Pathogens are disease-causing Microorganisms like protists, viruses, Bacteria, and Fungi. Pathogens can infect everyone. They are spread between organisms through either direct contact with the pathogen via fomites like water or faeces; or through contact with another sick infected organism.
The most known and lethal pathogens are usually Bacteria like Mycobacterium Tuberculosis and viruses like HIV, but protists and Fungi are also very important groups of pathogens capable of causing serious illnesses, known as protist diseases.
Pathogens are Microorganisms that cause infectious diseases by spreading between organisms and infecting people, Animals or Plants.
Fomites are inanimate components that can transmit the disease when exposed to the pathogen.
What are protists?
The Protoctista or protists encompass a wide spectrum of eukaryotes, including protozoans ('simple Animals') and Algae, such as seaweeds. A protoctist is a type of eukaryote that isn't a fungus, plant, or animal. They are generally single-celled, or occur as clusters of similar Cells and have a nucleus. Some have animal-like cells (no cell wall) and are sometimes referred to as Protozoa, while others have plant-like cells (cellulose cell walls and chloroplasts) and are referred to as algae.
Protists are eukaryotic, usually single-celled organisms of the kingdom Protista that are not Plants, animals or fungi.
Eukaryotes are organisms with eukaryotic cells that have a nucleus and other structures in the cytoplasm which have membranes around them.
Read our article Eukaryotes and Prokaryotes to learn more about these two types of organisms!
Only a few protists are pathogenic, but the diseases they cause are serious like Malaria. Often, protists need a vector to transfer from one host to the next. Vectors are very important tools for the transmission of pathogens. For example, malaria is transmitted via the Anopheles mosquito vector.
Hosts are organisms that harbour the pathogenic agent.
Vectors are organisms that do not cause disease itself but that spread infection by transmitting pathogens from one host to another.
What are the different types of protists that can cause diseases?
Many organisms in this kingdom may have more in common with organisms from other kingdoms than they do with one another. An example of this is that there are compelling grounds for identifying Algae as plants rather than protists. For this reason, protists can be divided into three main categories:
• Plant-like Protists
• Animal-like Protists
• Fungus-like Protists
Plant-like protists
Plant-like protists live in soil, freshwater, saltwater or bark and produce oxygen. They include:
Animal-like protists
Animal-like protists are called “Protozoans” which translates to ‘first animals’. They are all unicellular and tend to be categorised by how they move:
Fungus-like protists
These have cell walls and reproduce by forming spores, like fungi. Examples include:
Protist diseases in humans
Protists can cause severe diseases in humans such as Malaria, Trypanosomiasis (African sleeping sickness), Giardiasis, and Amoebic dysentery. The most relevant and lethal today by far is malaria which remains one of the biggest causes of death in the developing world.
Example of protist disease: Malaria
Malaria is typically transmitted between infected and uninfected people via the female Anopheles mosquito. This vector insect bites humans to feed on their Blood to obtain the necessary Proteins. If the bitten person is infected with the malaria parasite, the mosquito will also become infected while collecting the blood. The parasite will then grow and multiply inside the insect. When the mosquito feeds again, it will also release the parasite into the bloodstream of the bitten individual.
Malaria is a disease spread by mosquitoes. Mosquitoes act as a vector and carry the pathogenic agent Plasmodium falciparum (animal-like protist) without being affected by the disease. Malaria is particularly prevalent in areas such as Africa, Asia, South and Central America due to the higher temperatures.
Malaria transmission, therefore, requires cyclical infection of both humans and mosquitoes. Plasmodium multiplies inside both hosts and increases the chance of infecting other mosquitos and humans. Plasmodium’s life cycle has two stages: a sexual stage in the mosquito and an asexual stage inside humans.
Upon infection, the malaria pathogen parasite enters the bloodstream and initially targets liver Cells followed by red Blood Cells, multiplying inside both. Most pathological manifestations of malaria result from the erythrocyte (red Blood cell) infection stage. In this step, the parasite repeatedly destroys the red Blood Cells it was growing inside and invades other red Blood Cells cyclically.
Malaria symptoms
Malaria infection can result in various symptoms, often dividing malaria into uncomplicated or severe (complicated) disease. Following the infective mosquito bite, there is an incubation period before symptoms appear that can range from a week to a year, but, on average, it takes 10-15 days. In some aggressive cases, malaria symptom onset can occur as fast as 24 hours after the infective mosquito bite. Shorter incubation periods are more common with the more dominant Plasmodium falciparum parasite.
The incubation period refers to the time taken for the clinical manifestations of a disease to appear after exposure to a pathogen.
Malaria symptoms include:
Fever
Sweats and chills
Headaches
Vomiting and diarrhoea
Malaria treatment
Malaria is a treatable and curable disease. Early diagnosis and treatment are important to reduce malaria incidence and deaths. Malaria treatments include the antimalarial drugs quinine and chloroquine, which prevent the parasite from growing and spreading within the body by inhibiting its Protein Synthesis. These drugs can also be used as prophylactics (preventive drugs) to stop an infection from occurring if a person gets bitten by an infected mosquito.
According to the WHO, the best treatment available is artemisinin-based combination therapy (ACT), especially for Plasmodium falciparum infection. ACT works by rapidly eliminating the malaria pathogen from the bloodstream, thus stopping disease progression.
Besides prophylactic drugs, two other important preventive measures play a significant role in helping the global effort to control malaria. These are vector control strategies and the malaria vaccine. Vector control strategies include efforts to either reduce the number of malaria vector mosquitos or avoid getting bitten by them. In summary, prevention techniques include:
Sleeping under mosquito nets.
Wearing insect repellent to avoid bites.
Removing standing water sources to help reduce mosquitos
Since late 2021, the WHO has recommended using the first approved malaria vaccine RTS,S (Mosquirix), among children. This vaccine acts against Plasmodium falciparum and significantly reduces severe malaria disease in children.
Example of protist disease: African Sleeping sickness
Like malaria, sleeping sickness disease is spread via a vector. The Tsetse fly is a species native to Africa and it spreads a protist disease called sleeping sickness, also known as Human African trypanosomiasis. This disease is caused by protists microorganisms of the species Trypanosoma brucei. (animal-like protist) The Tsetse flies are found only in sub-Saharan African countries, and it's the people who live in the rural parts of Africa that are most affected.
When the fly bites a person, the bite develops into a red sore and the person develops a fever, muscle and joint ache, swelling in the lymph glands, irritation, and headaches. It is a disease that attacks the central nervous system. Other symptoms include:
If these symptoms are not treated, the disease will worsen and can be fatal within months.
Example of protist disease: Giardiasis
Giardiasis is a diarrheal disease caused by a small parasite protozoan called Giardia. Giardia can be found on surfaces or in soil, food, or water that has been polluted by sick individuals or animals' faeces. If you swallow Giardia, you can develop giardiasis. Giardia is easily transmitted from person to person, as well as through polluted water, food, surfaces, and items. Swallowing contaminated drinking or recreational water is the most prevalent way for people to become ill.
Giardiasis can be treated with Antibiotics and other prescription drugs.
The main symptoms of giardiasis are:
smelly diarrhoea
tummy pain or cramps
farting (flatulence)
dehydration
weight loss
Example of protist disease: Amoebic dysentery
The protozoan parasite Entamoeba histolytica causes amoebic dysentery or amoebiasis. Like giardiasis, it's transmitted by contaminated drinking water or by coming into contact with anything that has touched the faeces of an infected person. It results in mild symptoms such as fever, chills, diarrhoea, and abdominal discomfort, and only 10-20% of people become ill after infection.
It can be treated by Antibiotics used alongside IV fluids and medication that control diarrhoea, nausea and vomiting.
Plant diseases caused by plants
Downy mildew is a protist disease affecting many plants. It's spread by spores that can remain dormant in the soil for several years. It affects the yield and quality of crops as well as spoiling the appearance of ornamental plants. It's caused by several different types of protists of the Peronospora genus (water moulds or fungus-like protists) that infect leaves. By practising crop rotation, it is possible to avoid reinfection. Some of the symptoms of downy mildew include:
Mould-like growth on the underside of the leaf.
Discoloured blotches on the upper leaf surface.
Severely affected leaves may shrivel and turn brown.
Severely affected plants are often stunted and in some cases, the plant may die.
Spores are special types of reproductive cells that certain organisms like some fungi, plants, bacteria and protists produce and can originate a new individual without fusion with other reproductive cells.
Controlling downy mildew can be difficult. No fungicide chemicals are available to gardeners for use against downy mildews, but several non-chemical control strategies can be used including:
- Practice crop rotation.
- Avoid watering plants in the evening.
- Pick off and dispose of affected leaves.
- Remove and destroy severely affected plants.
- Avoid dense planting to allow good air circulation around the plants.
- Avoid periods of high humidity in greenhouses by opening doors and vents to encourage air movement.
Protist Diseases - Key takeaways
- Pathogens are microorganisms like protists that cause infectious diseases by spreading between organisms and infecting people, animals or plants.
- Protists are a group of eukaryotic, usually single-celled organisms of the kingdom Protista that are not plants, animals or fungi. Protists can be animal-like, plant-like or fungus-like.
- The main diseases caused by protists are African sleeping sickness, Giardiasis, and Malaria.
- An example of a protist disease in humans is malaria.
- An example of a protist disease in plants is Downy Mildew
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