OSU Sheep Team

Dr. Bill Shulaw, OSU Extension Veterinarian

In the previous article we described the basic signs and causes of mastitis in ewes. Mastitis caused by bacteria can be acute or chronic. Acute mastitis often results in obvious signs in the ewe such as being off feed, feverish, and depressed along with changes in the milk. Chronic bacterial mastitis may not result in obvious signs or changes in the milk but may lead to loss of secretory tissue and scarring in the udder with reduced milk production. We also discussed the so-called “hard bag” syndrome caused by the virus that is considered the cause of ovine progressive pneumonia (OPP). In this syndrome, the ewe is often not visibly ill and the producer may not realize that she doesn’t have much milk until the lamb(s) is starving or grow(s) poorly. A common characteristic of flocks that are infected with the OPP virus is the presence of excessive numbers of orphan lambs.

It is not always easy to determine whether ewes that don’t seem to have enough milk have nutritional problems, chronic bacterial mastitis, OPP, or some other condition. Generally speaking, when several ewes in the flock that appear to be healthy, well fed, and in good body condition have firm or swollen udders and unthrifty lambs, mastitis should be suspected. Bacterial mastitis often affects only one half of the udder, although both halves can be affected. The milk may be off-colored and have flakes, but it may appear normal. The udder may or may not be swollen or hot.

If the OPP virus is the primary cause of a firm, swollen udder, both halves are usually affected. The udder is not hot to the touch, and it may not be really hard. Often it may actually appear normal, but there is very little milk in the udder. The ewe often has no signs of illness.

When more than the occasional ewe seems to be milking poorly, udder problems are suspected, or too many lambs are starving or unthrifty, it is time for some diagnostic work and help from your veterinarian. This situation is a flock problem, and examining several ewes is most helpful in determining what the problem might be. Laboratory culture of milk samples from typical ewes is useful in determining whether bacterial mastitis is present. Staphylococcus infections in sheep seem to be the most common bacterial cause of chronic mastitis and udder damage leading to reduced milk production.

Infections during the dry period can cause damage to the udder, and frequently the bacteria are gone by lambing. If typical bacteria are isolated from milk samples from several ewes, it may indicate that the primary problem is bacterial. Often these result from unsanitary environmental conditions where the ewes are housed or damage to teat ends from trauma or soremouth (orf) virus. Damage to the teat end makes it easier for bacteria to get into the udder to cause mastitis. Improving the sanitation in the ewes’ environment and careful management of their nutrition at weaning time can reduce this problem. Treatment of the udder at dry off with dairy cow mastitis preparations has been used on some farms. However, there are no FDA-approved products for sheep, and this practice can result in drug residues. It should only be done in consultation with a veterinarian. Lastly, it is questionable whether this practice can be economically justified as a routine practice.

While sampling the ewes for bacterial mastitis, it is usually a good idea to collect blood samples for testing for OPP virus infection. As this blood test detects antibodies to the virus in the blood, it can tell us whether the ewe is infected. Because infected ewes are infected for life, the presence of test-positive ewes indicates the virus is present in that ewe and on the farm. However, a positive blood test does not tell us that the firm udder we are seeing has been caused by the OPP virus. It only indicates the infection is present, but infected ewes in an infected flock often have no visible signs. If most ewes with hard udders are test-positive for the OPP virus and few of them have bacteria in the milk, it is very likely that OPP virus is the culprit. In many flocks, a close inspection of the flock will reveal other signs of the presence of OPP virus. These include thin ewes that tire easily and ewes with swollen, painful joints.

We have used another technique to help determine whether these udder problems are caused by the OPP virus. Your veterinarian can use a biopsy tool (we use the TruCut biopsy needle) to get a sample of udder tissue. This sample can be placed in formaldehyde and sent to a laboratory where it can be microscopically examined for changes that are characteristic of the OPP virus infection. The ewe’s teat should be thoroughly cleaned and disinfected, and the biopsy needle can be inserted through the teat canal and on into the udder tissue for taking the sample. A sample from a single ewe may not be diagnostic of the flock problem, but samples from several ewes will usually provide evidence for OPP infection as the cause of “hard bag” if the virus is present. Often a problem flock will have several cull ewes that have had these udder problems and which can be sampled.

It is possible to find both bacterial mastitis and OPP virus infections in the same flock; perhaps even in the same udder. However, if enough samples are taken it is usually possible to sort out the real troublemaker. In assisting several veterinarians working with flocks with hard udder problems, we have frequently observed the following when OPP virus is the main problem: high rates of test-positive ewes on random sampling – usually 60% of ewes or higher; microscopic changes in udder tissue that are compatible with OPP virus infection; and the presence of other signs of OPP virus infection such as some thin ewes and some cases of swollen, arthritic joints. Post mortem examination of some of these cull ewes may provide additional evidence for OPP virus infection.