In Costa Rica the two populations of Lachesis muta (Atlantic and Pacific versants) differ in their morphology and their behavior. Both are classified as L. m. stenophrys. In South America there are two varieties: L. m. muta and L. m. noctivaga, the latter exclusively from Brazil. An antivenom for the two Costa Rican varieties was prepared in horses to determine their neutralization levels and the immunoelectrophoretic patterns compared to a sample from L. muta muta from Colombia. They were also compared against an anti- L. muta serum from the Instituto Butantan of Brazil. The LD 50 values of the three varieties were very similar and our antivenom effectively neutralized L. muta venom from the two Costa Rican populations as well as that from Colombia. The Brazilian antivenom has similar activity. Immunoelectrophoretic patterns of the venoms of the Pacific population from Costa Rica and that of the Colombia specimen were almost identical. Both showed small differences with that of the Costa Rica Atlantic variety. These results apparently indicate that L. muta from the Pacific lowlands of Costa Rica is more closely related to the South American subspecies than to that from the Atlantic versant.
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