Conservation,  Featured,  Health,  Science

A Toxoplasma’s Journey: From Cats to Sea Otters

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.

Toxoplasma gondii
Photo Credit: AJ Cann, licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.

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 itAccording 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.

Southern sea otters (Enhydra lutris nereis)
Photo Credit: U.S. Fish and Wildlife Service

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?

Kelp forest
Photo Credit: Stef Maruch. licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.


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.



  • Z. Haqqani

    While I was aware of T. gondii and its effects on humans and rats, before reading this article I had no idea how deadly it was to other organisms, especially those off-shore. This is devastating! In search of a solution for this problem, a few of the above comments (by tbrown110, mashim331, and Sara Eisen) mentioned the possibilities of a vaccine for T. gondii and discussed who it should be given to. My question is, how effective would a vaccine really be in this case? Vaccines that have actually hindered the spread of, and/or helped us eradicate, particular diseases (eg. Smallpox)were successful because there were only particular species affected. With T. gondii, however, there doesn’t seem to be a boundary. It has spread from felines and rats to farm animals and now the ocean. Thus, from my perspective, a vaccine for T. gondii would not be as helpful or effective as it may seem.

  • Jonathan Rosenthal

    I first wanted to say how informative this article was. I had truly never studied or read much about T. gondii before I had read this article. It is frightening to think how a microscopic infectious organism can invade a wide variety of other organisms (humans, rats, cows, cats, sea otters, snails). If you think about it, it’s as if T. gondii has this plan from the beginning, in which it uses rats indirectly to infect sea otters. Clearly, medical personnel have a means of diagnosing T. gondii in humans, as well as other organisms. This leads me to the second subject I wanted to bring up: whether scientists or researchers have discovered (or are in the process) any sort of pharmaceutical or therapeutic means of combating T. gondii? Is there any sort of immunization that can be injected into either of the parasite’s hosts, be humans, cats, or even sea otters? I hope to read more articles in future regarding T. gondii and its effects in the multitude of organisms it uses as a host.

  • Sara Eisen

    In my microbiology class, I learned about Toxoplasma gondii, but only very briefly. I knew that it came from cat feces, but I didn’t know how this happened. After reading this article, it made sense that cats can contract this disease from infected rats, but sea otters contracting this disease was surprising. I first thought, how is this happening? A parasite can’t survive on its own when it transfers from land to sea. I then read about how the runoff is being carried to the ocean and infecting the otters food supply. I thought water was purified, so why are we having this problem? Maybe what’s happening is that the water is being purified, but the the toxoplasma oocytes are not dying. I think in order to help the sea otters we should be purifying our water correctly before dumping it into the ocean.

  • dpitts5

    This is a sad, but interesting story. Its sad to see a disease affect millions of people as well as marine life forms in a detrimental fashion. However I find it interesting that this protozoan does not have an adverse affect on cats. I think we should look into this more deeply to get an understanding as to why the cats seem to be unaffected. Does this parasites affect unborn kittens? What does T. Gondii do during the cats pregnancy? Answering these questions thoroughly might help us to understand how the parasite acts during the cats pregnancy, which may help us prevent adverse affects in first time pregnant women. This would be significant information in developing a vaccine. If we are able to make this revolutionary discovery I think it should be mandatory that women get a T. Gondii vaccine to prevent the spread to unborn children.

  • Nhan Ta

    The article mentioned that T.gondii has been around for a very long time, and that it infects a large number of people, but it didn’t go into detail about why the parasite has been so successful. Samantha Deochand had written about how the parasite lodges itself in tissues in order to evade the immune system, and I wanted to add that once it is in the tissue, it forms a cyst. In its cyst form, T.gondii does not elicit a host reaction, which explains why it can persist so long in its host without being detected. I’ve also read that sometimes parasites can break out of cysts, and infect new cells. In a healthy individual, immune cells can destroy these released extracellular parasites, thus preventing them from multiplying. However, in immunocompromised patients, the immune system is unable to get rid of the parasites, which explains why T.gondii causes severe clinical Toxoplasmosis in these individuals.

    Studies have also suggested that T.gondii injects a protein called ROP16 into the infected cell that prevents a strong host immune response. This is done to protect both the host and the parasite because if the immune response is too powerful, the results can be deadly. As a parasite, it wants to keep its host alive, and by using these methods, T.gondii can reside in an intermediate host for a long time until it encounters a cat, its primary host. It is tragic that this is not the case for infected sea otters.

  • tbrown110

    To know that one third of the human population have been infected with Toxo is eye raising. Reading this I assumed that this would reflect mainly on third world countries leaving developed countries such as the U.S. out of the mix, but that certainly is not the case. 60 million people in a country of 300 million is a significant number showing that one in five people carry the parasite; this is alarming to say the least. Another interesting aspect of the story is the parasite T. gondii itself. It is quite fascinating how it has the ability to control the rat and make it do something that is basically suicidal. This capability alone both fascinates and scares me. If it were able to do this with human beings with the masses it currently affects the consequences would be one of if not the most devastating pandemics ever seen by man. Hopefully these crafty bugs do not evolve and develop in such a manner. Also I believe mashim331 made a very good point with attempting to develop a vaccination for cats to prevent their infection with toxo. It may also be wise to administer the vaccine to farm animals along with a pesticide for vegetables because they are our prominent sources of food.

  • cns2392

    I find the ability of the Toxoplasma gondii parasite to invade the brain particularly interesting. In rats, the origin of the parasitic cycle, the brain is tricked to no longer fear cats and actually quite the opposite… Become attracted to the cat! Almost as if a reversal of inhibition, maybe this kind of parasite could have strange health benefits later down the road?

  • Sarah

    It’s interesting how T. gondii can spread from one species of organisms to another species of organisms so easily and so rapidly to the point where mammals, such as sea otters, that are expected to be at low risk for exposure to T. gondii are affected by this protozoan parasite. It is to no surprise that many people are exposed to the parasite. People in many countries around the world eat snails and kelp as a normal part of their diet and utilizes certain components of snails and kelps for other uses such as cosmetics. Without rigorous regulation, we are just as much as exposed as the sea otters, if not more. However, immunocompetent humans can live on without any symptoms and are able to live normally, indicating that humans have the ability to prevent the infection by T. gondii, something that the sea otters cannot do. Since both humans are sea otters are mammals, I feel that studying the immunological ability of humans may be applied to the sea otters and it may reduce the number of otters that develop meningoencephalitis.

  • mashim331

    Wow! It is scary how a parasite of cat- a land animal- can infect sea otters. All species, including human, are living in such a small world that each species interact with the other with or without knowing. Human are lucky because we have the knowledge to protect ourselves from diseases such as Toxoplasmosis. For instance, we ensure to cook meat thoroughly before eating, we wash off all dirt on vegetables, we wash hands after contact with cat litter, women even have toxoplasmosis test before or on pregnancy, etc. Despite many protections, we are still getting toxoplasmosis here and there nowadays. Then let’s talk about sea otters. They have no protection or whatsoever! In fact, sea otter can’t even stay away from Toxo. The only food they have is turban snails, which also get infected by T. gondi. So sea otters (if they could) technically have only two options: either die from food craving or die from Toxo! Also as the article mentions about T. gondi alters rat’s brain and makes it attracted to Toxo infected cat, there might be a possibility that sea otter actually enjoy infected turban snails more than the healthy ones. I really think human is the only one that sea otters can rely on from this lethal disease. We might start from the source of Toxo- the cat (a vaccine for cat maybe??) and definitely work on the water cleaning process. These are just my thoughts…

    • Sara Eisen

      I think you might be a little confused. The toxoplasma infected rats aren’t attracted to toxoplasma cats, they are just attracted to cats. The toxoplasma is then defecated from the cats because the cats ate infected rats. I hope my explanation helped a little.
      I do think you make a valid point about vaccines for cats. I have a cat and my vet has never said anything about a vaccine for toxoplasma. I then found a few articles on pubmed for the development on a vaccine. Turns out a big reason is that this vaccine is expensive. A lot of people, of course, don’t want to spend the money for expensive vaccines, especially since the cat doesn’t show symptoms. The main problem with the vaccine is that toxoplasma has a specific life cycle involving sexual reproduction in cats and a live vaccine is needed. This also means that the mutated vaccine could possibly go revert back to it’s previous harmful state. Overall, the vaccine is hard to control when making it, probably because this disease is from a parasite and not a virus.

  • Aaron Alcala

    It is very interesting to observe a parasite’s life cycle as it survives in various environments and lives off various hosts. In this case, Toxoplasma gondii is an obligate, intracellular parasite, which means the protozoan cannot reproduce outside the host cell. The parasite’s reproduction relies on the host’s intracellular resources. A big question arises from this – how did the protozoan evolve in the first place? T. gondii is able to asexually reproduce in various mammalian hosts, but is only able to sexually reproduce within the intestines of cats.

    It is interesting to think about how these parasites evolved to use mammals as their primary hosts. Furthermore, it is interesting to see how the oocysts are able to survive dramatic changes in environment and travel through various hosts, in order to repeat their life cycles once they finally reach a feline host. Studying a parasite’s evolutionary past could be useful, as it could reveal aspects important to their life cycles. This information could have applications in situations like this, where T. gondii affects many mammals dramatically, including humans.

    • mashim331

      I was also wondering why T. gondii only reproduce in cat as Plasmodium – which cause malaria – only reproduces in female mosquitoes. I guess these hosts make some sort of chemicals that trigger the meiosis of these protozoans that no other hosts provide. The making of oocyst is a smart move indeed. The thick wall of oocysts keep them surviving from desiccation and other harsh conditions such as stomach acid. Some oocysts are so strong that even fluorine in our tap water won’t kill them. Although T. gondi only reproduce in cats, they do multiply in number via mitosis throughout many stages of life and many hosts. This is another smart move to spread the disease fast.

    • Z. Haqqani

      You brought up some points there, Aaron! It is quite fascinating how these parasites have selected specific groups of animals as their hosts. Last semester, someone in my microbiology class asked a question that relates to this. We were talking about a protozoa that could take over a bullet ant’s body and use it for its own benefit, and my classmate asked, “How does this parasite, or any other parasite for that matter, know how to take over this organism? How does it know how to work the machinery of the body? Could it do the same thing in a human?” Our professor explained that it is all due to trial and error over the years. Each group of parasites tried many different methods to complete their life cycles and many different hosts, and whichever one worked for them, that’s the one they stuck to and perfected because that allowed them to stay alive and complete their life cycle.

      For the sake of completion, the answer to whether or not they could take over a human in the same way was, “No, it’d take millions of years, if not longer, to go from specializing in taking over one organism, to successfully taking over a human.”

  • mgnd88

    cat eats mouse, cat’s droppings and this toxo end up in the oceans, toxo gets stuck to kelp forests, snail eats toxo, sea otter eats snail and gets toxo…. who eats sea otter? is that the top of the chain? may be shark eats otter? if shark eats otter, man eats shark, and… and… and… when does it end?

    • tbrown110

      This is where the proper preparation of food comes into play if an individual undercooks or does not cook their seafood it would be very possible to continue the cycle and transmission of Toxo. Being the most cerebral animals on earth and having knowledge of the life cycle of T. gondii, methods of transmissions, pathology, epidemiology, etc. if we wish to stop the effects of the parasite we must take responsibility. This definitely includes food preparation as I mentioned earlier, but also the care of livestock and household animals. State mandated and regulated statutes can be implemented to maintain the health of the aforementioned animals. Ultimately the cycle ends when we actively attempt to bring an end to it.

  • wilsya

    WOW, sea otters are such cute marine mammals (all of them are, though!!!!) I could never think they are dying because of a parasite that is found in people. But then I have another WOW for the previous comments, I didn’t know anything about sea urchins and I have no idea what macrophages are, I guess it’s stuff for biologists may be???? I do care deeply about the environment though, and I feel very sad to find out that there are so many problems in the oceans. We do hear so often about plastic pollution of the oceans, but it’s rare to read about these parasites that kill marine life. The post says that the parasites get stuck to the sticky substance on the kelp, but then what else gets stuck there, it’s just this toxo or everything else that floats around? How much more deadly things are the sea otters eating, then?

  • Samantha Deochand

    I first learned about Toxoplasma gondii in my microbiology class. We discussed the parasite’s life cycle in detail, how it is transmitted to humans, and the effects it has on humans, pregnant women, and unborn babies. However, it did not occur to me that this parasite can also infect other animals, specifically marine mammals like the endangered southern sea otter. Tylah Hankerson explained the importance of the sea otter in the marine environment as a keystone species. As a parasite, T. gondii is expanding its victims from land and into the sea. I learned in microbiology that the parasite can be ingested by a macrophage but not digested. Therefore, it can continue to live inside the macrophage and uses the macrophage as a source of transportation. In addition, T. gondii evades the immune system by lodging itself in tissues. Because T. gondii has the ability to infect a vast amount of species, regardless of their habitats, more research needs to be conducted on the parasite’s interaction with the immune system. I would be interested in reading about reasons why marcrophages cannot digest the protozoan parasite and if the reasons are related to the fact that the parasite is a eukaryote.

    • Rebecca Park

      T. gondii avoids getting destroyed by macrophages by going inside the macrophage’s vacuole. This way T. gondii doesn’t get in contact with the lysosomes. Lysosomes are cellular organelles that contain acid hydrolase enzymes that dissolve waste. So I don’t think the macrophage can’t digest T. gondii because it is an eukaryote. I believe it’s because T.gondii are clever organisms that learned through trial and error where to hide throughout the years.

  • Tylah Hankerson

    I find this both sad and disturbing. I am very interested in marine life and how humans impact the sea. It is awful that these creatures are endangered and may become extinct due to this toxoplasma affecting them. I think as humans we tend to forget how our everyday activities can affect us and future generations. Sea otters are a very important animal in our ecosystem. Without these creatures, the kelp forests could be destroyed. Last semester I took a marine biology class and discovered that sea otters are a keystone species, meaning they are essential to the kelp forest ecosystem. Sea otters help keep the sea urchin population low. Sea urchins feed on kelp forest and if allowed to grow unchecked could destroy a whole forest. The destruction of a kelp forest could mean disaster for other marine animals and fisheries. Many marine animals call kelp forest home. The forests provide nurseries for the young, hiding places from predators, and food for many species of animal. The loss of these marine animals could harm those who rely on the presence of kelp forest for food. If runoff can be controlled along with preservation of wetlands, then there might be a chance of helping not only these wonderful and adorable sea otters but other marine life as well.

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