He gained many insights into the disease, which was previously thought to be absent or at least extremely rare here because of the unique pastoral dairy system, while doing his masters degree and doctorate.
Studies have beendone overseas but limited information was available on DD in NZ.
DD is the most important infectious cause of cattle lameness in confined systems, particularly in the northern hemisphere. Once in a herd DD typically becomes endemic and is eradicated from few herds.
It can result in loss of production and increased animal health management time and cost. It also raises concerns of animal welfare because it causes varying levels of discomfort and pain.
Yang’s interest was encouraged by hoof health expert Neil Chesterton.
“Neil brought a project to Massey when I was doing my masters, which was a pilot study of DD in Taranaki,” he says.
“I thought it was a very interesting study as no one else had really looked at this disease carefully in NZ. I took the project as my masters dissertation and then DD research became part of my life over the past five years.”
He found the disease is rare, at least in Taranaki. Farms where more DD cases were detected were generally related to loose biosecurity management.
However, there was no information about it in the other parts of the country and he was to discover the mystery of DD in NZ where the dairy system is different to other major dairy production countries.
Yang adopted novel methodological approaches and investigated the distribution, risk factors, climate effect and transmission dynamic of DD.
He visually assessed 59,849 cows from 127 herds in Waikato, South Canterbury, West Coast and Manawatu.
He chose those regions to provide a cross-section of farming systems: Waikato for its 3000-herds, South Canterbury to represent large herds and irrigated pastures, West Coast for its unique geography and predominantly self-contained farms and Manawatu for its proximity to Massey.
During milking he hosed the cows’ feet and visually inspected them, screen testing for DD lesions large enough to be seen without lifting the hoof.
He also asked farmers to complete a questionnaire on their management systems and biosecurity practices.
Yang found the disease is still in the early stage of establishment in NZ though it is widely present on most dairy farms.
Digital dermatitis is an animal welfare issue because it can be painful and results in a loss of production and increased animal health management time and cost.
Factors associated with increased cases are mainly hygiene and biosecurity related, including hoof-trimming equipment and animal movements. However, lameness is rarely associated with the disease, an insight he has yet to understand.
The strong overseas risk factor of housing cows is not associated with cases of DD here, even on farms with herd homes or stand-off pads. However, Yang says absence of evidence does not imply evidence of absence and that needs more investigation.
DD lesions in NZ are very different from classical presentations overseas. Yang saw very few active clinical cases but small, grey, rubbery lesions that disappeared within six weeks.
There was also evidence of faster transmission in early lactation with the rate decreasing over time. The curve flattened druing lactation, like the much-publicised covid-19 flattening of the curve, with the number of new cases eventually lower than recovered cases.
Finally, climate observations were also surprising. Overseas, cases were higher with humid weather associated with rain but in NZ cases decreased as rain increased.
Yang says the only explanation he has is increased rain appears to make hooves cleaner and the environment more hygienic.
The disease profile here is starkly different to overseas.
“In the northern hemisphere I don’t think they had comprehensive research of DD at its early stage of establishment,” he says.
“Some overseas experts say NZ today is like the Netherlands was 20 years ago.
“The only thing that didn’t surprise about the findings was the low number of affected cows in most of the herds, that pattern was consistent across all regions.”
Yang’s chief supervisor Professor Richard Laven interpreted all the research results and authored all the associated publications, Professor Cord Heuer supervised his masters degree, technician Megan Moss collected most of the Taranaki data, Dr Kristina Muller and Dr Carolyn Gates co-supervised and Professor Wesley Johnson and Professor Geoff Jones provided statistical insight.
Yang also worked closely with staff from Vetent, Totally Vets, West Coast Vets and Southern Rangitikei Veterinary Services.
Despite his research Yang is reluctant to make a prediction about the future of DD here.
He is, however, cautiously optimistic.
“In general I will say this disease is still at a manageable condition but we currently lack an efficient and accurate method to detect the disease.
“At this stage it is being overlooked and underreported and the consequence of this is that the future is uncertain.
“The good news, however, is that we can do something to possibly eradicate the disease or at least keep it at this manageable condition for years and years depending on the ambition of the dairy industry.
“We need to develop and use an accurate, rapid method of testing a herd four times a season to determine what sort of cows are at risk.
“The wise thing to do at this stage would be to at least detect the disease in dairy herds and keep an eye on it, monitoring its pattern over time. I think precaution is a good idea.”
