The maintenance of challenge strains used in the potency test for canine leptospira vaccines

The challenge test for leptospira vaccines required by most licensing autho rities is difficult to standardise and unreliable. One of the main contribu tory factors to this, is the difficulty in maintaining the virulence of the challenge strain. This paper describes work carried out to assess the prac ticality of storing challenge strains of Leptospira canicola and Leptospira icterohaemorrhagiae in liquid nitrogen. The effects of different concentra tions of glycerol and dimethyl sulphoxide on the recovery of virulent and a virulent strains of Leptospira canicola and Leptospira icterohaemorrhagiae were investigated. Concentrations of cryopreservant above 5% increased the time taken for the leptospires to grow after recovery from vials stored in liquid nitrogen. In addition, the virulence of five challenge strains were shown to be little affected after 18 passages in vitro.