The use of inbred strains as test animals can be further questioned on the basis that they often develop spontaneous tumors in organs where cancers are not frequent in humans. For example, incidences of mouse liver tumors in 2-yearold B6C3F1 mice has ranged from 17.8 to 46.9 percent. In contrast, the death rate from liver cancer in the United States is about 0.005 percent.
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It has been conventional practice to test potential carcinogens using highly inbred strains of rodents. The rationale was the supposed superior reproducibility of results compared with those obtained from wild-type animals. However, that assumption can be questioned. At least three examples of genetic drift of inbred strains can be cited... Lifetime expectation [in an inbred strain of mice] of developing one or another form of neoplasm ['spontaneous tumour'] had risen from 10 to 80 percent.
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The principal method of determining potential carcinogenicity of substances is based on studies of daily administration of huge doses of chemicals to inbred rodents for a lifetime. Then by questionable models, which include large safety factors, the results are extrapolated to effects of minuscule doses in humans... The rodent MTD test that labels plant chemicals as cancer-causing in humans is misleading. The test is likewise of limited value for synthetic chemicals. The standard carcinogen tests that use rodents are an obsolescent relic of the ignorance of past decades.
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Results of the animal studies raise questions about the validity of federal regulations that are based on ad lib-fed inbred strains of rodents. Are humans to be regarded as behaving biochemically like huge, obese, inbred cancer-prone rodents?
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The standard carcinogenicity bioassay, which involves treating two rodent species for a minimum of 2 years, at a range of doses, is acknowledged to be an insensitive tool because of the background `noise' of spontaneous disease. Most strains of rat used in such studies have high incidence of pituitary and mammary tumours; some inbred rat strains frequently develop leukaemia or testicular tumours; mice strains show high incidence of malignant lymphomas and liver tumours.
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