By Roberta Attanasio
Toxoplasma gondii is a single-celled parasite that infects most warm blooded animals. In 2012, it landed in the news because of its ability to hijack the arousal circuitry of rats — the parasite activates a part of the brain normally engaged in sexual attraction. Rats infected with it are not afraid to approach cats and behave as they would in the presence of a sexually receptive female rat.
What happens next? The cat easily catches and eats the infected rat and, in doing so, also catches the parasite — the parasite is happy as it reproduces sexually only in cats. The parasite’s oocysts — sometimes called “eggs” — are later dispersed in the cat’s droppings and, from there, get to infect other animals, including humans.
In the news or not, Toxoplasma gondii — T. gondii or Toxo for short — has been popular for a long time. Indeed, one-third of people around the world have been infected with it. According to the Centers for Disease Control and Prevention, more than 60 million men, women, and children in the U.S. carry T. gondii. Most people do not show any symptoms. However, the parasite causes significant damage to the unborn child if a woman becomes infected for the first time during pregnancy. It also causes severe disease in people with a compromised immune system.
Now, let’s add a few more elements to the cat-becomes-infected-and-infects-people story. Cats become infected by eating animals that harbor the parasite in their tissues, for example infected mice, rats and birds. Then, infected cats shed large numbers (hundreds of millions) of parasite oocysts in their feces for one or two weeks. Once in the environment, the oocysts infect not only animals such as mice, rats and birds, but also farm animals such as cows, sheep, pigs, and chickens. People may become infected by eating undercooked meat or unwashed vegetables or, as we’ll see later in this post, uncooked or undercooked seafood. In addition, people may become infected through other routes.
T. gondii also infects marine mammals and causes significant mortality in southern sea otters (Enhydra lutris nereis), an endangered species. How does T. gondii end up in sea otters? Researchers at the University of California, Santa Cruz and at the University of California, Davis, designed an elegant study to answer this question. The study — A New Pathogen Transmission Mechanism in the Ocean: The Case of Sea Otter Exposure to the Land-Parasite Toxoplasma gondii — was published last month (December 18, 2013) in the open-access, peer-reviewed scientific journal PLOSone.
Southern sea otters live in giant kelp forests along the California coast. Kelp forests — one of the most productive ecosystems in the world — are composed of dense strands formed by kelp (large brownish seaweeds of the order Laminariales) along rocky coastlines.
As we already know, cats shed enormous numbers of Toxo oocysts in their feces. Some oocysts may reach coastal waters via freshwater runoff. Degradation of wetlands, now common in many coastal areas, results in increased transport of Toxo from land to sea. Once in coastal waters, the oocysts may survive for at least 24 months and infect marine organisms that feed by filtering seawater through their gills — shellfish, such as mussels and oysters, and fish, such as anchovies and sardines. These shellfish and fish may then transmit T. gondii to marine mammals.
However, southern sea otters are picky eaters — they like to eat mostly turban snails and, indeed, they get infected with Toxo by eating these organisms.
Here comes the mystery — turban snails do not filter food particles out of the water as mussels, oysters, sardines and anchovies do — they scrape surfaces using a radula (a ribbon-like structure which has hundreds to thousands of backwardly pointing teeth) to ingest food particles that are attached to a substrate. How do they acquire T. gondii, then?
The researchers designed a laboratory experiment based on the use of a tank that contained a simulated kelp forest canopy along with minuscule spheres mimicking the Toxo oocysts. They found that oocysts attached to kelp surfaces — this attachment was made possible by the presence on kelp of complex, sticky substances. Such sticky substances are part of dense biofilms, which also include bacteria and a type of algae called pennate diatoms.
Thanks to this experiment, now we understand how Toxo goes from the water to the otters. Here is the short story — when the Toxo oocysts enter the kelp forest canopy, they attach to the sticky kelp biofilms. Snails graze the biofilms and, while doing so, catch the Toxo oocysts– in turn, the sea otters that prey upon snails get infected with the parasite and develop meningoencephalitis, a serious condition that often leads to death.
Results of this study are important not only because they explain how southern sea otters become infected with Toxoplasma gondii, they are also important because of the implications for other waterborne pathogens — these pathogens may be transferred to invertebrates that feed on biofilms and, from there, infect marine wildlife and human populations that eat the biofilm-feeding invertebrates.