Protozoan Diseases - An Overview on Protozoal Infections

Table of Contents

What are Protozoans?

Protozoans are eukaryotic, unicellular, heterotrophic organisms that can be either parasites or free-living. They exhibit an enormous range of morphologies and lack cell walls, for example, Entamoeba, Plasmodium, Paramecium, etc.

Protozoa are not easily defined as they are diverse and often only distantly related to each other. Because of their extreme diversity, the only feature common to all protozoans is that they are unicellular eukaryotic organisms. Thus they have eukaryotic membrane-bound cell organelles that can exhibit various tasks.

Protozoan Diseases

Many important human and animal diseases are caused by protozoans. In addition to being known as parasites, some protozoa are considered opportunistic pathogens that primarily affect immunocompromised individuals.

Human diseases – African sleeping sickness, malaria, amoebic dysentery, etc.

Animal diseases – Leishmaniasis (dogs, humans and rodents), Babesiosis, etc.

Plant diseases – Phloem necrosis (coffee), sudden wilt of oil palm, etc.

Parasitic protozoans exhibit complex life cycles with various morphological forms involving multiple hosts and vectors. Under favourable conditions, many of them exist in a dynamic feeding and growing form called the trophozoite. Under unfavourable conditions, the trophozoite can convert into a dormant cyst and survive the harsh conditions. These protozoan forms multiply and infect the blood or tissues of humans and induce sickness.

Also, Read: Infectious Diseases

List of Protozoan Diseases




Entamoeba histolytica


Contaminated water or food

Giardia lamblia


Contaminated water or food





Visceral disease

Sand flies

Trichomonas vaginalis

Inflammation of urogenital tract

Sexually transmitted disease

Trypanosoma brucei gambiense

African sleeping sickness

Tsetse fly


Watery diarrhoea

Waterborne outbreaks

Toxoplasma gondii


Ingestion of undercooked meat


It is a mosquito-borne protozoan infectious disease that affects both animals and humans. The causative protozoan is Plasmodium, and it is spread by Anopheles mosquitoes. The symptoms usually include tiredness, vomiting, fever and headaches. The common species that cause this disease are – Plasmodium falciparum, P. ovale, P.vivax, P. malariae and P.knowlesi.


The term denotes several diseases caused by protozoan trypanosomes. In humans, it causes American trypanosomiasis and African trypanosomiasis. The former affects the Latin American population, and the latter is seen in the regions of Sub-Saharan Africa. Trypanosomiasis typically results in the swollen lymph gland, aching muscles, fever, pain in joints, etc.


This infection is caused by protozoan Entamoeba histolytica. Amoebiasis can result in severe weight loss, diarrhoea, abdominal pain, colonic ulcerations and even bloody diarrhoea. Excessive bleeding can also lead to anaemia.


This protozoan disease is caused by more than twenty Leishmania species. They are transmitted by the protozoan infected sandflies (vector). The three main types of leishmaniasis are – Visceral leishmaniasis or kala-azar, Cutaneous leishmaniasis and Mucocutaneous leishmaniasis. The common symptoms include fever, skin ulcers, enlarged liver and low red blood cells.

Protozoan Parasites in Animals

Parasitic protozoans are considerably large in number, and thus the number of organisms they affect is also higher. These protozoans can cause significant morbidity and mortality in domestic animals and thus provokes a substantial economic loss. Some protozoan animal diseases that have a considerable economic impact are –

  • Coccidiosis caused by Eimeria spp infects animals like goats, sheep, rabbits, and horses.
  • Ovine theileriosis is caused by Theileria lestoquardi in goats and sheep.
  • Amoebiasis is caused by Neoparamoeba perurans in salmon raised in fish farms.
  • Equine piroplasmosis is caused by Theileria equi and Babesia caballi in donkeys, horses and mules.
  • Trypanosomiasis is caused by Trypanosoma brucei, T. vivax and T.congolese in cattle, pigs, goats, sheep and camels.

Prevention and Control of Protozoan Diseases

Parasitic protozoans are considerably much larger and more complex pathogens than viruses or bacteria. They use various mechanisms to evade the host immune system. It is difficult to develop vaccines for such protozoan diseases. Thus preventing protozoan infection is an easier option than treating them.

  • Most protozoan diseases are transmitted through oral-faecal routes. Thus improvement in personal hygiene and sanitation can considerably reduce the spread of protozoan infections.
  • Control of disease-causing vectors is also a preventive method.
  • At present, using antiprotozoal drugs is the most preferred treatment for such diseases. Metronidazole, benznidazole, dehydroemetine and atovaquone, hydroxychloroquine are some prescribed antiprotozoal drugs.

Protozoans often use antigenic variations to escape the immune response of the host. Also, they are difficult to culture in vitro, and thus mass production of successful vaccines seems a major task. At present, usage of drugs, controlling vectors and improving sanitation in combination have led to the control of most protozoan diseases in humans as well as domestic animals.

Frequently Asked Questions

What are some water-borne protozoan diseases?

Amoebiasis is a classic example of water-borne protozoan infection. The other diseases are cryptosporidiosis, giardiasis, naegleriasis and cyclosporiasis. Untreated water and sewage contamination are the major causes of such infections. Most common symptoms of water-borne infections are vomiting, abdominal discomfort and diarrhoea.

What causes encephalitis?

Any inflammation in the brain is termed encephalitis. The symptoms include fever, vomiting, headache and confusion. It can be caused due to any agent such as protozoans, viruses, bacteria and fungi. Certain protozoan infections like malaria, naegleriasis and toxoplasmosis can also result in encephalitis.

What are some protozoan diseases in plants?

Most protozoan plant infections are caused by a genus called Phytomonas (trypanosomatids). They infect host plants and are typically transmitted by insect vectors. Examples – Phytomonas leptovasorum infects coffee plantations by causing phloem necrosis, and Phytomonas staheli causes wilt of oil palms and coconut.

What are fungal and bacterial diseases?

Fungi are microbes that have chitin in their cell walls, while protozoans lack a proper cell wall. Examples of fungal infections are – athlete’s foot, candidiasis, ringworm, etc. Bacterias are also tiny microbes that cause diseases like cholera, gonorrhoea, tuberculosis, diphtheria, etc. They can also be transmitted through vectors, contaminated water, air and body fluids.

Also, Check: Bacterial Diseases in Humans

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