HERD MIXING AND HERD MENTALITY

In one of the more impressive history books I have read, W.H. McNeil points out that among the conscripts of the 19^th century French army, robust peasant lads had a far higher death rate than undernourished weaklings from city slums, because the latter had developed immunity by exposure to disease in the filth of the cities, whereas the peasants had been leading the healthy country life. This is known to epidemiologists as "herd mixing:" a disease shows an increased incidence among a population where newcomers are exposed to infections to which the oldtimers are immune.

Herd mixing has been proposed as an explanation of the puzzl-ing rise of child leukemia in England and Wales since 1931, for leukemia deaths have been more frequent in "rural" (newly estab-lished) towns where the population mixing is greater than in "overspill" towns or suburbs of established cities.

But in a recent issue of Nature (1/3/91), Prof. Wolff of University College, London, contradicts this explanation by pointing to the more than 1 million children evacuated from the cities to the country during WWII. Had the herd mixing effect been important, it would have led to a surge in leukemia deaths during the war years, but this was not the case either during the war or the reloca-tions following it. Instead, Prof. Wolf suggests an environmental factor, namely the benzene in automobile exhausts, which in-creased with rising car use, noting that studies in Denver, Colo., show a higher likelihood ratio (probability of yes to probability of no) in higher residential traffic density.

The reason why this is of interest to this publication is that it is now fashionable for Sunday supplements and their resident scholars to blame child leukemia on either nuclear plants or near-by power lines. There were no nuclear plants in England or anywhere else in 1931, and the man who published the car traffic / child leukemia data from Denver was none other than Prof. David Savitz, who is often quite incorrectly invoked as having proved a link between power lines and child leukemia.

[More: Wm. H. McNeil, Plagues and Peoples, Anchor/Doubleday, New York, 1976; S.P. Wolff, "Leukaemia and wartime evacuation," Nature, 1/3/1991, p.23; D.Savitz and others, Work Environm. & Health, vol .15, pp. 360-363 (1989)].