Is there a silver lining to New Zealand?s Mycoplasma bovis outbreak?
Carl Eden, General Manager, Commercial, and Christoph Egli, Bovine Product Line Manager, of IDEXX Laboratories, argue that the Mycoplasma bovis crisis provides an opportunity to hone our response to other animal diseases, including BVD.
New Zealand has an admirable and ongoing record when it comes to animal disease control. While the current effort surrounding the control and eradication of Mycoplasma bovis has the attention of the industry, the opportunity to consider our response to other diseases of economic importance should not be sidelined. This discussion aims to focus on the challenges with respect to animal disease control from a holistic standpoint, using bovine viral diarrhoea virus, (BVD) as the discussion point. The authors objective is to stimulate dialogue regarding optimisation of animal disease control across multiple modalities and stakeholders.
BVD is unique in that it is transmitted almost exclusively by persistently infected (PI) animals. PI animals were themselves exposed to the BVD virus in utero. Either their dam was a PI herself, or, more commonly, she became infected with the virus while pregnant.
When exposed between one and four months of gestation, the calf?s developing immune system mistakenly assumes that the virus is ?normal?. If the calf survives to birth, its immune system will continue to ignore the virus. They are born and remain persistently infected with the BVD virus for life.
From birth, PI calves continually excrete enormous amounts of virus. All animals they come in close contact with will be exposed to the virus, resulting in a high proportion becoming transiently infected (TI) with BVD.
TI animals typically mount an immune response, neutralise the virus and recover. Some TI animals will show mild signs of illness. However, most exposed animals suffer transient immune dysfunction, reducing their ability to fend off other infectious agents. Transiently infected animals are not a major source of the spread of virus to other animals (Australian Cattle Veterinarians, 2015).
The BVD virus may be transmitted between animals in one of three ways:
- By direct contact between animals (?horizontal? transmission). The virus can be spread in all body fluids, including saliva, nasal discharge, urine, semen and faeces. Nose-to-nose transmission over boundary fences, and the unknowing purchase of a PI animal are common infection and introduction routes.
- By contact between an animal and the body fluids of an infected animal no longer present. For example, calf pens can remain infectious for days after PI removal if no disinfection is carried out. Or transmission can be via ?fomites? ? ie, objects that have come into contact with infected cattle (clothing, gumboots, scanners, trucks, etc.).
- By crossing the placenta and infecting the fetus when a cow is pregnant (?vertical? transmission?). This is the only way PI animals are produced. The dam would have had to be infected between the 1st and 4th months of gestation, or already be a PI. Animals exposed within the first 30 days of pregnancy usually suffer early embryonic death. However, abortion or still birth is a risk at any stage of pregnancy.
BVD outbreaks are often associated with the introduction of new animals to existing management groups. However, many endemically infected farms harbouring PIs sporadically suffer significant and noticeable losses when groups of animals become exposed at critical times (Australian Cattle Veterinarians, 2015) ? for example, during the crucial first four months of pregnancy.
BVD is widely acknowledged to have significant financial impacts in infected herds. Losses stem from ill-thrift PI animals, reproductive loss, decreased production, poor growth and increased incidence of other diseases (due to immune suppression). Epidemic outbreaks of BVD in na?ve herds can have dramatic consequences and typically result from the introduction of the virus (usually a PI animal or animals) into a na?ve, and therefore highly susceptible, population (Reichel et al., 2018). The variability in the cost of BVD relates to the timing of the exposure.
Losses in New Zealand dairy herds have been estimated at between $35 and $87 per cow per year in a then average size (n=393) infected dairy herd. Estimates of the annual losses for New Zealand dairy farmers are around $127 million, with predicted annual losses of an average of $70,000 for each average size infected herd (Reichel et al., 2018). In beef herds, economic losses are associated with ill-thrift in young stock, and an average five percent increase in empty cows. The cost of this has been estimated to be $3,000?$9,000 per 100 cows in infected herds (BVD Steering Committee, 2011).
The prevalence of exposure to BVD has been estimated in New Zealand at 35% of dairy herds and 65% of beef herds actively infected with the virus (BVD Free). The prevalence of PIs within a herd is not well defined in New Zealand, but reasonable estimates of up to 2% have been suggested.
BVD and Mycloplasma bovis: the opportunity to develop muscle memory
Challenges that are common to all animal disease control programmes include animal identification, biological variation, movement and traceability, prior history, farm management practices, the appetite between the farmer and their veterinarian to effect change, and political and socioeconomic pressures. With regards to BVD, in New Zealand there has been a voluntary control approach (non-mandated but purposeful reduction of disease prevalence).
Compare this to the current eradication effort for M bovis (mandated and purposeful reduction of specific disease to the point of continued absence of transmission and identification within a defined geographical area), and the differences between likelihood of successful eradication and living with either disease are stark.
Regarding specific challenges related to BVD, the current surveillance and risk assessment framework recommended by both MPI and the NZVA to manage producers? expectations surrounding the national response to M bovis provides opportunities for veterinarians and farmers to re-engage in farm productivity and profitability consultation when they choose to adopt a holistic approach to animal identification, traceability, infectious disease, biosecurity, nutrition and fertility at a farm management level.
For example, during an on-farm M bovis assessment where a minimal risk of M bovis is identified, there is an opportunity to demonstrate to the farmer the value of the veterinary profession?s contribution to the health and wellbeing of the people and livestock of New Zealand. When faced with minimal risk of M bovis, the opportunity to strengthen voluntary control of other risks on farm not only provides opportunity to boost farm productivity and profitability, but also increases veterinarians? involvement in New Zealand?s readiness to respond to other, potentially far more serious animal disease incursions.
Diagnostic options: The PI hunt
BVD testing is often found to be confusing, but can be broadly summarised as below:
- Antibody detection in milk or serum identifies herds with recent infection.
- Bulk milk virus detection identifies milking herds with active infection.
- Individual animal testing for virus detection identifies TI and PI animals (using blood, milk or tissue).
- Further testing of other mobs, and aborted and dead calves is important to clear herds and avoid missing Pis.
Identifying and subsequently making informed choices with regards to controlling the source of infection is the mainstay of any animal disease management strategy. Regarding BVD, diagnostic testing is the only way to identify PI animals. In almost all scenarios, early testing is critical so that PI animals can be removed before they spread infection. Removing Pis is accepted as the best way to reduce overall production loss and impact of medical costs on a herd. In addition, BVD-tested calves that have been demonstrated to be BVD antigen negative will likely bring added value to the herd. When coupled to the effective use of New Zealand?s National Animal Identification and Tracing (NAIT) system, opportunity exists for producers to leverage an individual animal?s BVD status for financial rewards.
Testing options in New Zealand include polymerase chain reaction (PCR), virus isolation, ImmunoHistoChemistry (IHC) and enzyme-linked immunosorbent assay (ELISA). Interpretation of results must be made considering information from the farm, including biosecurity risk assessment, who was tested, when they were tested and how they were tested. Subsequent action on the information is mandatory when identifying a PI, with elimination the preferred option for managing confirmed PIs.
Current BVD testing options employed in New Zealand primarily use a combination of individual ELISA testing methodologies and pooled PCR. Both ELISA and PCR have extremely high sensitivity. However, due to many factors, including those of biological and individual variation, one should ask questions regarding testing methodology when considering which test to select.
For example, BVD is an RNA virus that is extremely prone to mutations, (an estimated two base pair mutations every BVD virus replication). Some have described this phenomenon as a swarm of viral mutants. Therefore, to avoid the inherent variability of the virus, tests to identify BVD antigen or genetic material need to target structural or genomic targets that are highly conserved (eg, the envelope glycoprotein Erns). The high shedding of PIs lends itself to using pooled testing methodology (for example, bulk tank milk) to identify where to look for individual PIs. Consideration then needs to be given to the amount of viral shedding when looking for an individual PI. A PI is typically regarded as a virus factory, with a viral load and shedding far higher than that of a TI. Although PCR is extremely sensitive, this is a complex method that can be relatively expensive and sometimes slow to provide a final result at the individual animal level. Therefore, questions have been raised as to its suitability for use as a tool to identify individual PI animals. Choosing the right tool for the job at hand (at the right price point) to confidently identify a PI at the individual animal level in consideration of the viral shedding of the PI animal lends itself to using a highly sensitive ELISA test, which targets highly conserved structural antigens of the BVD virus.
It is generally accepted that confident identification of Pis and elimination is highly beneficial, as it removes the source of any infection from the herd. However, the needs of the farmer and differentiating a TI animal from a PI animal based on only one result has potential welfare, social and economic impacts to the farm and New Zealand?s brand. The use of ELISA ear notch samples in calves less than 35 days old has shown promise in identification and differentiation of TIs from PIs based on a single sample. Given the importance of individual animal testing for New Zealand?s herd, and the practicality demonstrated in overseas eradication efforts of using ELISAbased tests in calves less than 35 days (in Ireland, Switzerland, Germany and Belgium), it is clear that more work is required for New Zealand to demonstrate the applicability of the globally accepted practice of ELISA testing all calves using ear notches obtained in the first 35 days post-calving.
The benefits of good record-keeping and individual calf testing
As opposed to pooled testing, a calf testing negative to BVD antigen at the individual level with appropriate management has the benefit of reducing immediate and future testing required for adult cows in the herd. Every negative calf must have come from a negative dam, and any positive calf provides clear guidance as to which cows to target for testing in the adult cohort.
Is there a silver lining?
Much work has been completed assessing the approach to managing BVD across the world. While parties agree that the primary goal is the identification and elimination of the PI from the herd, not all schemes are alike. However, when considering BVD control and the current efforts to eradicate M bovis, it will be the holistic learnings regarding the approach to farm risk management, including biosecurity, traced animal movements, diagnostics and record keeping, that may provide a silver lining for New Zealand.
Australian Cattle Veterinarians (a special interest group of the Australian Veterinary Association). BVD: Bovine Viral Diarrhoea Virus, Management Guide, Dairy Edition. 20.07.2015
BVD Steering Committee http://controlbvd.org.nz
Reichel MP, Lanyon SR, Hill FI. Perspectives on current challenges and opportunities for bovine viral diarrhoea virus eradication in Australia and New Zealand. Pathogens 7, 14 doi:10.3390. 2018