Mycobacterium avium subsp. paratuberculosis (Map), the cause of Johne's Disease (JD), is defined as an obligate parasite of animals, meaning its survival outside of animals is finite. Transmission between animals is usually by the faecal-oral route so the length of survival in the environment is an important factor in both the risk and the rate of disease transmission. Whittington et al (2004) demonstrated that the survival of the S or sheep strain of Map in the Central Tablelands of NSW ranged from a maximum of 55 weeks in fully shaded locations to a low of 2 weeks in unshaded plots and he proposed that daily temperature variation may be the factor correlated with the lack of shade that reduces the environmental survival of Map.
The prevalence of OJD varies considerably across NSW being highest in the higher, wetter south east and lowest in the drier, hotter western districts. Prevalence is also low in the north east encompassing the New England region. A possible explanation for this uneven distribution is that Map does not survive for extended periods in the environment typical of the low prevalence regions hence reducing the chances of disease being established in the destination flock. In addition, if the environmental survival of Map is low then risk management procedures following an introduction of at-risk sheep could be simplified or removed.
Given that the original trial of Whittington et al (2004) was limited to the high prevalence tablelands region, the aim of this study was to quantify the survival of Map across different environmental and prevalence regions of NSW.
Beginning in September 2010, sheep and cattle faeces containing known concentrations of Map were placed on soil in open polystyrene boxes at 4 sites across NSW. These were Bathurst (temperate, high prevalence of ovine JD), Armidale (temperate, low prevalence of JD), Condobolin (semi-arid, low prevalence of JD) and Broken Hill (arid, low prevalence of JD). At each site boxes were either shaded (70% shade cloth) or left unshaded (4 replicates per treatment). Faeces were sampled fortnightly for 6 months and monthly for the next 6 months, and cultured using BACTEC.
The maximum survival time was 14 weeks. The initial concentration of Map was lower in cattle faeces compared to sheep faeces and this was reflected in the survival times observed. At all sites and for both strains, survival was significantly greater in faeces in the shaded treatments. There was a non-significant trend for survival time to be longer for the 2 temperate sites compared to the 2 warmer sites. Reduced diurnal temperature range was recorded in shaded compared to unshaded sites and this is more likely to explain increased survival than reduced exposure to UV radiation (Whittington et al 2004; Gumber and Whittington 2009). During summer, temperatures in excess of 60 degrees Celsius were recorded especially in the exposed boxes and these temperatures have been shown to kill 90% of Map in 35 minutes (Whittington et al 2010).
The 2010/11 season across NSW is likely to have been favourable for environmental survival due to above average rainfall and below average temperatures. Despite this, survival of Map in the shaded Bathurst site in this study was much less than the 55 weeks observed by Whittington et al. (2004) in the same environment. A major difference between the studies is that Whittington provided 100% shade compared to our 70% and in the field it would be unlikely that Map would be deposited in a completely shaded location.