Origin of Parasites and Parasitic Diseases

We have already discussed diseases caused by viruses and bacteria.

In today's essay, l shall write on parasites and parasitic diseases. We shall provide detailed information similar to what we covered for viruses, and bacteria. Let’s begin with the origin of parasites and their classifications.  

Origin of Parasites

Parasites have been part of the evolutionary process for hundreds of millions of years. They are part of the evolution of life on Earth. They evolved from free-living organisms that adapted to exploit host organisms for survival, often forming complex life cycles. Parasites include organisms from diverse kingdoms: Protozoa, Helminths (worms), and Arthropods (e.g., ticks, mites). Some evolved to infect humans, animals, or plants, deriving nourishment and shelter from their hosts while causing harm in varying degrees.

But how are these parasites classified? Parasites are broadly classified into protozoa, helminths, and arthropods. Each group has distinct features and ways of infecting hosts.

First, we have the protozoa (single-celled organisms) that includes for example, the amoebas that move using pseudopodia (false feet), e.g., Entamoeba histolytica.

Second, there are the flagellates that move using whip-like flagella, e.g., Giardia lamblia, Trypanosoma brucei. The ciliates move using hair-like structures called cilia, e.g., Balantidium coli.

Third, there is also the Sporozoa (Apicomplexa) that are non-motile, often intracellular parasites, e.g., Plasmodium species (causing malaria).

The fourth group of parasites are the helminths (parasitic worms). Examples are the nematodes (roundworms). They are the cylindrical worms, e.g., Ascaris lumbricoides (giant roundworm). We also have the cestodes (tapeworms). These are flat, ribbon-like worms, e.g., Taenia solium (pork tapeworm). Next are the trematodes (flukes). These are leaf-shaped flatworms, e.g., Schistosoma species (blood flukes).

The fifth group are the ectoparasites (arthropods). These groups live on the body surface, such as ticks, lice, fleas, and mites. They can cause diseases directly or act as vectors for other pathogens (e.g., mosquitoes transmitting malaria which possibly is the most important parasitic disease in Malaysia from my research findings, and in other tropical countries).

Pathogenicity of Parasites

Parasites exhibit varying degrees of pathogenicity, which depends on their life cycles, mode of transmission, and host interactions. Some parasites cause minimal harm, while others lead to severe diseases. Pathogenicity arises from these four main factors given below:

1. Nutrient Depletion: Parasites consume nutrients meant for the host, leading to malnutrition.

2. Mechanical Damage: Larger parasites (e.g., worms) can obstruct organs or damage tissues. 

3.Immune Evasion: Parasites can alter their surface proteins or hide within host cells to evade the immune system.

4.Toxin Production: Some parasites produce harmful substances or trigger immune responses that cause damage.

Examples of pathogenic parasites and diseases they cause are the protozoa such as Plasmodium spp. (Malaria). The classification for malarial parasites (MP) is Apicomplexa. The transmission of malaria is through the bite of Anopheles mosquitoes. The symptoms for malaria are fever, chills, headache, anaemia, organ failure in severe cases, Malaria is treatable with antimalarial drugs (e.g., chloroquine, artemisinin). It is preventable through vector control (example: mosquito nets).  

Possibly the second most prevalent parasitic disease other than malaria in Malaysia is elephantiasis. My colleagues and I have clinical experience on this disease when we were working in the rural areas of Malaysia. This disfiguring disorders also occurs in other countries in the tropical and subtropical regions caused by the bite of mosquitoes. Lymphatic filariasis causing elephantiasis is where mosquitoes transmit the infection. Microfilariae (larval forms of the parasite) and their causative agents, such as Wuchereria bancrofti, Brugia malayi, and Brugia timori, play a central role in this condition. This disease has significant public health impact in endemic areas.

The drug of choice in the early stages of elephantiasis is diethylcarbamazine citrate (DEC) and ivermectin. These drugs are effective treatments for the microfilariae especially noting that while DEC can target both the microfilariae and adult worms, Ivermectin primarily affects the larval stage and doesn't impact the adult parasites. Incorporating microfilariae in these discussions of parasitic infections highlights the comprehensive understanding of parasitic diseases, especially when considering the early treatment and challenges with chronic lymphatic filariasis and elephantiasis. 

In the late stage once elephantiasis is established, anti-parasitic treatment has no effect on the chronic condition. The management focuses on preventing secondary infections, improving lymph flow, and maintaining hygiene.

Let me show you a photo of an Indian male, aged 56, with a 4-year history of elephantiasis. His condition is no longer curable as it was already in the advanced stage. The swelling on his legs was due to the microfilariae worms totally blocking his lymphatic system on his right legs causing swelling and chronic leg wounds.  I photographed his condition in Kuala Lumpur Hospital on Friday, 20 September 2024. Click for the photo below:

https://photos.google.com/search/_tra_/photo/AF1QipNmwuk86FQ9QcVAM7VxZ1gKzlh4Gvy1IXXQloIY

Let us now look at Entamoeba histolytica (Amoebiasis). This is caused by the amoeba. The route of transmission is contaminated food or water.

The symptoms for amoebiasis are diarrhoea, stomach pain, liver abscess. Amoebiasis is treatable with metronidazole or other anti-amoebic drugs.

In African countries they have the African Sleeping Sickness, caused by Trypanosoma brucei. The parasite involved is the flagellate transmitted by the tsetse fly bite. I have not encountered this parasitic disease in Malaysia. But from the literature the symptoms for African Sleeping Sickness are fever, joint pain, confusion, sleep disturbances, coma if untreated. In term of curability, this disease is treatable with medications (e.g., pentamidine, suramin), but treatment depends on the stage of the disease.

Much more common in rural Malaysia especially among children from studies I have conducted, are the Helminths (worms). The most common is Ascaris lumbricoides (Ascariasis). They are the nematode (roundworm). The transmission is through ingestion of eggs from contaminated food or soil. In ascariasis, the symptoms are normally abdominal pain, malnutrition, intestinal blockage. Fortunately, Ascaris is easily treated with anthelminthic drugs (e.g., albendazole, mebendazole (Zentel).

Next in our discussion is the Schistosoma spp. (Schistosomiasis) caused by Trematode (fluke). The transmission is through contact with contaminated freshwater (larvae penetrate the skin). The symptoms presented are rash, fever, abdominal pain, liver and spleen enlargement. Schistosomiasis is treatable with praziquantel.

Another common parasitic worm is Taenia solium (Cysticercosis/Taeniasis) caused by cestode (tapeworm). The transmission is through ingestion of undercooked pork containing larvae. Patients presented with symptoms of cysticercosis are seizures, muscle pain, neurological issues; taeniasis leads to digestive disturbances. Cysticercosis is treatable with anti-parasitic drugs (e.g., praziquantel, albendazole).

Now let us talk a bit about ectoparasites such as Sarcoptes scabiei (scabies). Scabies is caused by an arthropod (mite). Its transmission is direct skin-to-skin contact. The symptoms for scabies are intense itching, rash, skin sores. Scabies is treatable with topical scabicides (e.g., permethrin).

Some people have Pediculus humanus capitis (head lice). This is caused by an arthropod (louse). The transmission for scabies is direct contact with infected person or contaminated items. In this infestation, there is itching, scalp irritation, but treatable with insecticidal shampoos (e.g., permethrin).

Rarely in Malaysia is Ixodes spp. caused by tick’s vectors for Lyme Disease.  The causative parasite is an arthropod (tick). Tick bite (may transmit Borrelia burgdorferi bacteria causing Lyme disease). The symptoms in Lyme disease are rash, fever, joint pain, neurological symptoms.  Lyme disease is treatable with antibiotics (e.g., doxycycline), but untreated cases may result in chronic symptoms.

But why are some parasites pathogenic? Let me list some of the reasons below.  

Host-Parasite Coevolution: Parasites have evolved sophisticated mechanisms to exploit host resources while minimizing immediate host death, allowing them to reproduce and spread.

Immune System Manipulation: Some parasites can suppress or evade the host's immune response, prolonging infections (e.g., Plasmodium species can hide inside red blood cells).

Direct Tissue Damage: Larger parasites, like worms, may physically damage tissues, while microscopic parasites (e.g., Entamoeba histolytica) invade and destroy host cells.

Toxin Secretion: Certain parasites produce toxins that harm the host (e.g., some helminths release substances that impair immune function).

Not all parasites are harmful; some may live in a host without causing significant disease or damage. For instance, there are commensal protozoa.  Some protozoa inhabit the gut without causing harm (e.g., Entamoeba coli). Second, there are also mutualistic relationships with some parasitic organisms that benefit the host, such as certain gut bacteria that play a role in digestion, though bacteria are not typically classified as parasites.

Curability of Parasitic Diseases

Parasitic diseases are often treatable, but the success of treatment varies based on the parasite and the stage of infection. We have anti-parasitic medications like albendazole, praziquantel, and artemisinin-based therapies used to treat helminthic and protozoan infections.

However, we also have challenges. Some parasitic diseases, especially in their chronic or advanced stages (e.g., cysticercosis), are more difficult to treat, and prevention through hygiene, sanitation, and vector control is often crucial.

Prevention of parasitic infections are improved sanitation. This prevents transmission of intestinal parasites (e.g., by reducing exposure to contaminated water or food).

Then we need to implement vector control. We can prevent diseases that are spread by insects and arthropods (e.g., mosquito control for malaria). There is also ongoing vaccination research aimed at developing vaccines for parasitic diseases like malaria.

ju-boo lim

Formerly, Senior Researcher

Division of Rural Health Research and Community Medicine

Institute for Medical Research

Malaysia