By Keith Woodford*
Currently, there is a fervent ‘behind-the-scenes‘ debate as to whether eradication of Mycoplasma bovis from New Zealand is feasible.
It is well over a month, possibly close to two months, since the international Technical Advisory Group (TAG) voted six to four in favour of eradication being feasible. This would have been based on information supplied to them by MPI and assessed over a telephone hook-up. New evidence since then provides further complexity and concerns.
First, there is extensive evidence from overseas that Mycoplasma bovis can transfer between species and that it can infect sheep, goats, pigs, deer and even poultry. Strictly speaking, this is not new evidence as it was sitting there all along in the scientific literature and easily found. However, the implications of this within the New Zealand environment have not been considered to date.
Second, there is a new line of evidence (not yet published) of a likely outbreak of Mycoplasma bovis in the South Island back in 2014. Both of these issues have potential to complicate the eradication campaign.
In this article, my focus is on the potential for cross-species infection. I will be writing about the new evidence for earlier arrival in a follow-up article.
I was recently alerted to the importance of the cross-species issue by a briefing paper undertaken by retired Wanaka vet Gilbert van Reenen. This paper was commissioned by a concerned third party for presentation to the Minister of Agriculture.
To quote van Reenen’s words: “there is now abundant reliable, scientific evidence that M. bovis has crossed the host species barrier from cattle to deer, bison, buffalo, goats, sheep, pigs and poultry in a range of [overseas] environments”.
I have gone back and checked the scientific references that van Reenen has used. The evidence does stack up. The only room for debate is the risk of this occurring in the New Zealand environment.
One paper describes how wild pigs were the transmission source in alpine Austria. This paper is particularly chilling in the New Zealand context, given the ubiquitous presence of pigs in our New Zealand back country. Evidence it can also occur in poultry further illustrates the robustness of this bacteria to survive and thrive under specific conditions in very different animal species.
Wild pigs are unlikely to be found on most dairy farms, but that cannot be said for many of the beef farms that grow out the bull and steer calves from dairy farms. An increasing number of New Zealand’s infected farms are beef farms. Wild pigs love dead carcasses and so anything that dies from Mycoplasma bovis will have been scoffed up by any resident pigs.
The evidence for deer relates mainly to whitetail deer, but there is also evidence for what we call Canadian elk which is a subspecies of Cervus elaphus, which we know as ‘red deer’. Indeed, many of our farmed red deer are actually crosses between European red deer and Canadian elk.
There is also evidence that the bacteria can survive for up to eight months in calf-shed bedding. Until now, MPI has worked on the basis of disinfection plus two months maximum life will see the bacteria destroyed. The good news is that Mycoplasma bovis hates disinfectants.
Before going into panic mode, it is worth reminding ourselves that Mycoplasma bovis has been around for many thousands of years – it did not suddenly arrive from outer space. It is generally regarded internationally as a relatively minor disease that can be managed but not eradicated.
The evidence that Mycoplasma bovis crosses the species barriers has been known by some of the MPI scientists for many months. For example, the MPI Pathways Report prepared in November 2017, but not released publicly until March 2018, gives consideration to species other than cattle being the way the disease came into New Zealand.
MPI’s judgement, which in this case seems very reasonable, is that it is unlikely for Mycoplasma bovis to have come into New Zealand via these other species, but they cannot be totally discounted as the source. More importantly, now that the disease is in New Zealand, these other species have potential to provide a disease reservoir. This is something that MPI has not publicly discussed.
The situation with sheep is more complex than some other species, with sheep being particularly susceptible to the very closely related Mycoplasma agalactiae. Until the mid 1970s, Mycoplasma bovis and agalactiae were considered the same species.
It is evident that Mycoplasma bovis and agalactiae have the same common ancestor, but each has evolved to now have a favoured host. However, Mycoplasma bovis can definitely survive in sheep and goats and thereby provides a viable disease reservoir. When Mycoplasma bovis infects sheep, it may or may not show symptoms, just as occurs in cattle.
It also needs to be understood that there are many Mycoplasma species other than Mycoplasma bovis, including some others that are found in cattle here in New Zealand, and many that are also found in humans.
Have we actually done the right thing in placing Mycoplasma bovis on a pinnacle of ‘Dreaded Disease Number 2’ (after foot and mouth)? There is no easy answer to that one.
Let’s remind ourselves that Mycoplasma bovis is typically a secondary disease that shows up when stress is present. The New Zealand practice of feeding calves with non-pasteurised ‘hospital cow milk’, and perhaps some other veterinary practices, may well have greatly facilitated its spread.
Let’s also be grateful that we don’t cull humans when they too go Mycoplasma positive. Some of the readers of this article will themselves in all likelihood be Mycoplasma positive, although highly unlikely to be the bovis species of Mycoplasma.
There are at least 125 Mycoplasma species and at least 15 of these have humans as the primary host. They cause a range of human diseases and pass directly between humans without need for other animals as intermediary hosts.
If this all sounds real scary, then just remember that humans, cattle and Mycoplasmas have all been in this world for a long time. Mycoplasmas do cause health problems but most of the time they stay below the radar and we all get along just fine.
As for the specifics of Mycoplasma bovis transferring to humans, that is highly unlikely. It does not particularly like humans, and in any case it is totally destroyed by milk pasteurisation and cooking of meat. There are hundreds of other diseases that are a great deal more important for humans.
*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.