What Are Antiparasitic Drugs?

Antiparasitic drugs are like superheroes in the world of medicine, fighting against tiny invaders that can make us or our furry friends unwell. These medications are specially crafted to tackle parasites - those pesky organisms that can cause infections in both humans and animals. Imagine these drugs as your body's defenders against unwanted guests. 

Antiparasitic drugs are medicines designed to seek out and eliminate parasites that decide to set up camp in our bodies or our pets. They work in different ways, disrupting the parasites' day-to-day activities, stopping them from having babies, or even breaking down their tiny homes. Knowing how these drugs work is like understanding our secret weapons against these microscopic foes. In places where healthcare might not be readily available, understanding these antiparasitic drugs becomes essential in the battle against diseases caused by parasites.

Why does antiparasitic treatment matter?

Parasites are one of the most common causes of infections in the world, predominantly in developing and underdeveloped countries. According to the WHO, over 25% of the world’s population (over 1.5 billion) is suffering from some form of parasitic infection.¹ Most of them are intestinal parasites occurring in the residents of developing countries although developed countries are not immune to parasites either.  Anyone can be at risk of acquiring parasites which usually can be treated. It's not just about feeling better after being sick; it's about the big picture. Parasites can be a big deal, especially in places where they're common. They can slow down entire communities, making it harder for people to work and thrive. That's why having effective antiparasitic treatments is like having a powerful tool to break the cycle of illness and poverty. This introduction aims to give you a sneak peek into the world of antiparasitic drugs and why they're so crucial in keeping us and our communities healthy.

Types of parasites

Generally, there are three types of parasites which account for a significant number of infections in humans.² They are:

Protozoa

These are unicellular, microscopic organisms that are usually transmitted to humans from food, soil and contaminated water resources. Protozoa are the most common intestinal parasites in the developed part of the world. These parasites show a high predisposition to immunocompromised people such as those with AIDS. Some examples of protozoa are Amoebas - causing dysentery and colitis, Trichomonas - causing trichomoniasis (a sexually transmitted disease), Leishmania - causing visceral, cutaneous and mucocutaneous leishmaniasis, Toxoplasma - causing toxoplasmosis, Plasmodium - causing malaria, and Trypanosoma - causing sleeping sickness and Chagas disease.

Helminths

These are also known as worms. They are usually transmitted via ingesting food and water containing larvae, vector transmission and skin penetration. Common diseases caused by worms include schistosomiasis - caused by flukes worms and flatworms (trematodes), cysticercosis - caused by tapeworms (cestodes), and enterobiasis, ascariasis - caused by roundworms (nematodes).

Ectoparasites

These are the group of parasites that live externally on the skin of the host and feed primarily on blood. A common symptom of ectoparasites is itching and pruritis. Organisms such as lice, mites, ticks, fleas and bedbugs are common ectoparasites. Ectoparasites can cause diseases such as scabies, pediculosis and myiasis.

Common antiparasitic drug classes

There are more than dozens of antiparasitic drugs. However, based on the types of parasites, it can be divided into three different classes.

Antiprotozoal drugs 

There are eight different kinds of antiprotozoal agents depending upon the type of parasites.

• Antimalarial agents

These agents are used for the treatment of malaria caused by plasmodium species. There are different types of antimalarial drugs available for use and their choice is dependent upon the type of plasmodium species causing malaria and the severity of the infection. Some examples include drugs such as chloroquine, primaquine, mefloquine, quinine and atovaquone-proguanil.³

• Antiamoebic agents 

Antiamoebic drugs are useful for the treatment of infections caused by species such as Entamoeba histolytica. These include amoebiasis, amoebic colitis and amoebic dysentery. Some of the common drugs in this category are metronidazole, tinidazole and paromomycin sulphate.²

• Antigiardial agents

Giardia lamblia is the causative agent for giardiasis and metronidazole is the drug of choice for treatment. However, alternatively, drugs such as tinidazole, albendazole, ornidazole and secnidazole are also used.^2,8

• Trypanocidal agents

Trypanosoma can cause diseases such as sleeping sickness and Chagas disease with or without central nervous system involvement. They can be treated using drugs such as nifurtimox or benznidazole, pentamidine and suramin.²

• Antileishmanial agents

Leishmania is one of the prominent parasites causing leishmaniasis, a common neglected tropical disease. The drug of choice for leishmaniasis is sodium stibogluconate, however, alternatives such as paromomycin, pentamidine, and amphotericin B can also be used.⁴

• Antibabesial agents

Babesiosis is caused by the babesia parasite and transmitted to humans through tick bite. Treatment includes drugs such as atovaquone and azithromycin or clindamycin plus quinine in severe disease.⁵

• Anti-toxoplasma agents

Toxoplasmosis is a serious condition and mostly affects immune-compromised persons. It can also cause severe congenital defects in a foetus if the mother is infected during pregnancy. Toxoplasma gondii is the causative agent and can be found in uncooked meat, contaminated water and foods. It can also be found in cats and be transmitted to the foetus if comes in contact with cat faeces. Drugs such as sulfadiazine combined with pyrimethamine are the first-line therapy. Alternatively, sulfamethazine and sulfamerazine are also sometimes used.⁶

• Antitrichomoniasis agents

Trichomoniasis primarily affects female reproductive organs. Its causative agent is Trichomonas vaginalis which is transmitted sexually. Metronidazole is the drug of choice for trichomoniasis.² Alternatively, doxycycline and tinidazole can also be used.

Antihelminthic drugs

Antihelminthic agents are the drugs that are used to treat worms in our bodies. It works by either killing the worms entirely or expelling the worms out of the bodies. Different antihelminthic drugs classified based on the parasites they act upon are listed below 

• Antitrematodal drugs

These include drugs such as praziquantel, a broad-spectrum drug of choice for infections caused by flukes such as Schistosoma sp., Clonorchis sinensis, and Paragonimus westermani. Alternatively, metrifonate, oxamniquine, and bithionol can also be used.²

• Anticestodal drugs

Praziquantel is also used for the management of infections caused by different species of tapeworm (Taenia saginata, Taenia solium, Diphyllobothrium latum, and Hymenolepis nana). Another broad-spectrum antihelminthic agent known as albendazole is a drug of choice for hydatid disease and cysticercosis.²

• Antinematodal drugs

Antinematodal drugs include albendazole - the drug of choice for infections caused by roundworms such as ascariasis, trichuriasis, trichinosis, cutaneous Larva migrans, and hookworms. Another drug known as ivermectin is a drug of choice for onchocerciasis, also known as river blindness and diethylcarbamazine is the drug of choice for filariasis.²

Ectoparasiticides

The most common infections in humans caused by ectoparasites include scabies, head lice and pubic lice. Drugs such as permethrin, lindane, benzyl benzoate, topical or oral ivermectin, malathion and pyrethrins are used either individually or sometimes combined in cases of resistant infections to treat ectoparasitic infections.^2,7

Side Effects and Considerations

Side effects of antiparasitic drugs vary with type and depend upon the dosage and pre-existing conditions of the person. Some may have mild side effects whereas others might even lead to effects ranging from renal failure to nervous system breakdown.² Some of the common side effects include

• GI disturbances including diarrhoea, vomiting and nausea
• Fever
• Rash
• Headaches including dizziness 
• Fatigue
• Photosensitivity
• Candida vaginitis can be caused by doxycycline use

Some antiparasitic drugs can cause severe side effects such as 

• Changes in heart rate 
• Hallucinations
• Psychosis
• Insomnia
• Renal toxicity and hepatotoxicity
• Central nervous system toxicity leading to seizures and encephalopathy

Contraindications

Antiparasitic drugs are contraindicated in some cases and hence should only be used under the supervision of healthcare professionals.² Some of the contraindications of antiparasitic drugs are 

• Pregnancy - Antiparasitic drugs such as primaquine, diloxanide furoate, metronidazole, emetine, and praziquantel can cause adverse effects to both mother and foetus and should be avoided.
• Pre-existing conditions - In cases of preexisting medical conditions such as heart conditions, seizure disorders, renal and thyroid disease warrants caution when it comes to antiparasitic use. Some examples are

Summary

In conclusion, antiparasitic drugs are essential defenders against widespread parasitic infections affecting humans and animals globally. Their diverse classes, ranging from antiprotozoal to antihelminthic drugs, provide a nuanced approach to combatting various parasites.

While these medications are crucial tools, their potential side effects emphasise the need for careful administration under healthcare supervision. Contraindications, especially in pregnancy and pre-existing conditions, emphasise the importance of responsible drug use.

Beyond individual health, antiparasitic treatments contribute to breaking the cycle of illness and poverty in communities. As indispensable allies in global health, these drugs play a vital role in fostering equitable well-being.