Update on treating digital dermatitis

Most of us in farm practice see cows affected by digital dermatitis (DD) day in, day out (Figure 1). This pernicious disease has been in the UK for around 30 years and not many dairy farms have escaped infection.

We don’t seem to be getting any better at controlling it, either. A mind-boggling number of different treatments exist that vets, foot trimmers and farmers use (Relun et al, 2013).

So, with all these treatment options and “secret recipes”, why does it still seem to be such a big problem? This article looks at rational approaches to treatment and control.

Some DD facts

We have learned many new things about DD over the years, for example:

l It is involved in the aetiology of toe necrosis, estimated to affect around 2% of the adult dairy herd and responsible for a huge amount of pain and suffering (Atkinson, 2012).

• Infections on claw horn lesions, such as sole ulcer and white line disease, can cause them to be non-healing (Evans et al, 2011; Figure 2).
• It is possibly involved in ulcerative mammary dermatitis (Evans et al, 2010) and teat necrosis.
• The disease cycles through different stages of lesion progression and regression in individual cows over time (Krull et al, 2016; Döpfe et al, 2012).
• Affected carrier cows, particularly with active lesions, are the main reservoir of the infection (Carter et al, 2009; Evans et al, 2012).
• Environmental pathogen loads are usually nil or low – therefore, the significance of slurry in the aetiology is less likely as a source of infection than the damaging effect it has on the skin’s natural immune defences (Carter, 2014; Figures 3 and 4).

Four control principles

When it comes to DD control in a herd, four basic tenets exist:

• Reduce the infection pressure – that is, treat carrier cows to dampen the pathogen load.
• Improve foot skin health – keep feet dry (for example, by improving lying times and maintaining floor surfaces to reduce pooling) and have good slurry management (Figure 5).
• Reduce immunosuppression – control concurrent infectious diseases, ensure good nutrition and transition cow management, and avoid overcrowding.
• Biosecurity – farms lucky enough not to have DD need to exercise strict quarantine or maintain a closed herd, farms with DD need to exercise similar care as several different strains and new strains can be brought on farm by stock, equipment or people.

It may be surprising the author does not include foot bathing. This hasn’t been forgotten, but challenges the dogma DD control is all about foot bathing. Foot bathing has an important role to play on many farms, but it must be done well and viewed in context.

When done well (beyond the scope of this article), foot bathing can be a valuable aid in reducing infection pressure; when done badly, it simply wets and damages the skin of feet and possibly contributes to the spread of infection.

Individual treatment of affected cows is a valuable way of reducing infection pressure. So a double benefit exists – you reduce lameness in that individual and prevent a further “x” number of cows becoming infected and going lame.

For individual treatment to work best, it needs to be done early on and effectively (Döpfe et al, 2012).

Early detection, prompt effective treatment

Early detection, prompt effective treatment (EDPET) needs to be drummed into the minds of every dairy farmer and dairy vet. As a means of lameness control, it is unbeatable, but every word of EDPET is important.

Early detection

In practice, early detection means regular mobility scoring. Lame cows are masters at remaining hidden and that is why you need to look for them. We don’t need to mobility score to detect the score-three cows – farmers already know who these ones are as they will be at the back of the herd – but we do need to do it to detect the score-two cows hidden in the mob.

One day, technological advances will no doubt make mobility scoring less necessary, but, at the moment, a person’s eyes beat any technical device on the market at finding lame cows.

Prompt

Then, there’s the word “prompt”, which means shedding a lame cow, getting her in a treatment crush and lifting her feet needs to be quick, safe and easy. One cow, one person, one minute – that’s our target. It is possible (Figure 6).

Effective treatment

The next words are “effective treatment”. Now we get to where I wanted to begin. In other words, how to treat individual cows with DD.

While DD treatment (including foot bathing) is probably one of the most thoroughly investigated topics relating to lameness in dairy cattle, many questions exist about best practice (Potterton et al, 2012).

For example, despite studies comparing different topical antibacterial agents and/or systemic antibiotics, a great degree of uncertainty exists over the value of prior foot trimming, lesion debridement, lesion cleaning/drying, use of bandages, aftercare and expected recurrence rates of treated infections.

Evidence certainly exists that a plethora of topical antibacterial agents have some treatment effect, and maybe this is why everyone seems to have their own preferred method, which all seem to work to some degree. It has been suggested, in fact, many treatments may bring about a clinical improvement due to suppressing secondary infections by the many opportunist pathogens, such as Staphylococcus species and Streptococcus species rather than the DD treponemes per se (Carter, 2014).

Evans et al (2016) commented the most effective antibacterials against treponemes are often not being used and this is contributing to misuse and overuse of antibiotics.

For example, lincomycin is often used (under the cascade) and this has relatively poor in vitro activity against DD treponemes.

Responsible use of medicines

The widespread misuse (illegal use) of antibiotics to treat DD, whether by topical application or by use in foot baths, causes the author concern. In part, this is because usually no scientific justification exists – to some extent due to the increased risk of antimicrobial resistance.

This is partly because I see vets taking totally unnecessary professional risks by disregarding the cascade rules when prescribing (selling) unlicensed products – particularly when not giving specific advice for recording and the minimum withdrawal periods.

Licensed products

No antibiotics are licensed for footbaths. In the author’s opinion, little justification exists for using antibiotic footbaths for the treatment or control of DD so he suggests, in this day and age, you don’t even go there.

Cefquinome is the only systemic antibiotic specifically licensed for the treatment of DD, but several others have broad indications, which might include DD infections (for example, tetracyclines, amoxicillin, tylosin and cefalexin). The work by Carter et al (2009) found DD treponemes were most sensitive to penicillin in vitro, although only a limited range of antibiotics were tested.

A logical rationale certainly exists for using systemic antibiotics to treat at least certain difficult cases, as the DD treponemes infect the deeper layers of skin, which topical antibiotics may not penetrate.

Most DD treatments, however, involve topical application of an antibiotic or non-antibiotic antibacterial agents (often containing copper). Several licensed oxytetracycline and chlortetracycline aerosol sprays exist. Various studies have shown topical tetracyclines to be effective with approximately 40% to 70% cure rates, depending on the study and definition of cure (Potterton et al, 2011).

Two non-antibiotic antibacterials are licensed for DD treatment, both containing chelated copper and zinc. Work by Holzhauer et al (2011) indicated chelated copper/zinc products may be at least as good as topical tetracycline treatment. Note, however, these products are not the same as copper sulphate powder, which is not licensed and still has very poor evidence for efficacy when used as a topical treatment (Thomsen, 2015).

A non-tetracycline topical antibiotic aerosol spray has been licensed for DD treatment, containing thiamphenicol. This is interesting, as thiamphenicol has a specific indication for treponemes and anaerobes. It has been used in human medicine for many years to treat syphilis (and other sexually transmitted diseases), which is a treponema phenetically closely related to DD treponemes. Thiamphenicol is also very lipid-soluble, which allows deeper penetration of the skin.

To bandage or not to bandage?

Two distinct camps exist on whether it is better to bandage DD lesions. The author is firmly in the “no bandage” camp for the following reasons:

• Bandages are usually used to hold in place unlicensed products, such as antibiotic powders or home-made pastes, for which there is little justification.
• DD treponemes favour anaerobic, moist conditions – exactly those that bandages create.
• For DD infection to occur, skin must first be damaged (“macerated”). Experimentally, this has been reliably replicated by wrapping feet in bandages (Gomez et al, 2012).
• Despite the best of intentions, bandages are too frequently left on feet until they “rot off”. This does nothing to improve skin hygiene or skin condition.

The devil is in the detail when it comes to DD treatment and, in this respect, it is difficult to make evidence-based decisions. Logically, I believe prior cleaning and debriding of the lesion is important before application of topical treatment (Figure 7). I prefer dry cotton wool for this and, occasionally, surgical debridement of proliferative lesions.

This should be done under local anaesthesia, which is easy enough to achieve using IV regional anaesthesia, but will require veterinary intervention.

Summary

We have not made the progress on DD control or treatment one might have hoped for in nearly 30 years of experience with this infection. Possibly, we have followed too many blind alleys with poor choice of antibiotics/unlicensed antibacterials and over-reliance on foot bathing. A clear role exists for individual treatment, but that must be done effectively and with rigour. Using licensed products and combining systemic antibiotics with topical treatments, where necessary, is a logical approach.

Future developments might yet see a vaccine for this disease, which would be a very welcome addition to our armoury.

Meanwhile, EDPET is strongly advocated to reduce the infection pressure within herds. Devising sensible treatment strategies and following up cases to review treatment success is something vets can do, which adds immense value to farmers’ businesses.