The CATastrophe that is Toxoplasma gondii

By Claire Nagel

What is it?

            Toxoplasma gondii is a protozoan parasite that causes Toxoplasmosis, a very common parasitic infection distributed all over the world.  Cats and other felines are its definitive host, meaning the parasite reaches sexual maturity and reproduces in the feline.  Rodents and other animals are intermediate hosts, while humans are a secondary host.  It can be transmitted through vertical transmission, fecal-oral transmission, iatrogenic transmission (medical procedures), and trophic transmission.  Toxoplasmosis mostly occurs in immunocompromised individuals, such as those infected with AIDS, or children who got it from their mothers.  Symptoms include fever, fatigue, muscle pain, headache, and sore throat, and can result in encephalitis and/or ocular disease.  It can even cause sudden abortions in sheep.   T. gondii is found globally in most continents, including North America, Europe, and South America.  It is estimated that in the United States and United Kingdom, 16-40% of the population are infected, while in Central and South America and Continental Europe, estimations range from 50-80%.

Clinical History

            T. gondii was first discovered accidently in 1908 by Charles Nicolle, a scientist who was working on the disease Leishmania in the North African rodent, Ctenodactylus gundi.  T. gondii was also identified by Alfonso Splendore at around the same time, but in the rabbit population in Sao Paulo.  It was not until 1948 though that Albert Sabin and Harry Feldman published their discovery on the ‘dye test’, a way to diagnose the infection in humans through the use of serum samples.

How is it Transmitted?

            T. gondii can be transmitted by various routes.  It can be transmitted vertically from mother to baby.  If a woman gets infected while pregnant, the infection can be passed on to the developing fetus.  Diseases of this range from mild to severe.  T. gondii can also be transmitted through fecal-oral transmission by consuming tissue cysts from uncooked or undercooked meat, or from food or water contaminated with oocysts from cat feces.  Trophic transmission occurs when a rodent or bird is eaten by another animal, typically a feline, and iatrogenic transmission can happen during blood transfusions and organ transplants.

Why is it so Special?

            T. gondii is an interesting parasite because it changes the behavior of rodents who are infected.  When a rodent is infected with the parasite, they have a reduced distaste to cat odor.  Uninfected rodents flee when they smell cat urine, but infected rodents are not afraid of the smell and do not run away, increasing the likelihood that they will be eaten by a cat.  Reports show that this attraction to cats can be in part due to the parasite activating the sexual arousal pathways in rats.  It is similar to a male rat smelling a female rat in heat, but he has no idea that what he is actually attracted to is cat urine.

Treatments Available

            There are currently no treatments that completely cure an individual of T. gondii, but there are drugs that are effective at treating it, such as sulfadiazine, pyrimethamine, and spiramycine.  These drugs target the parasite during its tachyzoite stage (rapid multiplication), but have limited effectiveness against the tissue cyst stage of T. gondii.  In most cases, animals and humans who have been infected with Toxoplasmosis develop immunity against the disease in the future.  There are no vaccines currently against the parasite in humans, but there is one vaccine used to prevent the Toxoplasma-induced abortion in sheep.

Conclusion

            Overall, T. gondii is a very weird parasite that has many different effects on their hosts.  It causes flu-like symptoms in humans, abortions in sheep, and an attraction to cat pee in rodents.  Discovered in 1908 in North Africa, T. gondii is now prevalent all over the world.  There currently is no cure to the disease, but with further research, it can be done.

Figure 1. The lifecycle of T. gondii (Hill et al., 2002).

References

Hill D, Dubey JP. Toxoplasma gondii: transmission, diagnosis and prevention. Clinical microbiology and infection: the official publication of the European Society of Clinical Microbiology and Infectious Diseases. 2002;8(10):634-640.

Innes EA. A Brief History and Overview of Toxoplasma gondii. Zoonoses & Public Health. 2010;57(1):1-7. doi:10.1111/j.1863-2378.2009.01276.x.

McConkey GA, Martin HL, Bristow GC, Webster JP. Toxoplasma gondii infection and behaviour - location, location, location?. J Exp Biol. 2013;216(Pt 1):113–119. doi:10.1242/jeb.074153