Richard Ehrhardt, Michigan State University Extension Specialist, Small Ruminants
(Previously published on MSU Extension, Sheep & Goat: January 28, 2020)
Foot rot is arguably the costliest disease in the sheep and goat industry in high rainfall areas of the USA (>30 inches per year) and has contributed greatly to the view that sheep and goat production are labor intensive. Animals become severely lame when infected and cannot graze easily or get to the feed bunk. This results in poor growth, poor conception and greatly increased risk for metabolic diseases such as pregnancy toxemia. Foot rot-free status provides producers options to sell replacement breeding stock for high value. Conditions for successful eradication improve as the soil dries during the summer and early fall. Eradication efforts also require a significant labor investment, so one should be sure to plan for this for the program to succeed.
Interplay between foot scald, foot rot and foot abscess
Foot rot, foot abscess, and foot scald are interrelated but different conditions. The schematic below shows the interplay between these conditions.
Foot scald typically presents as red irritated skin in the interdigital area between the toes. Foot scald refers to both a disease and clinical symptom, however, so it is confusing. Foot scald as a disease refers to infection of the foot with soil-borne bacteria, usually Fusobacterium necrophorum. Foot scald as a clinical condition refers to interdigital dermatitis, which can be caused by either Fusobacterium necrophorum alone or in combination with a relatively benign strain of foot rot bacteria, Dichelobacter nodosus. Dichelobacter nodosus strains vary in their pathogenicity. Virulent strains invade hoof tissue extensively, whereas benign strains generally present as foot scald symptoms and generally result in extensive invasion of the soft keratin of the hoof only under ideal environmental conditions for D. nodosus growth (warm and wet conditions). Benign and virulent strains of D. nodosus differ in a gene that codes for a protease (an enzyme that degrades protein) in the bacterium. Virulent strains express a very invasive form of the acid protease 2 gene called called AprV2, whereas benign strains express a far less intense variant of this protease called AprB2 (this is caused by a 2-base pair substitution of the acid protease 2 gene at position 661).
The working model of foot rot infection is that the soil-born organism F. necrophorum causes irritation of the interdigital zone of the hoof, generally under wet and warm conditions. If the sheep/goat specific bacterium D. nodosus is present, it can then invade hoof tissue, causing hoof rot. If not present, only foot scald will occur. So, F. necrophorum is considered the cause of true foot scald disease and the facilitator of foot rot disease. In foot rot, the invasiveness of D. nodosus varies considerably according to strain, resulting in relatively mild or intense infection symptoms.
The causative organism of foot scald lives in the soil for extended periods outside the animal. The causative organism of foot rot, Dichelobacter nodosus, has a more limited life span outside of the animal such that soil that has not had infected sheep or goats on it for 2 weeks is considered clean. This 2-week isolation period is an important factor to understand when implementing an eradication plan.
Foot abscess differs considerably from foot scald and foot rot in that it is caused by bacteria that gain entry into the hoof via puncture injury or via delamination of the hoof laminae (white line), allowing entry of bacteria to deeper recesses of the hoof. This creates a pocket of infection that can be surprisingly small or sometime large. Foot abscesses that occur just above the hoof sometimes enter the joint of the foot, causing severe lameness that is often slow to heal.
Preventing foot rot
The first line of defense against foot rot is a rigid biosecurity protocol that calls for isolation and foot soaking of new arrivals in a saturated, 15+% (weight/volume) zinc sulfate solution. Foot rot cases typically walk on to farms on 4 legs, so it is imperative to trim and soak (15+ minutes) all feet upon arrival to limit potential contamination of your farm’s soil with the causative bacteria, D. nodosus. This should be repeated again 2 weeks later. Any animals developing lame feet should be inspected closely and isolated. Those passing 2 rounds of soaking and remaining asymptomatic for 2 weeks following the second soaking can be added to the flock.
Another line of defense is to treat all new entries with a long acting antibiotic of the macrolide family. These are prescription drugs (Zactran® and Draxxin® are in this family), so it is required to work with a DVM to obtain and receive guidance in the use of these drugs. Boots should also be washed thoroughly between farms, although the risk of transferring the causative organism with dirty boots is far less than that transferred from the infected feet of live sheep, to the soil, and then to another sheep.
Foot soaking procedure
To set up a foot soaking bath, you need to calculate the volume (width x length x depth in inches divided by 280 = gallons). To create a 20% solution, multiple the number of gallons by 2 to calculate the pounds of zinc sulfate required. Zinc sulfate monohydrate can be purchased from many livestock and feed supply stores. It is preferred over copper sulfate, as sheep are very sensitive to copper toxicity. And copper sulfate will severely corrode certain metal surfaces (I have seen the floors of fancy aluminum stock trailers destroyed when copper sulfate foot baths have been used in trailers). The zinc sulfate soak solution should also include laundry detergent to aid in tissue penetration (divide the total volume of the bath in gallons by 30 to obtain the number of cups of dry laundry detergent to add).
A convenient foot soaking bath can be created by building wooden (treated lumber with silicon caulk applied to seams) or purchasing plastic trays to place in the bottom of a handling chute. Wool is commonly added to the trays to limit splashing of the soak solution. Ideally, feet should be soaked for a minimum of 15 minutes. Long handling chutes will obviously make this a much more labor efficient procedure.
Sort into 2 groups: Examine all feet and sort animals into 2 groups, those with healthy sound feet and those without. Virulent foot rot has a very characteristic foul smell and is usually accompanied by discharge. The bacteria appear to literally dissolve the hoof tissue; symptoms are typically not subtle. Collect the animals with symptoms together into the infected group and isolate them away from the group without symptoms (call this the exposed group). Place them on dry ground (hard-surfaced lots work great) or less optimally, on a dry pack indoors.
Exposed group treatment: Trim hooves on all animals in the exposed group and soak in bath as described (15+ min), then run animals onto clean ground (ground that has not seen infected animals for 2 weeks). Repeat procedure 2 weeks later. Place any animals developing symptoms into the infected group. If no animals are symptomatic after 2 soakings you can stop. Continue to monitor the group carefully.
Infected group treatment: Thoroughly trim feet down to healthy tissue to expose bacteria to the air. Do not over trim feet, however, as this can result in much slower recovery. The idea is not to remove all infected tissue but rather to allow better access of the foot bath solution to the hoof and to open it up to create an aerobic environment. Treat infected animals with a suitable, long acting antibiotic (the macrolide family of antibiotics is expensive, but has been shown to have superior therapeutic value in treating D. nodosus. – consult with a food animal DVM on this). Soak the infected group for a full hour at weekly intervals and keep off wet ground. As stated, it is best to do the infected group treatment on dry lot conditions so as to minimize the potential for reinfection. Re-examine and re-trim feet every month. Those not responding after 8 weeks should be culled.
The approach described is a sound approach. However, keep in mind that it can take several years to clean up foot rot in a flock, underscoring the importance of the need for a very dedicated approach to achieve eradication status. A successful practice for one flock with virulent foot rot was simply to soak all hooves of the flock during a routine management point in the management calendar (just prior to shearing during late pregnancy). If you are dealing with this challenging disease, consider if this could be a feasible time point in your management calendar to incorporate this practice. Ideally, soaking the entire flock every 6-8 months (perhaps late fall and before spring turn-out in annual lambing flocks) would be a sound practice for working toward eradication or, at the very least, a high degree of control.
Vaccination and other approaches
Vaccination and antibiotic therapy approaches can speed up the eradication process and be part of a comprehensive program combined with foot trimming and soaking, but should not be used as stand-alone approaches. The vaccines are quite variable in efficacy, probably due to strain variation of D. nodosus (in addition, vaccine supply has been inconsistent over the past 5 years in the USA). Soaking strategies using formaldehyde/formalin solutions are also commonly employed to battle foot rot. This approach can be successful but is risky to flock owners (formaldehyde/formalin is a known carcinogen) and is very harsh on sheep feet (sheep are run through the bath-never soaked).
It is possible to eradicate foot rot, but it requires a dedicated and disciplined approach of isolation, soaking, and trimming. This approach has been used for successful eradication in large flocks. Eradication of benign foot rot is more challenging than virulent foot rot ironically because the infection symptoms are more mild and easier to miss. The labor savings in flock care, the increased welfare status and productivity of the flock, and the increased value of breeding stock sales will pay for the eradication effort.