FURTHER EVIDENCE OF RESISTANCE TO WARFARIN IN RATTAS RATTAS DIARDII

Y.M. LAM

SUMMARY

Studies' were conducted in the laboratory to determine the efficacy of 0.025% warfarin against laboratory-bred warfarin-naive and wild-caught R. rattus diardii from a suspected warfarin-resistant population. A base-line for the susceptibility of warfarin-naive R. rattus diardii to 0.025% warfarin was determined. The median lethal feeding period (LFP₅₀) for males and females was 1.78 (95% fiducial limits 0.85-2.63) and 1.84 (1.11-2.56)days respectively. The LFP₉₅ for malesa nd females was 9.01 (5.52-30.00) and 6.41 (4.32-1489) days respectively. The pooled LFP₅₀ and LFP₉₅ for both sexes was 1.81 (1.25-2.35) and 7.59 (5.45-13.45) days respectively. The estimated LFP, indicated that a 14-day feeding on 0.025% warfarin is a suitable test for the detection of warfarin resistance. For the wild-caught males and females the LFP₅₀ was 4.73 (3.16-6.11 ) and 7.56 (5.96-9.28) days respectivelyl. The LFP₉₅ for wild-caugh males and females was 16.10 (11.23-36.3) and 19.03 (13.91-40.11) days. The pooled LFP₅₀  and LFP₉₅ for both sexes of wild-caught rats was 6.02 (4.92-7.10) and 18.91 (14.35-30.82) days respectively. A resistance factor (R) was calculated and the R values indicated that the wild-caught rats were 3.3 times less susceptible to 0.025% warfarin when compared with warfarin-naive rats. In conclusion, warfarin resistance has been confirmed in the wild populationo f R. rattus diardii studied.

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