MPI response incident controller Eve Pleydell, speaking at the Federated Farmers Mid Canterbury M. bovis meeting, said the earliest positive trace, so far, was in the first quarter of 2016.
Backing that up was quite a lot of national surveillance outside the network of infection, she said.
That national surveillance included MPI working with veterinarians and their clients around the country who fit the bill in terms of farm management; 89 were identified, none came up positive, Dr Pleydell said.
Also, nationally, any milk samples sent for mastitis testing, were also sent for further M. bovis testing. She said the only properties that came up positive during that testing were properties they already knew about.
There was also the national milk, bulk milk tank and
the discard milk surveys which brought up two or three other farms, but they were already on the list of farms of interest.
''The main transmitted pathway on to your farm is an infected cow not showing clinical signs.
''The majority of farms may not see any clinical signs. Those are the animals then taking it around the country.''
Dr Pleydell said if M. bovis was here a long time, it would be more widespread.
Another piece of evidence was genetic fingerprinting.
''On the infected farms we try and grow the bacterium in the lab, and then we extract the DNA and we sequence that; we genetic fingerprint it.''
From that fingerprint, MPI could work out how long it had been on the farms and its origins.
So far tracing went back to the end of 2015-start of 2016, which is why the disease was considered ''a relatively recent importation'', she said.
Genetic fingerprinting also showed it was a single strain.
And compared to other strains circulating around the world, it was most like those from Europe and the United States.
-By Toni Williams
