New treatments for digital dermatitis
Whilst scientists and product developers still do not know all the details about the causal organisms of this persistent disease, treatments are evolving.
BY TREENA HEIN, INDEPENDENT CORRESPONDENT D
igital dermatitis (DD), also called hairy heel warts, strawberry foot rot, raspberry heel, foot wart and hairy heel wart, is a globally recognised cause of dairy cow lameness. It was first discovered in Italy
in the mid-1970s, arriving in North America and beyond in the years that followed. A vaccine is not expected any time soon, and as dairy farmers well know, control of the disease is very difficult. Once estab- lished in a herd, digital dermatitis (DD) “typically becomes en- demic,” noted a group of New Zealand scientists recently, “and very few herds are able to completely eradicate the disease due to the multifactorial and complex interactions between the bacteria, animal and environment.” The results of their efforts to model DD transmission were published in February 2020, including a caution that in infected dairy herds, “on-going monitoring of DD is highly recommended.”
How the disease works As Dr Anne-Sofie Vermeersch notes, “researchers possess some pieces of the ‘DD puzzle,’ but they still need to figure out how to put everything together in the right order. The presence of treponemes [one type of causal bacteria] is important, but we need to assess which role exactly they play.” With her University of Gent (Belgium) veterinary school colleague Dr Geert Opsomer, Vermeersch recently published a two-part overview of the factors contributing to DD development. They concluded that because current treatments are mainly directed against the treponemes and do not cure the disease, it’s therefore uncertain whether treponemes play the main role. All the following factors – hygiene, bacteria, immunological mechanisms and genetic predisposition – and perhaps more contribute to the development of DD lesions, note Vermeersch and Opsomer, but the extent of each factors’ role and at which point it’s crucial in the disease process is yet to be discovered. It’s also not yet known how the disease
22 ▶ COW HEALTH | AUGUST 2020
spreads between cows and between farms, they note, but the leg and foot conformation of individual cows can affect susceptibility.
The immune system Beyond leg/food conformation of a cow, genetics has been shown to play a role in DD in terms of immune system function. It was determined for example a few years ago that cows with a high level of antibody-mediated immune response (IR) had significantly lower prevalence of DD than cattle with an average or low IR. Conclusions of this research were eventually incorporated into the Immunity+ breeding platform owned by global genetics firm Semex. Genomic programme manager at Semex Canada Dr Steven Larmer notes because cases of lameness in most commercial herds are not always well-characterised, he and his colleagues have grouped together all incidences of lameness in internal company studies. “We have seen in study of 9500 genotyped cows from commercial herds a relative decrease in lameness of 39% in ‘High’ IR animals relative to their herdmates that tested either average or low [in IR],” he reports. “We know that DD is the primary infectious cause of lameness, and so we believe that decrease to mainly be decreased cases of DD.” Dr Raphaël Guatteo, professor in Bovine Health Management at BIOEPAR-Oniris-INRA in France, notes that there is already, at least in that country, “the possibility to choose bulls with breeding index for infectious and non-infectious foot lesions.” However, Vermeersch (whose research focus is the deviant response of the cow immune system) notes that “the first line of defence and further propagation of an immune response do not seem to protect the animal. Researchers have yet to discover why the lesions can become chronic or why a cow might relapse.”
Newest treatment strategies After all this time, Vermeersch and Opsomer point out in their paper that knowledge of DD still “remains scarce,” and “without knowledge of the exact pathogenesis, it is challenging to establish an adequate, preventive and curative treatment.” Vermeersch adds that “lots of people tend to focus on giving supplements, but I think it is of crucial importance that the main feed ration is balanced correctly according to the needs of the herd/individual cows. There should not be any shortage or overabundance of certain elements in the
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