The tapeworm Taenia solium may be eradicable, but overlapping individual and societal risk factors complicate finding an effective method to do so. Mapping a network of causes and relationships may reveal important targets for preventing infections.
Imagine a horror film where a worm digs holes in your brain. This scenario isn’t as far-fetched as it might sound – larvae of the tapeworm Taenia solium, also called the solitary worm, can do exactly that. This worm can devastate the lives of those affected in areas of the world where few people have access to toilets and pigs roam freely. In 2017 Welburn and Gripper estimated in their systematic review The Causal Relationship between Neurocysticercosis and the Development of Epilepsy that these worms are responsible for around 30% of local epilepsy cases in endemic areas of Latin America, Africa, and Asia. Medicines exist that can kill the tapeworms, but the seizures and other symptoms may remain for life. It would be better to prevent the worms from infecting people at all. My research project maps information from multiple sources to find the most promising paths for eradication.
The International Task Force for Disease Eradication identified T. solium as a possible target in 1993. However, the worm’s complicated life cycle both helps and hinders eradication efforts. In brief, the parasite starts as an egg in the soil. When a pig eats that egg, it hatches into a larva that can live in several of the pig’s organs. People become infected by eating undercooked pork containing larvae, which then develop into adult tapeworms that live in the person’s small intestine and lay eggs. These eggs leave the infected human when the person defecates. If there is no toilet available, then the contaminated feces can mix with the soil, where the eggs can survive for months while waiting to be eaten by a pig. While adult tapeworms usually do not cause symptoms, people can become hosts to the larvae if they accidentally eat parasite eggs originating from human feces. This is when it can invade the brain and cause seizures.
Many ideas have been tested to eradicate T. solium. Repeated mass treatment of humans and/or pigs might reduce the number of worms until they can no longer survive long enough to infect new hosts. Additionally, each of the parasite’s many transmission steps (pig eats egg, human eats pig, egg enters soil, human eats egg) creates points where a change in pigs’ or people’s behaviour could stop transmission. However, lengthy studies would be needed to test how well these control strategies work because the worms and eggs can survive for months. Instead, most studies survey a community once to determine whether infections are more or less likely among pigs or people with a specific behaviour. The behaviours that are most closely linked with infection may be the behaviours that, if stopped, would prevent the most new cases.
The simple one-time survey is quicker than more complicated study types, but it is also harder to determine whether the behaviour actually contributes to acquiring the infection. Think about this type of study as a photograph of a room – you can see what the room was like at the time the picture was taken, but you can only guess why it looks the way that it does. In this photo, the proposed cause and effect are both items that may or may not be in the room. If the items are both present or both absent in the photograph, then it may be that one caused the other to be there. If both items are present or absent when taking photographs of different rooms (conducting surveys in other communities), you can be more certain that the items are associated. However, the common presence or absence of two items is not enough to determine whether, and how strongly, one item causes another. If there are muddy footprints on the floor of the room and an umbrella at the door, that does not mean that the umbrella caused the muddy footprints (or vice-versa) – both are likely because it is raining, a fact not captured by the photo. These common causes are called confounders. To make sure that behaviours of interest are truly causing infection with T. solium, it is essential to measure potential confounders in research studies.
Unfortunately, there is not currently a common method to identify confounders when designing or analyzing studies of T. solium infections. Most studies use statistical definitions that check whether adjusting for the suspected confounder alters the results meaningfully, while others use their expertise to determine which variables could be important common causes. In my research, I intend to use a more systematic approach to identify confounders. By closely analyzing previous studies, I can draw a network of causes and the relationships between them to identify confounders that would need to be adjusted for during analysis. I can then use this network to analyze previously collected data and see how the results are impacted. Using a consistent approach will make it easier to compare the results of different studies and identify the most serious risk factors for these worms.
This article was produced by Ellen Jackson, PhD student in veterinary sciences, Département de pathologie et microbiologie, Faculté de médecine vétérinaire (Université de Montréal), with the guidance of Marie-Paule Primeau, science communication advisor, as part of our “My research project in 800 words” initiative.