The title of our new paper, in EcoHealth, is actually “The One Health Approach to Toxoplasmosis: Epidemiology, Control, and Prevention Strategies” but it is really about taking the control of the intracellular parasite that causes the disease toxoplasmosis seriously. The paper is an interdisciplinary effort that came out of a meeting organized by Grant Sizemore at American Bird Conservancy and led by A. Alonso Aguirre at George Mason University.
One Health is a transdisciplinary approach designed to unify efforts to address the health of people, domestic and wild animals, plants, and the environment. As a review, we cover the natural history of Toxoplasma gondii (it is a parasite that only reproduces in the gut of members of the cat family), then lay out the impacts on humans, domestic animals, and wild animals. The take-home message here is that the effects of toxoplasmosis are worse than you imagine and certainly worse than you would think if you listened to people who have a vested interest in downplaying its impacts. For example:
- Toxoplasmosis is the second leading cause of death among foodborne illnesses in the US;
- 1.1 million people are infected each year in the US;
- Infection rates globally reach up above 80% in some parts of the world;
- Acute infection poses risks to pregnant women, their babies, and the immunocompromised;
- Chronic infection, previously thought to be “asymptomatic” is now recognized to result in cognitive decline, increased overall disease, schizophrenia, depression, and suicide attempts.
The effects of chronic disease may come as a surprise, but the epidemiological evidence is growing and frightening. The mechanism makes sense once you know that the parasite lodges itself in the brain and messes with your brain chemistry. It makes mice attracted to cat urine, which is the parasite’s clever way of getting eaten by a cat so it can reproduce.
One of the key parts of our discussion of human infection is about the transmission pathways for the parasite. First, the route that is most well-known, is through the consumption of meat that has been infected with T. gondii. This has been the focus of most toxoplasmosis prevention efforts, which is to cook meat thoroughly and practice good hygiene when handling meat. And of course good animal husbandry which means keeping cat feces from contaminating food that could be consumed by livestock and away from any animals that could become food. The second route is transplacentally, from mother to child. The third route of infection for humans is through direction consumption of the parasite oocysts (kind of like eggs, but not exactly), which is now the major route for transmission.
We then review the significant impacts to both domestic and wild animals and then discuss the prevalence of toxoplasmosis across ecosystems. Because of their raw abundance associated with humans, domestic cats are the major source of ecosystem contamination. How to manage unowned and free-roaming cats is a challenge.
Even though it is not recognized by those making the decisions, animal sheltering policies that reject stray and feral cats at shelters as a means to reduce shelter euthanasia are responsible for increased outdoor cat abundance and associated risk to people, domestic animals, and wildlife from toxoplasmosis.
More research is needed on toxoplasmosis, especially in light of the increased importance of environmental transmission by oocyst as other transmission pathways are reduced through improved hygiene and cooking practices. We outline a list of research needs, including:
- Soil sampling on a global scale to understand the extent of ecosystem contamination and correlated with outdoor cat numbers;
- Investigation of the relationship of policies on unowned cats and environmental contamination by T. gondii oocysts;
- Vaccine development for livestock, humans, and wildlife;
- Improved treatment modalities for long-term impacts of infection in the eye, brain, and other organs; and
- Standardization of diagnostic tests for Toxoplasma infection.
Another necessary component of linking efforts across public health, wildlife health, and domestic animal health is education and cooperation in the associated professions. We see a need for human and veterinary health practitioners to improve communication so that even though the risks from tissue cysts are important, greater attention should be paid to reducing environmental transmission. For example:
- Prevent cats from accessing vegetable gardens, including community gardens;
- Keep children away from areas where cat feces may be found;
- Wash hands after working in any soil where cat feces may be found: and
- Wash hands after touching a pet that has access to the outdoors (infection from oocysts carried on dog fur has been reported).
Finally, we express concerns about veterinary programs that have developed externally funded “shelter medicine” programs that put out messages about management of stray and feral cats that undermine established science.
Progress in controlling toxoplasmosis will be difficult, especially given that programs promoting free-roaming unowned cats have attracted such significant funding and institutional support (although shelter medicine programs often are working at cross-purposes with researchers and veterinarians in their own schools). The stakes are high — human suffering from acute and chronic infection are at a scale that should prompt swift action and the future of some endangered species lie in the balance. A One Health approach provides a path forward that could break through to identify approaches that are necessary and nonnegotiable to improve health in humans, domestic animals, and wildlife.
The paper is Open Access, free to download and share.
Aguirre, A., T. Longcore, M. Barbieri, H. Dabritz, D. Hill, P. Klein, C. Lepczyk, E. Lilly, R. McLeod, J. Milcarsky, C. E. Murphy, C. Su, E. VanWormer, R. Yolken, and G. C. Sizemore. 2019. The One Health approach to toxoplasmosis: epidemiology, control and prevention in humans, animals, and ecosystems. EcoHealth, p. 1–13. ResearchGate link.