Flagellates: The 2 Classes That Are Medically Important

Flagellates are protozoans of the Mastigophora group. They are unicellular eukaryotes with organelles such nucleus, the kinetoplast, axoneme, and undulating membrane. They move by means of flagella and multiply by binary fission.

Flagellates of medical importance are broadly divided into two main groups;

1. Uro-intestinal flagellates

  • Giardia lamblia
  • Trichomonas vaginalis
  • Dientamoeba fragilis

2. Hemo-somatic flagellates

  • Leishmania species
  • Trypanosoma species

1. Uro-intestinal Flagellates

a) Giardia lamblia

Giardia lamblia, commonly referred to as the old man in goggles causes giardiasis characterized by the following symptoms:

  • Abdominal pain
  • Severe diarrhea
  • Vomiting
  • Maltose intolerance
  • Fat malabsorption
  • Impaired growth in children
  • The stool passed out is offensive, mucoid due to fat without any bloodstains

Giardia lamblia resides on the upper part of the small intestine majorly in the duodenum and jejunum, has both trophozoite and cyst stages, the trophozoite is pyriform in shape (pear-shaped), rounded anteriorly and pointed on the posterior end.

The trophozoite contains flagella for locomotion, the motility is progressive, tumbling and spinning often like that of falling leaf in fresh stool samples. The cyst has 4 nuclei.

Giardia has a direct lifecycle and multiplies by simple binary fission.

Man is the only host for Giardia lamblia.

The infective stage is a mature cyst while the diagnostic stage can be both the cyst or trophozoite stage.

b) Trichomonas vaginalis

Trichomonas vaginalis reside in the urogenital tract of both men and women but commonly of women mainly in the cervix, urinary bladder, prostate, and seminal vesicles. Trichomonas vaginalis has only the trophozoite stage. The trophozoite is pyriform (pear-shaped), has non-directional, jerky (on spot) motility. It has 4 flagella on the anterior side and a single flagellum on the posterior side.  Trichomonas vaginalis is transmitted by sexual contact and also through the sharing of clothes and toilet equipment.

Trichomonas vaginalis causes trichomoniasis which is characterized by:

  • Vaginitis
  • Urethritis
  • Chaffing of the vulva
  • Cervical erosion
  • Burning sensation
  • Yellowish discharge
  • Reversible sterility in men

c) Dientamoeba fragilis

Has only the trophozoite stage was originally classified as a Sarcodina but has organelles found in group Mastigophora. Transmission of D. fragilis is thought to be via helminth ova such as Ascaris and Enterobias species. Most cases of D. fragilis are asymptomatic and some can be symptomatic presenting with abdominal discomfort, diarrhea, weight loss, and flatulence.

2. Hemo-somatic Flagellates

Leishmania species are transmitted by sandflies of the phlebotomous species while Trypanosoma species are transmitted by either tsetse fly (glossina species) for African trypanosomiasis or the kissing bug (triatomid bug) for American trypanosomiasis. Transmission occurs through insect vectors and man is the definitive host. 

Both Leishmania and Trypanosoma produce by longitudinal binary fission.

There are several developmental stages of the two parasites depending on: the presence or absence of free flagellum, position of the kinetoplast relative to the nucleus and presence or absence of undulating membrane. The developmental stages are amastigote and promastigote, epimastigote, trypomastigote, and metacystic trypomastigote.

a) Leishmania Species

Leishmania species cause leishmaniasis.  Leishmania species contain the amastigote and promastigote stage only. Man is the definitive host.

The life cycle of Leishmania.

Promastigote is inoculated into the skin by a sandfly during a blood meal. The promastigotes are taken up by macrophages and they become amastigotes. Amastigotes multiply by binary fission. Amastigotes are ingested by the insect vector during a blood meal and become promastigote in the gut of the vector. Promastigotes multiply by binary fission and migrate to the proboscis of the sand fly.

Clinical diseases caused by Leishmania species are:

  • Cutaneous leishmaniasis caused by Leishmania tropica major, Leishmania tropica minor, Leishmania mexican complex, and Leishmania aethiopica.
  • Visceral leishmaniasis caused by Leishmania donovani also called kala-azar fever.
  • Mucocutaneous leishmaniasis caused by Leishmania braziliensis complex.
Cutaneous Leishmaniasis

Cutaneous leishmaniasis causes skin lesions. The commonest of the three clinical diseases caused by Leishmania species

Leishmania tropica minor

Causes dry, urban, chronic old-world oriental sore.  At the site of bite by the vector, there is a painless dry ulcer that is self-limiting after about 1-2 years leaving a disfiguring sore. After the infection heals it confers long-lasting immunity to reinfection.

Leishmania tropica major

Causes a moist/wet, rural, acute old world cutaneous leishmaniasis. Has a wider geographical distribution compared to Leishmania tropica minor. The sore is self-healing after 3-6 months. After healing it confers a longlasting immunity against reinfection and infection from Leishmania tropica minor. There is sporadic human infection due to Leishmania tropica major.

Leishmania Aethopica

Found in Kenya, Ethiopia and some parts of Yemen. It causes old world cutaneous leishmaniasis and diffuse leishmaniasis.

Visceral Leishmaniasis

Caused by Leishmania donovani.  The Amastigotes reside in the reticuloendothelial cells of visceral organs such as spleen, liver, bone marrow, kidney, lungs, brain, and intestines. Visceral leishmaniasis is characterized by fever, chills, hepatomegaly, splenomegaly, lymphadenopathy, and hypopigmentation of the skin. It can also be transmitted by blood transfusions, sexual contact and congenitally.

Mucocutaneous Leishmaniasis

Caused by Leishmania braziliensis complex. Its habitat is in the reticuloendothelial cells of mucocutaneous tissues of the nose, mouth, lips, and larynx.

The risk factors for acquiring leishmaniasis are poverty, urbanization, malnutrition, and deforestation.

b) Trypanosoma Species

In humans, it involves African trypanosomiasis also called sleeping sickness and American trypanosomiasis also called Chagas disease.

African Trypanosomiasis

Caused by Trypanosoma brucei gambiense which is responsible for about 98% of the cases and Trypanosoma brucei rhodesiense accounting for 2% of the cases. Humans are the main reservoir host for Trypanosoma brucei gambiense while domestic animals and wild animals are the main reservoir host for Trypanosoma brucei rhodesiense.

Trypanosoma brucei gambiense causes a chronic illness with onset symptoms after a prolonged incubation period. Mostly found in tropical countries of west and central Africa. While Trypanosoma brucei rhodesiense causes an acute illness with onset symptoms within a few weeks of infection. Most found in eastern African and extends to Botswana.  Trypanosomes have trypomastigote and metacyclic trypomastigote stage only.

Symptoms of sleeping sickness are:

  • Severe headache
  • Insomnia
  • Enlarged lymph nodes
  • Anemia
  • Rash
  • Weight loss
  • Sometimes the central nervous system is involved. Determines the treatment regime.
American Trypanosomiasis

Caused by Trypanosoma cruzi transmitted by triatomine or kissing bug. It can also be transmitted by blood transfusion and congenitally since the parasite can cross the placenta.  The kissing bug is a stercorarian parasite as it is transmitted to man by contamination of bite area with faeces from the vector. 

It causes a chronic illness with myocardial damage leading to cardiac arrhythmias, cardiac dilation and intestinal involvement with enlargement of colon and esophagus. In children, it’s acute with chronic manifestations later in life.

Trypanosoma cruzi has amastigote, promastigote, epimastigote, trypomastigote, and metacystic trypomastigote stages.