensitivity results whenever possible.

The prognosis for acute contagious mastitis depends on the causative agent, the severity of the infection, and the promptness of treatment. For S. agalactiae, the prognosis is generally good with appropriate antibiotic therapy, as the organism is highly susceptible to penicillin and other beta-lactams. However, if left untreated, it can become chronic and cause significant milk loss. For S. aureus, the prognosis is guarded to poor, especially in chronic cases, due to the formation of microabscesses and the ability of the bacteria to survive intracellularly. Cure rates are often low, and many cows become chronic carriers with elevated SCC. For Mycoplasma bovis, the prognosis is poor, as the organism is resistant to many antibiotics and can cause severe, irreversible damage to the mammary gland. Affected cows often have permanent loss of milk production in the affected quarter and may need to be culled. The short-term prognosis for survival is good unless the cow develops severe systemic signs or gangrenous mastitis, which can be fatal. Long-term, affected cows are at increased risk of culling due to reduced milk yield and reproductive performance. Negative prognostic indicators include involvement of multiple quarters, severe systemic signs, and lack of response to treatment within 48-72 hours.

Follow-up care for cows with acute contagious mastitis is essential to monitor recovery and prevent recurrence. Cows should be re-examined 48-72 hours after initiation of treatment to assess response. Milk samples should be collected for culture 2-3 weeks after treatment to confirm bacteriological cure. SCC should be monitored monthly to ensure that it returns to normal levels. Cows that do not respond to treatment or become chronic carriers should be considered for culling to reduce the risk of transmission to other cows. Herd-level follow-up includes monitoring bulk tank SCC and clinical mastitis incidence. Milking equipment should be inspected and maintained to ensure proper function. Milking procedures should be reviewed, including teat disinfection and hygiene. Cows with chronic infections should be milked last to prevent spread. A comprehensive mastitis control program, including dry cow therapy and vaccination, should be implemented. Nutritional management should be optimized to support immune function. Regular monitoring of transition cows and early lactation cows is important to detect new infections early.

Clinical pearls: 1) In acute mastitis, always check for systemic signs such as fever, depression, and dehydration, as these indicate a need for aggressive supportive therapy. 2) Use the California Mastitis Test (CMT) as a rapid screening tool for subclinical mastitis; a score of 2 or 3 indicates high SCC. 3) For Mycoplasma mastitis, suspect when multiple quarters are affected and the milk has a sandy appearance; confirm with PCR. 4) In cases of S. aureus mastitis, consider systemic antibiotics in addition to intramammary therapy, as the organism can invade deeper tissues. 5) Always culture milk samples before starting antibiotics to guide therapy. Pitfalls: 1) Do not use antibiotics without culture and sensitivity, as this can lead to antimicrobial resistance and treatment failure. 2) Avoid using anti-inflammatory drugs in cows with severe dehydration without fluid therapy, as this can worsen renal function. 3) Do not milk affected cows into the same line as healthy cows, as this can spread infection. 4) Do not ignore the importance of milking hygiene; failure to disinfect teats and equipment can lead to herd outbreaks. 5) Do not treat chronic S. aureus infections indefinitely; culling may be more cost-effective.

Current drug protocols for acute contagious mastitis are based on Plumb's Veterinary Drug Handbook and AABP guidelines. For systemic signs, flunixin meglumine (Banamine) is administered at 1.1-2.2 mg/kg IV or IM, once daily for up to 3 days. Meloxicam (Metacam) is an alternative at 0.5 mg/kg SC, once. For fluid therapy, isotonic crystalloids (e.g., lactated Ringer's solution) are given at 20-40 mL/kg IV over 1-2 hours. For S. agalactiae, intramammary penicillin (e.g., 100,000 IU per quarter) or ceftiofur (e.g., 125 mg per quarter) is administered every 12 hours for 2-3 days. For S. aureus, intramammary ceftiofur or amoxicillin-clavulanate may be used, but systemic antibiotics such as oxytetracycline (10 mg/kg IV or SC) or ceftiofur (2.2 mg/kg SC) are often added. For Mycoplasma, oxytetracycline (10 mg/kg IV or SC) or tylosin (10 mg/kg IM) is recommended, but treatment is often unsuccessful. Withdrawal times must be observed: for flunixin meglumine, milk withdrawal is 36 hours and meat withdrawal is 4 days; for ceftiofur, milk withdrawal is 0 hours if used as labeled, but meat withdrawal is 3 days; for oxytetracycline, milk withdrawal is 96 hours and meat withdrawal is 28 days. Always consult the label and a veterinarian for specific protocols.

Evidence-based literature supports the importance of contagious mastitis control programs. The National Mastitis Council (NMC) recommends a five-point plan: 1) proper milking procedure, 2) teat dipping, 3) dry cow therapy, 4) treatment of clinical cases, and 5) culling of chronic cases. Studies have shown that implementation of these measures can significantly reduce the prevalence of S. agalactiae and S. aureus. A landmark study by Barkema et al. (1998) demonstrated that herds with low SCC had lower incidence of clinical mastitis and better milk quality. Research on Mycoplasma mastitis has shown that it is highly contagious and difficult to treat, with a high culling rate. A study by Fox et al. (2005) found that Mycoplasma mastitis outbreaks are often associated with introduction of infected cows and poor biosecurity. Meta-analyses have shown that vaccination against S. aureus can reduce the severity of clinical mastitis but does not prevent infection. The use of internal teat sealants in dry cows has been shown to reduce new infections. Overall, the evidence emphasizes the importance of prevention through hygiene and management, as treatment of contagious mastitis is often ineffective, especially for S. aureus and Mycoplasma.