Mastitis in Dairy Cows: Causes, Symptoms, Treatment and Prevention

Introduction

Mastitis in Dairy Cows is one of the most common and costly diseases in dairy farming worldwide. It is an inflammation of the udder, usually caused by infection, that reduces milk yield, damages udder tissue and can threaten the life of the cow in severe cases. Both visible (clinical) and hidden (subclinical) forms of mastitis in dairy cows affect milk quality, fertility and farm profit, making early detection, proper treatment and strong prevention programs essential for every herd.

This article explains what mastitis in dairy cows is, the main causes of mastitis in dairy cows, the clinical signs and symptoms of mastitis in dairy cows, how to diagnose mastitis, how to treat mastitis in dairy cows and practical strategies for prevention and control. It is written in clear, simple English for farmers, students and animal owners, while following the latest veterinary guidance on udder health.

What is Mastitis in Dairy Cows?

Mastitis in dairy cows is an inflammation of the mammary gland and udder tissue. It is most often caused by bacteria that enter through the teat canal, multiply inside the udder, and trigger an immune response. This immune reaction leads to swelling, pain and changes in milk, and in severe cases can cause systemic illness and even death.

Mastitis is usually divided into two main forms: clinical mastitis and subclinical mastitis. Clinical mastitis shows obvious signs such as udder swelling, heat, redness and abnormal milk (watery, clots or flakes). Subclinical mastitis does not show visible changes in the udder or milk, but there is inflammation and an increased somatic cell count (SCC), leading to reduced milk yield and quality. Subclinical mastitis is very common and often causes more total losses than clinical cases, because it can affect many cows without being noticed.

Economically, mastitis in dairy cows is considered the single most costly disease of the dairy industry because of lost milk, treatment costs, discarded milk due to antibiotic withdrawal times, reduced fertility and premature culling. For animal welfare, it is also a major concern, as cows with mastitis suffer pain and systemic illness in moderate and severe cases.

Causes of Mastitis in Dairy Cows

Most cases of mastitis in dairy cows are caused by bacterial infection. The bacteria can be broadly grouped as contagious (spread mainly during milking from cow to cow) and environmental (picked up from bedding, manure, soil and dirty equipment). Common contagious pathogens include Staphylococcus aureus and other staphylococci, which are major causes of clinical and subclinical mastitis and can persist in udders despite treatment. Environmental pathogens often include coliforms such as Escherichia coli and other environmental bacteria that enter the teat canal from dirty housing, wet bedding or poor hygiene.

Staphylococcus aureus is one of the most important contagious causes of mastitis in dairy cows. It lives mainly in infected udders and spreads via milking machines, liners and milkers’ hands if hygiene is poor. This pathogen can cause both clinical mastitis (with visible signs) and subclinical mastitis (high SCC, reduced yield). It often leads to chronic infections with damage such as fibrosis and abscesses in the udder, and is difficult to eliminate once established.

Management and environmental factors strongly influence the risk of mastitis. Poor milking hygiene (no teat disinfection, dirty cloths), worn or poorly maintained milking machines, overcrowded or dirty housing, wet or contaminated bedding, and inadequate nutrition all increase the chance of infection. Stress, early lactation, high milk yield and the dry period are critical times when cows are more susceptible to mastitis in dairy cows because of changes in immunity and the teat canal. Inadequate dry-cow therapy and failure to cull chronically infected cows also allow infection to persist in the herd.

Clinical Signs and Symptoms of Mastitis in Dairy Cows

The clinical signs and symptoms of mastitis in dairy cows vary from very mild to life‑threatening. In clinical mastitis, the udder may show swelling, heat, redness, hardness or pain when touched. Milk from the affected quarter often looks watery or discoloured and may contain clots, flakes or pus. These changes in milk are frequently the first signs noticed in the milking parlour.

Cows with mastitis may also show systemic signs. In mild cases, only the udder and milk are affected. In moderate cases, cows may have reduced appetite, decreased milk yield, sunken eyes, reduced mobility due to udder pain and signs of dehydration or diarrhoea. In severe acute mastitis, cows can appear very ill with fever, depression, weakness, rapid heart rate, and may go into shock; these cases can be fatal without rapid treatment.

Subclinical mastitis presents differently. The udder and milk look normal to the eye, and the cow usually appears healthy, but there is inflammation and an increased somatic cell count in the milk. The main “symptom” is reduced milk production and poorer milk quality over time, which may only be detected through milk recording, SCC testing or on‑farm diagnostic systems. Because subclinical mastitis shows few visible signs, it often goes unnoticed and can account for the majority of total mastitis losses on a farm.

How to Diagnose Mastitis in Dairy Cows

Diagnosis of mastitis in dairy cows begins with observation. Clinical mastitis is often identified from visible changes in the udder or milk, such as swelling, heat or clots in the first streams of milk during fore‑milking. Many farms use a strip cup or dark surface for fore‑stripping to make flecks or clots easier to see, which is a simple and effective on‑farm screening tool. Palpation of the udder to feel for heat, hardness or pain is another key step, along with checking the cow for signs of systemic illness.

For subclinical mastitis and herd monitoring, laboratory and on‑farm tests are essential. Somatic cell count (SCC) in milk is the main indicator of subclinical mastitis; high SCC reflects inflammation and infection in the udder. Bulk tank SCC gives an overview of herd udder health, while individual cow or quarter SCC identifies specific animals with subclinical infections. Regular SCC testing—monthly or more often—helps detect issues early and evaluate the success of control programs.

Bacteriological culture of milk samples is recommended to identify the causative bacteria and guide targeted treatment and control plans. Culture and sensitivity testing show which antibiotics are likely to be effective, and whether selective treatment is appropriate. Newer on‑farm diagnostic technologies and near‑cow systems can provide rapid results, supporting selective treatment and improved antimicrobial stewardship. Veterinarians and mastitis consultants use these data, combined with clinical signs and herd history, to design effective mastitis management programs.

Treatment of Mastitis in Dairy Cows

How to treat mastitis in dairy cows depends on the severity of clinical signs, the pathogen involved and farm treatment protocols. In many cases of clinical mastitis, treatment involves intramammary or systemic antibiotics to eliminate the causative bacteria from the infected quarter. Treatment duration typically ranges from 2–3 days for mild cases to 5–8 days for infections with pathogens such as Staphylococcus aureus, which are more difficult to clear. A clinical response—improvement in udder and milk appearance—should be seen within 5–7 days.

Current guidelines emphasise selective rather than blanket antibiotic treatment for mastitis in dairy cows, in order to reduce antimicrobial use and resistance. For mild clinical mastitis, milk samples should be collected for culture and susceptibility testing. Treatment may be delayed until results are available, and quarters with “no growth” or infections that resolve quickly without systemic signs may not need antibiotics, unless clinical signs persist beyond three days or the cow becomes unwell. This selective approach has been shown to reduce antibiotic use without harming cure rates, milk yield or cow survival.

Supportive therapy is vital, especially in moderate and severe cases. Cows with systemic illness may require anti‑inflammatory drugs to reduce pain and fever, intravenous or oral fluids to correct dehydration and shock, and intensive nursing care. Milk from treated cows must be withheld and discarded for the full withdrawal period to prevent antibiotic residues in the food chain. In chronic cases—particularly those due to Staphylococcus aureus with repeated flare‑ups and poor response to treatment—culling may be the most humane and economically sensible option. Decisions on how to treat mastitis in dairy cows should always be made with a veterinarian, based on culture results, severity grading and herd history.

Prevention and Control of Mastitis in Dairy Cows

Prevention of mastitis in dairy cows is more effective and cheaper than treatment. Successful control programs are built around hygiene, milking routine, cow environment, dry‑cow management and herd‑level monitoring. One long‑standing and proven strategy is the “five‑point plan” for mastitis control: recording and treating all clinical cases, post‑milking teat disinfection, dry‑cow therapy at drying off, culling chronically infected cows and regular maintenance of the milking machine. Many modern control programs build on this foundation.

Milking hygiene is central. Key steps include pre‑milking teat preparation to remove dirt and manure, use of individual clean paper towels or well‑managed cloths, fore‑stripping to detect early clinical cases, and proper milking machine attachment and removal to minimise teat end damage. Post‑milking teat dipping with an effective disinfectant reduces the spread of contagious pathogens like Staphylococcus aureus between cows. Regular inspection and servicing of milking equipment help ensure correct vacuum levels and pulsation, lowering teat stress and infection risk.

Housing and bedding management are equally important for prevention of mastitis in dairy cows caused by environmental pathogens. Clean, dry and comfortable bedding, frequent removal of manure, good drainage and ventilation reduce bacterial load around the teat ends. Overcrowding should be avoided, and sick or high‑cell count cows should be milked last or in a separate group to limit spread. A balanced diet and good body condition support immune function and udder health, helping cows resist infection.

Dry‑cow management is a critical period for both treatment and prevention. Selective dry‑cow therapy—using intramammary antibiotics only in quarters or cows known or strongly suspected to be infected, often combined with internal teat sealants—treats existing infections and prevents new ones at the start of the dry period. Teat sealants protect the teat canal when the natural keratin plug has not yet formed, blocking bacterial entry. Vaccines against some mastitis pathogens are available and may reduce the severity of disease, but they should be used together with, not instead of, good management and hygiene. Continuous herd monitoring with SCC, milk recording and regular veterinary review turns mastitis control into a proactive, long‑term program.

Q: What are the most common symptoms of mastitis in dairy cows?

A: The most common symptoms of mastitis in dairy cows include swelling, heat, redness, hardness or pain in one or more quarters of the udder, and changes in the milk such as watery appearance, clots, flakes or pus. Cows may also show reduced milk yield, poor appetite, dehydration and, in severe cases, fever and depression. Subclinical mastitis shows no visible changes but causes increased somatic cell count and lower milk production.

Q: How do you diagnose mastitis in cows on the farm?

A: On the farm, mastitis is first diagnosed by observing the udder and milk: fore‑stripping to check the first streams of milk for clots or colour changes, and feeling the udder for heat, swelling or pain. For subclinical cases, somatic cell count testing of individual cow or quarter milk samples is used to identify inflammation even when milk appears normal. Milk samples may be sent for culture and sensitivity to identify the bacteria and guide treatment and control measures.

Q: How to treat mastitis in dairy cows safely and effectively?

A: Treatment of mastitis in dairy cows usually involves intramammary or systemic antibiotics for clinically infected quarters, chosen and dosed according to veterinary guidelines and culture results. Mild cases may be treated for 2–3 days, while infections with pathogens like Staphylococcus aureus may require 5–8 days of therapy. Supportive care such as anti‑inflammatory drugs and fluids is critical in moderate and severe cases. Milk from treated cows must be discarded for the full withdrawal time to avoid antibiotic residues. Selective treatment based on culture results helps reduce unnecessary antibiotic use without harming outcomes.

Q: What is the best way to prevent mastitis in dairy cows?

A: The best prevention of mastitis in dairy cows combines strict milking hygiene, good housing and bedding, effective dry‑cow management and regular monitoring. Key steps include pre‑ and post‑milking teat disinfection, proper teat preparation and drying, fore‑stripping to detect early cases, and maintenance of milking machines. Clean, dry bedding and well‑managed housing reduce environmental bacteria. Selective dry‑cow therapy and use of teat sealants at drying off help treat existing infections and prevent new ones. Monitoring SCC and working with a veterinarian or mastitis consultant to design a herd‑level control plan are also essential.

Q: Can mastitis in dairy cows affect milk safety and human health?

A: Mastitis can affect milk quality and potentially food safety. Milk from cows with mastitis has a higher somatic cell count and microbial load and may contain foodborne pathogens and their toxins. Pasteurisation generally kills bacteria, but toxins and, in raw milk products, live pathogens can remain a concern. In addition, milk from cows under antibiotic treatment may contain drug residues if withdrawal times are not observed, so such milk must be discarded until tests confirm residue levels are below regulatory limits. Proper mastitis control, milk handling and adherence to regulations protect both consumer safety and marketability of milk.

When to Call a Veterinarian

A veterinarian should be called whenever a cow with mastitis shows moderate to severe systemic signs, such as fever, rapid breathing, marked depression, dehydration, diarrhoea or signs of shock. These acute cases can deteriorate quickly and may require intravenous fluids, systemic antibiotics, anti‑inflammatory treatment and intensive care that only a veterinarian can safely provide. Any cow that does not respond to initial treatment within a few days, or that has repeated episodes in the same quarter, also needs veterinary evaluation to decide on further diagnostics or culling.

Veterinary input is also crucial for herd‑level control of mastitis in dairy cows. Vets and mastitis specialists can help set up regular SCC monitoring, interpret culture and sensitivity results, and design tailored milking, hygiene and dry‑cow programs. They can advise on selective treatment protocols that balance animal welfare, cure rates and responsible antibiotic use. Consulting a veterinarian early, rather than waiting until problems escalate, improves success rates and reduces long‑term losses from mastitis.

Conclusion

Mastitis in dairy cows is an inflammation of the udder, usually caused by bacterial infection, that remains one of the most damaging diseases in dairy farming worldwide. It occurs in both clinical form, with visible udder and milk changes, and subclinical form, with hidden inflammation and elevated somatic cell counts that quietly reduce milk yield and quality. Understanding the main causes of mastitis in dairy cows—contagious pathogens like Staphylococcus aureus and environmental bacteria from dirty housing or poor hygiene—is the first step toward effective control.

Diagnosis relies on careful observation of the udder and milk, regular SCC testing and bacteriological culture to identify the pathogens and guide therapy. Treatment of mastitis in dairy cows should follow veterinary guidance, increasingly using selective antibiotic therapy based on culture results, along with strong supportive care in severe cases and strict milk withdrawal to protect food safety. Long‑term success depends on prevention and control: excellent milking hygiene, clean and comfortable housing, well‑designed dry‑cow programs, culling of chronic cases and continuous herd monitoring with veterinary support.

By applying these evidence‑based strategies, farmers can greatly reduce the incidence of mastitis in dairy cows, improve animal welfare, protect milk quality and strengthen the economic sustainability of their dairy operations.