OSU Sheep Team

William P. Shulaw, Extension Veterinarian

(Originally Published in Sheep Team Newsletter April 2003)

Every year some flocks will have ewes that deliver a live healthy lamb(s) that subsequently 1) suffers starvation, 2) must be bottle raised, or 3) grows poorly.  There are several possible causes for this observation.  Poor mothering ability; poor nutrition of the ewe; and systemic diseases in the ewe, such as chronic bacterial pneumonia, are several of these.  However, udder disease is also a common cause of unthrifty lambs.

Mastitis is name given to inflammation of the secretory tissue of the udder.  The most common cause of mastitis is a bacterial infection, and the vast majority of these infections enter the gland through the teat end.  We usually classify bacterial mastitis as acute or chronic based on appearance of the signs, the duration of the infection and the severity of the disease.  Acute mastitis is often easily recognized.  Usually the udder, one or both halves, is swollen, warm to the touch, and painful.  The milk may be off color with flakes or chunks in it.  The ewe may also show signs that she doesn’t feel good.  She may have a fever, be off feed, and be depressed.  In some severe cases, the infection may progress to gangrene with loss of an udder section and sometimes death of the ewe.

Chronic mastitis often produces no obvious signs in the ewe.  The milk may appear slightly abnormal or grossly normal to the eye.  These infections may occur anytime but are common at the onset of lactation and near dry off.  These chronic infections, especially those in the dry period, may produce enough inflammation and scarring to reduce milk production in subsequent lactations.  In some cases, enough damage is produced that the udder produces little or no milk.

Treatment of ewes with acute mastitis is usually done with antibiotics, frequent milking, and oxytocin.  If the ewe shows signs of depression, a veterinarian should be consulted.  Chronic mastitis, by its very nature, often is not diagnosed until after damage has been done.  Although treatment of the udder at dry off (weaning the lambs) has been advocated by some in the past, there is little information to suggest that routine treatment is economically justified.  There are no products labeled for this use in sheep.

Another infection that may cause severely reduced milk production, as well as other signs, is ovine progressive pneumonia (OPP) virus infection.  This is a relatively common infection in sheep.  Data from the 2002 NAHMS study is not yet completely analyzed, but it is expected that it will show that >30% of US sheep flocks are infected.  This virus is often considered the cause of the so-called “hard bag” syndrome.

The virus infects certain cell types of the body including some found in the udder.  The body’s reaction to these infected cells results in gradual replacement of the secretory tissue with immune system cells that trying to get rid of the virus.  This usually takes place over several months and doesn’t result in obvious signs in the ewe.  At lambing time, the producer often sees a symmetrically full udder and assumes that it contains a lot of milk.  However, there may be very little milk actually produced, the milk that is present usually looks normal, and the ewe usually shows no sign of illness.

Although the virus that causes OPP affects other areas of the body such as the lungs, joints, and brain, it is frequently the occurrence of several ewes with the signs described above that signal the producer that something is wrong.  Although a blood test is available that shows whether a ewe is infected with the OPP virus, a positive blood test really can’t tell you that the udder disease you are seeing is the result of OPP virus infection.  Many animals that are infected have no obvious signs or detectable disease.  At this point a thorough investigation is warranted.  This usually involves a diagnostic effort involving your veterinarian and the veterinary diagnostic laboratory.  The next issue of the newsletter will discuss diagnosis of the causes of mastitis, including OPP.