Recent research has shown that Gudair vaccine reduced mortality and the prevalence of sheep that shed Mycobacterium paratuberculosis subsp. avium by 90% (Reddacliff et al. 2006). However, vaccine sales have declined in the OJD high prevalence area of NSW from a peak in 2003. A recent survey in the Central Tablelands of NSW (Neef et al. 2007) has shown that the vaccination of Merino wether lambs has declined from 98.9% of lambs in 2004 to 76.5% of lambs in 2006. Some owners, including this producer, opted not to vaccinate wether lambs as an interim drought economy measure. Flock owners may also plan to sell wethers at an early age before OJD impacts on the flock.
The owner of a mixed cropping and grazing property west of Young sought veterinary assistance after 30 of 225 (13%) of the wether component of a mob of 550 mixed sex 21 month old Merino hoggets died and a similar proportion were ill-thrifty. On this property, the owner vaccinated the ewe portion of the flock with Gudair at lamb marking in 2006 while the wether portion remained unvaccinated. The owner reported that a few ewes died but losses were within the normal range for sheep in drought conditions.
The owner has vaccinated with Gudair since 2002. One mob of aged ewes on the property has not been vaccinated and a proportion show clinical signs of OJD. The owner has vaccinated the wether portion of the flock from 2002 to 2004 but these mobs have been sold.
One of us (JE) examined five clinically affected wethers on 16 January 2007. All were emaciated with sub-mandibular oedema and pale mucous membranes. On post-mortem, there was an entero-colitis with lymphatic cording typical of OJD. The diagnosis was confirmed on histopathology (ON07/002989/GB).
The mob continued to be trail fed under drought conditions and was drought affected with average condition scores of 1.5 -2. By the 22 May 2007, the mortality rate in the wether portion of the flock was 70 of 225 (31%). Losses among the ewe portion remained negligible. The owner is planning to sell the wether portion for slaughter as soon as practical.
In my experience (JE), this syndrome is typical of heavy exposure as lambs. Peak losses occur in the most susceptible sheep at 2-3 years of age. Prior to an effective OJD vaccine Young RLPB veterinarians regularly encountered losses of 20-30% by three years of age in cell-grazing systems.
This case reflects both the importance and efficacy of Gudair vaccination in high prevalence flocks and the risk of not vaccinating the wether portion of such flocks. It also challenges to notion of intermittent flock vaccination as an economy measure when the flock mortality rate declines. Not vaccinating the wether portion may be an appropriate strategy if the owner plans to sell the wethers as lambs. However, it effectively precludes the option of retaining the wether portion in high prevalence flocks.