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BOVINE VIRAL DIARRHOEA VIRUS - POTENTIAL FOR CONTROL AND ERADICATION IN AUSTRLALIA

Kate Sawford, Braidwood, South East Local Land Services and Genevieve Hannaford, Charles Sturt University

Posted Flock & Herd April 2017

Introduction

Bovine viral diarrhoea virus (BVDV) is the most prevalent infectious disease of cattle worldwide. In Australia, BVDV is endemic throughout the cattle population with greater than 80% of herds surveyed showing some level of exposure to the pathogen.1,2 BVDV causes significant financial loss to the Australian cattle industry due to a variety of clinical manifestations of the disease. Indeed, the high prevalence of BVDV across Australian cattle herds, in combination with its impact on reproduction and overall health status of affected herds, results in significant economic losses.1,3

BVDV control strategies at a regional or national level can be described as systematic and non-systematic. A systematic control strategy includes elimination of persistently infected (PI) animals from infected herds, biosecurity aimed at preventing incursion of the pathogen, and surveillance to monitor the progress of the control program and detect pathogen incursion into uninfected herds. Non-systemic approaches lack one or more of the components of systematic control approaches and are carried out on an individual herd basis with no coordinated simultaneous actions in other herds.4 Systematic BVDV control programs in cattle-producing countries are becoming increasingly common, particularly in the EU, and many have demonstrated success and financial benefit.5,6 The first large systematic control programs were launched in 1993-1994 in the Shetland Islands, Denmark, Finland, Norway, and Sweden. While differences in the estimated prevalence of PI animals were dramatic between countries, it took all Scandinavian countries approximately 10 years to reach the final phase of disease eradication.4 In contrast, uptake has been shown to vary considerably with non-systematic approaches and no program to date has been shown to be successful in controlling disease.4,7 The data in Australia supports this observation where there has been little change in cow- and herd-level prevalence in 45 years in spite of increased knowledge of the disease and the introduction of vaccination.2

Therefore, with Australia ranked as the tenth largest cattle inventory in the world, the concept of a systematic national BVDV control program warrants consideration.8 This report reviews the factors that must be considered prior to the development and implementation of a systematic national BVDV control program as outlined by Thrushfield et al. 2005.9

LEVEL OF KNOWLEDGE

The level of knowledge in regards to BVDV has increased dramatically in recent times.1 In 2015, the Australian Cattle Vets Association released guidelines for BVDV management in beef and dairy herds respectively.10,11 These documents include a series of decision-making tools and recommendations designed to help beef and dairy producers, with the assistance of their veterinarian, control and manage the disease.10,11 The strategies outlined are aimed at preventing foetal infection in early gestation and the removal of PI animals.10,11

While cattle are the natural host for BVDV, the virus has the ability to infect a range of species, including cattle, sheep, goats, pigs, bison, alpacas, llamas, and white-tail deer.12 BVDV can be transmitted from cattle to sheep and vice versa,13 however a serological study in the Australian sheep population concluded that transmission between sheep and cattle was likely infrequent under field conditions.1 Wild buffalo are known to coexist with cattle in the Northern Territory of Australia and a study completed between 1993 and 2001 in the Northern Territory detected BVDV antibodies in the Australian water buffalo (Bubalus bubalis) population, both farmed and feral, however no persistently infected water buffalo were identified.14 It is important to note that other hosts of BVDV have not proven to be a threat to BVDV control programs in Europe.1,13

VETERINARY INFRASTRUCTURE

Veterinarians play an integral role in Australia's infectious animal disease surveillance network. They apply their experience and expertise during outbreak investigations and report suspect cases of notifiable and exotic disease. There is a network of animal health laboratories that works in partnership with Animal Health Australia.15 Australia has demonstrated its ability to both eliminate disease and maintain disease freedom via the Brucellosis and Tuberculosis Eradication Campaign.1

DIAGNOSTIC FEASIBILITY

The wide range of diagnostic tools available has allowed successful BVDV control and eradication schemes to become a reality. Diagnostic testing is available for the detection of the viral RNA, BVDV specific antigen, and BVDV specific antibodies.5,16 These tests are generally reported as being very reliable, rapid and inexpensive. Control programs must be designed to fit into the structure of the cattle population at hand, and to handle the level of infection present in a population adequately. The most successful results for BVDV control have been achieved through active intervention where PI animals are systematically identified and removed using these testing methods.4

ADEQUATE SURVEILLANCE

Australia has developed the National Animal Health Information System (NAHIS) database to assist in disease surveillance or control programs.15 This database is capable of handling the amount of data generated through a national disease control program.1 Individual cattle movements can be monitored through the National Livestock Identification System (NLIS). The NLIS is underpinned by State/Territory legislation, which forms the regulatory framework for the system, making it compulsory that all cattle movements are uploaded to the system. The NLIS system could be used to identify the BVDV status of individual animals and herds and control movement of stock.1

During a control program the national herd would become more susceptible to BVDV infection, the prevalence of PI animals would decline, and other routes of virus reintroduction to susceptible herds would become more important. Experimentally it has been shown that pathogen transmission from transiently infected animals, contaminated equipment, and other infected species is possible.16 The role of other potential transmission routes under natural conditions in Australia would become clear as PI animal prevalence declines. Therefore there would be a need for ongoing monitoring of herds cleared of the disease during a control program.

Very large herds in pastoral Australia present a logistical challenge. Many of these very large herds are only mustered annually and mating is uncontrolled.1 Therefore identification and removal of PIs may not be sufficiently timely to ensure pregnant, previously naïve cattle were unexposed. Compartmentalisation and unique monitoring programs may need to be developed to fit this particular circumstance.1

Australia is fortunate to be free of many serious animal diseases in large part due to its geographical location and high biosecurity standards. Prevention of exotic animal disease incursion helps to ensure Australia's market access, farm productivity, public health and wildlife biodiversity. Were BVDV to be controlled the infrastructure already exists to maintain freedom. Further, Australia only imports semen and embryos from cattle - there is no importation of live cattle to Australia.17 This restriction means that the risk of reintroducing BVDV to Australia through importation of PI cattle or dams carrying unborn PI calves would be nonexistent. Testing requirements could be put in place to ensure infected embryos or semen were not imported into the country.

That is not to say that were BVDV to be eradicated from Australia surveillance could be eliminated. BVDV is a common contaminant of foetal calf serum, which is used in vaccines, diagnostic reagents and kits, stem cells and other products of the bioindustry.18 For example, a compulsory cattle vaccination program for bovine herpesvirus in the Netherlands resulted in the spread of virulent BVDV type 2 via contaminated live vaccine.18 There is an inactivation process to ensure biological products do not contain active pestiviruses.18

AVAILABILITY OF REPLACEMENT STOCK

The availability of replacement stock in the current Australian cattle market is, according to the Meat and Livestock Australia (MLA), one of “tight supplies”. Indeed the MLA released a 2017 analysis indicating that during 2017 the Australian cattle herd is expected to increase for the first time in three years.8 During the past ten years parts of Australia have experienced severe periods of drought and as such cattle numbers, particularly those in the beef industry, have reduced dramatically.8 According to the MLA it seems unlikely that the full Queensland beef herd will recover to pre-drought levels until 2021.8 Dairy cow numbers have also decreased as a result of the milk crisis of 2016, particularly evident in herds across southern New South Wales and Victoria.8

In spite of a short supply of replacement stock, any control program would only require removal of PI animals and therefore the impact on the supply of breeder cattle in the short term would be minimal. In fact, control of BVDV would likely enable quicker growth of the Australian cattle herd due to a decrease in reproductive losses associated with infection of previously unexposed pregnant cattle.

PRODUCERS' AND SOCIETY'S VIEWS

The views of producers and society play an integral part of a disease control and eradication program. Calf producers are perhaps the most important component to a BVDV control and eradication program as their commitment will influence the end result.6 A recent survey in South Australia found that while interest in BVDV was high amongst farmers, awareness was low with approximately 30% of respondents indicating that they do not know any facts about BVDV.1 A recent review of the European control programs found that systematic, well-coordinated programs have the most success.4 It was also found that the farming community must be invested in the program and producers must be well informed.4 Further, veterinarians, the farming press and farm advisers were reported as the most influential persuaders of the farming community.4 Therefore, for any systematic control program to be effective there must be buy-in from a variety of industry grounds and these groups need to deliver simple consistent messaging. Finally, an aggressive short to medium program is likely to bring the most success.4

In regards to the view of society, an important additional benefit of a systematic control and eradication program is improved animal welfare though the avoidance of acute infection and mucosal disease in particular.19

THE DISEASE'S PUBLIC HEALTH SIGNIFICANCE

Antimicrobial resistance is a growing public health threat and therefore the use of antimicrobials in both human and animal medicine is increasingly being scrutinised. In feedlots, cattle that are comingled with PIs have higher morbidity rates from bovine respiratory disease complex (BRDC) compared to cattle that are not comingled with PIs, and pens of cattle containing PIs have higher rates of treatment for BRDC.20 Therefore, control and eradication of BVDV would likely serve to reduce antimicrobial use by the Australian cattle industry.

Agriculture is a significant contributor to greenhouse gas emissions.21 Improved health and fertility of the Australian cattle herd could decrease the kilograms of greenhouse gases produced per kilogram of beef and dairy products.

THE EXISTENCE OF SUITABLE LEGISLATION

BVDV is now a listed disease by the OIE and therefore transnational animal trade regulations could be introduced on the basis of a country's BVDV infection status.22

Experience in Europe has shown that to achieve country-wide eradication legislative support is ultimately needed.4 To date eight European countries have made BVDV a notifiable disease: Austria, Belgium, Denmark, Finland, Germany, Norway, Sweden and Switzerland.4 Australia has previously implemented legislation to uphold livestock health standards. However, more specific legislation for the control and eradication of BVDV would be required to ensure a national result was achieved. Animal Health Australia (AHA) facilitates innovative partnerships between governments, major livestock industries, and other stakeholders to protect the health of Australia's livestock. This company is well positioned to bring together various stakeholders on the topic of BVDV control and advocate for government support.

THE POSSIBLE ECOLOGICAL CONSEQUENCES

It has been argued that infectious disease control and eradication could free up a previously occupied niche that could then be inhabited by a more virulent pathogen.9 However, experience with global Rinderpest eradication has not provided support for this argument as no disease has taken its place. As BVDV is an RNA virus mutations occur readily, which leads to genetic, antigenic, and pathogenic variation between strains. In 2004, an atypical pestivirus named D32/00_ 'HoBi' was isolated and described from Brazilian FCS.18 To date at least nine atypical 'HoBi'-like pestiviruses have been described, eight probably from Brazil and one of Thai origin.18 It is hypothesised that these viruses have existed long enough in the cattle population to spread and evolve and have not come to occupy a niche previously occupied by BVDV species.18

ECONOMIC COSTS AND THE AVAILABILITY OF FUNDS FOR THE PROGRAMME

The beef and dairy cattle industries contribute 11.8 billion AUD to the Australian economy every year.2 The total cost of BVDV across Australia is estimated at 114.4 million AUD per annum.2 The net gain from implementing control programs and reducing BVDV in herds to a lower exposure of disease is estimated at $98.5M.2 Therefore economic benefits would be achieved if successful BVDV control programs were initiated nationally. Experience in other countries has demonstrated that BVDV eradication programs are economically beneficial. For example, the Norwegian eradication program cost 52.4 million NOK from 1993 until 2003 (8.1 million AUD based on the current exchange rate).23 A cost-benefit analysis estimates that the project has saved the Norwegian dairy industry alone losses of between 50 million NOK (7.7 million AUD) and 200 million NOK (30.7 million AUD) annually.23

Australia has succeeded previously in garnering funding for the eradication of Brucella abortus and Mycobacterium bovis.1 This campaign was funded by an industry levy (50%), as well as state (30%) and commonwealth (20%) contributions.1 Cost sharing has been a common feature of BVDV control and eradication programs in Europe: in Switzerland, farmers covered approximately one-third of the costs of BVDV eradication while in Norway costs were shared between the cattle industry (19%), the Norwegian Animal Health Authorities (27%), the National Veterinary Institute (11%) and cattle owners (43%).1 In Australia, Animal Health Australia manages the Emergency Animal Disease Response Agreement that includes cost sharing arrangements in the event emergency animal disease event.24 Perhaps this agreement could be used as a model in the development of a cost sharing arrangement for BVDV control and eradication.

Conclusion

This review lends support for a systematic BVDV control program in Australia, though it would not be without logistical challenges. It is the opinion of the author that the biggest impediment at present to introduction of such a program is a general lack of understanding of BVDV pathogenesis and associated costs and clear, consistent messaging from the veterinary community and industry groups about the most effective means of mitigating the impacts of this disease. The recent guidelines from the Australian Cattle Vets Association are an important step to addressing these challenges. One of the next steps in garnering support for such a systemic BVDV control program would be seeking support from major industry groups, such as Dairy Australia and NSW Farmers. Veterinarians are well positioned to provide these groups with up-to-date technical information and summaries of international experiences with similar programs. Were eradication to be achieved, the biggest challenge would be to prevent reintroduction as with all infectious diseases exotic to Australia.

References

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