Parasites can cause nasty infections, but they are actually very similar to our own cells. In this lesson, learn about the types of drugs commonly used to eradicate eukaryotic pathogens.
Parasites: Protozoans and Worms
Parasites, such as protozoans and worms, or helminths, are some of the freakiest pathogens around. Is it because they're bigger than bacteria and can look like miniature animals that they're so gross? Is it because just the idea of worms living in our body is disgusting? Or, is it the terrible symptoms they can cause, such as severe intestinal problems, skin lesions and major damage to organs such as the brain, heart, liver, eyes and lungs?
At any rate, we don't want parasites in our bodies. So, we can kill them off with drugs, right? Since protozoans and worms are eukaryotes like us, there are limited options for treating parasitic infections without harming the host too much. In this lesson, we'll learn about some of the most common antiparasitic drugs and how they work. You're going to hear a lot of drug names in this lesson. For many of you, it may be more important just to learn the general ways that parasites can be treated rather than remember all of the drug names.
You've probably heard of malaria, an important disease caused by a protozoan parasite called Plasmodium. Malaria is estimated to affect 300 to 500 million people in the world and is especially prevalent in Africa, where a child dies about every 30 seconds from this devastating disease.
The most widely-used and cheapest drug for malaria is Chloroquine, which is derived from a plant-based traditional medicine called quinine. Chloroquine causes the buildup of a toxic molecule called heme inside the parasite, and it is also thought to block the parasite's DNA synthesis. However, nowadays, there is widespread resistance to this drug, so alternatives are needed.
One alternative to chloroquine is Artemisinin Combination Therapies (ACTs). Artemisinin had been used for a long time in traditional Chinese medicine to control fevers before it was discovered to be effective against malaria. It inhibits the parasite's nucleic acid and protein synthesis, essentially shutting down the factories that produce the molecules the parasite needs to reproduce. ACTs are expensive compared to chloroquine, which has unfortunately led to cheap counterfeit versions that contain very little of the actual drug. The main result of these counterfeit drugs is to increase resistance to ACTs, which is very bad news for the future of malaria treatment.
Other Antiprotozoan Drugs
Have you ever heard of Giardia? This waterborne parasite causes severe intestinal disease, as does the related parasite Cryptosporidium. These two bugs are particularly problematic because they are somewhat resistant to chlorine, so they can sometimes contaminate drinking water and swimming pools. Amoebas can also cause a nasty intestinal disease called amebic dysentery.
There are a few drugs that are commonly used to treat these kinds of protozoan diseases. The first one, Metronidazole, fights protozoans by damaging their DNA and inhibiting DNA synthesis. It is selectively toxic because it's a pro-drug, meaning that it starts off as a harmless molecule that must be converted into an active drug. Lucky for us, this conversion doesn't happen in our own cells, only inside of protozoans and also anaerobic bacteria.
Nitazoxanide is another antiprotozoan drug that disrupts the anaerobic metabolism that these parasites use to make energy, so treatment with this drug is like turning off the parasite's electricity source.
Next, we'll talk about the drugs that are used to treat helminth infections, that is, parasitic worms. Niclosamide is a drug that pulls the plug on worms' energy source by stopping production of ATP, a major energy carrier in all cells. Mebendazole also cuts off worms' energy, this time by preventing them from absorbing nutrients from their environment. Can you imagine if a drug prevented you from eating? You wouldn't last long, would you?
The last two drugs we'll talk about are kind of cool to think about because they actually paralyze worms before they kill them. Ivermectin does this by interfering with the worm's chloride ion channels. Praziquantel changes the permeability of worm cells' plasma membranes, causing spasms and paralysis. Interestingly, praziquantel also makes worms more susceptible to our immune system by exposing surface molecules that our antibodies can bind to.
In this lesson, we learned about a variety of drugs that are used to treat parasites, such as protozoans and worms, or helminths.
Malaria is one of the most important protozoan diseases globally, and we learned about Chloroquine and Artemisinin Combination Therapies (ACTs), two types of drugs that are used to treat it. Chloroquine, which inhibits the parasite's DNA synthesis, is cheap and accessible around the world, but there is widespread resistance to it as well. ACTs inhibit the parasite's nucleic acid and protein synthesis, but they are more expensive and are often counterfeited.
Next, we learned about two drugs that are used to fight protozoan parasites, such as Giardia, Cryptosporidium and amoebas. Metronidazole is a pro-drug that is transformed into an active drug inside the parasites and also inside of anaerobic bacteria. It damages DNA and inhibits DNA synthesis. Nitazoxanide blocks the anaerobic metabolism that the parasites need to make energy.
Finally, we learned about antihelminthic drugs that are effective against parasitic worms. Niclosamide pulls the plug on worms' energy source, preventing them from making ATP. Along the same lines, Mebendazole prevents worms from absorbing nutrients from their environment, cutting off their energy in that way. Finally, Ivermectin paralyzes and kills worms by interfering with their chloride channels, and Praziquantel changes worm cells' plasma membrane permeability, causing spasms, paralyzing the worms, and exposing surface molecules that our immune system can attack.
Following this lesson, you will be able to:
- Identify drugs used to treat malaria and explain how they work
- Name some other protozoans that can harm us and summarize how drugs used to treat their infections work
- Explain some of the different ways antihelminthic drugs work