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This article was published in 1993
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Johne's Disease in the Bega District
S. Jagoe, District Veterinarian, Bega
31 properties in the Bega District have a history of, or are suspected as having infection with M. paratuberculosis. These properties were sampled as part of the statewide survey from September 1992 to February 1993.
Results are indicated below.
| No. of herds | No. of cattle sampled | No. ELISA positive | No. ELISA inconclusive |
|---|---|---|---|
| 31 | 7341 | 51* | 78 |
* Four ELISA positives retested negative.
Inconclusive reactors were retested with the following results.
| No. tested | No. ELISA positive | No. ELISA inconclusive |
|---|---|---|
| 46 | 5 | 3 |
The majority of reactor animals have now been slaughtered under compensation. Special reference has been given to foetuses of these animals to determine the degree of in utero infection. Results to date (1 August, 1993) are:
| No. submitted | No. culture positive | No. culture negative | % positive |
|---|---|---|---|
| 29 | 10 | 19 | 35% |
One animal was a clinical case, and its foetus was culture positive. Not all results are finalised yet.
Of the 51 ELISA positive animals, 7 have entered from Victoria in the last 12 months. This implies some threat to the district, as several thousand dairy cattle from Victoria have crossed the border in the last 2 years, due to dairies increasing their size.
OBSERVATIONS
1. The rate of positive animals was lower than expected at 0.75%, given the history of many of the properties.
2. There is good correlation between ELISA reactivity and infection status. False positive rates (ELISA positive, culture negative) averaged 10-15% based on this small sample.
3. It appears that foetal infection of sub-clinically infected animals occurs frequently enough to be of concern. Whether or not infection status is maintained through to calving is unknown. in utero infection may hinder Johne's Disease control programs, and emphasises the importance of removal of reactors as well as their offspring.
4. Johne's Disease doesn't appear to spread very well in the Bega Valley. Extremes of seasonal conditions may account for this with drought removing reservoirs of infection on dairy farms (both contaminated environment and infected cows). High rates of replacement occurring in local herds may mean removal of subclinical infections for other reasons e.g. low milk production, mastitis, reproductive problems.
5. Victorian cattle act as a means of introduction of infection into the district, as well as a reservoir of other diseases (e.g. Mastitis, Salmonellosis, Leptospirosis, Vibriosis which are more important economically). The only regular dairy cattle sale occurs at Warragul, an area which has a farm infection rate of approx. 15%. Some movement restrictions are required and it is disappointing to see that after at least 5 years of Departmental discussion, no policy currently exists.
6. There appears to be strong producer interest in accreditation scheme, particularly by stud owners.
DISCUSSION
Each year, only 2-3 clinical cases of Johne's Disease are diagnosed in the Bega District, even though private practitioners are attuned to the disease. Many other diseases have much greater economic significance. Work needs to be undertaken to describe production losses due to subclinical disease under Australian conditions.
Adult to adult transmission appears to be occurring on at least one dairy - this herd is the largest and most intensively operated in the district. This may have implications for disease spread as the dairy industry further intensifies. It may also be part of the reason for the high infection rates in Victoria, where stocking rates tend to be higher than in NSW.
The environment of the farm appears to interfere with ELISA reactivity and may have implications for disease eradication. Flat farms with areas of inundation and low lying water tend to have a fluctuating level of ELISA positives and inconclusives. This may be similar to problems of TB testing that occurred on the Nowra river flats in past years, where saprophytic mycobacteria may interfere with reactivity. This phenomenon has been documented in New Zealand.
There currently exists no accreditation scheme in NSW, despite producer interest in such a scheme. Overseas schemes are based on 3 negative tests at defined intervals. This regime does not appear to account for the vagaries of the tests used in cattle, which appear to have differing levels of sensitivities and specificities in different laboratories. An accreditation scheme based on a combination of statistics and local disease behaviour would be more appropriate under the relatively extensive nature of the dairying industry in Australia compared to overseas.
Herd management has a role in Johne's Disease management. Animals culled for other conditions such as mastitis, infertility or poor production are probably more likely to be infected. Owners should be encouraged to remove these animals in the absence of regular testing.
Restriction of entry of cattle from Victoria is a means of slowing the spread of Johne's Disease in NSW. A five year certification has little meaning in the context of this disease (although it is a start). The Queensland model may be more appropriate, although with expansion of the dairy industry in southern NSW over the last few years, the horse may have already bolted.
In conclusion, Johne's Disease is a disease that currently does not cause significant economic loss to the dairy industry in the Bega District. However, with intensification, it is likely that the disease will increase in importance. A Johne's Disease policy must be formulated in NSW that meets the needs of producers and farm advisors which takes into account the limitations of current diagnostic tests and allows for the variable behaviour of the disease in different environments. The policy must also recognise that other diseases currently cause much greater economic loss to cattle industries and therefore must be sympathetic to the occurrence of the disease on properties of differing management.