Survival of Mycobacterium paratuberculosis

Summary

Reducing the survival of Mycobacterium paratuberculosis (Mptb) outside the host animal has long been attributed to environmental factors such as elevated temperature, pH, ultraviolet light and dryness. However, there is no direct scientific evidence of the contribution of these factors to the reduction of Mptb survival. The experiment reported here evaluates these four factors: ultraviolet radiation, soil temperature, pH, moisture plus organic matter, for their effect on the survival of Mptb from soil as measured by proportional recovery using the Whitlock double incubation and BACTEC culture method.

In this study, soil moisture and soil temperature were the most significant environmental factors affecting the survival or death of Mptb. Ultraviolet radiation appeared to have no effect and, different soil types and variable sensitivity of culture obscured the effect of pH.

Results for different environmental factors were evaluated on the low organic soils where the recovery of organisms in culture was not affected by soil type. Sand of acid pH and low organic matter had no loss in analytical sensitivity compared to the Mptb contaminated faeces with which it was inoculated. Fewer organisms were recovered from other soil types mixed with the same faeces. Clay soil of acid pH and high organic matter had a detection limit of 103 colony-forming units per gram. Tray trial results from the two low organic soils showed distinct effects between treatments, however, differences were inconsistent in high organic soils due to the lower analytical sensitivity and low total number of Mptb isolations.

The results were statistically analysed by logistic regression and a model developed which provides predicted recovery estimates for each combination of treatments. Results are presented in terms of the predicted mean recovery with standard errors of the predicted means. The conditions applied were within the range of environmental exposure and results of the treatments are directly applicable to field situations. The rate of death of organisms may have been accelerated in high temperature treatments because the temperature was maintained constantly rather than with diurnal variation.

Soil dryness and high temperature resulted in shorter survival times for Mptb in low organic soil. After 8 weeks, there was approximately 100% survival in wet, low temperature (10 degrees C) acid sand, survival in the same soil with cyclic moisture was reduced to 74% and in dry soils to 32% of initially detectable organisms. At higher temperature (30 degrees C) there was an 81%, 37% and 10% survival for wet, cyclic and dry treatments respectively. The alkali loam low organic soil demonstrated a lower culture sensitivity but the same survival trends as for the acid sand for moisture and temperature were observed.

Alkaline soil pH indicated a weak influence with shorter survival in wet and dry soil at both high and low temperature. There was no effect by UV light at the levels used, which may have been too low in intensity or exposure to ultra violet light may not be effective in killing Mptb in soil due to low penetration of the soil.

From these results it should be concluded that dry soil, high soil temperature and possibly alkaline pH are significant in reducing survival of Mptb in soil and should be used to best effect when implementing control procedures on properties.

In contrast wet soil at low temperature due to protection from sunlight and possibly with acidic pH are conditions where Mptb is likely to survive for longer periods. This study provides only semi-quantitative estimates of the death of Mptb under different environmental conditions. Further work quantifying the log reduction death of Mptb influenced by soil temperature and soil moisture in acidic low organic soil is recommended to assess the level of risk associated with restocking properties undertaking Johne's disease control.

Recommendations

Further work is recommended to quantify the log reduction rate of death of Mptb under critical moisture and temperature conditions to predict appropriate de-stocking intervals. The markedly different effect of moisture and soil temperature on the death of Mptb provides an opportunity to design de-stocking strategies over different land systems.

It is recommended that studies of death rates of Mptb should be conducted in low organic sandy soil due to the high sensitivity of culture in this soil compared with soils containing clay and organic matter.

Further work to elucidate the factors involved in the loss of culture sensitivity in clay and high organic soil types is needed. This will determine if Mptb culture procedures for soil need to be made more sensitive and improve the interpretation of results based on soil type. The low sensitivity of culturing Mptb from loam and high organic soils may mean contaminated areas may go undetected by current culture techniques.

A detailed case controlled study across dairy herds looking for association of herd JD prevalence with soil, environmental and management factors would help determine the influences of moisture and temperature plus local and regional differences and management practices.