Interdigital Phlegmon in Cattle
(Footrot, Foul in the foot)
Footrot is a subacute or acute necrotic infection that originates in the interdigital skin, leading to cellulitis in the digital region. Footrot has a worldwide distribution and is usually sporadic. It may be endemic in intensive beef units or in cattle at pasture. The incidence varies according to weather, season of year, grazing periods, and housing system. Footrot is presented less commonly today than decades ago, partly because more cases are treated by farmers and partly because dairy cows spend much less time at pasture. However, on average, footrot accounts for up to 15% of foot diseases.
The appearance of footrot in loose housing systems has been reported.
Injury to the interdigital skin provides a portal of entry for infection. Maceration of the skin by water, feces, and urine may predispose to injuries.
Fusobacterium necrophorum is considered the major cause of footrot. It can be isolated from feces, in which it may survive as a saprophyte, which may explain why control is difficult. It can also survive in moist soil.
F necrophorum is a gram-negative, nonspore-forming, nonflagellated, nonmotile, pleomorphic anaerobic bacteria. It has a lipopolysaccharide endotoxin capable of necrotizing activity. There are three subspecies and a number of genotypes, each of which targets different tissues. When PCR assays become more widely available, the genotypes will be more readily identifiable.
Other organisms, such as Dichelobacter nodosus, Staphylococcus aureus, Escherichia coli, Trueperella pyogenes, and possibly Bacteroides melaninogenicus can also be involved.
Research suggests that the incubation period of footrot can be a week. The fore- or, more commonly, the hindlimbs can be affected, but more than one foot is rarely involved at the same time in mature cows. However, footrot can occasionally develop in several feet in calves. The first sign is swelling and erythema of the soft tissues of the interdigital space and the adjacent coronary band. The inflammation extends to the pastern and fetlock. Typically, the claws are markedly separated, and the inflammatory edema is uniformly distributed between the two digits. The onset of the disease is rapid, and the extreme pain leads to increasing lameness. In severe cases, the animal is reluctant to bear weight on the affected foot. Fever and anorexia are seen. The skin of the interdigital space first appears discolored; later, it fragments with exudate production. As necrosis of the skin progresses, sloughing of tissue is likely to follow. A characteristic foul odor is produced.
If the disease proceeds unchecked, weight loss is severe and milk yield is significantly reduced. Milk production may not recover during the current lactation. Open lesions can be infected with secondary invaders. If the necrotic lesion is located in the anterior region of the interdigital space, the distal interphalangeal joint can become infected.
Hematogenous infection of the tissues of the interdigital space may account for peracute cases of footrot, which are referred to as either “blind” or “super foul.” This form of footrot is characterized by the initial absence of a skin lesion, extreme pain, and the tendency to progress despite aggressive therapy.
Most treated animals recover in a few days. Good results are obtained with penicillin G, IM, for 3 days. Treatment should be administered as soon as signs are observed. However, the label dosage may be inadequate to effect a rapid resolution, and increased dosages may be needed, requiring increased withdrawal times. Treatment of “super foul” must be particularly aggressive. Early cases respond well to single doses of long-acting oxytetracycline.
Sodium sulfadimidine solution IV or trimethoprim/sulfadoxine, IV or IM, bid for 3 days, can also be used. A single oral administration of a long-acting bolus containing baquiloprim/sulfadimidine may be suitable to treat beef cattle.
High concentration of an agent in the target tissues can be achieved by a regional IV injection (see Distal Digital Anesthesia for Diagnostic and Surgical Procedures in Cattle). Positive results have been obtained with penicillin or oxytetracycline.
Local treatment is essential for some longstanding cases and in all instances in which the anterior region of the interdigital space has been compromised. The lesion must be thoroughly cleansed, but it is inadvisable to curette or otherwise remove necrotic tissue surgically. The dorsal pouch of the distal phalangeal joint is very superficial at this point. A nonirritant bacteriostatic agent (such as nitrofurazone or a sulfa preparation) should be applied as a topical dressing. The application of gauze, cotton batting, or bandages is contraindicated. However, the lesion can be protected and immobilized by binding the digits together with a bandage. The entire digital region can be protected from contamination if it is enclosed in a plastic bag fixed in place with an adhesive bandage. However, prolonged protection is not advocated, because the enclosed lesion tends to macerate further. Bandages, if used, should be replaced daily.
Animals actively shedding infectious organisms should be isolated until signs of lameness have disappeared. If this is not possible, a waterproof dressing or protective boot should be applied; however, animals wearing protective boots should be monitored carefully to avoid additional damage. Boots should be disinfected between use.
Because busy traffic areas are invariably heavily contaminated, steps should be taken to ensure that areas around drinking troughs, gateways, and tracks are adequately drained. Animals at pasture might be moved to a clean, dry area, or possibly housed during periods of heavy rainfall. Contaminated concrete must be frequently cleaned and scraped free of manure.
Preventive use of a footbath with an antiseptic and astringent solution (eg, copper or zinc sulfate [7%–10% in water]) has given beneficial results. Formaldehyde solution (3%–5% in water) can also be used. Ethylenediamine dihydroiodide has been used as a feed supplement for prevention, but the results are extremely uncertain. Vaccines against F necrophorum have failed because of the weak immune response to the bacterium. High levels of zinc fed as a supplement have a beneficial effect by improving epidermal resistance to bacterial invaders.