Abstract :
It is believed that so-called ‘S’ strains of Mycobacterium avium subsp. paratuberculosis, the
causative agent of ovine paratuberculosis, were introduced into Australia in Merino sheep
from New Zealand in the 1960s. The disease was first diagnosed in Australia in the central
tablelands of New South Wales in the early 1980s, but caused few problems until the
mid 1990s, when the current epidemic of the disease emerged to become one of the most
important endemic diseases in Australia. Paratuberculosis continues to spread well beyond
the original infected region in New South Wales, with prevalence rising in all states of the
country, except South Australia and Queensland. A research project on GudairTM vaccine,
which led to the licensing of the product in 2002, identified that numbers of vaccinated
sheep that died of the disease, or shed the causative agent declined by 90% in the first generation
of vaccinated sheep. Vaccination and a risk-based trading scheme known as the ABC
scheme (Assurance Based Credit points accrued for evidence suggestive of disease absence,
currently including points for paratuberculosis vaccination) that uses a national sheep vendor
declaration for sale of sheep, are now the main tools used for the control of the disease
in most Australian states. However, vaccination for paratuberculosis remains controversial
in some parts of the world, largely based on concerns relating to the efficacy of the vaccine
in decreasing the risk of infection and transmission of the organism. The Australian experience
has shown that vaccination against paratuberculosis quickly eliminates the significant
mortalities in infected, high prevalence flocks. However, our applied research has shown
that shedding of M. avium subsp. paratuberculosis from vaccinated sheep may persist for
many years after commencing of vaccination programmes. In a longitudinal study of 11
flocks over 6 years, where 3- and 4-year-old animals were monitored every 2 years, we
observed a substantial reduction in the prevalence of microbial shedding of >75% following
vaccination, although in four flocks such a reduction was not evident. Further, in a study
of 41 flocks where vaccination of lambs had been applied for 6 years, shedding persisted
in the majority of flocks (81) and risk factor studies in these flocks indicated a number of
management factors may be important in persistence of the disease. Stray sheep, failure to
vaccinate wethers, introduction of unvaccinated sheep into the flock and use of commercial
‘contractors’ to do the vaccinations were associated with increased prevalence of paratuberculosis
in vaccinated flocks. Improvement of farm biosecurity and correct vaccinationschedules of all sheep in a flock are suggested as important management interventions
to optimize the protection offered by GudairTM. A recent study on Kangaroo Island has
confirmed that where a majority of these practices are rigorously applied, eradication of
the disease is potentially possible. Despite the persistence of shedding in vaccinates in
many parts of the country, vaccination to control paratuberculosis in Australia has been
of enormous benefit to the sheep industry and rural communities and this needs to be
recognized internationally.
