Dr. Bill Shulaw, OSU Extension Veterinarian
Green grass is beginning to peek through the brown plant residues on many Ohio pastures. If our weather pattern is typical this spring, we will soon be enjoying warmer, but wetter, weather. Although we will welcome the flush of new forage that this weather will bring, this is the major transmission season for one of the most common of sheep diseases: contagious footrot. Warm wet weather softens the hoof and soft tissues between the toes making the foot more susceptible to infection. It also favors the transmission of the causative bacteria, Dichelobacter nodosus (formerly Bacteroides nodsus), from the hooves of carrier sheep to the hooves of unaffected animals. For a review of the causes of virulent and benign footrot in sheep, as well as “scald,” the reader is referred to the appropriate section in the latest edition of the SID Sheep Production Handbook (available from ASI at: http://www.sheepusa.org/test-sph).
When footrot appears in a flock, it often seems to go away, or become less severe, as the weather gets hotter and dryer in mid to late summer. Sometimes this appears to be a positive response to a treatment effort, and the shepherd thinks he/she has “cured” it. Unfortunately, eradication of footrot usually takes a planned and concerted effort in most flocks, and many shepherds find that what they thought had disappeared, reappears at this time of year when weather conditions increase the irritation to the foot and favor transmission of the bacteria. If your flock experienced a footrot problem last year, you might wish to begin considering how to deal with it again this year to limit the labor and production costs it often creates. Scientists usually advise that attempts at eradication of footrot during times of high probable transmission is unwise, but that attempts to control the disease should help limit the number of animals affected and the numbers of cases that progress to severe disease. There are two main methods for controlling footrot during the transmission period – vaccination and topical treatment with a footbath.
Footvax® is used for vaccination for D. nodosus in the US. This product contains ten strains of this organism in an oil-based carrier. When one of these strains is present in a flock, the vaccine may stimulate a strong immune response that provides protection from infection in many sheep and a significant reduction in severity of foot damage in many others. Unfortunately, some strains of this bacteria exist in the US that are not present in the vaccine, and the lack of complete cross-protection across strains often leads to disappointing results. There currently is no easy way to determine which strain is present in a flock, but if footrot has been a recurring problem, it may be worth using the vaccine to evaluate its potential usefulness. The vaccine requires a primary series of two injections at least six weeks, and not more than six months, apart. Booster vaccinations can be given at six-month intervals or annually just prior to an expected period of transmission. It is important to try to stimulate the highest immune response to precede, or coincide with, an expected period of transmission. Because the vaccine contains an oil base, users should carefully follow the label precautions and avoid accidental human injection.
Zinc sulfate in a 10% solution (8 pounds of zinc sulfate powder [hepta hydrate] in 10 gallons of water) is the most commonly recommended solution for use in footbaths. A small amount of powdered laundry detergent, about a half cup, is usually added to improve the penetration of hoof horn. Although formalin (formaldehyde) solutions have been recommended for footbaths in the past, they are very irritating if inhaled or splashed in the eyes, are quite irritating to the skin between the hooves, can cause the hoof to become hardened and difficult to trim, are less penetrating than zinc sulfate, and formalin is now classed as a probable human carcinogen. Research has shown that footbathing in a control effort helps primarily in the early stages of infection where it is limited to the skin between the toes and before serious damage to the horny tissue has begun. Therefore, if it is likely that the flock has footrot, footbathing should start early in the transmission period to limit the number and severity of new cases. Numerous recommendations for the length of time animals should spend in the footbath, from walk-through baths to one-hour soaks, can be found in the scientific literature. Although one-hour soaks can result in higher “cure” rates and longer protection against re-infection, they can be difficult to accomplish with large groups of sheep. Some researchers have gotten good results with 2-5 minute soaks, but these will probably need to be repeated weekly during seasons of high transmission. It is generally agreed that footbathing success is improved if the sheep have relatively clean feet when they enter the footbath and if they are placed on a clean dry surface for a couple of hours after bathing.
It is believed that the combination of vaccination and footbathing is more effective than either administered alone. Footbathing may be especially useful if vaccine use was delayed until the onset of the transmission season.
Contrary to popular belief, the bacteria that cause contagious footrot do not survive outside the sheep’s foot for more than seven days. This fact is critical to the success of eradication efforts (which are most successful during dry periods when transmission is unlikely). However, because neither vaccination or footbathing are likely to be 100% successful in the short term, and because many flocks will not have enough separate pastures to allow a contaminated paddock to rest seven days, spelling pastures during a transmission period is seldom possible or recommended. The goal of footrot control during a transmission period is the reduction of the number and severity of cases, not eradicating the disease.
Another common misconception is that foot trimming prevents footrot. Foot trimming does not prevent sheep from getting footrot. Foot trimming will improve the success of footbathing in sheep which have more advanced disease with hoof separation, or under-running, of the horny tissues of the sole and wall. Foot inspection and appropriate paring are critical to an eradication effort, but inspection and trimming of every animal’s feet may not be necessary for control efforts during seasonal periods when the risk of transmission is high. Excessive trimming can reduce the effectiveness of footbathing and can lead to permanent damage to the hoof. It is usually adequate to remove only the excessive horny tissue to expose pockets of infection to the footbathing solution.
Severe cases of footrot are costly and may create an animal welfare concern. A review of a flock’s past history and a little planning can reduce the impact of this disease and is particularly relevant as we approach warm wet weather this spring.