The Deworming Question: A Good Idea? Pointless? Detrimental? The Evidence Mounts…

Last month, a meta-analysis paper came out searching the medical literature through the year 2018, looking for evidence of the benefits promised by the worldwide efforts at deworming school children, recommended by all global advocacy organizations including WHO.  The authors, from the Department of Public Health and Policy at the University of Liverpool, UK,  state their objective is, “To summarize the effects of public health programmes to regularly treat all children with deworming drugs on child growth, haemoglobin, cognition, school attendance, school performance, physical fitness, and mortality.”[i]  Their conclusion is not what you  might expect…at all:

“Public health programmes to regularly treat all children with deworming drugs do not appear to improve height, haemoglobin, cognition, school performance, or mortality… Studies conducted in two settings over 20 years ago showed large effects on weight gain, but this is not a finding in more recent, larger studies. We would caution against selecting only the evidence from these older studies as a rationale for contemporary mass treatment programmes as this ignores the recent studies that have not shown benefit.”

(This, by the way, was an update from their 2015 paper…which concluded the same.  There is no health or cognitive benefit to deworming programs.  They did not, of course, look at whether there are any detrimental health effects of such programs, as that was beyond the scope of the paper.)

I’ve been considering sharing that paper for the last few weeks and after reading another really interesting paper on the effects of deworming today, I decided that the time has come.

The paper I finished reading this morning dates back to this past February, and these scientists – most of whom are at the National Institute of Health – looked at the effects of deworming on the bacterial microbiome.[ii]  There were a few really interesting points that I want to convey.

In this study the researchers collected stool from 5 villages in Kenya prior to administering anti-helminth drugs, then again 3 weeks after the drugs were administered and then 3 months after the drugs were administered.  They considered the presence of only 2 kinds of helminths:  Ascaris lumbricoides and Necator americanus (hookworm). They found that neither “…significantly altered the overall diversity of the microbiota.”  (They point out that it is likely that all participants at some point were colonized by helminths, even if not at the exact time of testing.  This likely plays a role in the fact that significant variability was not found among the participants.  In other papers, like the one I discuss in this post, the biomes of native populations with helminths was compared to people in industrialized countries and the differences in microbiomes was vast.)

Following treatment with the anti-helminth drug, “…there were significant increases in the proportion of the microbiota made up by Clostridiales and reductions in the proportion made up by Enterobacteriales…” and even significant changes in those individuals who did not have helminths on board at the start of the trial.  (This is how mass deworming programs work:  everyone is treated, without first testing to see who is colonized.)  However, they do conclude that clearing helminths (in this case, the N.americanus more than the A.lumbroicodies) from the human body does lead to alterations in the microbiome…and that the health effects of this must be considered going forward.

As far as we know as of this moment, it appears that changes to the bacterial microbiome depend on the species of helminth.  (Since most studies are done in native populations where helminths are still endemic, the exact size of the colony inhabiting the individuals is unknown.  Do larger numbers of helminths induce more changes?  We don’t yet know.)  The authors acknowledge this:  “…there is still no clear consensus of the impact of STH [soil transmitted helminths] on microbiota diversity and composition.  This could be in part because of differences in the STH species, prevalence, and intensity of infection in these different locations…”

In a section the authors entitle “Importance,” they state:  “Intestinal worms may have an important impact on the composition of the gut microbiome.  Without a complete understanding of the impact of deworming programs on the microbiome, it is impossible to accurately calculate the cost-effectiveness of such public health interventions and to guard against any possible deleterious side effects.”  They agree that “…the presence of  helminth infections has been linked with increases in microbial diversity.”  In their discussion, they also mention a study in which pregnant women in Africa with helminth colonies were given anti-helminth drugs and their children showed an increase in allergy risk. They conclude that this deleterious effect was the result of the drug use during pregnancy but neglect to mention that those authors also state, “The detrimental effects of treatment suggest that exposure to maternal worm infections in utero may protect against eczema and wheeze in infancy.”[iii]

These scientists conclude their paper stating, “…the next step will be to understand the impact of the identified differences on human health…” of mass deworming.  Prior to posting this, I have 58 articles on this blog regarding the established health benefits of the human macrobiome. Anyone want to put money on what those future studies will find?


[i] Taylor-Robinson, DC, Maayan, N, Donegan, S, Chaplin, M, Garner, P.  Public health deworming programmes for soil-transmitted helminths in children living in endemic areas.  Cochrane Database of Systemic Reviews. 2019.  doi: 10.1002/14651858.CD000371

[ii] Easton, AV, et. al. The impact of antihelminthic treatment on human gut microbiota based on cross-sectional and pre- and postworming comparisons in Western Kenya. mBio: American Society for Microbiology. 2019;10(2).   DOI: 10.1128/mBio.00519-19

[iii] Mpairwe H, Webb EL, Muhangi L, Ndibazza J, Akishule D, Nampijja M,Ngom-Wegi S, Tumusime J, Jones FM, Fitzsimmons C, Dunne DW,Muwanga M, Rodrigues LC, Elliott AM. 2011. Anthelminthic treatmentduring pregnancy is associated with increased risk of infantile eczema:randomised-controlled trial results. Pediatr Allergy Immunol 22:305–312.

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