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This article was published in 1993
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Bovine Johne's Disease
A Southern Perspective
DD Salmon, District Veterinarian, Deniliquin
INTRODUCTION
Johne's Disease has developed over the past twenty years from an obscure disease of Channel Island breeds, with most cases traceable to importations from Britain, into a disease with a modest but increasing prevalence causing modest but increasing production losses but producing an immodest and increasingly acrimonious debate.
The Riverina has a relatively high incidence of Johne's Disease. There are several reasons for that:
1. Irrigation leads to intensive production and year-round green feed, favouring survival and spread of M. paratuberculosis.
2. A significant dairy industry, increasingly this is including economic refugees from coastal NSW and Victoria, both areas having a high prevalence of Johne's Disease.
3. Proximity to the heavily infected Goulburn Valley of Victoria.
HISTORY
Prior to 1974, Johne's Disease had not been diagnosed in the Riverina.
During 1974 and 1975 Johne's Disease was diagnosed in a Jersey cow bought from the Goulburn Valley of Victoria, a Murray Grey bull bought from the Murray Valley of Victoria, a Shorthorn bull bought from the Sydney area and a Shorthorn cow bred in the Riverina. Johne's Disease has since been found to be endemic in the two Shorthorn herds.
Since 1975, clinical Johne's Disease has been diagnosed on a further eight properties in the Deniliquin and Moulamein Pastures/Rural Lands Protection Districts.
All clinically affected animals have been Friesian cows. Half the initial cases were in animals which had been introduced from Victoria. The other half were in animals which had been bred in the Riverina.
On five of the properties, clinical or serological and pathological evidence of Johne's Disease was found in animals other than the index case.
On five of the properties Johne's Disease was diagnosed subsequent to AH Circular 92/1 outlining the new policy with respect to Johne's Disease.
AH CIRCULAR 92/1
This change of policy with respect to Johne's Disease was aimed at reducing the prevalence of the disease within New South Wales.
This new policy was a quantum leap from a low level advisory policy with a minor regulatory component to a full blown quarantine, test and slaughter program.
The change of policy with respect to Johne's Disease appears precipitate if compared with the long timescale over which Brucellosis and Tuberculosis were advanced to full scale regulation. It also compares unfavourably with the extensive industry consultation and economic analysis prior to the implementation of the Footrot Strategic Plan.
Subsequent to the release of this Circular it has become obvious that Johne's Disease is more common in New South Wales than was previously believed to be the case. This has created some problems with the implementation of the policy.
EPIDEMIOLOGICAL CONSIDERATIONS
It remains to be demonstrated that we have the capacity to eradicate Johne's Disease using current technology.
There are three stages of infection with M. paratuberculosis. A preclinical stage during which the infection is establishing in the intestinal mucosa. If, as seems to be universally accepted, infection is mainly contracted by young calves then this preclinical phase is at least two and sometimes ten years. Next is a subclinical infection where M. paratuberculosis is detectable in the faeces, then clinical disease develops.
The identification of subclinical infection is a substantial challenge which has been addressed worldwide, and significant progress has been made. The identification of preclinical infection is much more difficult. The long preclinical phase of Johne's Disease infection means that unless a test is capable of identifying preclinical infection eradication will take a very long time, and confirmation of eradication will take even longer.
Johne's Disease spreads readily from dam to calf during the periparturient period. Given this factor, it is unusual that it should be confined to a particular geographic area. While lateral spread could be expected to be climate dependant the same could not reliably be expected of periparturient spread. There is insufficient data to say whether geography influences infection or disease and whether infection in the absence of disease will disseminate.
Conventional wisdom states that infection inevitably progresses to clinical disease and death. The fact that some animals do not express disease until they are fairly old (10 years plus) means that some animals do not have long enough for their infection to manifest itself. This poses the question as to whether or not these animals shed large numbers of M. paratuberculosis and whether they pass infection to any or all of their offspring.
The distribution of clinically diagnosed Johne's Disease at any point of time does not necessarily give any indication as to the prevalence of M. paratuberculosis.
There have been no reports published of surveys which attempted to establish the prevalence of clinical, bacteriological or serological evidence of Johne's Disease anywhere in Australia except Victoria and Western Australia.
In the Deniliquin and Moulamein Rural Lands Protection Districts, recognised to have high prevalence of Johne's Disease, 7 of the 67 dairy herds have had clinical Johne's Disease affecting 11 of the 15,158 cattle on those farms during the past 20 years. 3 of the 1009 beef cattle herds have had cases with 34 of the 103,000 cattle being affected. Thus 10% of dairy herds and 0.3% of beef herds are infected with an annual case rate of 0.01% for both groups.
VICTORIAN DATA
Information collected by the Victorian Department of Agriculture (Gill, pers. comm.) in the Shepparton District indicates that Johne's Disease prevalence has increased substantially over the period 1978 to 1990 inclusive.
Prior to 1978, 19 dairy herds and 5 beef herds had clinical Johne's Disease diagnosed.
During the period 1978 to 1990 Johne's Disease was diagnosed for the first time in 283 of the 1450 dairy herds (20%) and 27 of the 1000 beef herds (3%) in the District.
Of the 334 herds on which Johne's Disease was diagnosed, the first case in 178 herds was bred on the farm and in 66 herds the first case was in an animal bought from a herd where Johne's Disease was not previously known or suspected. Thus 73% of the initial cases of Johne's Disease would have been cattle for which the owner and the District Veterinary Officer would have been in a position to make a declaration that the herd of origin had been free from Johne's Disease for the previous five years. The total number of cases in these herds during the study period was 845.
ECONOMIC CONSIDERATIONS
There is no good economic data on the cost of Johne's Disease because there is no epidemiological data.
Collins (1990) estimated the cost of subclinical Johne's Disease in a dairy cow to be $300.00 in the year after infection. If the clinically diagnosed Johne's Disease within the Deniliquin and Moulamein Districts represented 10% of the subclinical cases, that would put the cost at [$]33,000.00 for the dairy industry of the two Districts over a 20 year period.
In most cases the cost of Johne's Disease is the cost of movement restrictions.
THE JOHNE'S DISEASE DILEMMA
1. We do not know the true prevalence of Johne's Disease.
2. We do not have a reliable best guess of the prevalence of Johne's Disease.
3. We do not have even a rough approximation of the cost of Johne's Disease and the source of that cost.
4. We do not have reliable techniques to eradicate Johne's Disease from individual herds.
5. Using the rate of diagnosis of clinical disease to measure the prevalence of Johne's Disease does not give reliable results.
6. Herd history, even when verified by Government veterinarians does not have good predictive value.
7. Much of the current demand for Johne's Disease free cattle is for cattle destined for slaughter.
8. There is no money available to address any of the previous seven problems.
The problems presented by Johne's Disease do not have an easy solution. If we are to make a genuine attempt to control the spread of Johne's Disease we must have a better starting point than we have at the moment. The steps which would be a necessary starting point are:
1. Survey of dairy herds in particular, but also beef herds, to determine true distribution of M. paratuberculosis infection.
2. Long term studies into the use of serology and management to eradicate M. paratuberculosis infection from dairy and beef herds.
3. Establishment of some form of accreditation so that herds free from M. paratuberculosis infection can be identified. This would allow the market to place a value on freedom from infection.
REFERENCE:
Collins, M (1990) - The Control of Johne's Disease in Dairy Herds.