The number of somatic cells in goat milk is a key indicator for evaluating the quality of fresh goat milk and reflecting the health of the mammary glands of dairy goats. Abnormally elevated somatic cells can reduce the quality of goat milk and affect the economic benefits of dairy goat farming. It is an important signal of mammary gland health problems in dairy goats. In practical production, the excessive somatic cells in goat milk are not caused by a single factor, but are the result of the combined effect of multiple factors.
Breast infection (especially mastitis) is the core reason for the increase in somatic cells. Due to the stimulation of pathogens on the body's immune response, a large number of white blood cells (mainly neutrophils and macrophages) gather in the breast tissue, while the number of epithelial cells in the breast increases, ultimately leading to a significant increase in somatic cells.
1. Bacterial infections (most common)
Gram-positive bacteria: such as Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus (including Group A Streptococcus, Group B Streptococcus), etc., are the main pathogenic bacteria for subclinical mastitis.
Staphylococcus aureus: can colonize the epithelial cells of the mammary ducts, causing chronic infections. The somatic cells tend to increase moderately and are prone to recurrent occurrences.
Group B Streptococcus: mainly spreads through milking, and the inflammatory response in the breast after infection is significant.
Gram-negative bacteria: such as Escherichia coli, Klebsiella, etc., mostly cause acute clinical mastitis.
Bacteria release endotoxins, causing severe inflammation of the mammary gland (redness, swelling, fever, and curd-like milk), and the somatic cells can suddenly increase to over 5 million/mL, even triggering systemic infection (sepsis).
Other bacteria: such as mycoplasma (without a cell wall, resistant to conventional antibiotics), Mycobacterium tuberculosis (chronic granulomatous inflammation), etc., after infection, the somatic cells show a persistent increase and are difficult to be eradicated.
2. Non-bacterial Infections
Fungi/ Yeast: Such as Candida, usually caused by long-term misuse of antibiotics, resulting in imbalance of the breast microbiota. After infection, the somatic cells increase moderately, and white flocculent substances appear in the milk.
Viruses: Such as Sheeppox virus, Goat Arthritis-Cerebral炎 Virus (CAEV)， can indirectly damage breast tissue， trigger an immune response， leading to a slight increase in somatic cells， often accompanied by other systemic symptoms (such as joint swelling， neurological symptoms).
II. Non-infectious factors: Breast tissue damage or physiological abnormalities
The increase in somatic cells caused by non-infectious factors usually lacks evidence of pathogen infection. It is mainly related to mechanical damage to the breast tissue, changes in physiological stages, or metabolic abnormalities.
1. Mechanical damage
Improper milking operation: For instance, abnormal vacuum level of the milking machine (either too high causing breast congestion or too low resulting in milk residue), poor fit between the milk cup group and the breast (causing friction on the nipple or the base of the breast), can lead to an increase in the shedding of breast epithelial cells and a short-term rise in somatic cells.
Physical trauma: Such as when the breast is bumped or squeezed (common in situations with high population density), causing bruising or bleeding of the breast tissue, and the milk may contain blood.
2. Physiological stage changes
Dry milk period and early lactation: During the dry milk period, the apoptosis and shedding of breast epithelial cells increase, resulting in an elevation of somatic cells; in the early stage of lactation (within one week after giving birth), due to hormonal fluctuations (increased prolactin and estrogen), the breast tissue undergoes remodeling, and somatic cells may also temporarily rise, but without inflammatory symptoms.
Late lactation: As breast function declines, the shedding of epithelial cells increases, and somatic cells gradually rise, which is a physiological aging process.
3. Metabolic abnormalities
Nutritional imbalance: Such as vitamin E and selenium deficiency (decreased antioxidant capacity, making breast cells more susceptible to free radical damage), or calcium and phosphorus metabolism disorders (affecting the stability of breast epithelial cells), can lead to increased fragility of breast tissue and a slight rise in somatic cells.
Toxic injury: For example, feeding moldy feed (containing aflatoxin B1), the toxin directly damages breast epithelial cells, causing a rise in somatic cells, accompanied by symptoms such as decreased appetite and jaundice.
III. Management Factors: Improper Human Operations or Environmental Control
Management lapses serve as the "catalyst" for the increase of infectious or non-infectious somatic cells, especially in large-scale livestock farming, where their impact is significant.
1. Milking Hygiene Defects
Insufficient breast cleaning: Before milking, the breasts were not thoroughly cleaned (residual feces, bedding impurities remained), allowing pathogens to enter the mammary glands through the nipple ducts, increasing the infection risk by 3-5 times and the median somatic cell count to over 1 million/mL.
Milking equipment contamination: The milk pipes and tanks were not thoroughly disinfected (such as using insufficient chlorine disinfectant concentration), leading to cross-contamination of pathogens and triggering a rise in group somatic cells (especially Streptococcus and Escherichia coli).
2. Poor Environmental Management
Poor pen hygiene: Wet bedding and accumulated feces result in an increase in pathogen concentrations (such as Escherichia coli and Staphylococcus) in the environment, significantly increasing the probability of infection after nipple contact.
Stress factors: High temperature and humidity (common in summer), transportation and group transfer, sudden changes in feed, etc., cause a decline in the immunity of sheep (cortisol levels rise, inhibiting immune cell function), making the mammary glands more susceptible to infection, leading to an increase in somatic cells.
3. Lack of Health Monitoring
Subclinical Mastitis Missed Detection: Subclinical mastitis has no obvious symptoms (normal milk appearance), but if not regularly tested (such as the California Mastitis Test CMT or somatic cell counter), infected sheep continue to carry the pathogen, causing the group's somatic cells to gradually increase.
Inadequate isolation of infected sheep: Clinical infected sheep were not milked separately or isolated, and the pathogen was transmitted through the milking operator's hands and tools, triggering a widespread infection.
IV. Genetic and Breed Factors: Individual Susceptibility Differences
There are genetic differences in the resistance of different breeds or individuals to breast diseases, resulting in different basal levels of somatic cells:
Breed differences: Generally, the basal level of somatic cells in Saanen dairy goats is lower than that in Nubian goats, which is related to the structure of mammary epithelial cells and the expression of immune genes (such as TLR4).
Individual heritability: The number of somatic cells is a moderately heritable trait (heritability 0.2-0.3). If the somatic cell count of the parents is consistently high, the offspring are more likely to exhibit a high somatic cell phenotype.
Summary: Interactions of multiple factors lead to an increase in somatic cells
High somatic cell counts in goat milk are the result of the combined effects of multiple factors (infection - core, damage - direct, management - inducement, genetics - basis). In actual research, a comprehensive judgment needs to be made by combining clinical symptoms (such as milk quality, breast appearance), laboratory tests (pathogen isolation, dynamic monitoring of somatic cells), and management records (milking process, environmental parameters), in order to accurately identify the cause.
