|ecoglobe news (20 August 1999)|
By Maggie Fox
The laboratory mice most favoured by scientists for testing chemicals, drugs and other things may not be so good for the purpose, scientists said on Thursday [19 August 1999].
The mice are bred so that they produce large litters -- but this very breeding makes them resistant to certain environmental chemicals, Jimmy Spearow, a reproductive geneticist at the University of California Davis, said.
Spearow has found the problem specifically affects environmental oestrogens -- chemicals used in a range of products from insecticides to plastics which are blamed by some researchers for disrupting hormones in animals and humans.
Controversy is raging over whether such "endocrine disrupters" are affecting human fertility and health -- but Spearow says the tests being used to determine this may not be accurate.
"This study and a related study in rats potentially explain why doses of estrogenic chemicals resulting in endocrine disruption in fish and wildlife failed to disrupt reproductive development in previous laboratory animal studies," Spearow said in a statement.
He said many of the tests using mice to show the chemicals are not dangerous used the strains of mice that produce large litters.
"The reason they produce large litters is because they are resistant to estrogens," Spearow said in a telephone interview.
Writing in the journal Science, Spearow and colleagues said the differences were genetic. That means that human beings probably have individual reactions to the chemicals, as well.
"You are going to find that different people react differently, too," Spearow said.
And that will have implications for setting standards for how much of each chemical is safe.
"Do we want to protect just the most resistant or do we want to protect everybody, including the most sensitive?" Spearow asked.
Scientists started getting concerned about chemicals that act like hormones after reports of animals with deformed genitals -- such as fish and alligators.
Last year the Centres for Disease Control and Prevention (CDC) found a doubling in the numbers of cases of hypospadias, a birth defect involving the penis, and childhood cancer rates are rising by about 1 percent a year in the United States.
Sperm counts among men may or may not be falling, depending on which report is cited, and there is a reported trend toward early puberty in girls -- all things that could be caused by chemicals in the environment, or by something else.
The Environmental Protection Agency (EPA) has set up an office that is studying whether chemicals are to blame. It will screen thousands of suspect chemicals.
Spearow and colleagues recommend that the EPA watch which laboratory animals are used in the tests.
His team put oestrogen implants into various strains of male mice and found big differences in their response. In some mice, the testes were unusually small, and in others there was barely any effect. The mice bred to have large litters were especially unlikely to be affected by the oestrogen.
Spearow says obviously more study is needed.
"I propose to look at other stages of development," he said. This includes animals exposed to the chemicals while still in the womb. He also wants to find the genes involved and find out how they work.
"This has much broader implications than simply endocrine disruption," Spearow added.
"It has got indications as to what doses of contraceptives we want to use for different individuals. It may be very important to individualize that dose. The same thing may be true for oestrogen replacement therapy in women and how we treat prostate and breast cancer."
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