How does deworming affect cognitive function?

In The Partnership for Child Development’s MAKWAMI project, in the Bagamoyo area of Tanzania,  a sample of 1200 children who were followed for 18 months to examine effects of worm infection on children’s educational achievement, cognitive abilities and learning potential.  Each child was given a battery of tests three times during this 18-month period and their attendance and achievement at school was recorded. Children’s health, nutritional and socio-economic status were also measured.  This study involved a larger sample of children, considered a wider range of explanatory variables, and followed children for a longer period of time than previous studies, in order to address this complex addition.

Another innovation of this study was the expansion of the range of psychometric tests used. Novel ‘dynamic’ tests were developed by our collaborators, Sternberg and others, in the PACE centre, Yale University, and used for the first time in this study . These tests involve three stages: First the child is asked to complete a psychometric test (typically a problem-solving task). After completion of the test the child receives instruction on how to improve performance on the test. Finally, the test is re-administered in a parallel form. One of the objectives of this test is to measure how well children respond to instruction, by comparing the performance before and after the teaching intervention and by observing how the performance improves over time.

In the study 273 children, aged between 9 and 14, were moderately-heavily infected with hookworm (>400 epg) or S. haematobium (>50 eggs/10ml urine) and were given treatment (albendazole for hookworm and praziquantel for S. haematobium) at baseline and at 3 months and 16 months after baseline. Four hundred and fifty similarly infected children and 194 uninfected children, all aged 9-14, were given placebo (vitamin B) at the same time points. Immediately before each treatment, children’s performance was assessed on three dynamic tests as well as a battery of 11 tests of cognitive function. Anthropometric measures and haemoglobin levels were also measured at each time point.

Figure 1 below shows the results of one of the dynamic tests. On the y-axis is a combined measure of performance on the pre- post-tests at each time point. The figure shows that children uninfected with helminths were able to learn; their scores improved steadily over time. However, those children still infected with worms improved to a lesser extent (p < .005). Similar results were found for the other two dynamic tests.

Figure 1.  Improvement in learning potential following de-worming in Bagamoyo, Tanzania (MAKWAMI Project):  Changes in syllogisms test scores in Treatment (N=273), Placebo (N=450) and Uninfected (N=194) children, aged 9-14.

These results should be contrasted with those in Figure 2. This shows the effect of treatment on a more traditional test of cognitive function (the digit span task) at the three time points. Clearly, the scores of the three groups change at the same rate over time, and there was no significant treatment effect on any of the 11 standard cognitive tests.

Figure 2. Scores in the digit span test at baseline and at 3 and 16 months after initial treatment for hookworm and S. haematobium.

These results show that treatment for helminth infection alone is not sufficient to improve cognitive function but does increase the ‘potential to learn’. This study supports the conclusion that the most effective way to improve the cognitive function of children infected with parasitic helminths is to combine anthelmintics with an effective teaching intervention to exploit enhanced learning potential.

In this study, treating children for S. haematobium and hookworm did not improve their performance on standard cognitive tests but did lead to an improvement in their potential to learn.

Reference:   Sternberg’, R.J., Grigorenko, E.L., Ngorosho, D., Tuntufye, E., Mbise, A., Nokes, C.A., Jukes, M.C.H., & Bundy, D.A.P. (2002)  Assessing intellectual potential in rural Tanzanian school children. Intelligence, 30, 141-162.