This isn't the only evidence which has suggested that parasite load has a substantial impact. Of course, there's the fundamental issue that this provides a very neat explanation for the Flynn effect since not only has IQ gone up across the board, the Flynn effect hit diminishing levels in the most wealthy countries, while still going on in the less well-off ones. In the United States at a state by state level, there's a correlation between parasite load and lower average IQ. That link also discusses how this effect remains even after one controls for wealth and education.
This is interesting, I wonder if there's anything to it: International variation in IQ – the role of parasites (paper) by Christopher Hassall of U. Carleton.
It strikes me as the sort of thing that could be as big an issue as lead in the environment. Raise the sanity waterline: improve health!