Category Archives: Disease

Friday Fellow: Downy Mildew

by Piter Kehoma Boll

Last week I introduced a serious plant pathogen, the gray mold, that attacks many crops and has a special role as either a bad or a good guy in wine grapes. But a plant that is never happy with an infection by the gray mold is certainly the lettuce. And in this case our juicy vegetable has an enemy that makes it susceptible to the mold, and I’m bringing it to you today.

Named Bremia lactucae, this organism is a oomycete, thus belonging to a group of organisms that was formerly classified as a fungus, but that currently is known to be more closely related to brown and golden algae. This species attacks lettuces and closely related plants, causing a disease called downy mildew.


A lettuce leaf with downy mildew. Photo by Gerald Holmes.*

The downy mildew is the most important disease affecting lettuce worldwide. The disease itself is not the main problem, although it decreases the quality of the crop. Its main problem is that it makes the vegetable more vulnerable to other infections, such as those by the gray mold, and also increases the risk of contamination by human pathogens, such as intestinal parasites.


A branch of the downy mildew under the microscope. Photo by Bruce Watt.*

The usual forms of controling the spread of the downy mildew is by using fungicides and developing mildew-resistant lettuces by hybridization with wild and naturally resistant varieties. However, as usual, the downy mildew eventually adapts to this, giving rise to fungicide-resistant strains, as well as strains able to neutralize the resistance of lettuce lineages. It’s one more evolutionary arms race.

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Beharav, A., Ochoa, O., & Michelmore, R. (2013). Resistance in natural populations of three wild Lactuca species from Israel to highly virulent Californian isolates of Bremia lactucae Genetic Resources and Crop Evolution, 61 (3), 603-609 DOI: 10.1007/s10722-013-0062-5

Parra, L., Maisonneuve, B., Lebeda, A., Schut, J., Christopoulou, M., Jeuken, M., McHale, L., Truco, M., Crute, I., & Michelmore, R. (2016). Rationalization of genes for resistance to Bremia lactucae in lettuce Euphytica, 210 (3), 309-326 DOI: 10.1007/s10681-016-1687-1

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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 <;. Access on March 6, 2017.

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Friday Fellow: Amphibian chytrid fungus

by Piter Kehoma Boll

Today I’m bringing you a species that is probably one of the most terrible ones to exist today, the amphibian chytrid fungus, Batrachochytrium dendrobatidis, also known simply as Bd.


Several sporangia of Batrachochytrium dendrobatidis (spherical structures) growing on a freshwater arthropod. Photo by AJ Cann.*

The amphibian chytrid fungus, as its name says, is a chytrid, a fungus of the division Chytridiomycota, which include microscopic species that usually feed by degrading chitin, keratin in other such materials. In the case of the amphibian chytrid fungus, it infects the skin of amphibians and feeds on it. It grows through the skin forming a network of rhizoids that originate spherical sporangia that contains spores.

The infection caused by the amphibian chytrid fungus is called chytridiomycosis. It causes a series of symptoms, including reddening of the skin, lethargy, convlusions, anorexia and excessive thickening and shedding of the skin. This thickening of the skin leads to problems in taking in nutrients, releasing toxins and even breathing, eventually leading to death.


An individual of the species Atelopus limosus infected by the amphibian chytrid fungus. Photo by Brian Gratwicke.**

Since its discovery and naming in 1998, the amphibian chytrid fungus has devastated the populations of many amphibian species throughout the world. Some species, such as the golden toad and the Rabb’s fringe-limbed treefrog, were recently extinct by this terrible fungus. This whole drastic scenario is already considered one of the most severe examples of Holocene extinction. The reason for such a sudden increase in the infections is unknown, but it may be related to human impact on the environment.

We can only hope to find a way to reduce the spread of this nightmare to biodiversity.

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Fisher, M., Garner, T., & Walker, S. (2009). Global Emergence of Batrachochytrium dendrobatidis and Amphibian Chytridiomycosis in Space, Time, and Host Annual Review of Microbiology, 63 (1), 291-310 DOI: 10.1146/annurev.micro.091208.073435

Wikipedia. Batrachochytridium dendrobatidis. Available at <;. Access on March 4, 2017.

Wikipedia. Chytridiomycosis. Available at <;. Access on March 4, 2017.

Wikipedia. Decline in amphibian populations. Available at <;. Access on March 4, 2017.

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Friday Fellow: Flounder Glugea

by Piter Kehoma Boll

While looking for flatfish you may eventually find one with some grotesque growth on the body, like the one in the picture below:


A xenoma caused by Glugea stephani on a flatfish Limanda limanda. Photo by Hans Hillewaert.*

This sort of tumor is called xenoma and, in flatfish, is caused by a microscopical and parasitic fungus named Glugea stephani, or the flounder glugea.

The flounder glugea is part of a group of fungi called Microsporidia that until recently were classified as protists. They are unicellular and parasite other organisms, especially crustaceans and fish.

Once inside a flatfish, the flounder glugea enters an intestinal cell and starts to develop. They induce the host cell to increase in size and may give rise to the xenomas, which are the most extreme stage in the development of the disease. The proliferating and active stage of the glugea are free in the cytoplasm of the host cell, but they may change into a spore-like form called sporoblast that remains inside a vacuole.


Image of electron microscopy of an intestinal cell of winter flounder (Pseudopleuronectes americanus) infected by flounder glugea (Glugea stephani). The S indicates sporoblasts inside the vacuole (SV) and the P the proliferating organisms inside the host cytoplasm (H). Image extracted from Takvorian & Cali (1983).

Fortunately most infections are mild and do not compromise the fish health, at least not very much…

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Takvorian, P. M.; Cali, A. (1983). Appendages associated with Glugea stephani, a microscporidian found in flounder. Journal of Protozoology, 30(2): 251-256.

Wikipedia. Xenoma. Available at: < >. Access on September 17, 2016.

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Friday Fellow: Beggar’s tick

ResearchBlogging.orgby Piter Kehoma Boll

What if the cure for cancer has been living in your garden all this time and you have been trying to get rid of it because it is an annoying weed?

I cannot assure you that the answer lies in today’s Friday Fellow, but it certainly has a good potential. Its name is Bidens pilosa, commonly known as beggar’s tick, beggar ticks, black jack, cobbler’s pegs or Spanish needle.

Not extravagant, but discrete. This is Bidens pilosa. Photo by Wibowo Djatmiko.*

Not extravagant, but discrete. This is Bidens pilosa. Photo by Wibowo Djatmiko.*

Native from the Americas, where it grows in open fields and forest glades, the beggar’s tick is now found worldwide, from Eurasia and Africa to Australia and the Pacific Islands. At first it does not call much attention while growing among other weeds. It grows up to 1.8 m tall and has small discrete flowers in a daisy-like head, with a handful of white ray florets and a small disc of yellow florets.

The problem with this fellow happens when you have to pass among them after the flowers have turned into fruits.

The terrible evil infructescence of the beggar's tick. Photo by

The terrible evil infructescence of the beggar’s tick. Photo by Wibowo Djatmiko.*

The fruits of the beggar’s tick are small, stiff, dry rods with about 2–4 small heavily barbed awns at the end. They are arranged in spherical infructescences are are eager to stick on any passing animal. The small barbed awns catch onto fur and clothes and the fruits are easily dispersed to other areas. It is a classical example of zoochory, i.e., seed dispersal by animals. If you live in an area where this plant is common, you most likely have had the experience of finding your clothes full of those prickling seeds, especially after playing, working or simply walking through a field.

But the beggar’s tick is much more than a dull and annoying weed. In Subsaharan Africa, it is one of the most widely eaten plants. Its leaves are edible when cooked, but have a strong and unpleasant taste.

Furthermore, the beggar’s tick is used in traditional medicine in South America and several studies have found out that it is indeed a powerful medicine. Extracts from the plant have shown several medicinal properties, including:

  • Antibacterial and antifungal activity
  • Antimalarial activity
  • Anti-herpes simplex activity
  • Ability to reduce tumoral and leukemic cells
  • Immunosuppressive and anti-inflammatory effects

If this were not enough, the beggar’s tick has the ability to bioacumulate cadmium in its tissues, so that it can be used to depollute cadmium-contaminated soils.

The next time you find your clothes full of beggar’s ticks, remember that it is more, much more, than simply an annoying weed.

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Brandão, M., Krettli, A., Soares, L., Nery, C., & Marinuzzi, H. (1997). Antimalarial activity of extracts and fractions from Bidens pilosa and other Bidens species (Asteraceae) correlated with the presence of acetylene and flavonoid compounds Journal of Ethnopharmacology, 57 (2), 131-138 DOI: 10.1016/S0378-8741(97)00060-3

Chang, J., Chiang, L., Chen, C., Liu, L., Wang, K., & Lin, C. (2001). Antileukemic Activity of Bidens pilosa L. var. minor (Blume) Sherff and Houttuynia cordata Thunb. The American Journal of Chinese Medicine, 29 (02), 303-312 DOI: 10.1142/S0192415X01000320

Chiang, L., Chang, J., Chen, C., Ng, L., & Lin, C. (2003). Anti-Herpes Simplex Virus Activity of Bidens pilosa and Houttuynia cordata The American Journal of Chinese Medicine, 31 (03), 355-362 DOI: 10.1142/S0192415X03001090

Deba, F., Xuan, T., Yasuda, M., & Tawata, S. (2008). Chemical composition and antioxidant, antibacterial and antifungal activities of the essential oils from Bidens pilosa Linn. var. Radiata Food Control, 19 (4), 346-352 DOI: 10.1016/j.foodcont.2007.04.011

Kviecinski, M., Felipe, K., Schoenfelder, T., de Lemos Wiese, L., Rossi, M., Gonçalez, E., Felicio, J., Filho, D., & Pedrosa, R. (2008). Study of the antitumor potential of Bidens pilosa (Asteraceae) used in Brazilian folk medicine Journal of Ethnopharmacology, 117 (1), 69-75 DOI: 10.1016/j.jep.2008.01.017

Oliveira, F., Andrade-Neto, V., Krettli, A., & Brandão, M. (2004). New evidences of antimalarial activity of Bidens pilosa roots extract correlated with polyacetylene and flavonoids Journal of Ethnopharmacology, 93 (1), 39-42 DOI: 10.1016/j.jep.2004.03.026

Pereira, R., Ibrahim, T., Lucchetti, L., da Silva, A., & de Moraes, V. (1999). Immunosuppressive and anti-inflammatory effects of methanolic extract and the polyacetylene isolated from Bidens pilosa L. Immunopharmacology, 43 (1), 31-37 DOI: 10.1016/S0162-3109(99)00039-9

Sun, Y., Zhou, Q., Wang, L., & Liu, W. (2009). Cadmium tolerance and accumulation characteristics of Bidens pilosa L. as a potential Cd-hyperaccumulator Journal of Hazardous Materials, 161 (2-3), 808-814 DOI: 10.1016/j.jhazmat.2008.04.030

Wikipedia. Bidens pilosa. Available at < >. Access on July 31, 2016.

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Zika virus and the negligence towards health research in poor countries by Piter Kehoma Boll

About a year ago, almost nobody on the whole world was aware of the existence of a virus named Zika virus and the illness it may cause in humans, the Zika fever or Zika disease. But is this a new, previously unknown virus? Where did it come from and why is it suddenly of so much concern?

The Zika virus, or ZIKV, is a virus in the genus Flavivirus, which also include other viruses, such as the ones responsible for the dengue fever and the yellow fever. The name Flavivirus means “yellow virus” in Latin, due to the yellow fever. All the three diseases are transmitted to humans throughs mosquitoes, especially the widespread Aedes aegypti.

The mosquito Aedes aegypti is currently the main vector of the Zika virus. Photo by James Gathany.

The mosquito Aedes aegypti is currently the main vector of the Zika virus. Photo by James Gathany.

The Zika virus was discovered in 1947 in Uganda in a febrile rhesus monkey in the Zika Forest, hence the name. From 1951 on, serological studies indicated that the virus could also infect humans, as antibodies against the virus were found in the blood of humans in several African and Asian countries, such as Central African Republic, Egypt, Gabon, Sierra Leone, Tanzania, Uganda, India, Indonesia, Malaysia, the Philippines, Thailand and Vietnam.

In 1968, in Nigeria, the virus was isolated from humans for the first time. During the following decades of the 20th century, the virus was detected via serological evidence or isolated directly in many humans. However, despite the confirmation of this virus in humans, research developed very slowly, most likely because the affected countries don’t have enough resources to conduct the necessary studies and richer countries are not at all interested in the health of the poor ones.

There was a small increase in concern over the virus after it was detected outside Africa and Asia for the first time, in 2007, in the Yap Island, Micronesia. After that, some epidemics occurred in several archipelagoes in the Pacific.

Since last year, the Zika virus has been dectected in South America and started to spread rapidly across the countries. It was suggested that the virus reached Brazil in 2014 during the World Cup. (Thanks, FIFA!). By November 2015, the disease has reached Mexico, which means it is about to reach the United States! Now suddenly it started to be of a major concern worlwide.

Currently known distribution of the Zika virus in humans. Map of the United States Centers for Disease Control and Prevention.

Currently known distribution of the Zika virus in humans. Map of the United States Centers for Disease Control and Prevention.

Common symptoms of the Zika fever include mild headaches, fever, joint pains and rash. It was not considered a serious disease, as it usually fades quickly after a week, until recently, when it was linked to the development of microcephaly in fetuses of mothers infected by the virus during the first trimester of pregnancy.

I wonder how many children were born with microcephaly in Africa and Asia during the last decades because there was no investment to study the virus. Now that it suddenly became a worldwide threat, there is no vaccine, no adequate treatment and most physicians are unable to identify the illness through the symptoms.

And there are a lot of other viruses forgotten in poor tropical countries just waiting for the right opportunity to spread and scare North America and Europe. No one cares while they remain among poor African and Asian people, but global warming is here and tropical diseases love it more than anything else.

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Gatherer, D. & Kohl, A., (2015). Zika virus: a previously slow pandemic spreads rapidly through the Americas Journal of General Virology DOI: 10.1099/jgv.0.000381

Hayes, E. B. 2009. Zika Virus Outside Africa. Emerging Infectious Diseases, 15(9): 1347-1350.

Vasconcelos, P. (2015). Doença pelo vírus Zika: um novo problema emergente nas Américas? Revista Pan-Amazônica de Saúde, 6 (2), 9-10 DOI: 10.5123/S2176-62232015000200001

Wikipedia. Zika virus. Available at: <;. Access on January 25, 2016.



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