Toxoplasmosis: In Defense of the CatsteemCreated with Sketch.

in science •  3 months ago

Mountain lion larry moats us fish and wildlife public.jpg

The picture shows a mountain lion peering out at a photographer. Mountain lions go by many names, including puma, cougar and panther. All of these cats belong to the concolor species,. The animals may be found wandering wild (though no longer in great numbers) across regions in Canada and the United States. According to a report issued by the US Geological Survey, a high percentage of concolors test positive for the parasite T. gondii. This is the organism that causes the zoonotic disease, toxoplasmosis.

The picture is credited to Larry Coats of the US Fish and Wildlife Service (public domain).

cat George Chernilevsky public.jpg

A domestic cat
This image was released into the public domain by its author, George Chernilevsky.

Recently I read some thoughtful blogs on Steemit about toxoplasmosis (authors credited in "Sources" at the end of this post). I did a little research and came up with a few ideas I'd like to add to the discussion. Here goes:

If you search for 'toxoplasmosis' on Steemit, you're bound to come up with alarming information about cats. These furry household favorites, it seems, are sometime purveyors of an insidious parasite,T. gondii. An infected cat may shed T. gondii in the family home, and potentially spread the disease, toxoplasmosis, among family members.

When the process of disease dissemination is described, one fact is often repeated: It is in the cat, and only in the cat, that the parasite T. gondii reaches sexual maturity and completes its life cycle. A search on Google, for "toxoplasmosis and cats", will likely yield the same result. From these search results, it would seem reasonable to assume that, if there were no more cats, there would be no more toxoplasmosis.

Not exactly. T. gondii is an inventive survivor and has devised a neat way to get around its cat problem.

Asexual Reproduction

ToxoTachyzoites Green  Formalin-fixed T. gondii tachyzoites stained by immunofluorescence cdc public.jpg

The image above shows rapidly replicating T. gondii tachyzoites. The tachyzoite is an intermediate stage of T. gondii asexual reproduction. Tachyzoites mature into bradyzoites. The growth of T. gondii slows down during this stage. The bradyzoite is encapsulated in a cyst. In this state, the parasite can usually resist a number of assaults, including those from the host's native immune system.

Image source: United States Center for Disease Control and Prevention: public domain.

T. gondii yearns to be in a cat's intestines, where it can mature and "complete its life cycle". However, the parasite has alternative strategies. It reproduces asexually if it can't find a cat. It does this in an "intermediate" host. In this less-desired host, the parasite can continue to infect warm-blooded animals without ever touching base with a cat. According to a research paper published in Microbiology and Molecular Biology Reviews, asexual reproduction of T. gondii outside its preferred host (the cat) can proceed "ad infinitum". The parasite reproduces by cloning. The only downside to cloning for T. gondii is possible frustration :) and loss of genetic diversity, which sexual reproduction enables. Of course, genetic diversity is very important in evolutionary adaptation.

KownloonPigeon1Citycat at wikimedia toxo.jpg

The beautiful bird in the picture is a pigeon from Kowloon, China. The bird is a delicacy on restaurant menus in Kowloon. It also is a potential source of toxoplasmosis. A 2012 article published in the online journal, Parasites and Vectors, reported that T. gondii was discovered in tissue samples taken from chickens, ducks and pigeons in Lanzhou, China. The animal specimens were purchased from a market where local people shopped for dinner. The authors of the article conclude that the prevalence of T. gondii in consumer meat "poses a potential risk for T. gondii infection in humans." Please note, the only way a cat would enter this picture is if it managed to get a piece of undercooked meat from the dinner table. Image credit: Citycat at Chinese Wikipedia, who has released the photo into the public domain.

As far as finding a suitable "intermediate" host, just about any warm-blooded animal will do. Birds, people, dogs, cats, mice, and many others, will serve. So it's not just cats that carry and introduce T. gondii into the environment. We all do. If every cat on the planet disappeared tomorrow, T. gondii would still thrive.

There's another problem with looking to cats in order to stop the spread of toxoplasmosis. When we speak of cats and T. gondii, we have to think big. Really big. Like lion and leopard. For T. gondii likes all felids, not just cats we find in our homes or out in the street. Blaming the house cat, or the "feral" cat for toxoplasmosis isn't a very productive exercise.

Beijing Zoo Panda WT-shared Clute88 at wts wikivoyage released public.jpg

In 2015, the journal, Parasite, reported a fatal T. gondii infection in a giant panda at the Beijing Zoo. This was the first case of "clinical toxoplasmosis" in a giant panda recorded by the journal. This photo was released into the public domain by its author, Clute88.


T. gondii is "at least 10 million years old". It has survived, apparently, because it is extraordinarily adaptable. Numerous strains and subspecies have evolved over time.

The relationship between the mouse, the cat and the human seems to have occasioned a leap in the parasite's evolutionary development, and this occurred about 11,000 years ago, with the introduction of farming. Before this time, mice weren't much of a presence in human activity. However, with the growth and storage of grain, mice proliferated in human environments. Cats followed. The presence of cats was encouraged by humans because the felines controlled the proliferation of mice. With the establishment of a cat/mouse/human nexus, T. gondii was offered an opportunity, which it exploited brilliantly. Therefore, as is true in many cases where invasive species find a niche, humans played a significant role in the spread of toxoplasmosis.

toxoplasmosis cyst in mouse brain 4.0 Parasite journal.jpg

Toxoplasmosis cyst in a mouse brain. From the journal Parasite. Authors:Carmen Anca Costache, Horaţiu Alexandru Colosi, Ligia Blaga, Adriana Györke, Anamaria Ioana Paştiu, Ioana Alina Colosi & Daniel Ajzenberg. The image is used under a Creative Commons Attribution-Share Alike 4.0 International license.

mouse pinyon parkservice employee public.jpg

Research indicates that mice infected with T. gondii show no fear of cats. This behavior suggests to some researchers that the parasite is actually manipulating the mouse so that it is more likely to be consumed by a cat. In this way, the parasite may complete its life cycle inside the cat's intestine.

This image of a pinyon mouse is in the public domain because it was prepared by an employee of the United States Park Service.

Perils of Toxoplasmosis

Toxoplasmosis is one of the most widespread diseases on earth. A 2018 report suggests that 30% of the world's human population may be infected. The parasite can be fatal in immunocompromised individuals and can cause birth defects if a mother suffers an acute infection during pregnancy.

Female goat Public_Domain Lisafern.jpg

A female goat.

One route of transmission from animals to humans is raw (unpasteurized) goat's milk.. This image was released by its author Liserfern under a CC1.0 universal public domain license.

Effects on the Healthy

Generally, healthy people who are infected with T. gondii harbor the parasite with little consequence. However, a number of studies have suggested a connection between psychiatric syndromes and infection with T. gondii. The strongest association, it has been suggested, exists with schizophrenia. The studies that showed this association were not longitudinal. Which means there may have been discrete factors that influenced the results--for example, stressed people might be more inclined to own cats and therefore more likely to be infected with T. gondii (just an example of how outcomes might be distorted). Fortunately, there is at least one longitudinal study, carried out in New Zealand, and published in Plus One Journal that looked at the connection between behavioral issues and T. gondii. This study took a cohort of individuals, male and female, from diverse socioeconomic backgrounds (1,307 subjects) and followed them from birth until age 38. Over those years, the participants were periodically evaluated. The outcome of the study: "little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations or neurocognitive impairment".

Of course, that is just one study, but it was one that followed standard study protocols.

Another, recent study (published 2017 in the journal Pathogens), with the suggestive title, Is Taxoplasma gondii a Trigger for Bipolar Disorder?, outlines the strong associations between various psychiatric disorders and T. gondii. However, buried in the middle of this article is one telling sentence: "...the role of infectious agents remains controversial, and to date, no clear cause-effect etiopathogenetic relation can be established." What this means is, association and correlation do not equal causation.

That said, T. gondii is prevalent in brain and muscle tissue of its hosts. In immunocompromised individuals (those with HIV, for example), toxoplasmosis can cause a vicious type of encephalitis. This, however, does not make the case that T. gondii causes either bipolar disorder or schizophrenia.

The Monk Seal

Hawaiian monk seal Monachus_schauinslandi U.S. Government work public.jpg

This picture of a Hawaiian Monk Seal was taken by Dr. James P. McVey for NOAA. As a work of the U. S. government, the picture is in the public domain.

One of the most emotionally resonating issues in the discussion of cats and T. gondii is the possible extinction of the Hawaiian Monk Seal. This marine mammal is listed as an endangered species. Recently, several of these seals have been found dead off the coast of Oahu from what seems to be toxoplasmosis. Cats on the island have been blamed for these deaths, because T. gondii is excreted in cat feces, and runoff containing feces washes into the sea. Therefore, the T. gondii contaminated runoff is presumed to have come in contact with seals. Many people believe, if cats were removed from the island, contamination of the water with T. gondii would cease and the endangered Monk Seal would be protected. This suggested course of action needs scrutiny.

As we have already established, T. gondii exists in many warm-blooded animals, besides the cat. These intermediate hosts include the rat, mongoose, mouse and bird. All of these are likely heavily infected with T. gondii. Of particular interest is the bird, because, according to OSHA, not only cat, but bird feces can carry T. gondii. Avibase ("the world bird database"), suggests that there are 233 bird species on Oahu.

Many sources exist that document contamination of beaches and parks with bird feces. One that addresses Hawaii in particular is entitled Microbial Load from Animal Feces at a Recreational Beach. In a book published by the University of Hawaii Press, Hydrology of the Hawaiian Islands, the authors state that duck feces and pigeon feces contaminate runoff from the land and beach in Hawaii.

It's probably true that runoff contaminated with cat feces in Hawaii carries T. gondii. But cats are not the only likely source. They are, though, the obvious source and the easiest target. An example of how they can be mistakenly targeted may be found in two Canadian episodes.

Port Alice July_2008 B. C. 3.0 attribution Sflostrand.jpg

Port Alice, British Columbia, Canada. Author Sflostrand. Used under a Creative Commons Attribution-Share Alike 3.0 International license.

A 1998 toxoplasmosis outbreak occurred in British Columbia. The source was water contamination, but, it was discovered cats were not the only culprits. Cougars were at least equally responsible. And, again, in 2018, deceased marine mammals in the Pacific Northwest were found to be contaminated with T. gondii, but the source was not just cats. It was also opossums.

So, while it is true that cats are the preferred ("definitive") host for T. gondii, they are hardly the exclusive candidates for spreading this parasite--on land or on sea. Perhaps, instead of indicting cats for carrying and spreading toxoplasmosis, we should target the parasite. This would seem to be a more effective approach. There is research underway to do just this, and the research has met with some success. Although there is no vaccine for humans, there are some that show modified utility in cats and mice.

The Tragic Story of Lyall's Wren

XenicusInsularis John Gerard Keulemans public domain expired.jpg

Sketch of a Lyall's Wren (also called Stephens Island Wren), by John Gerard Keulemans, 1895. The image is in the public domain because the copyright has expired.

The story is told of how Tibbles the cat extinguished the last Lyall's Wren on Stephen's Island, New Zealand. The story is true, sort of. Tibbles didn't single-handedly wipe out this species. There were other cats on the island. But none of these cats hired a boat to take them to Stephens Island. Humans introduced the cats to a pristine habitat, a habitat where flightless birds cavorted free of native predators.

According to fossil remains, Lyall's Wrens were once widely distributed across New Zealand. It is guessed that they were hunted into extinction by humans, as this predatory species expanded into new areas--everywhere except Stephens Island. In 1891, this refuge also became unsafe, when lighthouse construction was begun on the island. Shortly thereafter, humans brought cats to the preserve. This was the last gasp for the wren.

In light of this history, it's not reasonable to indict one cat, or a colony of cats, for the extinction of Lyall's Wren. The process of extinction was long and complex, and was due mostly to human activity. A recent National Geographic headline, New Zealand Announces Plan to Wipe Out Invasive Predators, highlights how the plight of the wren is similar to that of many native species. Endemic species in New Zealand have been in the bull's eye of invasive predators for generations. Most of these predators were introduced by, you guessed it, humans.

cat prompt toxo2.jpg

Well, there it is, my defense of the cat. While it is true that I love cats, circumstances in the past have also put turtles, birds, fish, dogs, and even mice in my care. It might be said that I'm pretty much, though not entirely, species neutral.

I'd like to credit three excellent blogs that got me thinking about this topic:

  1. Toxoplasmosis is the Leading Disease-caused Death for the Endangered Hawaiian Monks Seals (Neomonachus Schauinslandi), Showing How Much Damage Cats Can Indirectly Do to the Wildlife, by @valth
  2. Will Toxoplasmosis Make You Like Cats? Let's Take a Closer Look at the Disease and the Parasite That is Responsible for It!, by @valth
  3. Mind Bending Effect Caused by Toxoplasmosis Might Have Made You Interested to Pursue Business!, by @conficker

Some Direct Sources for My Blog:
Hydrology of the Hawaiian Islands,

*The purplish cat image at the end of the blog was created with a Microsoft 3D Paint program.

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Wow, this is quite the in-depth analysis. Most Toxiplasmosis posts focus on the unusual life cycle or the studies that show it influencing the behavior of the host. This covers a ton of stuff many don't consider about this microbe.


I tried to add something. I thought there were some general misconceptions about cats, and the parasite, so I did my best to address them. My concern was that maybe I overdid it, for steemit. I'm glad some readers found the research worthwhile. Thanks so much for the feedback.


You didn't overdo it. I always find that your articles look big but once you start they're quite easily read - some articles by other posters look shorter but take more effort and time to read. So it's an illusion, and hopefully as more people give your posts a chance, your 'brand' will grow!


You've been one of voices on Steemit that encourage me to do a proper job. Plus, I read your posts and see how thoroughly you cover a subject. Sets a good example. You never fudge on the science. Thanks for the feedback.

What an awesome post @admoore!!! So well written and informative! It's something this cat lover had never heard of before. And the history of the wren extinction is a sad one indeed, but as usual, the blame can be traced back to the dumbest species around :)

As I was reading this (I found it of course through asapers), I was thinking how I wish I'd seen it sooner to submit to curie, then I saw your post payout! Wow! Congratulations :) In all of my curating travels, I have not seen a post with this high a payout in a very long time! You should be very proud of yourself ;)


I am very, very pleased and surprised. I worked hard and tried to do a good job. I guess this feedback means I must have.
It's very kind of you to get back to me. I read your UPDATE and know you have important stuff going on right now in your life. I'm leaving a comment on that post.
Thanks, as always for your support.


You certainly did do something right! And I appreciate you understanding my life right now too. Truthfully, steemit is my outlet and I really enjoy getting on here and kind of forgetting everything else once in a while. Brian is currently sleeping, so it's the perfect time for me to do just that, instead of sitting and stewing about things.

You're welcome for the support too :)

Another awesome article @agmoore. Always such detail. It's one of those things that if 'we' can pinpoint to 1 thing, great problem solved. Then we don't have to look at other effects from said problem.

The part about the mouse with the behavior change to entice cats, or the possibility of being related to schizophrenia explains a lot. I hope some research is done to see what kind of psychological effects there are.


Thank you! I can be a little tedious with detail, but I like to check my facts so readers can depend on them. Anyway, I learn a great deal.
I appreciate your reading the blog and commenting on it.

I got drawn in by the first picture where it looks like he checks who knocked on his door, but I found myself reading the entire article! very interesting! well done.
It seems to be something so close to us, but yet so unknown amongst most people, so that makes an interesting topic :D


Thank you! Your opinion means a lot to me.
I loved all the pictures, but especially that one. Have a soft spot for animals so picking the pictures was fun:)

Amazing post! Thanks for this. I've often wondered about the maligning of the house cat for this reason. I've seen scare articles shared on Facebook (even unfriended someone over his radical beliefs in this area.) I always felt it was a threat blown way out of proportion, but I never had any science to back that up. Thanks so much!


I appreciate that. I really went to bat for the cats. They get a raw deal on this one. About 3/4 of toxoplasmosis infections in the world are due to asexual (non-cat) reproduction. We should really look to ways of controlling the parasite and practicing good hygiene.

I can spot a cat lover a mile away :) Me too...and dogs, and mice, and birds, and....


Yep! I'm totally with you there. I love them all (except mosquitoes - I hate mosquitoes... and biting flies - in fact, anything that bites me and makes me swell up...)


Very detailed and informative post! I have 4 cats, and was told all my life to be careful with toxoplasmosis. I personally think that as long as I don't lick my cat's litterbox, I'll be fine =) Maybe I already have it....oh well!


I think you'll be fine if it hasn't gotten you already :) Thanks so much for appreciating my post. I hope the information persuades a few people to give cats a break and have a more enlightened attitude toward the parasite.

Thank You, @agmoore.
Well written and documented post.
The case of the Monk Seal is very interesting, it really puts into perspective how strong these micro organisms are. Mountain lions are on their way to extinction too, yet major contributors...


I really appreciate that. I tried to check every fact. I didn't know a lot to start with. Just had a few bits of information. Wanted to make a case that was anchored so it would be credible. I'm glad I did that. Grateful for the few readers, like you, who made it to the end. Thanks.

Excellent information. As usual, Man f*cks it up!

Namaste, JaiChai

Congratulations @agmoore, your post has been selected by the @asapers for a resteem and a feature in our brand new curation post. Issue 87

What does this mean for you? Well first an upvote from some members of the team, we are no @curie or @ocd but who is going to be unhappy with some extra upvotes. Also each post featured in the article will receive a 10% share of the SBD generated from the curation post.

Keep up the great work and please consider supporting the @asapers with an upvote and/or a resteem on the post you feature in. Please wait seven to ten days for payout.

Your friendly @asapers

Giving back A.S.A.P

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P.S. Always so well researched and often undervalued, I hope this helps @insideoutlet.


I love @asapers. One of the friendliest places on Steemit. Thank you for the support.

This post has been voted on by the steemstem curation team and voting trail.

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I am grateful. I love the upvotes--they're wonderful. But the appreciation really means a lot. It's the best sort of reinforcement and gives me incentive, because it means people are actually reading the blog :)
Thank you.

This kind of parasite blows my mind.

When micro-organisms can force behavior change for their benefit eg stopping fear response in mice or the cordyceps fungus hijacking ants to climb as high as possible before the fruiting body burst out to spread its self better.

That's just scary as hell and they would be the exact kinds of things some evil dystopian government would want to study to see if they could be used to control us.


Yeah, it is creepy, but they'd have to get a better handle on the parasite first to predict how it would control us. Maybe we could persuade some nice scientists, if they're going to put all that work in, to just get rid of the thing. Save the cats, save humanity :)

This post was shared in the Curation Collective Discord community for curators, and upvoted and resteemed by the @c-squared community account after manual review.

Thank you so much! That's very encouraging. I love to cover a subject in depth and it's nice to see the effort is noticed.

Though I'm a dog person, still, as a resident of the island on which the oldest human-cat burial site is found (no, it's not Egypt), and as a person whose mom owns at least 10 cats, I must say I approve your putting the record straight!

A lovely scientific post to show other STEMers how diligent research is done!

Congrats on the good amount of comments as well!


That's a fascinating article. Reminds me of horse burial (with humans) on the Eurasian Steppe or dog burial in Peru. Although the cats in your article don't seem to have been sacrificed, it is a possibility.

This post was as much a learning exercise as it was a writing exercise. That's why the research is so broad--wanted to make sure to get it right.

I love that you come from a cat family--although you are a "dog person". I seem to get stuck on whatever species is left at my door. There have been many. So far, fortunately, no snakes :)


We were initially a dog family, and then our dog died, and a friend of my mom's gifted her a cat, and it was downhill from there :( Now my mom takes part in competitions and we have triple European grand champions or whatnot :D


Downhill?? Champions sounds like uphill. Seems though, it's more love than competition:)