Footrot is caused by bacteria (Dichelobacter nodosus) that live on the feet of infected sheep, which may or may not be lame. The bacteria pass from one sheep to another via the surface the sheep are standing or walking on (e.g. the pasture or standing areas at gathering sites). D. nodosus can survive for a maximum of 7 –10 days on pasture and for up to 6 weeks in hoof horn clippings. The organism survives best in a warm, damp environment.
Scald (also known as strip or interdigital dermatitis) is caused by invasion of the surface layers of the skin between the claws (interdigital skin) by bacteria following damage by moisture (damp conditions underfoot), frost and/or mechanical damage from long grass, thistles, etc. It was previously believed that scald was caused by the bacterium Fusobacterium necrophorum, however we now believe that D. nodosus causes both footrot and scald. In some cases scald can progress to footrot as D. nodosus invades further causing separation of the hoof horn from the underlying tissue.
Whether or not scald cases progress to become footrot cases (under-running of the hoof horn) depends on:
- The virulence and dose of the D. nodosus
- The susceptibility of the sheep
- Whether the sheep is treated promptly before separation of the hoof horn occurs
We recommend that individual cases of scald in adult sheep are treated as an early stage of footrot (i.e. with an antibiotic injection and an antibiotic spray). Antibiotic sprays should clear most scald cases in lambs. Outbreaks of scald in lambs can be controlled by footbathing.
A more in depth explanation of footrot and scald with references can be found in the aetiology and pathogenesis section of this website.