Effect of Subclinical Mastitis on Milk Composition in Lactating Cows
Mastitis is the most economically significant disease affecting dairy herds due to its high prevalence, high medication costs, increased labor, loss of production and milk quality, and consequently early culling of animals. On dairy farms, this illness is the leading cause of antibiotic use .Antibiotic use may be linked to the development of antimicrobial-resistant bacteria that pose a public health risk, as evidenced by antimicrobial residues in milk and dairy products . The leading risk factors for mastitis incidence include inappropriate farm management , the application of dry-cow treatment , cows with high parity , and cows through physiological changes and stresses . Subclinical mastitis (SCM) is the most detrimental type of mastitis, with no noticeable changes in the appearance of milk and udder. However, reduced milk production, elevated somatic cell count (SCC) rise, the presence of pathogens in milk, and altered milk composition can be observed . Although many animals can have SCM, lactating cows are more susceptible to substantial changes in their milk constituents . Early detection of health issues in the udder is critical for dairy farmers and veterinarians to ensure the health of the animals and the quality and production of the milk. Regular bacteriologic testing of quarter milk samples is time-consuming and hampered by financial considerations . Somatic cell count is frequently used to monitor udder health and milk quality as an indicator of mastitis. Several studies have found that as SCC increases, the activity of milk proteolytic enzyme increases concomitantly. Immediately following the elevation of SCC in milk, the activity of proteolytic enzymes dramatically increases .
Mastitis is inflammation of the parenchyma of mammary glands characterized by bacteriological changes in milk, milk production and pathological alteration in glandular tissues. It is one of the leading diseases of dairy cattle that potentially affect farmers’ economy due to reduction in milk yield, milk discard after treatment, cost of treatment, and premature culling. Mastitis is categorized as clinical and subclinical mastitis based on signs and changes in milk quality and quantity of affected cows. The main features of clinical mastitis are edema, udder inflammation with constant and steady pain, and change in colour and consistency of milk, whereas only quality and quantity are reduced with no visible changes in udder observed in subclinical mastitis which make them complex in detection. So, diagnosis of subclinical mastitis (SCM) is difficult than clinical mastitis. When a cow is in heat, or there is a climate change, subclinical mastitis is converted into clinical mastitis (Saroj et al., 2015). On the other hand, many microorganisms are implicated in mastitis infection, few of them are environmental pathogens, and some are contagious (Ruegg, 2017). The milk from affected quarters seems to be normal, even when millions of somatic cells (SC) present in milk, and hence SCM can also be defined as when milk is normal but there may be a decrease in production and may have SCC level of 200,000 cells/mL or greater and can be detected through California Mastitis Test. Therefore, herd screening for SCM infection should be done regularly to prevent milk production loss (Halasa et al., 2009). In the current scenario, MALDI-TOF MS has been used in many countries to detect mastitis because of its speed and reliability (Nonnemann et al., 2019). Therapeutic and prevention strategies have been practiced over the years to improve the health and production of dairy animals which contributes to the economy of the country.
The mastitis is characterized by biochemical changes in composition of milk. These compositional changes reflecting the degree of physical damage are well marked and can be used as a basic indicator for diagnosis of subclinical mastitis. These changes not only alter nutritional quality of milk but also preservation quality. So, investigation on biochemical profile of subclinical mastitis is important. The quality and quantity of milk is affected to varying degrees. Mastitis is not only responsible for great economic losses to the dairy industry but also acts as one of the biggest obstacle in achieving the “White Revolution”. In mastitis, the economic losses are due to reduced milk production, poor quality milk, increased cost of Veterinary services and drugs, shortened productive life of animal and finally its replacement cost. Mastitis can be caused by physical or chemical agents but the majority of cases are infectious and usually caused by bacteria. Over 135 microorganisms have been isolated from bovine inflammatory infections and bacteria are one of the major etiological agents of mastitis . Mastitis is caused when pathogenic bacteria enter the sterile environment of the mammary gland, often as a result of disruption of physical barriers such as the teat, requiring prompt and appropriate host defenses to prevent colonization and subsequent disease pathology. In many countries the most common bacterial species causing mastitis are Staphylococcus aureus, Streptococcus dysgalactiae and streptococcus . Staphylococcus aureus, Streptococci and members of the Enterobacteriaceae are among the most common etiological agents in cows and in other animal species. Mastitis is derived from “matos” referring to the mammary gland, and its meaning inflammation .Mastitis describes an inflammatory reaction in the mammary gland. It is characterized by physical, chemical and bacteriological changes in milk and pathological changes in glandular tissues of the udder and affects the quality and quantity of milk . It is a complex disease associated with variable origin, severity, and outcome depending on the environment, pathogen, and host . Mastitis in dairy animals leads to economic losses in different forms viz reduction in milk production (70%), premature culling (14%), veterinary expenses (9%) and milk discarded or low grade (7%) . Bovine mastitis can be defined as a multi factorial disease and is one of the most costly and difficult to control diseases throughout the world . It affects both quality and quantity of milk, increases rate of culling and veterinary expenses . Bovine subclinical mastitis, which has higher rate of prevalence than bovine clinical mastitis in India is economically destructive , and clinically elusive due to its camouflaged clinical manifestations . The various diagnostic tests for detection of subclinical mastitis are CMT, WST, SFMT, electrical conductivity of milk, Cl- estimation in milk, Modified Aulendorfer Mastitis Probe (MAMP) test, somatic cell count and culture . The diagnosis of mastitis according to the International Dairy Federation (IDF) recommendations is based on the somatic cell counts (SCC) and microbiological status of the quarter. Though bacteriological culture of milk samples is the standard method for identifying mastitis, the logistic and financial considerations involved with sampling all fresh cows have precluded this technique from being widely adopted . California Mastitis Test (CMT) is a simple, inexpensive, rapid and highly sensitive test that accurately predicts the inflammatory cell counts in milk from individual quarters or pooled milk samples .The SCC is account used to screen epithelial cells that have been shed from the lining of the gland and white blood cells (leucocytes) that have entered the mammary glands in response to injury or infection .SCC is a useful predictor of subclinical udder infection, therefore, it is considered as an important component for assessing the quality and milk hygiene for mastitis control protocols .The leucocyte count is the basis for most indirect tests employed for diagnosis. Over135 different microorganisms have been isolated from bovine intra mammary infections (IMI), but the majority of infections are caused by Staphylococcus spp, Streptococcus spp. and gram negative bacteria .
Economic impacts of subclinical mastitis in dairy cow
Subclinical mastitis has a negative influence on the performance of dairy cows and buffaloes. It has a significant impact on production, nearly 10 – 20% decrease in milk production, as it causes an undesirable effect on the milk constituents, which affects its nutritional value and renders it unfit for processing and consumption (Iraguha et al., 2015). It includes reduced milk production, changes in milk quality and treatment cost. Rathod et al. (2017) estimated the economic losses due to subclinical form of mastitis were approximately in the range of INR 21,677 to INR 88,340 per animal for a lactation period. It has been reported that SCM causes threefold more production losses as compared to clinical mastitis leading to substantial economic losses of 60-70% all due to mastitis (De Vliegher et al. 2012; Sinha et al. 2014). Susanty et al. (2018) observed agro climatic effects on milk production and shows subclinical mastitis prevalence in dairy cattle. Another study revealed that the prevalence rate of mastitis in cows was high as environmental temperature increases significantly (summer) due to the rise in somatic cell score (Bouraoui et al., 2002; Khate and Yadav, 2010; Bertocchi et al., 2014). The temperature and humidity index (THI) can regulate the comfort zone of cows. The incidence of clinical mastitis progresses with an increase in the temperaturehumidity index (THI). In addition, increase THI is related to mastitis prevalence and somatic cell score (Hammami et al., 2013). A rise in somatic cell count in dairy cattle linked with mastitis also reported by Barkema et al. (2013). Any variation of the body temperature above the normal range suggests the level of heat stress in dairy cattle and also affected by wind speed, dry temperature, humidity and solar radiation (Dikmen and Hansen, 2009). It has been observed that room temperature below 23oC and humidity of 68% is most suitable for the highest production of milk by dairy cattle (Bohmanova et al., 2007). Hence, temperature and humidity play a crucial role in the production of dairy cattle.
Etiology of subclinical mastitis in dairy cattle
The foremost causes of SCM in dairy animals are due to many factors such as unhygienic management, nutritional deficiency and several pathogens. Multiple causative factors are involved in SCM occurrence and its severity depend on various variables of the host, pathogens, and the environment. Cheng and Han (2020) stated that multiple pathogens that contribute to causing mastitis included contagious organisms, environmental bacteria and opportunistic bacteria. Other organisms such as fungi are less frequently a cause of mastitis in dairy cows. The bacterial etiology are Streptococcus agalactia and Staphylococcus aureus, Nocardia spp., Mycoplasma spp. and environmental Streptococci. Another report showed that Staphylococcus spp., Streptococcus spp., Micrococcus spp., Trueprella pyogenes, Corynebacterium bovis and Bacillus spp. were causing bovine mastitis in 82 lactating cows (Mollalegn et al., 2010). Hande et al. (2015) identified the bacterial isolates of Staphylococcus aureus, Streptococcus dysgalactiae, Enterococcus spp. and Streptococcus agalactiae from the cases of subclinical mastitis in dairy cattle in the Samsun district of Turkey. Host factors and environmental factors play an essential role in the occurrence of the disease in animals. A wide range of microorganisms can enter the mammary gland through the teat canal and cause disease. Sources of environmental exposure are unhygienic managemental conditions such as contaminated manure, bedding, feeds, dirt, mud and water. Fahim et al. (2019) observed that E. coli strains are causing intramammary infections including subclinical mastitis. Banerjee et al. (2017) observed that Pseudomonas aeruginosa is one of the etiological pathogens of subclinical mastitis in dairy cows. Karabasanavar et al. (2019) reported that Staphylococci were isolated and confirmed using 16S rRNA genebased genus-specific PCR in 39.2% of SCM affected quarters and detected virulenceassociated nuc gene in 75% of Staphylococcus isolates suggesting their potential pathogenicity. The high prevalence of subclinical mastitis was associated with different factors, including herd size, milk marketing, agro-climatic conditions of a region, animal owner education level, variations in sociocultural practices, feeding methods and management (Joshi and Gokhale, 2006). Ndahetuye et al. (2019) reported the non-aureus Staphylococci (40.2%) most prevalent microbes of subclinical mastitis, followed by Staphylococcus aureus (22%). Another study showed that Streptococcus species (22.2%) were the most recorded pathogen for subclinical mastitis, followed by Staphylococcus aureus (15.7%), Pseudomonas aeruginosa (5.1%) and Enterobacter species (0.7%) (Mbindyo et al., 2020). A study identified the causative agents of subclinical mastitis in dairy cows as Staphylococcus aureus, Streptococcus uberis, Streptococcus agalactiae, S. dysagalactiae and S. pyogenes, E. coli, Proteus spp. and Klebsiella pneumoniae by using PCR techniques (universal 16S primers) (Sahoo et al., 2020).
Nutritional factors: The number of lactations, metabolic stress and protein-energy ratio may affect the immune system and induce the cause of mastitis in animals. Immunosuppression can be linked to metabolic disturbance and changes in the modulation of immune system, which may lead to mastitis and reduce milk production (Bronzo et al., 2020). Dey et al. (2019) stated that proper balance of nutrients prevents mastitis while nutritional imbalance or sudden change in diet may enhance mastitis incidence in the dairy cow. Vitamin A and zinc play an essential role in maintaining the physical defense barriers as well as epithelial health of the udder. The functions of phagocytic cells in cattle are influenced by vitamins (A and E), copper, zinc and selenium. Nutritional supplementation of dairy cows with vitamin E enhances the killing ability of immune cells (neutrophils). The nutrient imbalance may be occurring due to either excess or deficiency of essential nutrients in the diet of dairy animals (Haq et al., 2016).
Risk factors: The major risk factors of SCM include host, management practices, diet and pathogens. The host factor consists of age, stage of lactation, milking interval, breed, parity, milk yield capacity, udder and teat conformation, somatic cell count, udder defense, dry period, teat injuries and genetic resistance. Cows of certain breeds are more prone to mastitis. Among Swedish breeds, a high prevalence of mastitis has been recorded in Swedish Holstein cows. Crossbred cows are more susceptible to mastitis than local breeds (Persson et al., 2009). Managemental practices comprise shed and udder hygiene, poor teat condition, poor environmental hygiene, milking technique, sanitation and large herd size, use of handwash cloth, teat dipping and hygiene of milking machine.
Diagnostic test for subclinical mastitis
Microbiological status of the affected quarter and the somatic cell count (SCC) used to detect abnormal changes are mainly due to the inflammatory process (Sudhan and Sharma, 2010). Tresamol et al. (2019) found that EC estimation to be easier than CMT as a screening test. Langer et al. (2014) stated that many direct and indirect tests help to detect SCM. The direc test includes Somatic Cell Count, and the indirect test includes Modified California Mastitis Test (MCMT), Modified White Side Test (MWST), Cow Side Test, Screening Test [Electrical conductivity (EC) and pH test] and Laboratory Test [Methylene Blue Reduction Test (MBRT)]. These tests determine the quality of milk. However, in the absence of laboratory facilities, a suitable field-level test may be considered in detecting SCM (Sharma et al., 2008). Inflammation of the mammary gland is directly accompanied by an increase of SCC in milk (Rodriguez et al., 2000). Therefore, SCC is a standard test for SCM (Durr et al., 2008) and represents a direct test for detecting SCM. Reddy et al. (2015) reported that the quarterwise incidence of SCM as detected by CMT and EC was 27.83 and 19.95% respectively whereas the animal-wise incidence was 52.28 and 38.13% for CMT and EC respectively.
Treatment of subclinical mastitis in dairy cow
Several systematic antibiotics such as procaine penicillin G, amoxicillin, ampicillin and oxytetracycline are effective against mastitis (Preez, 2000). Salat et al. (2008) found that systemic treatment of subclinical mastitis with penethamate is effective. The bacteriological cure (BC) of infected quarters has a sustained positive effect on milk SCC during the treatment. Nutrition and feeding management can affect the outcome of mastitis, and some of these nutritional components are less specific to older cows due to differences in their nutrient requirements and metabolism (Saroj et al., 2015). The therapeutic use of oil extracts of Ocimum sanctum (tulsi) with Azadirachta indica (neem) and aqueous extract of Tinospora cordifola (giloy) gave good results for reducing bacterial load and enhancement of phagocytic capacity (Dash, 2001). Mastitis remains one of the most economically devastating diseases in dairy cows. Pandey and Kumar (2018) observed that herbal spray was effective against subclinical mastitis in dairy cows. Administration of tylosin along with benethamine penicillin, penethamate hydriodide and framycetin showed effective results in dairy cattle affected with subclinical mastitis (Ismail et al., 2018).
Prevention and control measures
The outbreak of SCM can be prevented either pre and post milking udder disinfection or by changes in milking technique like using machine milking (Mpatswenumugabo et al., 2017). The contribution of dietary management in the development of mastitis has been controversial and difficult to isolate from other confounding effects. Feeding high-concentrate diets to increase milk yield was a risk factor for mastitis and complexity in udder metabolism (Hamilton et al., 2006) but significant effects of nutrition on udder health which predisposes to mastitis and ketosis in dairy cow (O’Rourke, 2009). Dietary deficiencies of selenium and vitamin E cause of increasing incidence as well as the duration of mastitis. So, a balanced diet should be provided to dairy animals along with hygienic managemental conditions. Kumari et al. (2018) observed the efficacy of herbal preparations as an internal and external sealant against subclinical mastitis in dairy cow management. They found that TM or R herbal products along with the company as internal teat sealant helps in treating subclinical mastitis in dairy cows. McParland et al. (2019) observed that the use of teat sealant alone could be beneficial in dairy cows with an efficient mastitis control program by decreasing the subclinical mastitis cases. Dry cow therapy has shown effective results in eliminating the existing intramammary infections and preventing the occurrence of new intramammary infections; hence plays a vital role in the mastitis control programme. This therapy includes administering intra-mammary or systemic antibiotics during the dry period (2 weeks before parturition) (Kashif et al., 2016). Leitner et al. (2018) utilized acoustic pulse therapy for treating subclinical mastitis and estimated 70.5% of mastitis cattle return to normal milk yield than the control group (18.4%). National Mastitis Council (NMC, 2000) developed ten steps list to follow in a mastitis control program which includes: establish goals for udder health, maintain a clean, dry, comfortable environment, follow proper milking procedures, maintain and use milking equipment properly, keep good records, manage clinical mastitis during lactation appropriately, establish an effective dry cow management program, follow a biosecurity program against contagious pathogens, monitor udder health status regularly and periodically review your mastitis control program.
Effects on Milk Composition
- Milk Somatic Cell Count (SCC): Subclinical mastitis is associated with an elevated SCC in milk. SCC represents the number of somatic cells, primarily white blood cells, which are indicative of an immune response to infection. High SCC levels reduce milk quality and can lead to a decrease in milk yield.
- Milk Yield: Lactating cows with subclinical mastitis often experience a reduction in milk yield. The inflammatory response in the udder diverts resources away from milk production, resulting in decreased milk output.
- Milk Fat Percentage: Subclinical mastitis can alter the milk fat composition, causing a decrease in milk fat percentage. This can impact the quality of dairy products like butter and cheese, as fat is a crucial component for their production.
- Milk Protein Percentage: The milk protein content, particularly casein, can also be affected by subclinical mastitis. This can impact the quality and processing characteristics of milk for cheese production.
- Lactose Content: While less pronounced, subclinical mastitis can lead to changes in lactose content, which may affect the taste and processing properties of dairy products.
- Milk Hygiene and Bacterial Load: Milk from cows with subclinical mastitis may contain higher bacterial loads, which can lead to quality issues and pose a challenge for dairy processors. Additionally, the presence of certain bacteria may affect milk composition.
Economic Impact
The economic consequences of subclinical mastitis are significant. It results in lower milk production, reduced milk quality, and increased veterinary costs. Additionally, milk with high SCC and altered composition may be subject to penalties or price reductions when sold to dairy processors, impacting the income of dairy farmers.
Prevention and Management
Preventing and managing subclinical mastitis are critical for dairy farm sustainability. Strategies include:
- Routine Udder Health Monitoring: Implement regular udder health checks, including SCC testing, to identify subclinical mastitis cases early.
- Hygiene Practices: Emphasize proper hygiene during milking to reduce the risk of bacterial infection.
- Dry Cow Therapy: Implement dry cow therapy to prevent new intramammary infections during the dry period.
- Genetic Selection: Consider genetic selection for cows with lower susceptibility to mastitis.
- Nutrition and Immune Support: Ensure that cows receive a well-balanced diet to support their immune system.
Conclusion
Subclinical mastitis is a hidden menace in the dairy industry, affecting milk composition and quality, and impacting the economic sustainability of dairy farms. Early detection, preventive measures, and effective management practices are essential to mitigate its effects. As the industry continues to evolve, addressing subclinical mastitis is crucial for maintaining high-quality milk production and ensuring the well-being of dairy cows and the satisfaction of consumers.
Subclinical mastitis is a major health concern for dairy cows, as it causes silent economic loss without visible alteration in the milk. SCM causes 3-5 times more financial loss than clinical mastitis. SCM prevalence is also highly associated with the agroclimatic condition affecting milk production in dairy cattle. The following prevention strategies should be followed to overcome SCM, such as implementing good management in a dairy farm, regular checkup of animals for early diagnosis and proper treatment, which may reduce health-related problems and occurrence of concurrent diseases in dairy cows. The hygienic practice of milking has a more significant impact on lowering SCM incidence, which includes dry cow therapy, post milking teat disinfection, and the use of sterile gloves while milking. So, by following the above steps, we can prevent SCM and minimize economic losses by improving the quality and quantity of milk.
Compiled & Shared by- This paper is a compilation of groupwork provided by the
Team, LITD (Livestock Institute of Training & Development)
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