In low and middle income countries, about 1.2 billion people are infected with roundworm (Ascaris lumbricoides)
, and more than 700 million are infected with hookworm (Necator americanus
or Ancylostoma duodenale
) or whipworm (Trichuris trichiura
Infection with intestinal nematodes is linked with poverty because of its association with unsafe disposal of faeces, in which the infective stages develop.
Infection can occur in all age groups but is most common in school age children. Though infection can be fatal,2
the major burden of disease is due to its insidious effects on physical and cognitive development during childhood.3
Anaemia, for example, is commonly associated with infection and can impair cognitive ability.4
In areas of high prevalence of infection in East Africa, 15-25% of anaemia in schoolchildren is due to hookworm infection.5
In this week's BMJ
, a systematic review of randomised trials by Gulani and colleagues assesses the effect of routine deworming on haemoglobin concentrations.6
It finds that deworming increases haemoglobin by 1.71 g/l (95% confidence interval 0.70 to 2.73), which could translate into a small (5-10%) reduction in the prevalence of anaemia. However, some elements of the study design suggest that this may be an underestimate of the impact, and that the results may have broader implications for practice.
Most deworming programmes assume that it is unnecessary to diagnose the specific infection as the commonly used benzimiadazoles are effective against common worm species. This can make it difficult to assess the results of clinical trials because the species of worm profoundly influences the risk of anaemia.
Hookworms adhere to the gut mucosa, feed on blood, and leave areas of intraluminal microhaemorrhage when they detach. Daily blood loss due to A duodenale
is estimated at 0.2 ml per worm, equivalent to 100 ml in heavy infections. This is about 10 times greater than with N americanus
infection, and surveys in Africa confirm that anaemia is more common with A duodenale
Blood loss during whipworm infection comes mainly from the inflamed gut mucosa, and at 8.6 ml/day is less than with hookworm. Blood loss is not typical of roundworm infection, and it is unclear whether the low serum retinol and serum ferritin concentrations associated with fat malabsorption result in anaemia.8
Therefore, the review by Gulani and colleagues does not, or perhaps could not, differentiate between the effects of different worm species in contributing to anaemia and the potential impact of deworming.6
Not differentiating between species and anthelmintics also makes it difficult to assess the effects of specific drugs. The World Health Organization recommends the use of albendazole, mebendazole, pyrantel, and levamisole. Of the 14 studies included in the review, one used an anthelmintic that is no longer recommended (bephenium hydroxyl naphthoate), three used mebendazole, and 10 used albendazole. While both benzimidazoles have similar high efficacy against roundworm and moderate efficacy against whipworm, single dose mebendazole is much less effective against hookworm, with cure rates typically below 60%.9
In almost a third of the trials, treatment (dose and choice of drug) was less than optimal for the hookworm infection that would probably contribute most to anaemia. Thus, the review probably underestimated the effect of deworming on anaemia.
So what practical lessons does the review offer? Firstly, the number of doses of anthelmintic did not predict effectiveness. This suggests that less frequent and therefore cheaper approaches may be adequate; this should encourage a review of current guidelines on the frequency of anthelmintic treatment in the community. Secondly, analysis of the studies (around half) that gave iron supplements and anthelmintics found that coadminstration of iron significantly increased the size of the effect of deworming on anaemia. Available evidence suggests that removing the source of blood loss alone is unlikely to replenish iron stores in the short term,10
and the review provides more evidence of the value of combining deworming with iron supplements.11
The lack of iron supplementation in half the studies would also tend to underestimate the effectiveness of the approach in improving haemoglobin concentrations.
Because the review tended to underestimate the impact of deworming, and given the remarkably low cost of deworming and iron supplementation,12
combining the two approaches in programmes for young people should be encouraged. Given the high prevalence of both anaemia and worm infection in pregnancy,13
a similar review is needed in pregnant women.