A review of the cluster of farms infected with cattle disease Mycoplasma bovis in Mid-Canterbury has ruled it could be spreading via airborne transmission.
The Ministry for Primary Industries (MPI) has released the findings of an independent review into M bovis infection in Wakanui near Ashburton.
MPI, DairyNZ and Beef & Lamb New Zealand ordered the review in mid-2022 after it became apparent infection was circulating in a small geographical area despite the use of disease control measures.
The outbreak centres around the large ANZCO Fivestar Feedlot, which can house 19,000 cattle at any one time.
In May last year, the feedlot was the last remaining property with M bovis, but more properties began to test postive, resulting in MPI putting a controlled area notice (CAN) around the feedlot and nearby farms.
All eight farms in the high-risk area of the notice became infected but alongside the feedlot have since been depopulated.
The review carried out by independent epidemiologist Dr John Happold looked at how the disease could be spreading in the confined area.
He found it was most likely not being spread by sheep or deer, through infected milk or colostrum or through vehicles and equipment moving between farms.
"Overall, there is moderately strong evidence to support the hypothesis that the feedlot is the primary source of infection for other farms in the Wakanui area.
"This conclusion is based on the results of molecular epidemiological investigations the concentration of Confirmed Properties within a 1.5km radius of the feedlot, and the absence of support for alternative explanations for spread among Confirmed Properties in the area."
Happold noted airborne transmission of M Bovis had not been reported in published studies but said while not definitive, the evidence suggested local airborne transmission over 4-5km from a very large, dense and heavily infected population was possible.
"In the absence of compelling alternative explanations, this route of transmission would appear one of the most plausible."
Happold made 15 recomendations including exploring the potential for additional epidemiological studies on the transmission of M Bovis in New Zealand.
MPI said most of the recommendations had already been implemented.
M Bovis programme director Simon Andrew said the eradication effort continued to make good progress. The high-risk area of the CAN was now free of cattle and the CAN was on-track to be lifted in mid-March.
"It is expected all current Confirmed Properties are likely to be cleared within the first half of this year.
"While the job is far from over, we are as close to moving to the next phase of the eradication as we have ever been and the collective effort from farmers, industry and programme staff has helped us get to where we are today."