Loa loa, Onchocerca and Dracunculus
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Direct answer
Loa loa, Onchocerca volvulus and Dracunculus medinensis are tissue nematodes that cause subcutaneous or ocular disease rather than intestinal infection. Loa loa is spread by deer flies and causes transient calabar swellings, Onchocerca is spread by blackflies and causes river blindness, and Dracunculus is acquired by drinking water containing copepods. The key exam facts are the daytime periodicity of Loa loa microfilariae and the global eradication programme for guinea worm.
What you must remember
- Loa loa vector: Chrysops, the deer or mango fly, transmits Loa loa, and calabar swelling is a transient migratory subcutaneous oedema.
- Loa loa periodicity: Microfilariae of Loa loa show diurnal periodicity and appear in blood during the day.
- Eye worm: Adult Loa loa worms can migrate across the conjunctiva, which is the classic eye worm.
- Onchocerca vector: Blackflies of the genus Simulium transmit Onchocerca volvulus, and their larvae breed in fast-flowing rivers.
- Onchocercomas: Adult worms live in subcutaneous nodules that release microfilariae into the skin.
- River blindness: Microfilariae reaching the eye cause keratitis and optic nerve damage, which can lead to blindness.
- Diagnosis: A skin snip examined in saline shows microfilariae of Onchocerca, while Loa loa is detected on a blood smear.
- Dracunculus: Guinea worm is acquired by drinking water containing infected Cyclops, and no drug cures it, so the worm is extracted slowly by winding it on a stick.
Following Onchocerca from bite to blindness
These nematodes live in tissues, not the gut, so diagnosis and treatment depend on the organ affected. The pathway below follows Onchocerca because it links the vector, the skin lesion and the eye damage that exam questions often combine.
- Bite: A blackfly of the genus Simulium breeds in fast-flowing rivers and releases infective larvae onto the skin during a blood meal.
- Nodule formation: Adult worms live in subcutaneous nodules called onchocercomas for years, releasing microfilariae into the surrounding skin.
- Skin damage: Microfilariae cause chronic itching, depigmentation and skin atrophy, which lead to a visible loss of skin quality over time.
- Eye involvement: Microfilariae reach the cornea and retina, causing keratitis, sclerosing keratitis and optic atrophy that eventually cause blindness.
- Diagnosis: A skin snip examined in saline shows motile microfilariae, and excision of a nodule confirms the adult worms.
- Treatment: Ivermectin given in community programmes kills microfilariae but not the adult worm, so it is repeated over years.
Where the exam tries to catch you
Most errors come from mixing up the vectors and treating microfilariae in a way that causes harm.
- Trap: Choosing ivermectin for a patient with heavy Loa loa microfilaraemia Correct: ivermectin can cause fatal encephalopathy in heavy Loa loa infection, so microfilarial load must be checked first.
- Trap: Calling river blindness due to Wuchereria Correct: river blindness is caused by Onchocerca volvulus transmitted by blackflies.
- Trap: Treating Dracunculus with an anthelmintic Correct: no drug cures guinea worm, so the worm is extracted slowly and eradication relies on safe drinking water.
- Trap: Linking calabar swelling to Onchocerca Correct: calabar swelling is the transient swelling of Loa loa, not Onchocerca.
Frequently asked questions
Which vector transmits Loa loa?
Chrysops, the deer fly or mango fly, transmits Loa loa in West and Central Africa. Its bite is followed by transient calabar swellings in the subcutaneous tissues.
Why is ivermectin risky in Loa loa infection?
Ivermectin can cause serious encephalopathy in patients with high Loa loa microfilarial loads, because dying microfilariae trigger severe reactions. Microfilarial counts are therefore checked before ivermectin is given.
Which vector transmits Onchocerca volvulus?
Blackflies of the genus Simulium transmit Onchocerca volvulus, and their larvae breed in fast-flowing rivers. The disease is commonly called river blindness because of this association with river habitats.
How is Dracunculus medinensis acquired?
Dracunculus medinensis is acquired by drinking water containing Cyclops copepods infected with larvae. The adult female worm emerges through the skin, usually of the leg, roughly a year after infection.
Which treatment is used for guinea worm disease?
Guinea worm disease has no effective anthelmintic cure, and the worm is removed by slowly winding it onto a stick over several days. Prevention through safe drinking water remains the main control strategy.