Mycoplasma bovis has been identified in three new properties, bringing the number of infected farms up to six.

The Ministry for Primary Industries confirmed the findings today, saying the new properties had existing links to other infected farms, and that it was "an entirely expected pattern at this stage of response".

"We fully expect to find more infected properties as we continue our tracing and testing programme," MPI response co-ordinator David Yard said. "These detections are evidence of the programme working, not of unexpected disease spread.

"All detections to date have links to the original infected properties via animal movements and have been caused by close animal contact. What is encouraging is that, despite intensive testing, no adjacent properties have as yet been identified as infected.

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"We have no evidence of any means of disease spread other than close animal contact, at this stage. This includes the disease having jumped fences - which our scientists and vets tell us is highly unlikely to occur," he said.

Two of the newly identified properties are Van Leeuwen Dairy Group farms which were already under the Restricted Place notices under the Biosecurity Act.

The third property is a lifestyle block near Rangiora which had received a small number of calves from the third infected farm, which was confirmed last week.

MPI said it is not naming farms that do not wish to be named.

"MPI is prevented from doing otherwise by the Privacy Act," Yard said. "However, we do understand community concern about the disease and we are strongly encouraging farmers under controls or investigation to talk to their neighbours, customers and suppliers."