Interdigital phlegmon

What it is

Interdigital phlegmon (foot rot, foul in the foot, also called interdigital necrobacillosis) is an acute cellulitis of the interdigital space. The interdigital skin is injured by stones, stubble or mud, or macerated by moisture, and anaerobic bacteria enter through that portal: the main one is Fusobacterium necrophorum, often together with Porphyromonas levii, Prevotella spp. and Dichelobacter nodosus. The toxins of F. necrophorum necrotise the tissue and the infection spreads through the subcutaneous tissue of the digit.

Some herds have outbreaks with many animals affected and a peracute form (so-called super foot rot), with rapid, extensive necrosis that responds less well to treatment.

Who it affects

Cattle of any age, both dairy and beef, at pasture or housed, with more cases in wet and muddy environments, on stony tracks and in feedlots. It can occur as sporadic cases or as herd outbreaks.

Anatomy involved

• Claws (ungulae) and interdigital space: the infection lies in the skin and subcutaneous tissue between the two main claws (digits III and IV). The swelling spreads the claws apart and extends to the coronary band and the heels.
• Phalanges of the pelvic limb: the interdigital space lies between the proximal and middle phalanges of the two digits, joined by the interdigital ligaments. If the infection progresses, it can reach the distal interphalangeal joint, the sheath of the deep digital flexor tendon and the bone, causing septic arthritis, tenosynovitis or osteomyelitis of the phalanges.

Clinical signs

Sudden lameness, often severe, of a single limb, with symmetrical swelling of the foot above the coronary band that spreads the claws apart. A fissure with necrotic edges and yellowish tissue appears in the interdigital space, with a characteristic foul smell. Fever, reduced appetite and a drop in milk yield may occur. If lameness and swelling persist or become localised to a single claw after treatment, a deep complication should be suspected.

Diagnosis

• Clinical: clean and examine the lifted foot; the combination of sudden lameness, symmetrical swelling and a necrotic, foul-smelling interdigital lesion is characteristic.
• Exclude other causes of lameness and digital swelling: interdigital foreign body, sole or white line abscess with a draining tract, digital dermatitis and septic arthritis of the distal interphalangeal joint.
• Radiography or ultrasonography if the response to treatment is poor, to assess the joints, tendon sheaths and bone.
• Bacterial culture is not needed in typical cases, but can help investigate an outbreak.

Treatment and prevention

• Early systemic antimicrobials, with a product licensed for the species and indication; early treatment greatly improves the prognosis.
• Cleaning of the interdigital space, removal of necrotic tissue and topical treatment; some clinicians use intravenous regional limb perfusion.
• Anti-inflammatory drugs for pain. Cases with deep complications may need surgery, including digit amputation.
• Prevention: tracks and gateways free of stones and mud, drainage and cleaning of wet areas, footbaths in problem herds, and isolation and prompt treatment of cases during outbreaks. Vaccines against F. necrophorum are available in some countries.
• Drug use in food-producing animals is subject to national regulations and to withdrawal periods for milk and meat.

Key studies

• Kontturi et al. (2019), observational study in 19 Finnish dairy herds with outbreaks and 3 control herds, with samples from 106 cows with interdigital phlegmon, 58 with other foot diseases and 64 control cows. F. necrophorum subsp. necrophorum was clearly associated with interdigital phlegmon, and Trueperella pyogenes with the healing stage. 66.7% of samples from acute cases contained both F. necrophorum and D. nodosus, and D. nodosus was more common in herds with higher morbidity. The results come from outbreaks in a single country.
• Morck et al. (1998), randomised controlled trial in 308 feedlot steers with acute interdigital phlegmon, comparing two ceftiofur regimens with oxytetracycline. Success (no lameness on day 4) was 73% (94 of 129) with the high ceftiofur regimen and 68% (87 of 128) with oxytetracycline, with no difference between them, against 10% (5 of 50) with the low ceftiofur regimen. Porphyromonas levii and Prevotella intermedia predominated in biopsies. There was no untreated group.
• Apley (2015), narrative review of the available randomised trials with negative controls; no systematic review of higher quality exists. There is trial evidence for systemic treatment with ceftiofur, florfenicol, tulathromycin and oxytetracycline, but not for standard treatments such as penicillin G, sulfadimethoxine or tylosin.
• Torehanov et al. (2021), network meta-analysis of randomised controlled trials of antimicrobial treatment in cattle: 4 studies with 565 animals. No antimicrobial differed significantly from placebo in risk ratio, although ceftiofur ranked best. The network is very small and the abstract contains errors in dose units, so its conclusions are weak.
• Checkley et al. (2005), randomised, blinded field trial of a F. necrophorum bacterin in feedlot cattle in Canada. Overall foot rot incidence was 6.5%. In animals fed forage ad libitum, vaccinated cattle were less likely to be treated for foot rot (OR 0.18; 95% CI, 0.04–0.82), but there was no difference in those on a limit-fed grain diet. The effect therefore depends on the feeding system.