Additional Parasites

This section will cover filariae and flukes, two groups of parasitic worms that can cause a range of diseases in humans. We’ll explore their etiology, epidemiology, and transmission

Filariae (Filariasis)

• Etiology: Filariae are parasitic nematodes (roundworms) that infect the lymphatic system, subcutaneous tissues, or serous cavities. Major filarial parasites include:
  • Wuchereria bancrofti
  • Brugia malayi
  • Brugia timori (cause lymphatic filariasis, leading to elephantiasis)
  • Onchocerca volvulus (causes onchocerciasis, or river blindness)
  • Loa loa (causes loiasis, or African eye worm)
• Epidemiology
  • Geographic Distribution: Tropical and subtropical regions of Africa, Asia, and the Americas
  • Risk Factors
    • Living in or traveling to endemic areas
    • Exposure to mosquito or blackfly bites
    • Poor sanitation and hygiene practices
  • Chronic Disease: Filarial infections can cause chronic and debilitating conditions, such as lymphedema, elephantiasis, and blindness
• Transmission
  • Vector-Borne: Filariae are transmitted by the bites of infected mosquitoes (lymphatic filariasis and loiasis) or blackflies (onchocerciasis)
  • Life Cycle
    • Adult worms live in the lymphatic system, subcutaneous tissues, or serous cavities
    • Female worms release microfilariae (larval stage) into the bloodstream or skin
    • Mosquitoes or blackflies ingest microfilariae during blood meals
    • Microfilariae develop into infective larvae in the vector
    • Infective larvae are transmitted to humans during subsequent bites

Flukes (Trematodes)

• Etiology: Flukes are parasitic flatworms that infect various organs, including the liver, lungs, intestines, and blood vessels. Major flukes include:
  • Liver flukes
    • Clonorchis sinensis (Chinese liver fluke)
    • Opisthorchis viverrini and Opisthorchis felineus
    • Fasciola hepatica and Fasciola gigantica
  • Lung fluke
    • Paragonimus westermani
  • Intestinal flukes
    • Fasciolopsis buski
  • Blood flukes
    • Schistosoma spp. (as covered previously)
• Epidemiology
  • Geographic Distribution: Worldwide, with different species prevalent in different regions
  • Risk Factors
    • Consumption of raw or undercooked freshwater fish, crustaceans, or aquatic plants
    • Contact with freshwater containing infected snails
    • Poor sanitation and hygiene practices
    • Agricultural practices involving irrigation
  • Chronic Disease: Fluke infections can cause chronic inflammation, tissue damage, and cancer
• Transmission
  • Ingestion of Larvae: Liver and intestinal flukes are transmitted through ingestion of raw or undercooked freshwater fish, crustaceans, or aquatic plants containing metacercariae (encysted larval stage)
  • Skin Penetration: Blood flukes (Schistosoma spp.) are transmitted through skin penetration by cercariae (free-swimming larval stage) released from snails
  • Life Cycle
    • Flukes have complex life cycles involving one or more intermediate hosts (typically snails and fish or aquatic plants)
    • Adult flukes live in the liver, lungs, intestines, or blood vessels
    • Eggs are released in feces or urine
    • Eggs hatch in freshwater, releasing miracidia
    • Miracidia infect snails, where they develop into cercariae
    • Cercariae are released from snails and either penetrate human skin (blood flukes) or encyst as metacercariae in fish, crustaceans, or aquatic plants (liver and intestinal flukes)
    • Humans become infected by ingesting metacercariae or through skin penetration by cercariae

Key Takeaways

• Vector-Borne Transmission: Filariae are transmitted by insect vectors (mosquitoes and blackflies), while some flukes are transmitted through skin penetration by cercariae released from snails
• Ingestion of Larvae: Liver and intestinal flukes are transmitted through ingestion of raw or undercooked freshwater fish, crustaceans, or aquatic plants containing metacercariae
• Complex Life Cycles: Both filariae and flukes have complex life cycles involving multiple hosts and environmental stages
• Chronic Disease: Filarial and fluke infections can cause chronic and debilitating conditions, such as lymphedema, elephantiasis, blindness, and cancer

Key Terms

• Filaria: A parasitic nematode that infects the lymphatic system, subcutaneous tissues, or serous cavities
• Microfilariae: The larval stage of filarial worms found in the bloodstream or skin
• Fluke: A parasitic flatworm that infects various organs
• Trematode: Another name for fluke
• Metacercariae: The encysted larval stage of flukes found in fish, crustaceans, or aquatic plants
• Cercariae: The free-swimming larval stage of flukes released from snails
• Vector-Borne: Transmitted by an insect or other arthropod vector
• Endemic: A disease that is constantly present in a particular geographic region or population
• Lymphatic System: The network of vessels and tissues that drain fluid from the body and transport it back to the bloodstream
• Elephantiasis: A condition characterized by massive swelling of the limbs and other body parts due to lymphatic obstruction caused by filarial worms
• Onchocerciasis: A parasitic disease caused by Onchocerca volvulus, leading to blindness and skin lesions
• Loiasis: A parasitic disease caused by Loa loa, characterized by swelling and itching of the skin and migration of the worm across the eye
• Lymphedema: Swelling caused by lymphatic obstruction
• Miracidia: The ciliated larval stage of flukes that infect snails