From ancient remedies to modern antiparasitic drugs, some of medicine's most effective compounds are designed to exploit the differences between a parasite and its human host.
Parasites have evolved extraordinary ways of living inside the human body. Some live in the intestine, feeding on food passing through the digestive system. Others burrow into tissues, invade red blood cells, migrate through the liver or lungs, or live inside the lymphatic system. Some can remain in the body for years.
Yet despite their complexity, many parasites have vulnerabilities that scientists can exploit.
Modern antiparasitic medicines work by attacking those vulnerabilities. Some paralyse worms. Others disrupt their energy production, damage their protective surfaces, interfere with DNA or prevent parasites from reproducing.
The result is a remarkable collection of chemical weapons that can kill organisms living inside us while, when properly used, causing considerably less damage to the human host.
First, what counts as a parasite?
The word parasite covers several very different organisms.
Helminths are parasitic worms, including roundworms, hookworms, whipworms and tapeworms.
Protozoa are microscopic single-celled parasites. They include Plasmodium, which causes malaria, and organisms responsible for diseases such as giardiasis, amoebiasis and leishmaniasis.
There are also parasites that live on the outside of the body, such as mites and lice.
Consequently, there is no single "parasite killer". A compound that is devastating to one species may have little or no effect on another.
The World Health Organization lists medicines including albendazole, mebendazole, ivermectin, praziquantel and triclabendazole among important treatments for several major parasitic diseases.
Albendazole and mebendazole: starving the worms
Two of the most important anti-worm compounds belong to a family called benzimidazoles. The best-known are albendazole and mebendazole.
They work partly by interfering with tubulin, which is a protein parasites need to construct their internal cellular structures. This disruption affects the parasite's ability to absorb nutrients and maintain its cells.
In simple terms, the drug interferes with the worm's cellular machinery until the parasite can no longer function normally.
These compounds are particularly important against intestinal worms such as Ascaris, hookworms and whipworms. Their usefulness is partly due to something that is easy to overlook: selective toxicity.
A drug does not necessarily have to be harmless to every cell in the body. It needs to exploit a biological process that the parasite depends on more heavily, or that can be targeted at a sufficiently different level from the human equivalent.
That principle lies behind much of antiparasitic medicine.
Ivermectin: turning the parasite's nervous system against it
Few antiparasitic compounds are more famous than ivermectin. It belongs to a class called the avermectins and is particularly effective against several types of nematode and arthropod parasites.
Its principal target is a group of glutamate-gated chloride channels found in many invertebrates. When ivermectin interacts with these channels, it increases chloride ion movement through parasite nerve and muscle cells.
The result can be paralysis.
For a microscopic parasite, that can be fatal. Ivermectin is used in the treatment and control of diseases including onchocerciasis, commonly known as river blindness, and lymphatic filariasis. WHO includes it among the medicines used in large-scale programmes against neglected tropical diseases.
The compound also illustrates an important principle of pharmacology: the same chemical can be useful against multiple parasites because different organisms may share a vulnerable biological mechanism.
Praziquantel: a chemical attack on flatworms
If ivermectin is particularly important for certain roundworms, praziquantel is one of the major weapons against parasitic flatworms.
It is especially important in treating schistosomiasis and several tapeworm infections.
Praziquantel affects the parasite's calcium regulation. The drug causes an abnormal increase in calcium permeability in the worm's cells, producing powerful muscular contraction and damage to the parasite's outer surface.
The worm effectively loses control of its body. Its damaged surface can then become more vulnerable to the host's immune system.
Praziquantel has become one of the world's most important antiparasitic medicines.
Triclabendazole: targeting liver flukes
Some parasites occupy particularly difficult locations. Liver flukes, for example, can live inside the bile ducts.
One important drug against them is triclabendazole. Unlike many of the broad-spectrum drugs used against intestinal worms, triclabendazole has particular importance against Fasciola species, the liver flukes responsible for fascioliasis.
Its importance demonstrates another feature of antiparasitic medicine: sometimes the challenge isn't simply finding something that kills a parasite, but finding something that works against a particular parasite at the stages of its life cycle that occur inside humans.
Artemisinin: the parasite killer from a plant
Perhaps the most extraordinary story involves artemisinin. The compound originates from Artemisia annua, commonly known as sweet wormwood (qinghao).
Artemisinin is one of the most fascinating antiparasitic compounds ever discovered. It became one of the foundations of modern malaria treatment.
Malaria parasites belong to the genus Plasmodium and spend part of their life cycle inside human red blood cells during an important part of its life cycle. While there, it digests haemoglobin to obtain nutrients.
Haemoglobin contains haem, which contains iron. The leading model is that iron-containing chemistry helps activate artemisinin's peroxide bridge. Once activated, the molecule can generate highly reactive intermediates that damage important parasite molecules.
The parasite is therefore attacked at a chemical level rather than simply being paralysed.
Artemisinin derivatives, including artesunate, artemether and dihydroartemisinin, act rapidly against malaria parasites.
Would drinking absinthe work?
No — drinking absinthe would not be a reliable way to treat malaria or other internal parasites.
The confusion is understandable because both absinthe and artemisinin have connections to Artemisia plants, but they're very different things.
Artemisinin comes primarily from Artemisia annua (sweet wormwood).
Absinthe traditionally contains Artemisia absinthium (grand wormwood), along with other botanicals and, importantly, a substantial amount of ethanol.
Absinthe is not a standardised artemisinin medicine. The amount of artemisinin in an absinthe product would therefore not provide a predictable therapeutic dose.
The compound responsible for absinthe's historical reputation, thujone, isn't artemisinin and isn't an established malaria treatment.
Drinking enough alcohol to try to obtain a medicinal effect would expose someone to acute alcohol toxicity without providing any dependable antiparasitic treatment.
The parasite fights back
Killing parasites is only half the story. Parasites evolve.
Drug resistance occurs when genetic changes allow some parasites to survive exposure to a medicine. Those survivors can reproduce, increasing the proportion of resistant parasites in subsequent generations.
Malaria provides perhaps the clearest example. A WHO 2025 World Malaria Report identifies growing antimalarial drug resistance as a major threat.
Partial resistance to artemisinin has been confirmed in several African countries, including Eritrea, Rwanda, Uganda and Tanzania, while suspected resistance has been reported in several others.
Other chemical weapons
The list of antiparasitic compounds extends far beyond the best-known drugs.
Metronidazole
Metronidazole is active against certain anaerobic organisms and protozoan parasites. It is used against infections including giardiasis and amoebiasis, depending on the organism and clinical circumstances.
Its active metabolites can damage DNA and other cellular components.
Nitazoxanide
Nitazoxanide is another antiparasitic compound with activity against certain protozoa, including Giardia and Cryptosporidium in appropriate circumstances.
It interferes with energy metabolism in susceptible organisms.
Diethylcarbamazine
Diethylcarbamazine, or DEC, has an important role in treating certain filarial worm infections. It has been used extensively against lymphatic filariasis and other filarial diseases.
Atovaquone and proguanil
These compounds demonstrate another approach to malaria. Atovaquone disrupts the parasite's mitochondrial energy production, while proguanil interferes with parasite folate metabolism.
The combination is used for malaria prevention and treatment in appropriate circumstances.
So, why don't these drugs kill us too?
This is perhaps the most interesting question. A parasite living inside a human is using the same basic biological building blocks as its host: proteins, DNA, membranes and energy-producing systems.
So how can a chemical destroy the parasite without destroying the person?
The answer is biological differences.
Parasites may possess:
- molecular receptors humans don't have
- enzymes that humans lack
- different versions of shared proteins
- unusual metabolic pathways
- distinctive ion channels
- unique methods of obtaining nutrients
- different sensitivity to particular chemicals
A drug designer's goal is therefore to find a difference that can be exploited. The closer the parasite is biologically to humans, the more difficult this becomes.
This is one reason some parasitic infections are considerably harder to treat than others.
The extraordinary problem of parasites with complicated life cycles
Some parasites don't simply live in one place. They move.
A parasite may begin in contaminated soil, enter a person's body, migrate through tissues, mature in the intestine and eventually produce eggs that leave the body.
Others require an intermediate host.
Schistosomes, for example, have a life cycle involving freshwater snails. Malaria parasites move between mosquitoes and humans.
This complexity creates another challenge for medicine. A drug may be highly effective against one stage of a parasite but much less effective against another.
Consequently, treating a parasitic infection isn't always about finding one compound that kills everything.
Doctors may need to consider:
- the parasite species
- its location in the body
- its life-cycle stage
- drug resistance
- the patient's age and weight
- pregnancy
- immune status
- interactions with other medicines
The hidden danger: treating the wrong parasite
Antiparasitic drugs are not interchangeable. A person with a tapeworm does not necessarily need the same treatment as someone with malaria, and a drug effective against a roundworm may do little against a liver fluke.
Symptoms can also overlap. Abdominal pain, diarrhoea, fatigue, weight loss and fever can have many causes.
An ongoing chemical arms race
Humanity has developed an impressive arsenal against parasites:
- Albendazole disrupts worm biology
- Mebendazole interferes with cellular structures
- Ivermectin can paralyse susceptible parasites
- Praziquantel disrupts calcium regulation in flatworms
- Triclabendazole attacks liver flukes
- Artemisinin rapidly attacks malaria parasites
Other compounds interfere with parasite metabolism, DNA, mitochondria or reproduction.
But parasites are not static targets. They evolve. Reports that resistance to medicines used against several parasitic diseases is an increasing concern, including resistance involving malaria and some neglected tropical diseases.
The future of antiparasitic medicine will therefore depend on more than discovering powerful chemicals.
Scientists need new drug targets, new compounds, combination therapies, better diagnostics and continued surveillance for resistance.
The ultimate objective is not simply to find something that kills a parasite.
It is to find something that kills the parasite while leaving the patient alive and healthy — and remains effective long enough for the parasite to have nowhere left to hide.
Important note:
These medicines are powerful pharmaceuticals, and the appropriate drug depends on the specific parasite and circumstances. They should not be self-administered simply because symptoms appear consistent with a parasitic infection.
Read more on:
Nanoplastics: How They Get Into Your Body
Switzerland’s Pharmaceutical Machine: How a Small Country Became a Global Drug Powerhouse
Palantir and Your NHS Data: Questions You Might Be Asking
Can Wireless Technology Affect Tinnitus?
Comments
Post a Comment