New Zealand’s Mycoplasma bovis Eradication Programme
Mycoplasma bovis (M. bovis) is a bacterium that can cause serious disease in cattle, including mastitis, pneumonia, arthritis, and ill-thrift in calves. An inherent resistance to a number of antibiotics paired with growing acquired antimicrobial resistance is a concern for animal welfare and productivity. Up until 2017, infection occurred in all major cattle-rearing countries apart from New Zealand (NZ). In this article, Dr. Amy Burroughs explains teh New Zealand’s Mycoplasma bovis eradication programme.
What is Mycoplasma bovis?
Mycoplasma bovis (M. bovis) is a bacterium that can cause serious disease in cattle, including mastitis, pneumonia, arthritis, and ill-thrift in calves. An inherent resistance to a number of antibiotics paired with growing acquired antimicrobial resistance is a concern for animal welfare and productivity. Up until 2017, infection occurred in all major cattle-rearing countries apart from New Zealand (NZ).
The bacterium does not pose a risk to human health and it is safe to eat meat and drink milk from infected cattle. M. bovis is not considered a disease of relevance to trade by the World Organisation for Animal Health, and internationally there are no regulatory restrictions for meat and dairy products due to M. bovis. Infection is common in many food producing nations, and where infected cattle aren’t showing signs of infection they are processed for human consumption.
Mycoplasma bovis is confirmed in New Zealand
On 21 July 2017, samples were collected from dairy cattle in the South Island of NZ that were experiencing clinical disease consistent with M. bovis infection. The presence of M. bovis was subsequently confirmed. Following this detection, a national response commenced to define the extent of M. bovis infection and contain its spread.
The route of entry into NZ is unknown but is thought to have occurred around 2015. It is unlikely that we will know the original entry pathway, despite investigating the movements of risk goods onto the originally affected properties.
Eradication is achievable without knowing the original pathway, provided the likelihood of incursion is very low. One possible entry pathway is via imported bovine germplasm. A new Import Health Standard (IHS) for bovine germplasm took effect from May 2022 and it provided improved risk management.
Embarking on an eradication programme
On 28 May 2018, the decision to eradicate M. bovis from NZ was announced. The Ministry for Primary Industries (MPI) is in partnership with DairyNZ and Beef + Lamb New Zealand to eradicate M. bovis. The M. bovis Eradication Programme (the ‘Programme’) was set up and funded to run for 10 years. Eradicating M. bovis will support animal welfare, reduce farmer reliance on antibiotics, and protect NZ’s cattle industry.
Allowing M. bovis to spread would cause an estimated $1.3 billion in lost productivity in the first ten years.
The three goals of the programme are to:
- Eradicate M. bovis from New Zealand,
- reduce the impact of the disease and the Programme for everyone affected by M. bovis, and
- leave New Zealand’s biosecurity system stronger.
Situation to date
Since identification of the index case to 21 September 2023, there has been a total of 281 confirmed infected properties (CPs). The majority (148/281, 53%) of CPs are classified as primarily beef producing farms. Of the remaining CPs, 70 are classified as dairy and 63 as other property types (including lifestyle blocks, calf rearers, graziers, dairy support blocks). Most CPs are located in the South Island (211/281, 75%) where infection has been most prevalent in the Canterbury, Otago, and Southland regions. As of September 2023, one CP exists – a dairy farm in Canterbury – however further infected farms may still be detected. Figure 1 shows an epidemic curve ordered by date of detection.
Surveillance for Mycoplasma bovis
For M. bovis in NZ, our main diagnostic tests are the enzyme linked immunosorbent assay (ELISA) and the real-time polymerase chain reaction (RT-PCR). The ELISA can be applied to serum or milk samples and the RT-PCR is applied to a range of sample types including tonsil swabs collected at slaughter, milk samples, and joint fluid. Neither test is appropriate to detect infection at an individual animal level (mainly due to imperfect sensitivity), so these tests are used to make decisions about the infection status of a group of cattle.
A case of M. bovis is defined as a place where the herd-level serology is positive at two different times for at least one live cattle group and/or a place from which one or more samples collected from cattle test positive by RT-PCR with sequencing to M. bovis. Network surveillance. Transmission of M. bovis in NZ – between and within farms – is primarily via the movement of infected cattle and sharing raw milk/colostrum. Confirmed infected properties are found through a combination of network and background surveillance streams.
Network surveillance uses information gathered about cattle and milk movements on and off CPs, the proximity of CPs to other cattle farms (break outs, shared trading) as well as other associations including business owner relationships (because unrecorded movements between, may occur).
Farms assessed to be at risk enter the Programme and require on-farm sample collection and testing to determine infection status. Movement controls (legal notices) are often used to prevent onward transmission while the infection status of the cattle is determined.
The case finding ability of network surveillance relies on the availability of accurate information pertaining to cattle movements. Most CPs have been identified via network surveillance. When infection is confirmed on a property, measures to contain and control disease are put in place. Measures include:
- Movement controls restricting cattle moving on and off the farm;
- Depopulation (culling of infected groups of cattle);
- Cleaning and disinfection (e.g. dairy sheds, water troughs, vehicles and equipment in contact with cattle) (Figure 2);
- Standdown period (identified high-risk areas such as dairy sheds, calf sheds, and cow barns kept free of cattle for 60 days).
Background surveillance. Background surveillance streams have been developed by the Programme as part of the case finding process and work as an adjunct to network surveillance. The bulk tank milk (BTM) surveillance screens for infection in the dairy sector and national beef and dry stock cattle surveillance (NBCS) screens for infection in the beef sector.
To date, 21 dairy CPs have been found via BTM screening and one beef CP found via NBCS. The results of background surveillance to date supports that infection is not widespread in New Zealand’s dairy or beef sectors.
Background surveillance will play a fundamental role in collecting the negative data NZ needs to accumulate a high level of confidence that M. bovis no longer remains in the country.
BTM screening. Every dairy farm in NZ supplying milk for commercial processing has a BTM sample tested, at least monthly, using an ELISA to screen for M. bovis specific antibodies. Dairy farms that produce a BTM sample for which the sample-to-positive (S/P) ratio is ≥ 30, and farms where the S/P ratio is ≥ 20 in two consecutive samples, are followed up with on-farm blood sampling and herd-level serology.
The average monthly rate of detection is very low, with < 0.3 percent of dairy farms requiring on-farm blood sampling after BTM testing, and more than 95 percent of these are subsequently confirmed to be negative for M. bovis.
In July 2022, the Programme began a trial of an increased frequency of BTM screening to twice monthly for July, August and September (early spring lactation) to avoid missing infection. This was based on the findings from a latent class analysis that found diagnostic sensitivity of the ELISA was highest when milk samples were tested in early lactation. This also coincides with the introduction of heifers to the milking platform and the return of the milking herd from winter grazing, both of which can be the source of infection to dairy farms.
Twice monthly screening has shown to be useful as it detected the current active CP in Canterbury.
Beef and dry stock surveillance (NBCS). NBCS screens non-lactating cattle across the country using the ELISA. Blood samples are collected from:
- Beef cattle slaughtered at meat-processing plants (‘meat processing plant surveillance’, MPPS),
- cattle prior to entry at a feedlot (‘feedlot induction surveillance’, FIS),and
- beef cattle breeding herds and dairy heifers (‘on-farm beef and drystock cattle surveillance’, BBS).
Farms all over New Zealand are eligible for sampling under the NBCS. As of September 2023, a total of 1,010,000 cattle from approximately 26,000 farms had been sampled.
One CP has been identified through the NBCS to date. The property would have been identified through network surveillance, had it not been identified through the NBCS.
Passive surveillance. MPI operates an exotic pest and disease hotline (phone or online) through which farmers, veterinarians, and laboratory scientists can report any suspicion of an unwanted organism, including M. bovis. This is how the index case was identified. Since then, the Programme has continued to rule out M. bovis infection in situations where there are suggestive clinical signs.
Removal of Mycoplasma bovis from an infected feedlot and surrounding area. In mid-2018, a feedlot on the South Island was confirmed as infected with M. bovis. Restrictions were applied to prevent outward spread, and testing was performed on incoming stock to determine the ongoing risk of re-infection. The feedlot was permitted to continue operations as the risk of onward spread was thought to be extremely low. Depopulation of the feedlot was planned for when there was no (or very little) risk of re-infecton, preferably when the Programme was down to no CPs.
In October 2022, a Controlled Area Notice (CAN) was placed in the Wakanui area of Canterbury (Figure 3). This was the first time the Programme had used this type of legal notice and was in response to finding properties – containing cattle that surrounded a feedlot – that had become infected multiple times without an attributable source.
An independent review in October 2022 found moderately strong support that the feedlot could have been the primary source of infection for surrounding farms in the Wakanui area.
The CAN covered a geographical area including the feedlot and a buffer of surrounding farms that was divided into a high-risk zone and at-risk zone. The high-risk zone required all cattle to be depopulated in conjunction with the depopulation of the feedlot. The at-risk zone required all cattle on-farm to undertake a census and to be tested pre and post depopulation of the high-risk zone.
In December 2022, the feedlot completed depopulation, which was followed by cleaning, disinfection and a 60-day standdown in early 2023. The farms in the high-risk zone were simultaneously depopulated.
The CAN was lifted in March 2023 following the depopulation of all farms in the high-risk zone, and two rounds of negative test results from all farms in the at-risk zone. The depopulated farms are now in the process of repopulating with cattle. The feedlot is also undergoing additional testing post repopulation and results to date are negative.
Genomic analysis of Mycoplasma bovis isolates in New Zealand
Whole genome sequences are generated where possible from CPs. Genomic analyses enable MPI to:
- Support our timing assessment of the initial introduction into New Zealand,
- monitor for any subsequent incursions, and
- generate hypotheses as to likely infection sources for CPs where there is uncertainty in our attribution of source.
To date, we have at least one whole genome sequence for 122 of 281 (43%) CPs. Sequences for all but one of the CPs are sequence type 21 (ST-21). ST-21 is the original incursion strain and sequences group into one of four clades.
The genetic similarity of ST-21 sequences supports the hypothesis that for the original incursion, clonal expansion followed a single M. bovis introduction (or a small number of very closely related introductions) around 2015.
All ST-21 isolates obtained since spring 2020, belong to Clade 2. This suggests that disease control activities have been successful at removing infection caused by Clade 1, 3, and 4 isolates from the national herd.
Sequence type 29. In September 2022, a strain of M. bovis novel to New Zealand was detected on a dairy farm in Canterbury. The farm was found to be infected after a BTM surveillance detect result in April 2022 and has since been depopulated, cleaned and disinfected, and repopulated with cattle. There is no evidence of ST-29 on any other cattle farm in NZ. The most plausible source of the novel strain is imported bovine semen that came into the country prior to the current Import Health Standards (IHS). The current IHS for bovine germplasm took effect from May 2022.
The risk of transmission via semen is considered very low when it is collected and processed according to MPI’s requirements. As an extra precautionary measure to help protect the gains made to date, the Programme has implemented a bovine semen testing project to test semen imported prior to the current IHS that remains in storage.
Phases of the programme
To achieve eradication, the Programme will move through three phases (Figure 4):
The Programme is currently in the case finding (delimiting) stage, while also planning for the next phases of provisional absence and confidence of absence. Surveillance screening is currently being optimised for sensitivity so that we can:
- Identify any remaining infected farms,
- contain, control and eliminate infection, and
- move to the next phases where negative surveillance data (generated by the background surveillance streams) will be key to providing confidence of disease absence.
M. bovis is challenging to find as infection is frequently subclinical – only recorded cattle and milk movements can be traced – and cattle may not be captured by background surveillance until 24-36 months of age. Surveillance must continue for at least four years in the absence of finding infection to provide enough confidence that M. bovis is no longer present. Many pieces of evidence have helped optimise the design of the background surveillance streams to enable the Programme to achieve a high level of confidence that no unidentified infection remains. The Programme has built, and will use, a scenario tree model (STM) to combine the sensitivities of all surveillance streams and quantify our confidence of absence along the way.
Conclusion
Animal disease eradication programmes present an enormous challenge. They can be very distressing and challenging for farmers, industry, and those responding. The Programme to date shows good progress towards the goals set when Programme partners DairyNZ, Beef + Lamb NZ and MPI agreed on the path to eradicate M. bovis from NZ.
The successes so far have been possible thanks to the commitment of farming communities, industry partners and MPI to protect NZ’s biosecurity.
The response to M. bovis has provided many lessons for government and the cattle industries on how to better prepare for, and respond to, large-scale outbreaks of livestock diseases. These include the need for sustained investment in readiness and traceability systems, and the continued importance of on-farm biosecurity.
