Contagious footrot is caused by two anaerobic bacteria. The bacteria Fusobacterium necrophorum is commonly found in soil, manure, etc. and colonizes the soft tissue between toes of the sheep. This is followed by penetration of the skin by a second bacteria Bacterioido nodusus. Both bacteria have to be present with certain enviromental conditions to cause contagious footrot. There are different strains of B. nodusus, some being more infectious and/or more severe. Scald is a less severe form of footrot and happens with just Fusobacterium necrophorum present which is commonly found in soil and sheep manure since it is also housed in the sheep's gut. Some sheep seem more prone to footrot and scald. Sheep have different immune systems so their immune response to bacterial infections like footrot and scald can vary.
The B. nodusus organism has to be brought in on the property (Fusobacterium n. is already on the property in soil/sheep's gut/manure). Bringing in footrot infected sheep is a good way to spread footrot into your herd and onto your property. Carrier sheep are a real worry since they may not show signs of footrot at the time of purchase. When conditions are right the sheep has a footrot outbreak. As soon as it becomes infectious again in the sheep it is easily spread to other sheep and onto your property. Especially if your sheep are in mud where the bacteria can thrive in an anaerobic (without oxygen) state. Some articles I have read say the bacteria can live inside the hoof for up to 2 year in carrier sheep. Sheep have different immune responses so not all sheep will be affected the same. As of March 2015, Merck Vet Manual states...After apparent healing, D nodosus remains hidden in small pockets within the foot, where it is detectable only by extensive trimming, and becomes active again when moist conditions recur. These sheep are carriers and generally remain infected for life. Immunity to footrot after infection does not seem to occur, because relapses are the norm.
Environmental conditions conducive to outbreaks of footrot are warmth, moisture and an anaerobic (without oxygen) state. Spring and fall in Oregon are the best times for footrot outbreaks with warmer moist conditions. Keeping sheep in dryer conditions will also help prevent spread and outbreaks since footrot thrives in an anaerobic state in mud and manure.
We don't trim our sheep's hooves. A minuscule cut into the integral wall of the hoof opens it up to bacterial infections. Our sheep's hooves break off naturally with no trims. Over time a line will form where the hoof breaks off. Mother nature does an excellent job while we also selectively breed for good feet with compact hooves that maintain well on their own.
To avoid footrot/scald, quarantine all new animals, no exceptions. Make sure your quarantine is long enough to include proper enviromental conditions that are conducive to a footrot outbreak. Footrot is caused by the introduction of sheep into a flock carrying the disease, by carrier sheep that don't display the disease at time of sale but have a relapse when added to a clean herd they infect during their outbreak or by contaminated premises. Shepherds who track manure in housing footrot, also can expose their sheep to footrot. Since the B. Nodusus organism will live in soil for only 14 days, there are some arguments to whether it lives longer, the major means of contamination is by sheep to sheep contact. Footrot bacteria can live in cracks, crevices, etc. of sheep's hooves for an extended period of time (I have read up to 2 years inside the hoof) therefore sheep can serve as a carrier of footrot without showing symptoms. This makes it even more difficult to avoid footrot or eradicate it.
Not all shepherds agree with me on scald. In 20 years of raising sheep, we have never had a case of scald or footrot. So I believe there is some relationship between scald and footrot. In Oregon, I'm told by sheep breeders scald is just a mild infection between the sheep's toes and a plugged oil gland that starts the infection. But you have to realize I was told many versions of this story by many breeders years ago who diagnosed their foot problems as scald. Most had some form of infection around the tip or sides of the hoof or in several hooves. They told me it was due to the wet warmer weather. Sounds like footrot to me. Here's why. Our Dorsets would go down to a pond to drink in the lower pasture during spring and fall months with warmer very wet weather. Conditions are ideal for footrot related infections during this time. None of our sheep have ever had any foot infections or issues what so ever. Since there are about 20 strains of B. Nodusus (I read that on several reputable sites) and the severity varies in the different strains, I have to believe that scald is a milder form of footrot. We bought a footrot free herd back in 1989 to start out our foundation flock. We closed 2 years later once we found out we couldn't trust what different breeders were telling us and how to bring in new rams and avoid footrot. All of those breeders I contacted to bring in a new ram had footrot. To positively avoid footrot, we closed our herd. We have avoided footrot and scald for 35 years now in both our closed herds.