On 1 January this year the entire state of Western Australia moved from being a Low Prevalence Area for ovine Johne’s disease (OJD) to a Medium Prevalence Area. This worsening of WA’s ranking prevents the state’s farmers from bringing sheep into South Australia. The good news is that is there is a range of proven biosecurity measures they can undertake take to improve the OJD health status of their flocks.
The highest risk of OJD infections comes from introduced infected sheep, including agisted sheep. Most infected sheep are in the subclinical stage of infection and show no obvious signs of having the disease, but they can still spread the disease. However OJD may also enter a farm from infected neighbouring properties through straying sheep or washing of dung across boundaries, so maintaining stock proof fences and other barriers is important.
The Assurance Based Credit Scheme (ABC Scheme) was launched in 2004 as a national standard for assessing and assuring flocks for OJD. A consignment of sheep can have a score between 0 and 10. The higher the score, the better the assurance.
To reduce the risk of introducing infected sheep, a producer should firstly know his or her own flock’s ABC score and only buy sheep where the vendor has provided a completed National Sheep Health Statement that declares a score that is at least equal to but preferably higher than their own flock.
It is strongly recommended that graziers implement a long-term vaccination programme to minimise stock losses and reduce environmental contamination, including vaccinating all lambs being retained on the property past 12 months of age. In certain situations whole flock vaccination may be considered on the advice and recommendation of your veterinarian.
Overall, investigations show that vaccinating with Guidair® is effective in reducing mortalities and, in most flocks, reducing the level of bacterial shedding.
However, it is important to note that vaccination alone does not eliminate infection. In addition, introducing non-vaccinates from infected flocks or nil assurance flocks from the High Prevalence and Medium Prevalence Areas, or leaving some sheep unvaccinated, increases the pressure of infection on the vaccinated sheep.
OJD is primarily spread when sheep graze on pastures contaminated with the manure of infected animals. Meat & Livestock Australia studies show that grazing management can be used to reduce infection rates and deaths from OJD and is likely to be of value in both vaccinated and non-vaccinated flocks. The main practices to follow are as follows:
1. Prepare low-risk paddocks for lambing ewes and/or weaners. Remove all sheep that may be shedding OJD bacteria for at least 6–12 weeks during the summer, or six months in other seasons. Ideally graze with adult cattle or adult OJD-free sheep during this time or leave paddocks ungrazed.
2. Restrict joining to as short a period as possible (ideally five weeks – two cycles). A short joining period will ensure that weaning is not delayed by late-born lambs. Ram harnesses can be applied and the ewe flock divided into two groups based on crayon marks.
3. Remove all ewes showing signs of wasting shortly before lambing and again at marking. This will reduce pasture contamination for lambs.
4. Move the lambing flock onto low risk pasture shortly before lambing. Run the lambing flock at the lowest stocking density possible.
5. Wean lambs early onto a second low risk pasture. Weaning can occur when the youngest lambs are seven weeks old if pastures are highly nutritious. A green, legume-dominant pasture is generally satisfactory. Early weaning will separate lambs from the main source of infection (their dams and the pastures contaminated by them). If ‘safe’ pasture is scarce it should be reserved for weaners rather than lambing ewes.
For a comprehensive round-up of information on Ovine Johne’s Disease visit www.ojd.com.au.
Download the National Sheep Health Statement.
(Sources: AHA, MLA, ASHEEP Esperance)