Pork tapeworm: a parasite that afflicts poor and marginalized farmers and rural communities in 56 countries around the world. Matt Dixon examines how a One Health strategy could reduce infections and prevent a major cause of epilepsy in low- and middle-income countries, neurocysticercosis.
Colored scanning electron microscope image of the head of a pig tapeworm, showing the hooks it uses to cling to its host. Image courtesy of Steve Gschmeissner: https://theworldcloseup.com/
Endemic zoonotic infections, which jump between animals and humans and are present at a stable level of infection in the population, represent a substantial and lasting health and economic burden. They often have a disproportionate impact on marginalized communities in low-income countries; it is therefore essential that these endemic zoonotic infections are not overlooked. Pork tapeworm is one of these zoonotic infections, widely endemic in Central America, South America, sub-Saharan Africa, and Asia. Taenia solium.
the life cycle of T. solium is complex. It passes through pigs as a larval stage infection and humans, where the adult tapeworm grows and lives in the human intestine. Humans can also develop a larval stage infection, in which cysts reside primarily in the central nervous system and can lead to chronic infection and morbidity, a condition called neurocysticercosis (NCC). Several studies have identified NCC as the leading preventable cause of epilepsy in low- and middle-income countries, accounting for about one third of epilepsy cases in countries where T. solium is endemic.
T. solium is a complex One Health problem that develops through close interaction between pigs, humans and places.
One Health has been defined as:
“The recognition that the the health of humans, animals and ecosystems is interconnected“.
T. solium provides a clear example of how these three elements – humans, animals and ecosystems – are so closely linked (see figure). A typical endemic area includes pigs roaming free in rural farming communities, which exposes pigs to infectious stages of T. solium in the environment. In many communities where the disease is endemic, humans often practice open defecation, which subsequently results in the release of infectious stages into the wider environment. Poor sanitation also leads humans to ingest T. solium eggs and this ultimately leads to NCC infection. Ineffective meat preparation and minimal inspection of carcass meat results in human exposure to T. solium larvae in infected pork. Thus, although the disease system is clearly complex, the involvement of humans, animals and the environment in transmission processes also offers a range of different options for control.
Effective solutions, especially in the short term, will require a cross-sectoral approach involving interventions in pigs, humans and the environment.
The World Health Organization (WHO) New Neglected Tropical Disease (NTD) 2021-2030 roadmap sets key milestones for T. solium based on measuring the number of countries achieving “intensified control” in hyperendemic areas by 2030. Tools currently available for control include treatment of humans, treatment of pigs and vaccination of pigs. All these tools have proven effectiveness in the field. For example, a large pilot study in Madagascar demonstrated the ability of a three-year human mass treatment strategy using the drug praziquantel, to dramatically reduce the level of adult tapeworms that live in the human gut. These results are encouraging, but it is important to note that the infection rebounded quickly after stopping treatment. Combined treatment and vaccination of pigs in Tanzania has also dramatically reduce cysticercosis infection in pigs. Although interventions on pigs or humans separately are effective, achieving more ambitious control or elimination goals will likely be take a combination of these interventions in the hosts and in the environment.
A more intensive intervention program that can achieve the MTN roadmap T. solium short-term milestones might be a more attractive option for policymakers. Achieve these control efforts within a few years, or shift ambitions towards elimination rather than control (with elimination of transmission proven possible by northern peru) will likely require a One Health approach, targeting chemotherapeutic interventions in pigs and human hosts. Once disposal is achieved in specific geographic areas, further challenges may arise regarding the risk of re-importation of T. solium infection by movement of pigs and humans.
Implement a One Health strategy to T. solium does not necessarily restrict an approach to pharmaceutical interventions. Build on and combine efforts with ongoing activities in the area of water, sanitation and hygiene (WASH), focused on access to safe drinking water and encouragement of use latrines, will also provide an advantage T. solium control. The One Health principles also extend to the search for sustainable and innovative financing mechanisms to support control programs.
Existing large-scale NTD programs can also have an impact T. solium infection in co-endemic areas, such as human mass treatment programs that also use praziquantel to control schistosomiasis. Recent commitments to give treatments such as praziquantel, specifically for T. solium the fight against NTDs has raised the possibility of strengthening these existing NTD programs. We are currently evaluating the additional impact of schistosomiasis control programs in Uganda on T. solium, including the potential impact of additional praziquantel donations, which will provide insight into the effectiveness of this human-only strategy at country level, and whether a more comprehensive One Health strategy will be needed to meet the milestones in the scorecard. WHO’s road to NTDs.
The path to follow
To meet the challenge posed by a zoonotic infection, the One Health approach uses various strategies, such as:
- Understanding the complex system that keeps infections endemic
- Monitoring, control and disposal options
- Collaboration and sustainable financing structures
Take this approach to T. solium hopefully, will provide a more collaborative way to tackle this persistent global health problem in a sustainable manner and, as a result, improve the health and livelihoods of communities affected by the disease.