Cats, Bats, and Rats: Harbingers of Diseases of Disparity

On Monday, I attended a symposium, titled "One Health: Interface of Human Health/Animal Diseases", at the American Society for Tropical Medicine and Hygiene (ASTMH) annual meeting (aka #TropMed17). This was one of many sessions I attended, but one of the few that really stood out to me.

I love the concept of One Health, which emphasizes the interwoven nature of the environment, animals, and humans, where a change to one aspect of the world will affect the other surrounding environments and populations. This is a vital concept when it comes to global health, as we often focus on human disparities without working to improve the person's environment for sustainable health and change. As you can imagine, One Health is a critical component when describing the lifecycles of zoonotic diseases, which infect both animals and humans. I wrote a magazine article for The Biochemist about the impacts of One Health on disease emergence. You can find that here (e-zine) or here (PDF), for free.

Within the One Health symposium, each speaker outlined an important animal exposure for each infection, whether parasitic, viral, or bacterial. For example, the first speaker discussed an outbreak of hantavirus in Peru during the creation of a new interoceanic highway that runs across Peru and Brazil. Deforestation and construction created a new opportunity for exposure to hantavirus for construction workers and residents in the area. Here's some more info on that, if you are interested.

Image of bats emerging from the Swedagon Pagoda in Myanmar, borrowed from an article on Biodiversity via ResearchGate

Another speaker spoke about emerging pathogens in Myanmar due to exposure to bats through the use of caves for economic (mostly the creation of tours for tourists) or religions purposes (think shrines in caves). Myanmar apparently has 22% of world's species of bats, meaning it has a diverse population of bats, and they are everywhere.

The two talks that I really enjoyed covered parasitic infections that are a global issue affecting primarily low income populations, even those in the United States.

Beautiful illustration borrowed from The New Yorker

Angiostrongylus cantonensis, or "rat lungworm", is a parasitic nematode that is found throughout Southeast Asia and the Pacific basin. Rat lungworm causes eosinophilic meningitis in humans. Humans are exposed to A. cantonensis through ingestion of infested foods, like undercooked or seafood, fish, frogs, vegetables, or snails.

An apple snail, image borrowed from Hidden History

 Apple snails act as a reservoir for A. cantonensis throughout Southeast Asia. Upon investigation, the larval stage of the nematode can be found invading the lungs of the snail. Adult A. cantonensis live in the pulmonary arteries of rats, and larvae migrate to the pharynx, where they are swallowed by the rat and dispelled in the rat's feces. The larvae are then ingested by the apple snail, or other specific mollusk species.

The apple snail can be purchased to eat from stands along the road in Laos, Cambodia, and Thailand, or the larvae can infest other aquatic animals. Once ingested by a human, the nematode travels to the brain through the blood stream, and then dies shortly after. The presence of a foreign body in the central nervous system causes a flood of white blood cells, specifically eosinophils, which triggers inflammation of the meninges. While this can clear on its own, some people have been seen with significant neurological dysfunction, some of which are lifelong, or death. This parasite is most commonly associated with Southeast Asia, in the past few years, has popped up in Hawaii.

Another speaker detailed a project focusing on parasitic load in public parks throughout New York. Toxocara spp. can infect dogs (Toxocara canis) and cats (Toxocara cati), and can also infect humans if the eggs are ingested. I think a lot of people in the US believe that parasites are a problem in other countries, but there are many types of parasites that can infect (and infest) people in urban and rural areas of the US.

Toxocara roundworm image from the CDC

Toxocaraisis, or an infection caused by Toxocara spp., is a roundworm infection that causes symptoms depending on where the larvae migrate in the body. Visceral toxocariasis occurs when the larvae migrate to various organs, most commonly the liver or central nervous system, causing fever, fatigue, coughing or wheezing, or abdominal pain. When in the central nervous system, psychological symptoms and neurological dysfunction can occur. Ocular toxocariasis occurs when the larvae migrate to the eye, causing symptoms relating to the eye, such as vision loss, inflammation of the ocular tissues, or damage to the retina that can cause permanent blindness. The CDC estimates that up to 70 people are blinded each year from Toxocariasis. Many people clear a Toxocara infection without experiencing any symptoms, and have a strong immune response.

This particular project tested soil and sand samples from parks because children are frequently exposed by ingesting soil and sand from parks, and from putting contaminated hands in their mouths. Infection during childhood can affect childhood development, and most people are never diagnosed if they don't present with severe symptoms. Many psychological symptoms aren't linked to parasitic infection, so they aren't treated appropriately!

Some random kid from the internet eating dirt.

More importantly, the presenter emphasized how this is an infection that is more common in low income communities. For example, out of all the areas tested throughout New York, parks in the Bronx had significantly high contamination levels (something like 67% of parks tested in the Bronx had positive samples!). The CDC estimates that ~14% of people in the US have antibodies against Toxocara spp., suggesting that tens of millions of Americans have been exposed to these parasites at one or more points in their lives.

The presenter's data, combined with the CDC's estimates of exposure in the US, suggests that a majority of those exposed to Toxocara are from disadvantaged communities, where public health initiatives fall short. This means that health services are not doing their jobs to protect all people within a community. We can't just protect select groups. All people have a right to be informed of what they may be exposed to in their local environment, and what local organizations (whether nonprofits, government organizations, or research initiatives) are doing to minimize risk!

If you are interested in this specific project, here is an older youtube video of the speaker describing her research:

Its important to remember that the concept of global health affects people around the globe, especially children, no matter where they live or how developed their environment is. Public health interventions have helped reduce the number of diseases that many people in developing countries would be exposed to, but no one lives in a sterile environment!