Keith Woodford explains why the Mycoplasma battle is far from over; at best it is the end of the beginning

By Keith Woodford*

There is a widespread belief in both the rural and urban communities that Mycoplasma bovis is well on the way to being eradicated from New Zealand. My response here is that there is a still a long way to travel before any declarations of success are appropriate.

In December, Prime Minister Ardern, no doubt choosing her words carefully and based on official advice, talked of ‘substantial progress’.  However, the broader tone of both MPI and DairyNZ messaging has led to parts of the media and then the general public taking a further step and concluding that the battle is almost over.  

On 18 December 2018, MPI sent a three-page letter addressed and mailed individually to New Zealand’s pastoral farmers, which oozed confidence. It started with the lead-in that “I’m pleased to update you on progress…” and then went on to say that “We’re confident about how the program is tracking’’.

The letter did concede that “There is still a long way to go to eradicate M bovis”. But the message was strong that the investigators and eradication teams are travelling faster than the disease can travel. The letter said “we are[now] identifying many at-risk farms before cattle have been able to mix, thereby reducing the risk of spreading the disease to other cattle on the farm”.

The messaging from DairyNZ was even stronger. On 17 December 2018, they issued a media release saying that “Today will come as a relief to many farmers”. They reported “encouraging progress” and said “Thank you to everyone involved… This has truly been a team effort”.

All of this does indeed sound as if we are heading into the final straight.  I say again, let’s not get ahead of ourselves.

To understand where we are at in the eradication campaign, it is helpful to recall that the disease was first identified in July 2017 on a South Canterbury farm, officially known as IP1.  MPI was caught napping, with no prior plans for eradication in the event of an outbreak. The response controller has said that he had to look on the internet to find out what was this surprise disease.

By December 2017, it had been found in Southland and elsewhere, with the first Southland farm identified as IP10. At that stage MPI thought that all of the new cases through to and including IP10 led from IP1. It was only around April 2018 that MPI belatedly came to the view that IP10 preceded IP1.

Throughout 2018, MPI has been increasingly insistent that there is no evidence the disease arrived prior to December 2015 and that all infections lead back to IP10. This belief is based on modelling of the rate of genetic mutation in the analysed samples.

Throughout the first half of 2018, the number of herds identified as infected spread out like a fan, first in the South Island but also in the North Island, with many of the infected farms clearly being linked to IP10 by known animal movements.

However, there is another group of farms, mainly in Canterbury but also in North Otago, for which the links are not readily apparent.

Accordingly, although the messaging from MPI has consistently stated that all new infections are linked by traces back to IP10, that is not the full story. There are about 10 farms where the links remains unclear.

This is not to suggest that the farms with unclear links represent a different outbreak, but it is very challenging to see how these links could get back to a single point-source infection as late as December 2015.

Those of us who are outside the official MPI system, but who know something of the animal movements on these infected farms, believe the logistics requite a longer period back at least one year further and most likely two years. Otherwise we cannot make sense of how the disease managed to travel so quickly and jumped from one farm to another.

The alternative to earlier arrival is that both IP10 and some other farms were each struck with an infection from a common source of semen. This is a definite possibility, but even then it becomes challenging to find disease transmission pathways that started no earlier than December 2015.

At times the debates on these matters can get down to semantics, such as the difference between ‘known evidence’ and ‘unknown evidence’, what information should be trusted, and also what constitutes ‘proof’.

For example, the molecular clock gives an estimate based on specific assumptions which may or may not hold.

Similarly, MPI has not always obtained the detailed and correct information from farmers that it needs. Sometimes this was because MPI officials did not ask the right questions.

On other occasions, the information got muddled as it passed up the chain. Subsequently, MPI officials became resistant to correcting obvious mistakes as they did not trust farmer information.

For reasons of space, I will use only two puzzle examples here. They are representative of others.

One of these farms was identified positive from a milk sample in the autumn of 2018. Subsequently, it was found that 2017-born calves were also positive, which leads to the 2016 mating being the most likely time of infection. However, on this farm there are no suspicious semen links and the most likely source is service bulls born in 2014.  

There is no proven link back from this farm to IP10, but a scenario linking back to IP10 in 2014 is possible. Other scenarios are also possible, but there is no identifiable scenario that is consistent with IP10 being a single-source infection as late as December 2015.

The second example is a dairy farm that went positive in the weeks leading up to Christmas 2018 with no forward traces from other infected farms. Once again, the most likely infection source would have been service bulls born in 2014 or at the latest in 2015.

For this second farm, there is a back-trace for a bull calf that was sold in 2016, so MPI can correctly say that it was a trace farm. But there are over 5000 trace farms in New Zealand, and back-traces tell us nothing as to how the disease got to the farm. They only tell how the disease may have left the farm.

The challenge for eradication is that very few farms show clinical evidence of the disease. In most cases it fans out without visual evidence.   Also, the tests are not reliable. Where has it got to that we don’t know about?

Currently, the most likely source of ongoing transmission is via service bulls and heifer calves. Given the starting point was in the Friesian breed, that is where the greatest risk lies. But it has also spread elsewhere. The Five Star beef feedlot is further evidence of that spread to other breeds. There lies an interesting story. 

The bottom line has to be that New Zealand is still in the early stages of its eradication campaign. It will be many years of monitoring and slaughter before victory can be claimed.

*Keith Woodford was Professor of Farm Management and Agribusiness at Lincoln University for 15 years through to 2015. He is now Principal Consultant at AgriFood Systems Ltd. His articles are archived at http://keithwoodford.wordpress.com. You can contact him directly here.