This study tested the hypothesis that the recreational use of tobacco helps defend against parasites, namely helminths or parasitic worms. They investigated this relationship among the Aka, a remote population of foragers in Central Africa. They found that tobacco use was associated with lower worm burden (adverse health outcomes from parasites). They also found that populations who metabolized nicotine more slowly had lower burdens that those who metabolized them faster. The results support the hypothesis that substance use may help defend against parasites.
There is a history of plants and herbivores using plant neurotoxins to defend against parasites. Plants have toxins to deter or poison parasites. Herbivores self-medicate by eating neurotoxic plants to expel intestinal parasites. Similarly, humans have a long history of using plants for medicinal purposes such as protection against parasites. So, it is possible that the recreational use of psychoactive plant drugs can be partly attributable to their antiparasitic properties.
The impact of tobacco use on human helminthiasis, an infestation of parasitic worms, has never been investigated. Approximately one billion people are infected by one or more helminths. Nicotine, an anthelmintic that destroys parasites, is extensively used by the global population via smoking. Investigating the relationship between parasitic worms and smoking could be critical to the history of helminth diseases and the increase in smoking in the developing world. The researchers chose the Aka because they had locally available nicotine and a high prevalence of helminthiasis.
The sample was comprised of 206 men and 44 women. Saliva samples were used to measure nicotine exposure. Stool samples were used to measure parasitic worm burden.
97.9% of the total sample tested positive for at least 1 species of helminth infection. They found that high levels of nicotine were strongly negatively correlated with worm burden. This aligned with their hypothesis. Their results supported the idea that nicotine exposure was regulated in response to worm infection. Other causal mechanisms cannot be ruled out, of course. Smokers with slow-metabolizing CYP2A6 alleles (breaking down nicotine more slowly) should have higher nicotine exposure. This meant that they had lower worm burdens than those with normal or fast-metabolizing alleles.
Discussion & limitations
Tobacco use motivated, in part, by therapeutic benefits imply that signaling pathways exist between the immune and nervous systems that might be exploited to reduce smoking. Limitations and uncertainty about the mechanisms of this study must be kept in mind, of course. The project provides a new model system to test evolutionary theories of self-medication by humans and other primates. These results are the first to suggest an important relationship between smoking and helminthiasis, two of the world's most pressing health problems.