Subclinical Mastitis and Elevated Somatic Cell Count

Subclinical mastitis is an inflammation of the mammary gland that occurs without visible abnormalities in the milk or the udder, but is characterized by an elevated somatic cell count (SCC) in milk, typically above 200,000 cells/mL in dairy cattle. It is the most economically significant form of mastitis due to its high prevalence, reduced milk yield and quality, and increased risk of progression to clinical mastitis. The condition is primarily caused by intramammary infection (IMI) with contagious or environmental pathogens, leading to an influx of leukocytes, mainly neutrophils, into the milk. Subclinical mastitis can occur at any stage of lactation but is most prevalent in early lactation and during the dry period. It affects high-producing dairy cows more severely, with estimated milk losses of up to 10-15% per affected quarter. The economic impact includes reduced milk production, increased culling rates, higher treatment costs, and penalties for high bulk tank SCC (BTSCC). In beef cattle, subclinical mastitis can impair calf growth due to reduced milk quality and quantity. The condition is a major focus of udder health programs, with SCC monitoring being a cornerstone of mastitis control.

Subclinical mastitis is caused by a wide range of microorganisms, with the most common being bacteria. Contagious pathogens include Staphylococcus aureus, Streptococcus agalactiae, Mycoplasma bovis, and Corynebacterium bovis. Environmental pathogens include Escherichia coli, Klebsiella pneumoniae, Enterococcus species, Streptococcus uberis, Streptococcus dysgalactiae, and Trueperella pyogenes. Coagulase-negative staphylococci (CNS) are increasingly recognized as a cause of subclinical mastitis, particularly in heifers and early lactation. Fungal and yeast infections (e.g., Candida spp., Aspergillus spp.) are less common but can occur after antibiotic therapy. Viral agents such as bovine herpesvirus 4 and bovine leukemia virus have been associated with elevated SCC, but their direct role is debated. Parasitic infections are rare. The pathogenesis involves bacterial invasion of the teat canal, colonization of the mammary gland, and evasion of the host immune response. Virulence factors include adhesins, capsules, biofilm formation (e.g., S. aureus), toxins (e.g., alpha-toxin, leukocidins), and enzymes (e.g., proteases, lipases). Gram-negative bacteria release endotoxin (lipopolysaccharide, LPS), which triggers a strong inflammatory response. Management-related factors such as poor milking hygiene, improper milking machine function, and inadequate dry cow therapy contribute to the establishment of infection.

Subclinical mastitis is endemic in dairy herds worldwide, with a prevalence ranging from 20% to 80% of cows at any given time. The incidence is higher in high-producing dairy breeds such as Holstein-Friesian, due to increased milk yield and stress. Parity is a significant risk factor, with older cows (≥3rd lactation) having higher SCC due to cumulative exposure and udder damage. The transition period (2 weeks before to 2 weeks after calving) is a critical time for new infections due to immunosuppression and metabolic stress. Seasonality is observed, with higher rates in summer months due to heat stress and increased fly populations. Herd size and management intensity influence prevalence; larger herds with high stocking density and poor hygiene have higher BTSCC. The economic impact is substantial: each doubling of SCC is associated with a loss of approximately 0.4 kg of milk per cow per day. Subclinical mastitis is the most common cause of elevated BTSCC, which can lead to milk price penalties. In beef herds, subclinical mastitis is often underdiagnosed but can reduce weaning weights of calves by up to 10%. Morbidity is high, but mortality is negligible except in cases of severe clinical mastitis. The disease is a major driver of antimicrobial use in dairy cattle, contributing to concerns about antimicrobial resistance.

The pathophysiology of subclinical mastitis involves a complex interplay between bacterial invasion, host immune response, and tissue damage. Bacteria enter the teat canal and adhere to the epithelium of the teat cistern and mammary ducts. Once established, they multiply and release virulence factors that damage mammary epithelial cells and attract leukocytes. The initial innate immune response involves recognition of pathogen-associated molecular patterns (PAMPs) by toll-like receptors (TLRs) on epithelial cells and resident macrophages, leading to the release of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) and chemokines (IL-8, C5a). This results in vasodilation, increased vascular permeability, and recruitment of neutrophils from blood into the milk. Neutrophils phagocytose bacteria but also release reactive oxygen species and proteolytic enzymes, causing collateral damage to the mammary epithelium. The SCC in milk increases dramatically, often exceeding 200,000 cells/mL, with neutrophils comprising over 90% of the cells. Chronic infections, particularly with S. aureus, can lead to fibrosis and atrophy of the mammary tissue, reducing milk production permanently. In Gram-negative infections, LPS triggers a massive release of pro-inflammatory mediators, leading to systemic signs if the infection becomes clinical, but in subclinical cases, the response is localized. The blood-milk barrier is disrupted, leading to increased serum albumin and electrolytes in milk, which can be detected by electrical conductivity. The milk composition changes, with decreased fat and lactose content and increased whey proteins, affecting milk processing quality.

Intrinsic risk factors include high milk yield, which increases the diameter of the teat canal and the risk of infection; parity, with older cows having more exposure and udder damage; genetics, as some cows are more susceptible due to udder conformation (e.g., pendulous udders, wide teat sphincters); and immunosuppression during the periparturient period, due to negative energy balance and elevated cortisol levels. Extrinsic factors include poor milking hygiene, such as inadequate teat disinfection before and after milking, contaminated towels, and improper milking machine function (e.g., excessive vacuum, unstable vacuum, overmilking). Environmental factors include dirty bedding, wet and muddy conditions, and poor ventilation in barns. Management practices such as not using dry cow therapy, improper treatment of clinical cases, and lack of culling for chronic infections increase the prevalence. Nutritional deficiencies, particularly of vitamin E, selenium, and copper, impair immune function and increase susceptibility. Heat stress and overcrowding also contribute. The presence of other diseases, such as lameness and metritis, can increase the risk of mastitis due to reduced grooming and increased environmental contamination.

Subclinical mastitis is characterized by the absence of visible abnormalities in the milk or udder. The only detectable changes are an elevated SCC, typically above 200,000 cells/mL, and sometimes a positive California Mastitis Test (CMT). The milk may appear normal in color and consistency, but on closer inspection, there may be slight changes in viscosity or the presence of fine flakes, which are not visible to the naked eye. The udder is normal on palpation, with no swelling, heat, or pain. Systemic signs are absent; the cow has a normal temperature, appetite, and rumen function. However, subclinical mastitis can have subtle effects on milk yield, with a reduction of 5-10% per affected quarter, which may not be noticeable without individual cow milk recording. In herds with high prevalence, the bulk tank SCC may be elevated, leading to milk quality penalties. In beef cows, subclinical mastitis can result in poor calf growth due to reduced milk production and quality. The condition is often detected during routine milk recording or when investigating high BTSCC. It is important to note that subclinical mastitis can progress to clinical mastitis, especially if the cow is stressed or if the pathogen is highly virulent.

Differential diagnoses for subclinical mastitis include: 1) Clinical mastitis, which presents with visible milk abnormalities (clots, flakes, watery milk) and udder inflammation; 2) Elevated SCC due to non-infectious causes, such as trauma, injury, or irritation of the udder; 3) Milk fever (clinical hypocalcemia), which can cause recumbency but is not associated with SCC elevation; 4) Ketosis, which can cause decreased milk yield but does not elevate SCC; 5) Teat canal injuries or lesions that can cause secondary infections; 6) Mycoplasma mastitis, which can be subclinical but often spreads rapidly and may cause agalactia; 7) False-positive CMT due to recent milking or colostrum; 8) Contagious vs. environmental mastitis differentiation based on culture results; 9) Chronic S. aureus infections that may have intermittent clinical flare-ups; 10) Coagulase-negative staphylococci (CNS) infections, which are often subclinical but can elevate SCC. Definitive diagnosis relies on milk culture and SCC measurement.

The diagnostic approach for subclinical mastitis involves a stepwise process: 1) Herd history and milk recording data review to identify cows with elevated SCC (e.g., >200,000 cells/mL) or high BTSCC. 2) Individual cow examination, including udder palpation and CMT on each quarter. CMT is a simple cow-side test that provides a semi-quantitative estimate of SCC. 3) Aseptic milk sample collection for bacteriological culture from quarters with positive CMT or elevated SCC. Samples should be taken before milking, after teat disinfection, and using sterile containers. 4) Laboratory analysis: milk culture on blood agar and MacConkey agar, incubated aerobically for 24-48 hours. Identification of pathogens based on colony morphology, hemolysis, Gram stain, and biochemical tests. 5) SCC measurement using an electronic counter (e.g., Fossomatic) or flow cytometry. 6) In cases of high BTSCC, bulk tank milk culture can be performed to identify the predominant pathogen. 7) Additional tests such as PCR for Mycoplasma bovis or antimicrobial susceptibility testing may be indicated. 8) Monitoring of SCC over time to assess the dynamics of infection. 9) Evaluation of milking machine function and milking procedures to identify risk factors. 10) Implementation of control measures based on the identified pathogens.

Laboratory findings in subclinical mastitis include: Milk SCC >200,000 cells/mL, with a differential count showing >75% neutrophils. The California Mastitis Test (CMT) is positive, with scores of 1 to 3 (on a scale of 0 to 3) corresponding to SCC ranges of 200,000-400,000, 400,000-1,200,000, and >1,200,000 cells/mL, respectively. Milk culture yields the causative pathogen, with colony counts often >1,000 CFU/mL. In chronic infections, the milk may have increased pH (>6.8) and electrical conductivity. Blood parameters are usually within normal limits, but there may be a mild leukocytosis or neutrophilia. Acute-phase proteins such as haptoglobin and serum amyloid A may be elevated in milk. In cases of systemic involvement, which is rare in subclinical mastitis, blood fibrinogen may be increased. Rumen fluid analysis is not typically performed, but if the cow has concurrent metabolic disease, rumen pH may be altered. Milk composition changes include decreased fat and lactose, and increased whey proteins and chloride. The milk N-acetyl-β-D-glucosaminidase (NAGase) activity is elevated, indicating epithelial damage.

Imaging is not routinely used in the diagnosis of subclinical mastitis. However, ultrasonography of the udder can be performed to assess the mammary parenchyma and detect fibrosis, abscesses, or teat canal abnormalities. In chronic cases, ultrasound may reveal hyperechoic areas indicative of fibrosis. Thermography can detect increased heat in affected quarters, but this is not specific. Mammography is not applicable in cattle. In research settings, magnetic resonance imaging (MRI) has been used to evaluate udder tissue, but it is not practical in clinical practice. Therefore, imaging plays a limited role in the diagnosis of subclinical mastitis, which relies primarily on SCC and culture.

Cytological examination of milk from cows with subclinical mastitis reveals a high number of somatic cells, predominantly neutrophils. The presence of phagocytosed bacteria within neutrophils can be observed. Histopathological changes in the mammary gland include infiltration of neutrophils and macrophages into the alveolar lumen and interstitium, degeneration and desquamation of epithelial cells, and in chronic cases, fibrosis and atrophy of alveoli. The glandular tissue may show hyperplasia of the secretory epithelium in early stages, followed by loss of secretory function. In S. aureus infections, microabscesses and scar tissue formation are common. Histopathology is not routinely performed in live animals but is useful in postmortem evaluation. Biopsy of the udder is rarely indicated but can be done for research or in cases of suspected neoplasia.

Treatment of subclinical mastitis is challenging because the infection is often chronic and the efficacy of antimicrobial therapy is variable. The primary goal is to eliminate the infection and reduce SCC. Treatment options include: 1) Intramammary infusion of antimicrobials during lactation, but this is often less effective than dry cow therapy due to the presence of milk and the biofilm-forming ability of some pathogens. Common intramammary products include ceftiofur, amoxicillin, and cloxacillin. 2) Dry cow therapy (DCT) is the most effective approach, as it allows for higher concentrations of antimicrobials and a longer duration of action. Products such as cloxacillin, cephapirin, and penicillin/streptomycin are used. 3) Systemic antimicrobials may be used in cases of deep tissue infection, but penetration into the udder is limited. 4) Supportive therapy includes anti-inflammatory drugs such as flunixin meglumine or meloxicam to reduce inflammation and pain. 5) In cases of chronic S. aureus infection, culling may be recommended due to poor cure rates. 6) Teat sealants can be used at dry-off to prevent new infections. 7) Immunomodulators such as recombinant bovine IL-2 have been investigated but are not widely used. 8) Nutritional support with vitamin E and selenium may improve immune function. It is important to note that treatment of subclinical mastitis during lactation is often not cost-effective, and control measures such as improved hygiene and milking procedures are more important.

The prognosis for subclinical mastitis depends on the causative pathogen, the duration of infection, and the cow's immune status. Infections caused by environmental pathogens such as E. coli and S. uberis have a good prognosis for spontaneous cure, with SCC returning to normal within weeks. Contagious pathogens like S. aureus and S. agalactiae have a poorer prognosis, with cure rates of 20-50% for S. aureus and 50-70% for S. agalactiae with dry cow therapy. Chronic infections can lead to permanent udder damage and reduced milk production. The prognosis for milk yield recovery is guarded, as each affected quarter may lose 10-15% of its production. Cows with high SCC are at increased risk of clinical mastitis and culling. The long-term impact on reproductive performance is minimal, but the economic impact is significant. Negative prognostic indicators include infection with S. aureus, high SCC (>1,000,000 cells/mL), and multiple quarters affected. Positive indicators include infection with CNS, early lactation stage, and no previous history of clinical mastitis.

Follow-up for subclinical mastitis involves regular monitoring of SCC through milk recording, ideally monthly. Cows with elevated SCC should be re-tested after treatment to assess cure. Bulk tank SCC should be monitored to evaluate herd-level control. Individual cow SCC can be used to identify chronic shedders that may require culling. Herd-level interventions should be reviewed, including milking procedures, hygiene, and dry cow therapy protocols. Environmental management, such as bedding and ventilation, should be assessed. Milking machine function should be tested at least twice a year. Cows that fail to respond to treatment should be considered for culling. Additionally, monitoring of clinical mastitis incidence and culture results can help identify emerging pathogens. Regular review of the mastitis control plan with the veterinarian is essential.

Pearls: 1) SCC is a reliable indicator of subclinical mastitis, but it can be influenced by stage of lactation and age; use a threshold of 200,000 cells/mL for cows, but 100,000 cells/mL for heifers. 2) CMT is a quick and inexpensive cow-side test that correlates well with SCC. 3) Aseptic milk sampling is critical for accurate culture results; contaminated samples can lead to misdiagnosis. 4) Dry cow therapy is more effective than lactation therapy for curing existing infections. 5) Control of contagious mastitis requires strict milking hygiene, including teat dipping and milking infected cows last. 6) Environmental mastitis control focuses on clean, dry bedding and reducing exposure to manure. 7) High BTSCC can lead to milk quality penalties; aim for BTSCC <150,000 cells/mL. Pitfalls: 1) Treating subclinical mastitis during lactation without culture can lead to unnecessary antimicrobial use and poor cure rates. 2) Ignoring the role of milking machine function in mastitis transmission. 3) Failing to implement a comprehensive mastitis control program, relying solely on treatment. 4) Using contaminated sampling equipment, leading to false-positive cultures. 5) Overlooking the importance of dry cow therapy and teat sealants. 6) Not monitoring SCC trends over time, missing chronic infections. 7) Assuming that a negative culture means no infection; some pathogens are difficult to isolate.

For subclinical mastitis, dry cow therapy is the mainstay. Common protocols include: 1) Cloxacillin (benzathine cloxacillin) intramammary infusion at dry-off, 500 mg per quarter, single dose. Withdrawal time: 30 days milk, 28 days meat. 2) Cephapirin benzathine, 300 mg per quarter, single dose. Withdrawal: 42 days milk, 4 days meat. 3) Penicillin G procaine and dihydrostreptomycin, 1 million IU penicillin and 1 g dihydrostreptomycin per quarter, single dose. Withdrawal: 30 days milk, 30 days meat. 4) For lactating cows, intramammary ceftiofur hydrochloride, 125 mg per quarter, infused every 24 hours for 2-8 days. Withdrawal: 72 hours milk, 2 days meat. 5) Amoxicillin trihydrate, 62.5 mg per quarter, every 12 hours for 3-5 days. Withdrawal: 60 hours milk, 12 days meat. 6) Systemic therapy with oxytetracycline (10 mg/kg IV or IM, q24h) or ceftiofur (2.2 mg/kg SC, q24h) may be used for systemic infections, but penetration into the udder is limited. 7) Anti-inflammatory drugs: flunixin meglumine (1.1-2.2 mg/kg IV, q24h) or meloxicam (0.5 mg/kg SC, single dose) can reduce inflammation. 8) Supportive therapy with vitamin E (1000 IU/day) and selenium (0.3 mg/kg) may improve immune function. Always follow label directions and consult the veterinarian for appropriate withdrawal times.

Landmark studies have established the relationship between SCC and milk production losses. For example, a study by Hortet and Seegers (1998) found that milk loss increases with SCC, with losses of 0.4 kg/day for SCC between 200,000 and 400,000 cells/mL, and up to 1.5 kg/day for SCC >800,000 cells/mL. The National Mastitis Council (NMC) recommends a BTSCC goal of <150,000 cells/mL. Research on dry cow therapy has shown that blanket DCT reduces the prevalence of subclinical mastitis by 50-70%, but selective DCT is increasingly recommended to reduce antimicrobial use. A meta-analysis by Halasa et al. (2009) found that DCT is cost-effective in most herds. Studies on S. aureus mastitis show cure rates of 20-50% with DCT, and culling is often recommended for chronic cases. The use of teat sealants has been shown to reduce new infections during the dry period by up to 50%. The AABP (American Association of Bovine Practitioners) has published guidelines on mastitis control, emphasizing the importance of hygiene, milking procedures, and monitoring. Recent research focuses on the role of the microbiome and the use of probiotics and immunomodulators. Overall, the evidence supports a comprehensive approach to mastitis control, with SCC monitoring as a key component.