Mastitis in Sheep: Udder Infection in Ewes

Mastitis is inflammation of the udder, most often caused by a bacterial infection entering through the teat canal. In ewes, it can affect one half of the udder or both. Some cases are mild and mainly reduce milk quality and milk supply. Others are severe, fast-moving emergencies that damage udder tissue and make the ewe systemically ill.

In sheep, mastitis matters for both the ewe and her lambs. A painful udder can make nursing difficult, and reduced milk production can quickly lead to hungry, weak, poorly growing lambs. Merck notes that mastitis in ewes can contribute to lamb starvation, lower weaning weights, and, in severe cases, death of the ewe.

You may hear flock terms like blue bag and hard bag. Blue bag usually refers to severe, often gangrenous mastitis where blood supply to the udder is compromised and the skin may look dark or bluish. Hard bag often describes chronic scarring and firmness of the udder after inflammation, with long-term loss of milk production from that side.

Because mastitis can worsen quickly, especially around lambing and early lactation, a ewe with udder pain, swelling, or abnormal milk should be checked by your vet as soon as possible.

Most mastitis in sheep is caused by bacteria. Common organisms reported in ewes include Staphylococcus aureus, coagulase-negative staphylococci, streptococci, Escherichia coli, Mannheimia haemolytica, and Trueperella pyogenes. Different bacteria can cause different patterns, from mild inflammation to severe gangrenous disease.

Bacteria usually enter through the teat canal. Risk goes up when teats or udder skin are damaged by nursing trauma, muddy bedding, thorny pasture, mouth lesions in lambs, or poor hygiene around lambing and milking. Cornell also notes that preventing respiratory disease in lambs may help reduce some ewe mastitis cases because Mannheimia haemolytica is an important cause.

Timing matters. Mastitis is often seen soon after lambing, during peak nursing demand, or around weaning when udder pressure changes. Ewes with previous udder damage, chronic scar tissue, or poor teat structure may be more likely to have repeat problems.

Some ewes develop chronic, low-grade infection with little outward illness. These animals may have a firm, lumpy udder and reduced milk on one side. They can struggle to raise lambs well and may remain a source of ongoing flock problems.

Your vet usually starts with a physical exam of the ewe, udder, and lambs. They will look for swelling, heat, pain, asymmetry, abnormal milk, dehydration, fever, and signs that lambs are not getting enough milk. In many flock situations, the nursing behavior and body condition of the lambs are part of the diagnostic picture.

Milk evaluation is often the next step. Your vet may strip out a sample to check for clots, watery secretion, blood, or reduced flow. A milk culture can help identify the bacteria involved and guide antibiotic selection. In some cases, a California Mastitis Test may be used as a screening tool for inflammation, especially when subclinical mastitis is suspected.

If the ewe is systemically ill, your vet may recommend bloodwork to assess dehydration, inflammation, or organ effects. Ultrasound of the udder can sometimes help if there is concern for abscessation, severe tissue damage, or poor milk-out. Chronic cases may be diagnosed based on a firm, scarred udder with poor milk production and a history of weak lamb growth.

Because treatment choices in food animals depend on the ewe's condition, likely pathogen, and drug-use rules, diagnosis should always be guided by your vet.

Prevention starts with flock hygiene and close observation around lambing. Keep lambing areas clean, dry, and well bedded. Check udders regularly in late gestation, right after lambing, and during early lactation so swelling, asymmetry, or poor milk flow are caught early. If lambs seem hungry or one twin is falling behind, examine the ewe's udder and call your vet.

Reduce teat and udder injury whenever possible. Manage rough bedding, thorny areas, and overcrowding. Treat lamb mouth lesions promptly and work with your vet on flock respiratory disease control, since Mannheimia haemolytica can be linked to ewe mastitis. Good nutrition and minimizing stress also support immune function during the high-demand period after lambing.

Chronic udder problems deserve attention before breeding season. Merck recommends careful inspection of mammary glands before mating so ewes with chronic mastitis can be identified and, when appropriate, removed from the breeding flock. Ewes with a persistently hard, lumpy, or nonfunctional udder are more likely to have trouble raising lambs again.

If mastitis is recurring in your flock, ask your vet to help build a prevention plan. That may include udder scoring, recordkeeping on affected ewes, culture of clinical cases, lamb growth monitoring, and culling decisions based on repeated udder disease rather than waiting for another difficult lambing season.