CASE NOTES


ERADICATION OF ENZOOTIC BOVINE LEUCOSIS IN NATIONAL DAIRY HERD

Dr Diane Ryan, Department of Primary Industries, Elizabeth Macarthur Agricultural Institute, PMB 4008 Narellan NSW 2567

Posted Flock & Herd April 2013

INTRODUCTION

In the mid-1990s, a scheme to eradicate Enzootic Bovine Leucosis (EBL) infection from Australia's dairy herds began as voluntary State based programs. A National Dairy Enzootic Bovine Leucosis Eradication Program (NDEBLEP) was established in 2008. The NDEBLEP is managed and implemented by the Australian Dairy Industry Council, Dairy Processors, Australian dairy farmers, Australian Department of Agriculture, Fisheries and Forestry (DAFF), State animal health authorities, and the industry body- Dairy Australia. NDEBLEP aims for the Australian dairy herd to be EBL free by 2013.

THE DISEASE

Enzootic Bovine Leucosis (EBL) is a lymphoproliferative disease of cattle that is caused by an exogenous oncovirus in the retroviridae family, bovine leukaemia virus (BLV). Both beef and dairy cattle can become infected as can buffalo, sheep and, experimentally, camels. It can occur in cattle of any age and is distributed worldwide.

The vast majority of infections with BLV are subclinical. Only 5% of infected cattle will show clinical signs of lymphosarcoma. Approximately 30% of cattle that seroconvert will develop persistent lymphocytosis. In an infected herd, there can be economic losses from death of infected cattle, loss of milk production, costs associated with treatment and diagnosis, replacement costs from early culling and carcase condemnation at slaughter (Kirkland and Rodwell 2005).

EBL virus was first recognised in Australian cattle in 1966 (Clague and Granzien, 1966). It is a notifiable disease in all states and territories of Australia and is included in the World Organisation of Animal Health (OIE) list of cattle diseases.

DISEASE SPREAD

EBL infections result from the transfer of infected lymphocytes in blood or colostrum from infected to susceptible animals. Approximately 10% of calves born to seropositive cows have been infected in utero (Meas et al 2002). The virus does not persist outside of the host so there is no environmental reservoir for infection. Animal husbandry practices including gouge dehorning, ear tagging, tattooing and intensive management such as mass rearing of calves on colostrum can spread the disease in a herd. Natural mating has been implicated in transmission of the virus. The use of common vaccination guns and not changing gloves or cleaning Artificial Insemination equipment between cows can also spread the disease (Kirkland and Rodwell 2005).

EBL has been recovered from multiple blood feeding insects following a feed on an infected animal but no insect host has been proven to transmit the disease under natural conditions.

WHY CONTROL THE DISEASE

Justifications for controlling EBL include reducing economic loss, avoiding the potential for trade issues and the fact that the disease is feasible to control.

The highest prevalence of EBL infection during 1980's occurred in Queensland, with 71% of dairy herds and 14% of dairy cattle infected, and in NSW, with 25% of herds infected, especially in the Far North Coast and South Coast dairy regions. The prevalence of disease in the other Australian States was considerably less than 5% with negligible prevalence reported in Tasmania.

In the high prevalence states, economic loss occurred in affected herds. During the 1980's in NSW, it was normal to only have a few infected cattle within an infected herd. In some Illawarra stud herds, however, close to 100% infection was found. Over 50% of herds within the Illawarra region were infected in a 1985 study.

In low prevalence States, economic loss was not the main driver for control. The main concerns were the perception that milk contained a cancer causing virus (even though it posed no hazard to people consuming dairy products) and the potential impedance to trade of milk products.

The existence of an accurate and reliable test and the lack of an environmental reservoir meant that a successful control program was possible. The disease can only be introduced into a herd by an infected animal or animal product (milk and blood). It can subsequently be controlled effectively through individual animal and herd testing, removal of infected animals, herd introduction policy and management procedures that prevent the spread of potentially infective secretions from one animal to another.

CONTROL TO ERADICATION

The EBL control program started as a voluntary program within States. Depending upon the State, testing was paid for by government, an industry fund or the individual producer. Prior to 2000, there was tremendous progress in control of the disease in the State based programs.

Initial individual cow testing in the State programs was superseded by pooled sample testing and eventually bulk milk testing. Each change decreased the cost of testing for the program. Bulk milk testing was introduced as the preferred surveillance test into every State prior to 1997. The testing results from all States were compiled and reported in the Australian Disease Surveillance Quarterly from 1997 onwards.

After 2000, there was a rapid decline in dairy farmers as a result of deregulation but the majority of farms tested EBL negative at each round of testing (figure 1). As the number of infected herds decreased, the emphasis of the State programs changed from accreditation and control to eradication.

In 1998, the Australian Animal Health Committee appointed the EBL Working group. This group developed the standard definition and rules (SD&R) to reflect the key elements for eradication of EBL from the National Herd. The SD&R were finalised in 2002.

In 2002, the SD&Rs were adopted and replaced the existing State schemes. A change was also introduced in the reporting of EBL results. Previously 'EBL negative' indicated that the test result on a bulk milk or individual milk or blood sample was negative. After 2002, results were reported relating to herd status according to the definitions in the SD&Rs. Dairy herds were now 'monitored free', 'bulk milk testing' and 'non assessed' according to the number of sequential negative bulk milk tests assigned to the herd.

Two States were mainly affected by these changes in definitions. All herds in Tasmania were defined as 'non assessed' since regular bulk milk testing in that State had ceased. In Victoria, over 40% of herds were classified as 'bulk milk negative'.

The funding model for State EBL programs changed to a user pays model either directly through payments by the dairy processors or the individual dairy farmer or indirectly through Cattle Compensation funds (levies on the sales of dairy and beef cattle).

PROGRAM AT THE CROSSROADS

In the mid 2000's it became apparent that the EBL eradication program had lost its impetus, with on-going testing without any discernable end point. Two States (Victoria and Tasmania), seemed to be marking time, rather than progressing in determining the EBL status of their dairy herds.

There was concern that there was no apparent benefit for the eradication of EBL for the dairy industry, rather than for an individual farm. Only one overseas market, Russia, had EBL certification as an import requirement. This market only made up 0.5% of total dairy exports.

Funding sources, such as the cattle compensation funds, wanted a return for the funds spent on the program.` Since these funds received contribution from both dairy and beef sectors, the flow of money into one sector required justification for such funding to continue. The beef sector had no interest in EBL control. Similarly dairy companies were now large cooperatives or corporate milk buyers with shareholders requiring justification for resources used. In the mid 2000's, drought was continuing to erode farm income through decreased production and increased on-farm costs. The program was an extra cost of business without an obvious benefit.

At the same time, New Zealand, Australia's largest competitor for bulk dairy markets had achieved a potential marketing advantage by reaching the OIE definition of freedom from disease in its dairy industry before Australia. Extensive testing at the beef/dairy interface had been completed - where beef bulls are used in the dairy industry - and the result had show that this sector was effectively free from EBL. There was a perceived threat that New Zealand dairy products could be considered of higher quality that those products originating from Australia.

NEW DIRECTION FOR THE PROGRAM

A paper 'Eradicating Enzootic Bovine Leucosis from the Australian Dairy herd' was developed by the National EBL Working group and submitted to the Australian Animal Health Committee in 2007 . This paper proposed building upon the successes of the State based program, with the level of EBL herd infection now only 0.5%, to achieve National Dairy Industry Freedom from EBL. The benefits of this approach included intentional trade benefits (many European Countries were now free from EBL and New Zealand was close to announcing their country's freedom), domestic trade benefits with perception of disease free, higher quality dairy products, defining an endpoint for the current annual surveillance costs to industry and recognition that total eradication was achievable. The disincentives were that the approach would marginally increase costs to the dairy industry in the short term at a time when drought conditions had reduced supply and there was still no current strong market drivers for eradication.

The aim of the program was to achieve OIE freedom for the dairy industry compartment. The concept of compartmentalisation was introduced by the OIE in May 2010, to minimise the effect of existing diseases in one area of a country affecting international trade for non infected areas. The compartment had to be clearly and demonstrable separate animal population within a country based on management and biosecurity practices. Within the zone, all dairy herds were required to be tested free of EBL, using a test of 99% confidence of detecting infection at a prevalence of 0.2%.

One potential issue was that Australia used bulk milk sampling as a surveillance method and, although this method of testing exceeded the OIE requirements for freedom, it is not recognised as a surveillance tool by OIE.

Another issue was that the sensitivity of the EBL diagnostic test enabled one weak reactor to be detected in a sample of milk from 200 cows. Since deregulation, the average dairy herd size now exceeds 200 cows so statistically an EBL infected cow could exist in a herd with a history of negative bulk milk tests.

NEW DIRECTION FOR THE PROGRAM - MARK II

The National Dairy Enzootic Bovine Leucosis Eradication Program (NDEBLEP) was established in 2008 and the NDEBLEP 2008-2012 Business plan outlined the action plan to be conducted by each State. The program was now industry driven with support from the Australian Dairy Industry Council. The 'industry' component involved the dairy farmer organisations, dairy processors, Dairy Australia, herd recording companies, private veterinarians and private testing laboratories. The 'government' component involved Animal Health Committee, DAFF, all State Governments, (including LHPA) and government testing laboratories. Two committees were created to oversee the program and to provide technical assistance - the EBL National Dairy Program Advisory Group, chaired by an industry representative and the EBL Technical Work Group chaired by a representative from Animal Health Committee.

The issue of the test sensitivity was addressed by all herds over 200 cows being intensively bulk milk (IBMT) tested where the milk of individual cows were collected and tested in bulks of 50 cows. This involved 40% of dairy herds.

The Australian dairy herd as a group achieved Provisionally Free (PF) status in December 2009. At the beginning of 2012 all Australian dairy herds had achieved Monitored Free (MF) status. Australia was well on its way to meeting the December 2012 target of achieving unconditional EBL freedom (as defined in the SDRs) from the dairy herd.

However, the program will not achieve Provisional Freedom based on the OIE Terrestrial Animal Health code, as planned, because the new 2012 OIE requirements for a compartment cannot be met due to the structure of the Australian dairy industry and the strict movement and testing requirements for beef cattle within the 'dairy compartment'. There is no intention by the beef sector to monitor or eradicate EBL.

Although it is recognised that the Australian EBL program does deviate from the OIE code, the eradication and testing regime in Australia will achieve equivalence to 'compartment' freedom.

THE FUTURE 2013 - 2016

A new nationally coordinated, industry driven and funded plan has been developed to ensure the maintenance of EBL eradication within the dairy sector. Dairy farms will be bulk tested every three years. There will be a compartment biosecurity plan to management the risk of disease introduction from the beef cattle sector.

CONCLUSION

The EBL program started as a voluntary accreditation program between industry and government, with variable success, within the individual States and has concluded as a successful National Dairy Industry eradication program. The success factors for this program include:

REFERENCES

  1. Animal Health Australia 2006, Enzootic Bovine Leucosis, viewed 29 February 2012, Factsheet
  2. Clague DC, Granzien CK. 1966 Enzootic bovine leucosis in south east Queensland. Australian Veterinary Journal; 42:177-82
  3. DAFF 2009, Enzootic Bovine Leucosis, viewed 29 February 2012, EBL
  4. Kirkland, PD, Rodwell BJ 2005, 'Enzootic Bovine Leucosis - Australia and New Zealand Standard Diagnostic Procedures', SCAHLS website, viewed 29 February 2012 EBL
  5. Meas S, Usui, T, Ohaschi, K, Sugimotot, C and Onyma, M 2002. Vertical transmission of bovine leukemia virus and bovine immunodeficiency virus in dairy cattle herds. Vet Microbiol 84: 275-282

 


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