Environmental risk factors for infectious causes of lameness
1. Foot Rot is caused by specific pathogenic strains of Fusobacterium necrophorum and Baceroides melaninogenicus that enter through the interdigital skin. These bacteria persist in wet soil or slurry for very long periods and normally reside in the digestive tract of cattle. All strains are not necessarily pathogenic. Intact dry skin resists penetration of these organisms.
Conditions that produce breaks in the interdigital skin such as coarse sand or small stones may predispose to foot rot. These conditions occur in muddy cattle lane ways, around water sources, or in riparian zones. Traditional control methods have been to fence cattle away from riparian zones and mud holes. A new approach for cattle lane ways in the United Kingdom consists of a 40 inch wide roadway excavated to a depth of 12 inches (Dr. Roger Blowery from Gloucester). Eight inches of gravel or crushed stone is placed in the trench, covered with geotextile fabric and the remainder of the excavation filled with shredded bark. The lane way remains dry on the surface, stands up very well to traffic, and cows move comfortably.
Foot rot in housed dairy cattle is increased by abrasion of continuously moist interdigital skin. Manure removal practices influence both infection pressure and the interdigital skin integrity. Foot bathing with an antibacterial compound is used routinely to prevent new cases of foot rot.
2. Interdigital Dermatitis is a chronic superficial infection of the interdigital skin caused by Dichelobacter nodosus. Visible lesions are present in the majority of cattle, whether housed in free-stalls or tie-stalls. Pain and lameness are not present in most obviously infected cattle. Exposure to manure and urine predispose to infection and severity. Most lameness due to interdigital dermatitis is secondary either to skin (and possibly hoof sole) hypertrophy or to hoof fissures in the heel horn caused by the bacterial elastases that can cleave keratin. The main environmental risk factors seem to be manure contact with the skin and anaerobic conditions between the manure layer and the skin. Control is as for foot rot.
3. Digital Dermatitis is an infectious disease of the skin affecting cattle older than about 6 months of age anywhere from the vicinity of the dewclaws distally. The causal organism(s) have not been conclusively identified but response to therapy with antibacterial drugs and consistent observation of spirochetes in affected tissues supports a bacterial etiology. Environmental risk factors are the same as for interdigital dermatitis. Infection with Dichelobacter nodosus may facilitate establishments of the agents of digital dermatitis.
Dry conditions as might occur in dry lot or some pasture conditions seem to prevent spread of the infection but do not influence severity in already infected cattle. Control is with foot bathing or spraying with the antibiotics (oxy)tetracycline or lincomycin. These plus some antiseptic solutions are used successfully in foot baths for control.
Foot rot or interdigital philegmon
An acute disease caused by the bacteria Fusobacterium necrophorum and Bacteroides melaninogenicus with both required for disease to occur. Bacteria from the genus Prevotella may also be involved. The cow becomes lame over the course of a day or two with symmetrical swelling above the hoof. Pain may be severe with unwillingness to bear any weight on the affected limb. Rear limbs are more commonly affected. A fissure in the interdigital skin occurs with necrosis of underlying tissues. Usually this is a dry necrosis with no exudate but some very virulent strains of F. necrophorum may produce tissue liquefaction. Foot rot odor is strong and characteristic. Treat with corrective claw trimming, along with topical antiseptic to the interdigital space. Bandaging is strongly discouraged; air should reach the interdigital tissues. Parenteral antibiotics are the most important part of therapy. Many antibiotics are effective and the choice is unimportant.
In the United States, as of this writing, Ceftiofur is registered for foot rot and requires no milk discard. Some cases of foot rot are caused by multiple-drug-resistant F. necrophorum. These cases respond to Tylosin at label recommended dosages.
Treatment failure with the usual choice of antibiotic suggests the presence of drug resistant strains in a herd. If a sole ulcer caused the lameness, changing antibiotics will not help.
Interdigital dermatitis, heel horn erosion, heel cracks
Chronic interdigital dermatitis due to Dichelobacter nodosus is common in moist environments. Subclinically affected cattle have non-painful erosions or ulcerations of the interdigital skin. A moist, white exudate with characteristic odor distinguishes it from foot rot. The infection produces a mild irritation that results in underlying skin hypertrophy and sometime faster growth of the adjacent axial hoof wall. This can result in an interdigital fibroma or excessive horn accumulation along the axial wall. The axial wall may flare toward the interdigital space or cause an abnormally high region in the adjacent sole. Corrective trimming should remove the excessive horn and open the interdigital space to increase self-cleaning and air access. If the infection spreads across the heels it may erode the horny portion of the heel in irregular patterns or create a transverse crack at the sole heel junction.
Lameness results from interdigital dermatitis when the cracks in the heel combined with hypertrophy of heel bulb skin change the weight distribution to increase pressure on the heel. The discontinuity of tissues from sole to heel may also result in pinching of sensitive tissues beneath the crack. Lameness is not usually severe but cows may stand with their heel suspended over the manure gutter or off the rear of a freestall curb. Usually the problem is symmetrical in both limbs. Rarely, a crack at the sole heel junction penetrates to expose the corium.
Treatment involves removal of the flaps of overlying horn and opening the enclosed spaces to air. A hoof block helps. Topical disinfectants such as iodine or copper or zinc solutions are all effective in killing D. nodosus.
Digital dermatitis, mortellaro, heel wart
Digital dermatitis begins as a reddened circumscribed area typically just above the interdigital cleft on the plantar aspect of the pastern, the strawberry form. The most striking feature is the degree of pain expressed. Hairs at the periphery of the lesion are often erect and matted in exudate to form a rim. As the lesion progresses, focal hypertrophy of the dermis and epidermis leads to raised conical projections appearing much like wet, gray terry cloth. In even later stages, papilliform projections of blackened keratin may extend 10 to 15 mm from the surface, the hairy wart stage. Lesions may affect any of the limbs but rear limbs are more commonly involved. Lesion location is variable and includes interdigital skin, anterior and posterior margins of the interdigital cleft, and distinct lesions that do not touch the coronary band. Simultaneous infection with Dichelobacter nodosus are common and lead to significant erosion of heel horn. The hoof may be noticeably misshapen by abnormal wear due to altered limb use resulting in short rounded toes and exaggerated heel depth.
Interdigital fibromas, regardless of etiology, are commonly infected with digital dermatitis in endemic herds. In my experience, after digital dermatitis has been present in a herd for a year or so, most cases of lameness are found in the first lactation animals even though lesions may be formed on older cows.
I have recommended topical treatment of lame cows with oxytetracycline in the form of 5 to 15 cc of injectable 10% oxytetracycline applied on a cotton dressing with a flimsy wrap. Others use tetracycline powder under some form of bandage with or without a cotton pad. I currently recommend treating the cleaned lesion with oxytetracycline in propylene glycol without a wrap of any kind. It is quicker, cheaper, and seems equally effective.