Review finds South Australia’s OJD flock infection rate is underestimated

Terry Sim, February 19, 2018

An OJD-affected sheep among healthy animals.

OVINE Johne’s Disease management in South Australia should be de-regulated and OJD infections in sheep flocks would probably increase in high rainfall areas even if regulatory controls continue, a confidential review has concluded.

The ‘Review of the management of OJD in South Australia’ report prepared confidentially for Primary Industries and Regions SA and the government-appointed South Australian Sheep Advisory Group has concluded that OJD prevalence in south-east SA is significantly underestimated.

The report by then University of Adelaide Professor Kym Abbott  in 2016 has not been released generally to SA producers by PIRSA, despite it being the most comprehensive recent analysis of the producer levy-funded SA OJD Control Program and sheep producers having only until March 12 to submit comments on whether the National Ovine Johne’s Disease Management Plan should continue, be changed or dropped.

Since 2012, Primary Industries and Regions SA has run the industry-funded SA OJD control program as part of the NOJDMP, with the support of PIRSA’s Biosecurity SA and the South Australian Sheep Advisory Group (SASAG).

While other states have largely de-regulated the disease’s management, SA authorities have treated the entire state as a regional biosecurity area and tried to monitor and regulate the disease’s introduction and spread. This has been attempted through stopping the entry of OJD-infected sheep, the quarantining of infected flocks and properties via Property Disease Management Plans, the SheepMAP program for stud breeders, abattoir surveillance, Gudair vaccine subsidies, on-farm monitoring by veterinarians and producers, and producer education.

OJD prevalence under-estimated in south-east SA

However in his report, Professor Abbott said that the limited penetration and insensitivity of abattoir surveillance leads to the conclusion that the prevalence of OJD in the south-east region is significantly under-estimated (based on abattoir surveillance) at present.

“While infections on Kangaroo Island are manageable, new infections in the South-East and other medium/high rainfall regions will continue to occur, probably at increasing rates, even while regulatory controls continue.”

He said if regulatory controls are removed, it is expected that OJD infection will spread through the state’s medium and high rainfall zones at a higher rate and progress towards the levels of flock infection seen in Victoria, New South Wales and New Zealand.

“OJD will present a significant disease threat to producers in the medium and high rainfall regions, requiring preventive vaccination to control the disease.

“In the lower rainfall zones of SA, OJD will spread more slowly and will in some cases be manageable on farm without vaccination,” Professor Abbott said.

Deregulate to producer responsibility for biosecurity

In his report, Professor Abbott’s principle recommendation is for a de-regulation of OJD and a move towards individual responsibility for biosecurity.

“The current strategy in South Australia is providing only partial control (slowing, not stopping the spread) and is not providing good epidemiological data which could inform good strategies developed regionally and locally to control the disease.

“Information about the disease – particularly its regional prevalence – is limited and unreliable,” he said.

“There is substantial misinformation, particularly in regard to the existence of OJD on farms released from orders and the reliability of vaccine.”

SASAG sees no evidence to reduce regulatory controls

Chair of SASAG when the Abbott report was finalised, Leonie Mills, said despite Professor Abbott’s review and conclusions, SASAG decided that there was no scientific evidence or reason to lessen the current regulation of OJD in South Australia, although new measures for affected producers were being discussed.

“We can only deal with what we know is there.

“We are all convinced that we can keep it low for a very long time.”

Ms Mills said the review was conducted by PIRSA for SASAG, rather than for producers, to help the advisory group determine if the current SA OJD control program should continue. It was up to PIRSA whether to release the Abbott report or not, although it was her understanding it could be accessed if requested, she said.

OJD vaccine subsidies were cut back in South Australia last year, with SA producers with new detections or their at-risk neighbours and trace property owners, to receive a 50pc Guidair vaccination subsidy for the first year only.

Not all OJD infections are being detected

Abattoir surveillance for OJD in South Australia is carried out at two abattoirs, Thomas Foods International’s Murray Bridge and Lobethal plants.

“For the South-East regions, where the most new detections have occurred, 21pc, 41pc and 45pc of each region (lower, mid, upper respectively) of flocks were subject to abattoir surveillance in the four years 2012 to 2015,” Professor Abbott reported.

He said in 2015, an estimated 419 flocks in the South-East regions had sheep inspected at abattoirs, representing 22 percent of the flocks which had sheep activity in that year.

“Interpretation of the available data suggests that detection of five new cases of infected flocks in the South-East resulted from inspection of lines from those 419 flocks.

“One could assume therefore that a further 15-20 new cases were missed, because the remaining 78pc of flocks did not submit sheep to an abattoir where surveillance was occurring,” he said.

“Abattoir surveillance is not a highly sensitive tool for detecting OJD in low prevalence flocks.

“If one assumes that abattoir surveillance detected 50pc of the previously undetected flocks then it follows that a further 20-25 flocks with new infections in the South-East remained undetected in 2015,” Professor Abbott concluded.

“The combination of limited penetration and insensitivity of abattoir surveillance leads to the conclusion that the prevalence of OJD in the South-East region is significantly under-estimated (based on abattoir surveillance) at present.”

Professor Abbott also concluded that the prevalence of OJD in SA has remained low and that the current South Australian Control Program has been effective and has reduced the rate of spread of OJD within the state. However, he said the control program is dependent for its efficacy on abattoir surveillance to detect new infections and vaccination to eliminate infections from flocks.

“Both of these strategies are less than 100pc effective, allowing new infections to go undetected for extended periods, and for vaccinated flocks with undetectably low prevalences to resume trading.”

The two regions of SA with the highest prevalence of flocks detected with OJD are Kangaroo Island (26pc) and the south-east (2.6pc). Kangaroo Island has been subject to much higher levels of structured disease surveillance than other regions of SA.

Professor Abbott said over the past decade, most new detections have been in the south-east regions of SA.

“Of the 190 flocks detected in SA to date, 140 have been released from quarantine on the basis of a PDMP or PDEP and subsequent clearance test, based on testing of pools of faeces by HT-PCR, culture, or both.”

Levels of Gudair vaccination are low in SA

He said about 460,000 doses of Gudair vaccine are used in SA each year.

“Roughly one third of these doses are distributed by PIRSA under a subsidy arrangement with SASAG using the industry fund, and two thirds are used independently by producers outside the subsidy arrangement.

“The proportion of sheep bred each year and retained beyond 12 months of age which are vaccinated with Gudair is relatively low compared to Tasmania, Victoria and NSW,” he said.

“A crude estimate is that 14pc of the sheep for which vaccination is appropriate, based on their likely longevity, are vaccinated in SA.”

In the report, Professor Abbott said the current expenditure on control strategies – subsidised vaccine and farm investigations – is competing with the allocation of funds toward collection of better data, on which good management decisions depend.

“Currently, control of OJD in SA through regulation places responsibility for disease management on government agencies.

“Producers are neither encouraged nor sufficiently well-informed to be able to make good business decisions about their plans for management of OJD,” he said.

“If, as expected, there are marked differences in the prevalence of OJD-infected flocks between regions of the state, the most effective way to slow the spread of disease is to apply different strategies in different regions.”

Plan now for de-regulation in SA

Professor Abbott said a bio-economic model developed by AusVet (2006) indicated that, if one assumes a relatively low rate of spread of OJD across the state, de-regulation of OJD had a cost to the industry similar to the costs of continuing the current control approach.

“The approach proposed by Biosecurity SA includes a de-regulation of OJD, with some regulatory activity directed towards clinical cases of OJD.

“This approach should be tempered so that it is consistent with falls in line with the approach taken for other serious diseases managed under the Livestock Act and the Welfare Act.”

Professor Abbott concluded that planning for a transition to a de-regulated OJD environment in SA should begin now.

“OJD management funds should be directed away from vaccine subsidy and farm investigation costs towards a well-structured epidemiological survey to provide a reliable estimate of the prevalence of OJD-infected flocks within regions of SA.”

He recommended that better information about OJD should be provided to producers.

“Some of the funds currently spent on control strategies should be allocated to industry extension and education.”

He also recommended that producers in some areas should be encouraged, if appropriate, to create regional biosecurity areas or cooperative biosecurity groups in order to manage the disease-control status of their flocks and reduce the risk of OJD establishment.

“Some of the funds currently spent on control strategies could be allocated to supporting the creation of such groups.”

Professor Abbott also recommended that abattoir surveillance should be continued and expanded, and the data should be analysed and reported to the industry. He also advocated using the term ‘low prevalence’ rather than ‘low risk’ as descriptions of OJD assurance levels for flock managers.

Sheep Central has asked PIRSA why the Abbott review report has not been released generally to SA producers.

Click here to read Professor Abbott’s report.


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