Investigating the impact of polychlorinated biphenyls (PCBs) - carcinogenic compounds - and methylmercury (MeHg) in fish on rat pubs, the researchers at the University of Illinois at Urbana-Champaign found that these toxins might affect motor function.
"Because people are exposed to these toxicants by eating fish taken from ecosystems where these chemicals accumulate, our findings suggest that we should seriously consider the possible impact of their additive toxic effects on human health," said Susan L. Schantz, a professor of veterinary biosciences at the University of Illinois at Urbana-Champaign.
Previous laboratory studies had suggested that the two chemicals act together to impair nervous system function. A study in February's issue of the Journal of Pediatrics found that exposure to methylmercury causes heart damage and impairs brain growth.
The new study - published in the February issue of Toxicological Science - shows that motor skills were not significantly affected by methylmercury exposure alone, but when paired with PCBs the combined effect during development 'dramatically impacted the pups' skills in one of three motor tests'.
In the study, female rats were exposed to just PCBs or just MeHg or to both chemicals, beginning four weeks before breeding and ending when their pups were weaned. None of the female rats showed signs of toxicity, said Schantz.
At birth, the pups of mothers exposed to methylmercury alone did not differ in weight from the control group. The birth weight of pups whose mothers had been exposed to just PCBs or both chemicals was slightly, but significantly, less than the control animals.
At weaning, the weight of pups exposed to PCBs was as much as 11 per cent below that of control animals, while the pups in the PCB-MeHg exposure group weighed as much as 15 per cent less than controls.
Two months later, one male and female pup from each litter were tested for the next four weeks on their abilities to navigate vertical ropes, parallel bars and various speeds of rotating rods. At the end, female pups whose mothers had been exposed to MeHg were slightly impaired on the rope-climbing test, while their male counterparts actually had less hind-limb slips on the parallel bars. Overall, the results of the MeHg-exposed pups were not significantly different than that of the control animals.
However, on the rotating rods the impact of exposure to both PCBs and methylmercury became clear. As the speed of the rods exceeded 25 rpm, the pups, regardless of sex, that were exposed to both toxicants during their mothers' pregnancies slipped significantly more often than their control counterparts. Exposure to either of the chemicals alone did not significantly impact performance.
But Schantz said that the tests showed that PCB exposure contributed more than the methylmercury to the pups' deficits, although the low dosage of MeHg used in the study (1.20 milligrammes compared with the 2 to 18 mg used in previous studies of motor skills) may explain why, she added.
It could be, concluded the researchers, that the chemicals have independent mechanisms of toxicity or they each act by means of the same mechanism but with greater impact together.
PCB's hit the press earlier this year when a Canadian study published in the January issue of Science alerted the world to levels of cancer-causing toxins present in farmed salmon. Raising the alarm, the Canadian researchers found that carcinogens PCBs (polychlorinated biphenyls) and other environmental toxins are present at higher levels in farm-raised salmon than in their wild counterparts. The UK TDI (Tolerable Daily Intake) for dioxins and dioxin-like PCBs is 2 pg WHO-TEQ/kg bodyweight/day.
In a bid to reassure consumers, European Commissioner David Byrne told MEPs last month that the Commission is preparing to expand current legislation 'to integrate some PCBs with toxicological effects similar to dioxins.' The organochlorine pesticides detected in the study published in Science have, since long ago, been banned in the European Union.