Parasitology and Helminthology

Hemo flagellates: Trypanosoma spp.

• Trypanosoma spp. infective stage: Metacyclic trypomastigote
• Vector: Reduviid bug (Triatoma) for T. cruzi and Tse tse fly for T. brucei
• Reservoir hosts: Armadillos, opossums, and woodrats for T. cruzi, and domestic cattle, bushbuck, and hartebeest for T. brucei
• Morphology:
  • Amastigote stage: Found in tissues
  • Promastigote stage: Found in vector (V)
  • Epimastigote stage: Found in vector (V)
  • Trypomastigote stage: Found in blood
• Pathogenesis:
  • Chagoma: Local inflammation, reddish nodule, furuncle-like lesions associated with central edema, and regional lymphadenopathy
  • Romana's Sign: Periorbital swelling (edema of eyelid and conjunctiva) and unilateral swelling (only one eyelid affected)
• Disease manifestation:
  • Acute phase: Fever, lymphadenopathy, and diffuse or focal inflammation affecting myocardium
  • Chronic phase: No characteristic symptoms, but prominent lymphadenopathy and CNS invasion
• Diagnosis:
  • Thick and thin smears
  • Buffy coat concentration technique by Strout method
  • Culture by NNN medium
  • Serology: IFAT, CF test, ELISA, Western Blot, IHA

Trypanosoma cruzi

• Life cycle:
  • Bug bites an infected person and ingests trypomastigotes
  • Trypomastigotes transform into epimastigotes and multiply in the bug
  • Epimastigotes further transform into metacyclic trypomastigotes (infective stage)
  • Bug bites again and defecates near the bite wound
  • Metacyclic trypomastigotes enter the person through the bite or mucous membranes
  • Trypomastigotes transform into amastigotes (multiplying stage) inside cells
  • Amastigotes multiply and then transform back into trypomastigotes
  • Cycle repeats if another bug bites an infected person and picks up the parasite

Trypanosoma brucei

• Life cycle:
  • Similar to T. cruzi, but with a Tse tse fly as the vector
• Disease manifestation:
  • Gambian or West African Sleeping Sickness (chronic disease)
  • Rhodesian or East African Sleeping Sickness (acute disease)

Hemo flagellates: Leishmania spp.

• Leishmania spp. infective stage: Promastigote
• Vector: Sandflies (Phlebotomus spp.)
• Morphology:
  • Amastigote stage: Found in tissues
  • Promastigote stage: Found in vector
• Disease manifestation:
  • Cutaneous Leishmaniasis: Skin lesions, may be urban or rural
  • Mucocutaneous Leishmaniasis: Skin and mucous membrane lesions
  • Visceral Leishmaniasis: Kala-azar, Dumdum fever, or Post kala-azar dermal leishmaniasis
• Diagnosis:
  • Demonstration of lesions
  • Tissue and skin biopsies for amastigotes
  • Montenegro Skin Test for Leishmania donovani
  • Formol Gel Test for hypergammaglobulinemia

Leishmania spp. Life Cycle

• Sandflies bite an infected person and inject promastigotes
• Promastigotes enter immune cells and turn into amastigotes
• Amastigotes multiply inside these cells and spread to other cells
• Sandflies become infected by biting an infected person and taking in infected cells
• In the sandfly, amastigotes turn back into promastigotes
• The cycle continues when the sandfly bites another person### Liver Flukes
• There are two main species of liver flukes: Fasciola hepatica (Sheep Liver Fluke) and Fasciola gigantica (Tropical Liver Fluke)
• Both species have a large, broad, flat body with a cephalic cone and prominent shoulders
• They have intestinal ceca that are highly branched

Habitat and Hosts

• Fasciola hepatica infects the biliary passages of the liver, and metacercaria are found in aquatic plants (Ipomea)
• Fasciola gigantica is found in the bile ducts and liver
• Hosts include sheep and humans

Morphology

• Fasciola hepatica has a longer but narrower body, with a more developed cephalic cone and shoulders
• Fasciola gigantica has a shorter body with a less developed cephalic cone and shoulders
• Adult flukes have an attenuated anterior end

Disease Manifestation

• Acute stage: due to larval migration and worm maturation, causing fever, chills, fatigue, weakness, weight loss, liver enlargement, and jaundice
• Chronic stage: persistence of adult worms in biliary ducts, leading to obstruction and inflammation, and eventually fibrosis or cirrhosis

Infection

• Infection occurs through ingestion of undercooked or raw aquatic plants containing metacercaria
• Ingestion of livers (cooked) containing eggs can also lead to infection

Diagnosis

• Eggs are reported as Fasciola eggs

• Diagnosis can be made through stool examination (FECT, Kato Katz, Stoll’s dilution)

• Difficult to distinguish from F. buski, but can be differentiated through size and shape of eggs### Liver Flukes

• Opisthorchis felineus: Cat liver fluke, adult worms found in the gallbladder and bile duct, ventral sucker bigger than oral sucker, and eggs are dark brown with a thick shell and a large operculum.

• Chronic infection leads to periductal fibrosis, cirrhosis, and portal hypertension, and may cause cholangiocarcinoma.

• Humans are accidental hosts, and infection occurs through ingestion of raw or undercooked fish containing metacercariae.

Intestinal Flukes

• Dicrocoelium dendriticum: Lanceolet/Lancet fluke, adult worms found in the bile duct or liver, and eggs are dark brown with a thick shell and a large operculum.

• Humans are accidental hosts, and infection occurs through ingestion of ants or plants contaminated with metacercariae.

• Infection can cause periductal fibrosis, cirrhosis, and portal hypertension.

• Fasciolopsis buski: Giant intestinal fluke, adult worms found in the small intestine, and eggs are large and operculated.

• Humans are the definitive host, and infection occurs through ingestion of encysted metacercariae in aquatic plants.

• Infection can cause marked eosinophilia, malabsorption of vitamins, and inflammation at the site of attachment.

• Echinostoma ilocanum: Garrison's fluke, adult worms found in the small intestine, and eggs are straw-colored and operculated.

• Humans are the definitive host, and infection occurs through ingestion of second snail hosts raw.

• Infection can cause local inflammation at the site of attachment, diarrhea, and intoxication due to release of metabolites.

• Heterophyes heterophyes: Small intestine fluke, adult worms found in the small intestine, and eggs are light brown and operculated.

• Humans are the definitive host, and infection occurs through ingestion of encysted metacercariae in fish.

• Infection can cause local inflammation at the site of attachment, and heavy infections can lead to diarrhea and malabsorption.

Pancreatic Flukes

• Eurytrema pancreaticum: Pancreatic fluke, adult worms found in the pancreas, and eggs are ellipsoid and yellow-brown.
• Humans are accidental hosts, and infection occurs through ingestion of ants or grasshoppers contaminated with metacercariae.
• Infection can cause local inflammation, ulceration, and hemorrhage, and heavier infections can lead to abdominal pain, duodenal ulcer disease, and diarrhea.

Lung Flukes

• Paragonimus westermani: Oriental lung fluke, adult worms found in the lung, and eggs are reddish-brown and coffee bean-shaped.
• Humans are accidental hosts, and infection occurs through ingestion of undercooked crabs containing metacercariae.
• Infection can cause lung fluke disease, with symptoms including dry cough, blood-stained or rust-colored sputum, and may mimic signs of TB.

Blood Flukes

• Schistosoma spp.: Oriental blood fluke, eggs are oval with a recurved hook or knob, and are found in the mesenteric veins.
• Humans are the definitive host, and infection occurs through exposure to water contaminated with cercariae.
• Infection can cause acute phase - Katayama syndrome, and chronic infections can lead to hepatosplenic schistosomiasis, intestinal bilharziasis, and pulmonary schistosomiasis.