Digital dermatitis

What it is

Bovine digital dermatitis (Mortellaro's disease, hairy heel wart) is an infection of the skin of the foot, not of the claw. It is polymicrobial: spirochaetes of the genus Treponema are consistently found in the lesions, together with other anaerobic bacteria, and the disease spreads between animals within the herd. Moisture, constant contact with slurry and maceration of the skin favour infection, and the introduction of infected animals is the usual route into a free herd.

Lesions progress through stages described by the M classification (M0 healthy skin; M1 small early lesion; M2 active, ulcerated lesion larger than 2 cm; M3 healing lesion covered by a scab; M4 chronic, thickened, hyperkeratotic or proliferative lesion; M4.1 chronic lesion with a small active focus). Many lesions become chronic and reactivate later, which keeps the infection in the herd.

Who it affects

Mainly dairy cows housed on dirty, wet floors; it also occurs in replacement heifers and in feedlots. It is more common in Holstein cows and decreases with parity. Prevalence varies widely between herds: some are free, while in others most cows have lesions.

Anatomy involved

• Skin of the foot: the typical lesion lies on the plantar aspect of the pelvic limb, at the proximal border of the interdigital space, between the heel bulbs and next to the coronary band. It can also affect the dorsal aspect of the interdigital space. The pelvic limbs are affected far more often than the thoracic limbs.
• Claws (ungulae): the lesion borders the perioplic margin and the bulb of the claw. Chronic lesions can disturb heel horn production; the cow bears weight on the toe to unload the heels.

Clinical signs

Lameness of variable degree, often with toe-walking and lifting or shaking of the affected foot. The active lesion is a circumscribed ulcer, round or oval, with a red, granular ("strawberry") surface that bleeds easily and is very painful to touch, surrounded by long, erect hairs and with a characteristic smell. Chronic lesions are thickened, dark and have filiform or papillomatous projections. Some cows with active lesions are barely lame, so lameness underestimates the problem. Milk yield and body condition may fall.

Diagnosis

• Inspection of the lifted foot, after washing it with water, during functional trimming or in a trimming chute; this is the reference method.
• Inspection in the milking parlour with a torch and a mirror, useful for herd screening but less sensitive.
• Recording lesions with the M classification to follow progress in the individual and the herd.
• Differential diagnosis: interdigital dermatitis, interdigital phlegmon, interdigital hyperplasia (corn) and heel horn lesions.

Treatment and prevention

• Individual treatment: clean and dry the lesion and apply a topical treatment, mainly tetracyclines as a spray, paste or powder under a bandage; salicylic acid pastes are also used as a non-antibiotic alternative. Bandages must be removed after a few days. Systemic treatment is rarely indicated. The lesion should be rechecked one or two weeks later.
• Herd measures: hygiene and drying of alleys and beds, frequent slurry removal, early detection and treatment of active lesions, control of purchased animals and footbaths, although the evidence for their efficacy is limited (see Key studies). Some footbath products, such as copper sulphate or formaldehyde, raise environmental or occupational safety concerns and their use is regulated.
• Drug use in food-producing animals is subject to national regulations and to withdrawal periods for milk and meat.

Key studies

• Holzhauer et al. (2006), cross-sectional study of 22,454 cows in 383 Dutch herds, with data recorded by 20 hoof trimmers. Digital dermatitis was found in 21.2% of cows, and herd prevalence ranged from 0% to 83.0%. The Holstein-Friesian breed was at higher risk (OR 1.7) and the risk decreased with parity. Interdigital dermatitis and heel horn erosion, interdigital hyperplasia and interdigital phlegmon predisposed to digital dermatitis. It is an observational study and only the hind claws were examined.
• Caddey and De Buck (2021), meta-analysis of 6 16S gene sequencing studies comparing healthy skin and lesions. The genera Treponema, Mycoplasma, Porphyromonas and Fusobacterium were found in most lesions and best distinguished lesions from healthy skin. The individual study and the gene region sequenced influenced the results, and association does not prove causation.
• Cutler et al. (2013), randomised clinical trial in 214 Holstein cow hooves: tetracycline paste, tetracycline powder under a bandage for 2 days, or no treatment. At 8–12 days, 47.4% had healed with the paste and 57.1% with the bandage, with no difference between them, and no lesion in the control group had healed. Active lesions were painful to pressure, although 19% tolerated the maximum force. Follow-up was short.
• Ariza et al. (2017), systematic review with meta-analysis of collective treatments (footbaths and similar) against an untreated or placebo control: 13 publications with 24 treatment and 18 prevention comparisons. The overall quality of the evidence was low, with small trials and possible publication bias, and the analysis does not support a beneficial effect of these treatments on prevention or cure.
• Jacobs et al. (2019), systematic review and network meta-analysis of footbath protocols (14 publications). Only 5% copper sulphate used at least 4 times a week was superior to no footbath (OR 5.26; 95% credible interval, 1.27–28.8) and to a water placebo for curing lesions. No other protocol reduced digital dermatitis. Despite the widespread use of footbaths, the evidence for their efficacy remains weak because the studies are small.