By William P. Shulaw, Extension Veterinarian
(Originally Published in Sheep Team Newsletter August 2003; reprinted with permission)
Wet weather this spring and summer has certainly created ideal conditions for foot problems in sheep. Nearly continuous exposure to moisture softens the hoof’s horny tissues and makes it more vulnerable to irritation, injury, and infection. Those flocks that are affected by classic virulent foot rot (sometimes called contagious foot rot) have likely seen a surge in the number and severity of cases if they have not been attempting to control or eradicate this potentially devastating disease.
Those flock owners who have experienced milder forms of lameness in their flocks may assume that they don’t have foot rot but have a milder condition called “foot scald” or “scald”. Actually there are really two recognized conditions that are sometimes referred to as “scald”. At first the difference may seem academic, but for some producers, it may be more than that.
Virulent, or contagious, foot rot is caused by a synergistic infection with two organisms, Dichelobacter nodosus (formerly Bacteroides nodosus) and Fusobacterium necrophorum. This last organism is in virtually all sheep environments and sets the stage for infection with the organism necessary for foot rot to occur – Dichelobacter nodosus. This organism produces a powerful proteolytic enzyme that dissolves hoof horn and leads to the undermining of the sole, the severe lameness, the foul smell, and the abnormal hoof growth seen with classic virulent foot rot. About 20 different strains of D. nodosus are believed to occur in the US.
True foot rot does not occur in the absence of D. nodosus. However, a less persistent and generally milder condition in which only inflammation between the toes and a slight under-running of the hoof horn occurs in some flocks. Both D. nodosus and F. necrophorum can be isolated from these cases, but the strains of D. nodosus isolated seem have a reduced virulence or ability to produce disease. This condition is technically referred to as benign foot rot but has also been called foot “scald.” It is believed that the strains of D. nodosus that are involved are weak enzyme producers and are less able to produce the severe damage seen with virulent foot rot.
Another infectious and inflammatory condition which involves only the skin between the claws without significant undermining of the horny tissue has been termed ovine interdigital dermatitis (OID), and it has also been called “scald”. The bacteria associated with this condition are F. necrophorum and Actinomyces pyogenes; both of which tend to be common in typical sheep environments. D. nodosus is not involved. The disease usually only occurs when the weather and other conditions on the farm damage the skin and allow these bacteria to create skin infections. Because the term “scald” has been applied to both OID and benign foot rot, the use of that term can be misleading. Both conditions usually cause only mild or temporary lameness that may be obvious only in wet periods of the year. From a practical standpoint, the two conditions are difficult to distinguish from one another, and laboratory capability to isolate and serotype D. nodusus is generally not readily available. Foot baths and soaks with 10% zinc sulfate usually result in improved healing of either condition. Foot “scald” often disappears when the environmental conditions become dry.
So if the two conditions called “scald” are not easy to differentiate and they both tend to clear up with foot bathing or dry weather, why even bother to make the distinction? There are two reasons that may be important to some producers. The first is that all the strains of D. nodosus, whether they produce virulent or benign foot rot, are maintained in the flock by sheep that harbor the infection in cracks and crevices on their feet — carrier animals. That means that most likely, unless an attempt at eradication is made, benign foot rot will be back again as soon as the weather conditions favorable for foot softening and transmission reappear. Because some flocks have substantial numbers of sheep that are affected by benign foot rot when conditions are favorable, it can be an economically important problem.
Secondly, there appears to be some breed-related susceptibility to D. nodosus infections. Some breeds, such as Merinos, appear to be especially susceptible to this organism, and what appears to be a relatively mild problem in one flock may be more serious when the infection is introduced into another flock. In other words, what seems to be just “scald” in one flock, may be much more serious, and look like virulent foot rot, if it is introduced to another. OID is not considered a transmissible disease in that the bacteria that cause it are in most sheep environments and only cause trouble when environmental conditions are very favorable.
The word biosecurity seems to be used a great deal today — in reference to both human and animal populations. In the National Animal Health Monitoring System’s Sheep 2001 survey and report, foot rot had been diagnosed or believed to be present on 34.9% of US sheep operations. No data was collected on “scald”. It seems wise for producers considering addition of new sheep to their flock to question the seller about the presence of foot rot, “scald”, and other disease conditions in the source flock. And in light of the fact that D. nodosus infections seem to be common (both virulent and benign foot rot), it is also wise to use an isolation protocol and to rigorously inspect for signs of foot rot or scald before those animals are allowed access to the flock or the pastures the flock uses. The American Sheep Industry Association has an excellent fact sheet on biosecurity at www.sheepusa.org/news/animalhealth/biosecurity.shtml.