The long incubation period of Mycobacterium avium spp paratuberculosis, the causative agent of bovine Johne's disease (BJD) during which time none of the available tests have any predictive value makes eradication of established disease from grazing beef herds almost impossible.
This paper describes the eradication of BJD from a beef stud in southern NSW.
The herd was a successful stud. The BJD infection was most likely introduced with a bull imported from England in 1958 and that had died with signs suggestive of BJD. Of interest bulls from that same consignment were brought into other studs that subsequently were infected with BJD.
The first clinical case after that was in 1973. Over the next decade or so a significant amount of testing was undertaken including whole-herd serology and faecal cultures of every cow in the herd in the few days post-calving in anticipation of an increased chance of spreading MapTB due to the stress of calving.
Over that period a significant number of animals were identified as infected, most of these became clinically affected with BJD at about the same time that the infection was confirmed.
About 1-2% of the herd was affected by BJD annually.
There were also several cohorts of bulls where 10-20% broke down with BJD not long after starting to work.
When BJD was regulated in 1990 the herd owners decided to attempt to eradicate the disease while retaining their genetic material.
All cattle with any infection in their family line: either ancestor or descendant were culled for slaughter.
45 cows and 4 bulls that were over 5 years old and that had been seronegative on more than one occasion and that the owner believed to be genetically superior were removed from the herd and placed on agistment on four separate properties none of which had run any cattle in recent years. They were run in small groups so that if an animal turned up as infected there would be some cattle that had not been in contact with that individual.
The groups were subjected to the BJD ELISA every six months.
The cattle remaining on the home property were run as a slaughter-only operation until it appeared that the satellite herds were likely to remain negative to the BJD tests and then the holding was destocked and decontaminated by having crop and sheep only for 12 months.
After being consistently seronegative for four years the satellite herds were returned to the decontaminated home property and the herd enrolled in CattleMAP.
The herd progressed to MN3 and maintained that status for over 10 years.
Given the predictive value of the available tests test-and-cull programs for BJD have very limited effectiveness.
In dairy herds it is possible to limit the exposure of young, susceptible, animals to faeces from older animals that are likely to have begun to shed MapTB. This has made it possible to reduce the prevalence of BJD significantly and I am aware of one dairy herd that had endemic infection with BJD and may now be free from the infection.
This option is not available to beef herds.
Quite a few herds infected with BJD have been through programs approved by the Chief Veterinary Officer of NSW that will allow the herd to be classified as Not Assessed or Tested to Market Standard, but these have all been herds where it has been possible to identify animals that have not been exposed to the infection at a susceptible age.
This is the only herd of which I am aware where it has been possible to eradicate well-established endemic BJD. The cost was high but it allowed the owners to preserve the genetics that they valued.
Despite the success of this program it is unlikely to be repeated because embryo transfer technology makes it possible to salvage genetic material from infected herds in much less time. It is, however, a testament to the persistence of the owners to salvage the genetics that they valued so much.