Tag Archives: parasite

Friday Fellow: Toxo

by Piter Kehoma Boll

If I had to bet on a parasite that you who are reading this probably have in your body, I’d go for today’s fellow, the protist Toxoplasma gondii, sometimes simply called toxo.

Found worldwide, the toxo is one of the most common parasites in humans, with estimations that about half of the world’s population is infected. Fortunately, this creature usually occurs in a latent form and does not offer great risks, but eventually it may develop into a more serious condition called toxoplasmosis, especially in people with weakened immunity.

But let’s take a closer look at this tiny fellow.


Oocysts of Toxoplasma gondii. This is the form found in the environment and that can start an infection in your body.

The toxo is a protist belonging to the phylum Apicomplexa, a group of parasitic alveolates that also includes the agent that causes malaria. Although traditionally considered a protozoan, the apicomplexans are closely related to dinoflagellates (which are generally considered as a group of algae). They have a unique organelle called apicoplast, which they use to penetrate a host cell. The apicoplast is derived from a plastid (such as the chloropast), so in a certain way we can say that the apicomplexans are algae that evolved into intracellular parasites!


Tachyzoites of Toxoplasma gondii stained with Giesma from the peritoneal fluid of a mouse.

The life cycle of the toxo is kind of complex. Let’s start with the inactive form called oocyst, which may be found in the environment. If a warm-blooded animal ingests an oocyst, it will “burst” inside the gut of the animal and release several “quick-moving” forms called tachyzoites. The tachyzoites invade almost any cell of the body and multiply asexually inside it until the cell dies and release them, allowing them to infect more and more cells. When invading the brain, liver and muscles, the tachyzoites usually differentiate into cysts that become inactive. In this stage, the only thing that the toxo wants is that a cat (any species of the family Felidae) eats the host. It may even change the host’s behavior in order to make it bolder and more easily accessible to predators.


A cyst of Toxoplasma gondii that forms in the muscles, brain and liver of any warm-blooded anymal. All the cyst wants is to be eaten by a cat!

Now let’s assume that a cat ate the host (that was likely a bird or mouse). Inside the cat’s gut, the cyst burst and releases several “slow-moving” forms called bradyzoites. This form invades the epithelial cells of the cat’s intestine and multiply asexually inside them. Eventually, the bradyzoites differentiate into either tachyzoites or gametocytes (sperm- and egg-like cells). When two gametocytes fuse, they form a zygote that matures into an oocyst and is released into the environment, restarting the cycle.


The complex life cycle of Toxoplasma gondii. Credits to Mariana Ruiz Villarreal.

As always, the lifecycle of parasites is a wonderful adventure!

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Tenter, A., Heckeroth, A., & Weiss, L. (2000). Toxoplasma gondii: from animals to humans International Journal for Parasitology, 30 (12-13), 1217-1258 DOI: 10.1016/S0020-7519(00)00124-7

Wikipedia. Toxoplasma gondii. Available at <https://en.wikipedia.org/wiki/Toxoplasma_gondii&gt;. Access on March 6, 2017.


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Filed under Disease, Friday Fellow, protists

Friday Fellow: Corpse Flower

by Piter Kehoma Boll

ResearchBlogging.org I guess most of you already know Rafflesia arnoldii, the corpse flower, as it is quite popular for a lot of reasons. But sometimes it’s nice to show the classics too, right?

Described in 1822 by Robert Brown, the corpse flower is remarkable for having the largest flower among all flowering (and non-flowering too) plants. Its name honors its two discoverers, the statesman Sir Thomas Stamford Bingley Raffles and the botanist Joseph Arnold, who collected the first specimen in 1818. It is known from the Indonesian islands of Sumatra and Borneo, ocurring in secondary and primary forests. It is one of the three national flowers of Indonesia.

Rafflesia arnoldii. Photo by Henrik Hansson*

Rafflesia arnoldii. Photo by Henrik Hansson*

There’s a lot of other weird things about it yet to be mentioned. Its common name, corpse flower, is due to the fact that the flowers smell like rotten flesh to attract carrion flies from genera Lucilia and Sarcophaga that pollinate them. Besides that, Rafflesia arnoldii is also a parasitic plant, extracting all the nutrients it needs from the roots or stems of vines of the genus Tetrastigma, so that it doesn’t have neither roots nor leaves and passes most of its life hidden inside the parasitized plant. The only visible structure is the flower, which takes months to grow, but remains open only for a few days.

Currently, the corpse flower is not evaluated by IUCN, so that it is not labelled as threatened, but human disturbance in its natural habitat, including ecoturism, seems to be decreasing the number of flowers opening per year.

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Brown, R. 1821. XV. An Account of a new Genus of Plants, named Rafflesia. Transactions of the Linnean Society of London, 13 (1), 201-234 DOI: 10.1111/j.1095-8339.1821.tb00062.x

KEW Royal Botanic Gardens: Rafflesia arnoldii (corpse flower). Available at: <http://www.kew.org/plants-fungi/Rafflesia-arnoldii.htm > Access on February 8, 2013.
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