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If both test and control (comparator) animals in an experiment are consuming a back-ground level of toxins, any additional effect arising from the test-substance (a GM food or a pesticide, for example) will have to be massive to stand out above the noise.
The upshot is that regulators can wrongly conclude a substance under test is safe because the 'science' shows there's no significant difference between the exposed animals and the unexposed controls ... because they're both sick.
There's lots of evidence suggesting unjustified conclusions of safety and unjustified dismissals of harm have resulted from compromised chow.
For example, DuPont obtained regulatory approval for its GM oilseed rape using a feeding study with chow from Purina company. Purina feed was found on analysis to contain around 12.8% GM soya and 35.6% GM maize, and it wasn't labelled GM.
Time and time again, the German authorities dismissed evidence of birth defects in laboratory animals fed glyphosate (a weedkiller accumulated by most GM crops) on the basis that the levels fell within 'historical control values'. Since no one knows what combination of toxins these historical 'control' animals may have been fed, a wide range of ill-effects is to be expected, and treating poisoned rodents as 'normal' is simply wrong.
Professor Gilles-Eric Séralini's long-term rat feeding study on glyphosate-tolerant GM maize which raised concerns about cancerous effects, was criticised because the rat strain used has a reputation for 'spontaneous' tumours . It now seems more likely that there were other cancer-causing agents in the rat-chow, and that the glyphosate and/or GM maize tested had , indeed, an additive tumour-promoting effect.
Dr. Irina Ermakova's lambasted preliminary feeding study on rats during conception, pregnancy and lactation should be reviewed in a new light. This study was unusual in that the rats were fed standard chow plus soya paste in GM and non-GM form; thus, whatever background toxins were present in the chow were the same for both test and control rats. In this case, consumption of GM soya was so catastrophic to the rats that there was no third generation. Critics of Ermakova's study claimed the level of mortality in controls fed only on chow was an indication of poor husbandry. This may be true, or more likely may be a measure of the background toxins in the chow. UK regulators dismissed any significance in the Ermakova experiment on the grounds that another study with very different end-points and using mice demonstrated no harm: perhaps the mice were simply being fed an inferior quality of toxin.
As GM Watch pointed out:
"Claims of safety based on such badly controlled studies are not worth the paper they are written on".It further warns that there's a huge danger that, rather than doing some real, properly controlled, science, GM proponents and regulatory agencies who are finding their job too tough may "exploit the new findings to argue for abandoning Europe's currently mandatory requirement for animal feeding studies with GMOs on the grounds that it is too difficult to control the feed contamination content". Ditching animal-testing is an outcome GM-proponents have long pushed for, and would suit US representatives trying to negotiate the TTIP very nicely .
It's time to demand that feed-manufacturers who are supplying labs and breeders (and also your pets) clean up their act. If there's any suggestion of it being too difficult, remind them that feed from Italy was found to be uncontaminated by GM: if the Italians can do it, so can everyone else.
 POISONED LAB-RATS ARE NORMAL - August 2015
 GM MAIZE IS NOT SAFE TO EAT - October 2012
 TTIP IS ABOUT GM - August 2015
- Laboratory rat feeds contaminated with pesticides and GMOs, GM Watch 17.06.15
- Irina v. Ermakova, 2009, Influence of genetically Modified-SOYA on the birth-Weight and Survival of Rat Pups: Preliminary Study, Russian Academy of Natural Sciences 'Modern problems of science and education', www.mindfully.org