The Condition
Lameness in sheep flocks is one of the most common and persistent disease problems in the United Kingdom. It is not only a major welfare concern but will also have major economic consequences in flocks persistently affected by lameness. Persistent lameness in flocks may lead to reduced weight gain, metabolic diseases in pregnant ewes, reduced birthweight of lambs, mismothering and poor colostrum production by ewes (Harwood et al., 1997; Henderson, 1990).
Foot rot is caused by a synergistic infection of two anaerobic bacteria, Dichelobacter nodosus (Bacteroides nodosus) and Fusobacterium necrophorum (Fusiformis necrophorus). Fusobacterium necrophorum on its own causes a condition called scald, which affects the skin between the claws only. The skin is moist and painful, but scald is not invasive and causes no separation of the horn from deeper tissues. The damage to the skin, however, allows Dichelobacter nodosus to enter and colonise deeper layers. Once D. nodosus is established, F. necrophorum invades deeper tissues, where it may be responsible for a large proportion of the inflammation and tissue damage observed, caused by the action of its exotoxin. The damage can be observed externally by the under-running of the external layers of the hoof. This separation starts in the heel area and, depending on the strain of bacterium involved, may spread across the sole and eventually up the wall of the hoof (Hofstad, 1992; Winter, 1998).
Foot rot spreads more readily in warm, moist weather outdoors (Cross, 1978; Cross and Parker, 1981) and when sheep are housed (Henderson, 1990). Stocking density, soil pH and breed of sheep may also play a role in the infective process (Winter, 1998).
Recently, a new type of very aggressive foot rot has been observed in a number of flocks, causing severe inflammation of the foot. It usually affects both claws, the cleft and often the skin above the hoof. The horn may completely detach, but unlike ‘normal’ foot rot, the coronary band where new horn is produced may be permanently damaged, resulting in the cull of the animal (Harwood and Cattell, 1997; Harwood et al., 1997; Winter, 1997).
The white line is the site at which the horn of the wall of the hoof joins that of the sole. It is a naturally weak area in the horn and there are two problems that can occur here, both eventually leading to lameness. The first problem occurs when a toe abscess develops along the white line. Pus forms and the animal becomes acutely lame. Some animals suffer repeated attacks, probably due to a permanent defect in the horn.
The second problem is a more extensive degeneration of the white line. This is called shelly hoof, and is characterised by pockets, impacted with dirt and other debris, where the hoof wall has become separated. Mild cases, not necessarily leading to lameness, are very common. More severe cases get abscesses and become lame (Scott and Henderson, 1990; Winter, 1998).
Foot abscesses can form in the deeper parts of the hoof and often affect the pedal joint. The affected claw becomes swollen and very painful. Pus may burst out through the coronary band or between the claws. The deeper structures of the foot are often permanently damaged and the claw becomes chronically swollen and misshapen. The animal may become permanently lame (Winter, 1998).
A granuloma is a strawberry-shaped piece of proud flesh, which grows at the site of damage to the foot. The overlying horn is unable to grow back normally. It is often caused by over-paring which has led to bleeding, although other causes of injury to the foot may play a role. It usually causes an overgrown misshapen hoof, because the animal does not put its full weight on the foot due to chronic lameness (Scott and Henderson, 1991; Winter, 1998).
See also MAFF, 1992.