Ten curious facts about Toxoplasma gondii

Ten curious facts about Toxoplasma gondii

A whimsical look at a parasite that is most definitely not funny.

  1. A whole new meaning of “fatal attraction:” toxoplasma infected mice and rats seek out cats

One of the strangest scientific discoveries is recognising that parasites are able to alter the behaviour of the host.  Indeed, only recently are scientists beginning to recognise that even the bacteria which live in our guts can alter our behaviour!  If you have a sugar craving—is it you or is it the Candida in your gut making you eat those sweeties and cakes?

One of the most bizarre discoveries was that toxoplasma-infected rats and mice become attracted to their predator: the cat. House et al, 2011; Kaushik et al, 2014; Vyas et al, 2007; Voznessenskaya 2014.  It is believed that toxoplasma in the brain of the unfortunate rodent causes these suicidal tendencies because toxoplasma’s essential host is the cat: the toxoplasma parasite is only able to produce oocysts (i.e. eggs) in the cat, in all other hosts tissue cysts containing slowly dividing bradyzoites are formed in various organs, including the brain. Therefore, to complete the toxoplasma life cycle, the intermediate host has to be eaten by the definitive host, which is some species of cat.

Toxo turns chimps into chumps

It’s not just rodents whose behaviour changes: toxoplasma infected chimpanzees lose fear of their natural big cat predator, the leopard.  Poirotte et al, 2016

  1. You are more likely to become infected with Toxoplasma by eating meat, than by living with a cat

Toxoplasma gondii is a protozoan parasite that is responsible for approximately 24% of deaths attributed to foodborne pathogens in the United States.”  Guo et al, 2016

Various studies have looked at whether people who have cats, or who are cat sitters, Jung et al, 2017 are more likely to be infected with toxoplasma: most have found that they are not. The main source of infection for humans is eating or handling raw or undercooked meat. Belluco et al, 2016; Moshfe et al, 2018

In one study, people who knew to use separate chopping boards for raw and cooked food were significantly less likely to have been infected with T. gondii. Yan-Li et al, 2017

  1. Cats only shed T. gondii oocysts for 2 weeks, then never again: pregnant women need not worry about having a cat

Transplacental toxoplasmosis can be fatal to the unborn baby. The medical profession is always keen to blame cats for human toxoplasmosis, but actually in most cases it’s not the cat who infected the human – it was eating meat (see item 2 above). Admittedly the cat is the definitive host of T. gondii: the protozoan does have to replicate in the gut of a cat in order to complete its life cycle and produce oocysts.  However, serological prevalence studies show that around two thirds of cats have never encountered T. gondii in their lives—therefore, they have never shed toxoplasma oocysts.  Those cats who did experience infection only shed oocysts for up to two weeks, after which they became immune.  Re-shedding can occur if the cat becomes immunosuppressed, therefore it is prudent to do a toxoplasma antibody test in a cat prior to chemotherapy, or any other kind of immunosuppressive treatment.

  1. You are more likely to have a road traffic accident if you have been infected with Toxoplasma gondii

Worldwide, about one third of people have antibodies to T. gondii and many studies have found an association between being seropositive for T. gondii and having a road traffic accident. Flegr et al, 2002; Galván-Ramírez et al, 2013  Toxoplasma slows reaction time and can affect eyesight, both of which have been cited as possible causes for this phenomenon.

  1. The British athlete Sebastian Coe suffered from toxoplasmosis

Olympic gold medal winner, Seb Coe, now Baron Coe, famously suffered from toxoplasmosis in the 1980s.  So if he runs you over with his car—don’t blame him, blame Toxoplasma gondii!

  1. Clinical signs of toxoplasmosis in cats include uveitis, neurological signs and enlarged mesenteric lymph nodes

Most cats infected with T. gondii are asymptomatic, but for those cats who do develop clinical signs, toxoplasmosis can be lethal, especially if mis-diagnosed Cohen et al, 2016 and the cat given immunosuppressive treatment.  It has been speculated that cats, like humans, should possibly be given prophylactic clindamyin antibiotics prior to chemotherapy, Murakami et al, 2018 but testing for antibodies would identify which cats might be at risk of latent toxoplasma reactivation.

One of the most common mis-diagnoses for toxoplasmosis is feline infectious peritonitis (FIP), especially if the cat presents with uveitis, neurological signs, or enlarged mesenteric lymph nodes, but also if the cat develops jaundice, an effusion, or pyogranulomata in any organ.

If a cat is a hunter, or is fed raw meat, van Bree et al, 2018 ask for a toxoplasma antibody test if his eyes change colour, or the pupils become uneven sizes; if the cat has seizures; becomes jaundiced; or if an enlarged lymph node is detected in his abdomen.

  1. Toxoplasmosis in cats is diagnosed by a high antibody titre and a response to clindamycin antibiotic

Diagnosing toxoplasmosis in the living cat is not easy because about one third of cats have toxoplasma antibodies—so how do you know if the cat’s disease is due to toxoplasma or if the antibodies are simply there coincidentally?  In my experience the presence of a high toxoplasma antibody titre is very suspicious of toxoplasmosis, and I always recommend a one month course of clindamycin for sick cats with a high toxoplasma antibody titre.  In my experience IgG is more useful than IgM detection, although the presence of the latter does indicate a more recent infection. Antibody negative tests are very useful for ruling out toxoplasmosis—provided the test has good sensitivity.

Definitive diagnosis is by histopathology of post mortem, cytology of a fine needle aspirate taken from a lesion, Murakami et al, 2018 or biopsy samples.  Unfortunately biopsies often simply reveal pyogranulomatous inflammation of unknown aetiology.

  1. A toxoplasma infected man is less likely than an uninfected man to object to the smell of cat urine

I quote from Dr Flegr’s paper: “Thirty-four Toxoplasma-infected and 134 uninfected students rated the odour of urine samples from cat, horse, tiger, brown hyena and dog for intensity and pleasantness. The raters were blind to their infection status and identity of the samples. No signs of changed sensitivity of olfaction were observed. However, we found a strong, gender dependent effect of toxoplasmosis on the pleasantness attributed to cat urine odour (p = 0.0025). Infected men rated this odour as more pleasant than did the uninfected men, while infected women rated the same odour as less pleasant than did uninfected women. Toxoplasmosis did not affect how subjects rated the pleasantness of any other animal species’ urine odour; however, a non-significant trend in the same directions was observed for hyena urine.” Flegr et al, 2011

You really couldn’t make this up!  Either that such an experiment was done, nor the results!  I will never, ever, volunteer for one of Dr Flegr’s experiments! laughing Girls – still probably not a good idea to swap your perfume for cat wee!  Conclusion (mine, not Dr Flegr’s): if you’re a crazy cat lady whose house pongs of cat wee, you’d best marry a toxoplasma infected man who won’t object to the smell!

  1. Toxoplasmosis can give you obsessive compulsive disorder

People with latent toxoplasma infection were found to be 2.5 times more likely to have obsessive compulsive disorder (OCD) and 2.7 times more likely to have learning difficulties than uninfected people. Flegr & Horáček, 2017

Conclusion (mine, not Dr Flegr’s): men, if you’re going to marry a crazy cat lady, test her for latent toxoplasmosis – if so, she’s more likely to clean up the cat pee!

  1. Burmese cats are the breed least likely to have T. gondii antibodies

Birmans, Ocicats, Norwegian Forest Cats, and Persians are four to seven times more likely to be seropositive when compared with the Burmese cats. Must et al, 2017


Further  information

Dr Addie’s FIP website: www.catvirus.com

Free Continuing Professional Development films:  a series of five videos of a case study on YouTube entitled “Does Tommy Have FIP?” https://youtu.be/F_rRr6pZ1RE

European Advisory Board of Cat Disease website: www.abcdcatsvets.org



Belluco S, Mancin M, Conficoni D, Simonato G, Pietrobelli M, Ricci A. Investigating the Determinants of Toxoplasma gondii Prevalence in Meat: A Systematic Review and Meta-Regression. PLoS One. 2016 Apr 15;11(4):e0153856.

Cohen TM, Blois S, Vince AR. 2016 Fatal extraintestinal toxoplasmosis in a young male cat with enlarged mesenteric lymph nodes.  Can Vet J  57, 5: 483-486

Flegr J, Havlícek J, Kodym P, Malý M, Smahel Z. Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study. BMC Infect Dis. 2002 Jul 2;2:11.

Flegr J, Lenochová P, Hodný Z, Vondrová M. Fatal attraction phenomenon in humans: cat odour attractiveness increased for toxoplasma-infected men while decreased for infected women. PLoS Negl Trop Dis. 2011 Nov;5(11):e1389.

Flegr J, Horáček J. Toxoplasma-infected subjects report an Obsessive-Compulsive Disorder diagnosis more often and score higher in Obsessive-Compulsive Inventory. Eur Psychiatry. 2017 Feb;40:82-87.

Galván-Ramírez Mde L, Sánchez-Orozco LV, Rodríguez LR, Rodríguez S, Roig-Melo E, Troyo Sanromán R, Chiquete E, Armendáriz-Borunda J.  Seroepidemiology of Toxoplasma gondii infection in drivers involved in road traffic accidents in the metropolitan area of Guadalajara, Jalisco, Mexico.  Parasit Vectors. 2013 Oct 11;6(1):294

Guo M, Mishra A, Buchanan RL, Dubey JP, Hill DE, Gamble HR, Jones JL, Du X, Pradhan AK. Development of Dose-Response Models to Predict the Relationship for Human Toxoplasma gondii Infection Associated with Meat Consumption. Risk Anal. 2016. 36(5):926-38

Hartmann K, Addie D, Belák S, Boucraut-Baralon C, Egberink H, Frymus T, Gruffydd-Jones T, Hosie MJ, Lloret A, Lutz H, Marsilio F, Möstl K, Pennisi MG, Radford AD, Thiry E, Truyen U, Horzinek MC. 2013  Toxoplasma gondii infection in cats: ABCD guidelines on prevention and management. J Feline Med Surg. 15(7):631-7

House PK, Vyas A, Sapolsky R. Predator cat odors activate sexual arousal pathways in brains of Toxoplasma gondii infected rats. PLoS One. 2011;6(8):e23277.

Jung BK, Song H, Lee SE, Kim MJ, Cho J, Shin EH, Chai JY. Seroprevalence and Risk Factors of Toxoplasma gondii Infection among Cat Sitters in Korea.  Korean J Parasitol. 2017 Apr;55(2):203-206

Kaushik M, Knowles SC, Webster JP. What makes a feline fatal in Toxoplasma gondii’s fatal feline attraction? Infected rats choose wild cats. Integr Comp Biol. 2014 Jul;54(2):118-28.

Moshfe A, Arefkhah N, Sarkari B, Kazemi S, Mardani A.  Toxoplasma gondii in Blood Donors: A Study in Boyer-Ahmad County, Southwest Iran.  Interdiscip Perspect Infect Dis. 2018 Apr 15;2018:3813612.

Murakami M, Mori T, Takashima Y, Nagamune K, Fukumoto J, Kitoh K, Sakai H, Maruo K.  A case of pulmonary toxoplasmosis resembling multiple lung metastases of nasal lymphoma in a cat receiving chemotherapy. J Vet Med Sci. 2018 Nov 8.

Must K, Hytönen MK, Orro T, Lohi H, Jokelainen P. Toxoplasma gondii seroprevalence varies by cat breed. PLoS One. 2017 Sep 8;12(9):e0184659.

Poirotte C, Kappeler PM, Ngoubangoye B, Bourgeois S, Moussodji M, Charpentier MJ. Morbid attraction to leopard urine in Toxoplasma-infected chimpanzees.  Curr Biol. 2016 Feb 8;26(3):R98-9.

van Bree FPJ, Bokken GCAM, Mineur R, Franssen F, Opsteegh M, van der Giessen JWB, Lipman LJA, Overgaauw PAM. Zoonotic bacteria and parasites found in raw meat-based diets for cats and dogs.  Vet Rec. 2018 Jan 13;182(2):50.

Voznessenskaya VV. Influence of Cat Odor on Reproductive Behavior and Physiology in the House Mouse: (Mus Musculus). Editor: Mucignat-Caretta C. In: Neurobiology of Chemical Communication. Boca Raton (FL): CRC Press/Taylor & Francis; 2014. Chapter 14. Frontiers in Neuroscience.

Vyas A, Kim SK, Giacomini N, Boothroyd JC, Sapolsky RM. Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. Proc Natl Acad Sci U SA 2007; 104: 6442-6447.

Yan-Li G, Yi-Qing X, Yong-Gen Z, Da-Cheng XU, Wen-Wei XU, Yang D, Ming-Xue S. Zhongguo Xue Xi Chong Bing Fang Zhi Za Zhi. Infection status of Toxoplasma gondii and its related knowledge and behavior among special population in Changzhou City. 2017 Mar 20;29(4):498-501