Natural Diversity of Microbial Parasites
Updated: Sep 10
Special Guest Blog by Maria Martynova
While there are beneficial microbes that harmoniously coexist with humans, there are also microorganisms that have a parasitic lifestyle. Parasitism is a relationship between two organisms, a host and a parasite, in which a parasite benefits from this interaction, while a host is harmed. Even though some of them are truly scary and untamed, others were “domesticated” by scientists over decades and present less danger today. Let’s dive deeper and learn about some of these unique microscopic parasites that will make you gasp for air!
In this part, we will be discussing parasitic microorganisms that you have probably never heard of: Protozoa and Amoeba
“Fearless” victim - Toxoplasma gondii
Bacteria and fungi are not the only microbes that practice a parasitic lifestyle. Another type of organisms that need to have a host are protozoa. Historically, protozoans were viewed as “one-celled animals”, because they often possess animal-like behaviors, such as motility and predation. But, they are quite different from all other kinds of life you may have heard of, which is why they get their very own group.
Toxoplasma gondii is a parasitic protozoan that makes rodents reckless and fearless. Why would it be the problem, then? Well, such an unusual change in behavior makes these poor animals the unfortunate victims of cats and other predators, allowing tricky T. gondii to make their way to the next host’s body. There, the parasite fully develops and waits for its turn to be released into the environment.
Sometimes, however, this parasite can infect humans. It is estimated that more than 40 million people in the United States may be infected with the Toxoplasma parasite. Domestic cats are usually blamed for the spread, and many pregnant women avoid these beloved pets like the plague. But, recent studies demonstrated that the main source of infection with T. gondii for humans is undercooked or raw meat. An infected human also experiences a change in behavior (although not as drastic as in rodents). People have a slower reaction time, which has been connected to a higher number of road traffic accidents. One of the studies even demonstrated that infected people are 6 times more likely to get into a traffic accident.
2. Parasite as old as time - Trichomonas gallinae
This microorganism has been in the “parasitic game” since before the dinosaurs! And even today, it stays active and infects birds of prey, such as falcons and hawks. Protozoan Trichomonas gallinae causes lesions in the lower beak of these birds. Injuries of this type often lead to the development of a more serious infection, which prevents birds from feeding, leading to starvation and death.
Now, let’s think about this. If T. gallinae is an ancient, prehistoric parasite, and birds are considered as evolutionary descendants of dinosaurs, is it possible that this protozoan was infecting famous reptiles back in time? The answer is yes, and scientists have evidence to confirm this! They analyzed the remains of Sue, the most complete and best-preserved T. rex specimen. It was believed that Sue died in a battle with another dinosaur. Scars and lesions found on the skull were characterized as marks of struggle with another reptile titan. However, recent research shows that these wounds are the result of the protozoan infecting Sue’s mouth and throat. Moreover, the location of lesions in Sue’s jaws match the location of lesions found in modern birds infected with T. gallinae. The infection might have been so severe that the 42-foot-long dinosaur starved to death.
3. “Self-reliant” parasite - Giardia lamblia
Parasites are believed to be primitive in structure and composition, which requires them to live with a host. Although it is a true statement, it is not a universal case. Some parasites can survive in the environment without a host for a while. Giardia lamblia is one of them.
This protozoan parasite causes giardiasis, a disease that is directly associated with poor living conditions and low quality of drinking water. An estimated number of 200 million people worldwide are affected each year. It was formerly included in the WHO list of neglected diseases, because it mainly affects the world’s poorest populations, and, unfortunately, did not receive enough attention. However, every person can become infected, because this parasite can be passed to a human host through contact with infected animals, like cats and dogs.
While waiting for a suitable host, G. lamblia forms a cyst that protects and preserves a microorganism. The cyst remains viable for several months in water at temperatures below 10°C and several weeks at room temperature. Current treatment of giardiasis in humans consists of administration of antiprotozoals, and animals receive a vaccine called GiardiaVax. Yet, the veterinary vaccine often requires the administration of additional medications to provide maximum efficiency.
4. Buzzing danger - Plasmodium falciparum
Malaria is one of the most commonly known diseases, and it, undoubtedly, played a significant role in human history. Throughout centuries, people reported incidences of a sickness characterized by fever and chills. Later on, the disease started to be associated with mosquitoes and swamp areas. Although this correlation is correct, and mosquitoes, indeed, infect people, they are not the “bad guys” in this story. Mosquitoes are called vectors, because they serve as points of transmission of an organism, which is a real trouble-maker. (find out what mosquitoes want from you here)
Malaria is caused by a parasitic eukaryotic organism of the Plasmodium genus. Most deaths are caused by Plasmodium falciparum, whereas other species of the same genus cause milder infections. When an infected mosquito bites a person, an insect’s saliva comes in contact with human blood, allowing the parasite to enter the bloodstream and travel to the liver, where it matures and reproduces. In 2019, there were an estimated 229 million cases of malaria worldwide.
Researchers all over the world are working on medications and vaccines, which will be effective enough to eradicate malaria worldwide or offer some protection. Good news is that this month the WHO approved the administration of a new vaccine that stimulates a child’s immune system, thus preventing the development of the parasite. Director of the WHO’s global malaria program Dr. Pedro Alonso says that the development of such treatment is “a historic event”, and we couldn’t agree more!
5. Not all waters are safe - Naegleria fowleri
You are not going to believe this, but the parasites we described so far are not as deadly as the one we are about to discuss! Naegleria fowleri is almost invincible. It is a free-living, cyst-forming amoeba that is highly resistant to any environmental conditions. It can inhabit pretty much any warm body of water, and if water is contaminated with the parasite, it can enter an organism through the nose and olfactory nerves all the way to the brain. N. fowleri causes a rare infection called meningoencephalitis, which leads to severe brain inflammation and seizures. The neurological damage caused by the infection is so severe, that this organism is commonly known as brain-eating amoeba.
This parasite became known to the public in 1978, when a young girl contracted meningitis and died after swimming in the restored Roman Baths, well-preserved facilities for bathing in the city of Bath (Somerset, England). Water testing showed the presence of N. fowleri in the water, and the baths were, then, closed to the public. They remain closed even today, although visitors can tour the baths and museum.