|Species Differences in Metallic Poisoning|
|Metal||Type of Poisoning||Fraction of the Rat Toxic Dose proving poisonous to humans|
Then during the 1960s came the thalidomide disaster which left thousands of children crippled and disabled. The Australian obstetrician William McBride had first been alerted to the drug’s effects after seeing three babies born with very unusual birth defects. Unfortunately his warnings to the medical profession were delayed because he tried to ‘confirm’ his observations by testing the drug in mice and guinea pigs, both of whom proved resistant.(5) Only after seeing further human cases did McBride publish his findings.
Subsequent tests with thalidomide revealed ‘extreme variability in species susceptibility’(6): mice could tolerate 8000 times the dose which harmed human babies.(7) Even New Zealand White rabbits, one of the few species eventually found to mimic the birth defects observed in people, had to be dosed with 300 times the amount of drug.(8) Despite the failure of animal tests, public demand for improved safety led to a massive rise in animal experimentation. As an editorial in The Lancet explained, ‘The mirage of a truly "safe" drug has dominated public expectations, and governments have responded by demanding ever more costly and time-consuming screening of potential agents before tests can be started in man.’(9) Many other substances including pesticides, food additives and cosmetics also came under increased scrutiny: the result was more and more animal tests. Today toxicology accounts for approximately 20% of all animal experiments.’(1)
Increasing the use of animals will not solve the problems if the method is fundamentally flawed. Animal tests are especially undermined by species differences in the speed and pattern of metabolism, or the way in which a substance is processed by the body. These differences are the rule rather than the exception.(10) If animals metabolise a drug more quickly than us, they are likely to miss harmful effects resulting from longer exposure. The Table shows how great these differences can be.(10) >>
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