Treatment with Ca(OH)2 for inactivation of Salmonella Typhimurium and Enterococcus faecalis in soil contaminated with infected horse manure

Aim: To investigate the inactivation of Salmonella enterica serovar Typhimurium and the faecal indicator Enterococcus faecalis in horse manure:soil mixtures by application of hydrated lime (Ca(OH)₂). Methods and Results: In laboratory incubations, the inhibitory effect of different concentrations of Ca(OH)₂, as well as different application techniques, was tested. Other variables were horse manure:soil ratio, incubation temperature (6 and 14°C) and soil type (sand/clay). Bacterial enumeration by the plate count method in samples taken at increasing intervals revealed that Ca(OH)₂ effectively reduced Salmonella Typhimurium numbers. However, to achieve a sufficient reduction, the Ca(OH)₂ had to be applied at a sufficient rate, and the amount required varied because of manure:soil ratio and incubation temperature. The results showed that a pH above 11 was needed and that a high pH had to be maintained for up to 7 days. An appropriate application technique for the Ca(OH)₂ was also important, so that a high pH was obtained throughout the whole material to be treated. In addition, a high manure:soil ratio in combination with a higher incubation temperature was found to rapidly neutralize the pH and to increase the risk of Salmonella re‐growth. Conclusions: Application of Ca(OH)₂ can be an efficient method for treating a Salmonella‐contaminated horse paddock. A high pH is a key factor in Salmonella inactivation, and thus, monitoring the pH during the treatment period is necessary. To avoid re‐growth excess manure should be removed for separate treatment elsewhere. Significance and Impact of the Study: Persistence of Salmonella in horse paddocks poses a risk of disease transmission to healthy animals and people who come into contact with these animals. An efficient method to de‐contaminate a Salmonella‐contaminated soil would be a valuable tool for animal welfare and for public health.