Dracunculus medinensis. Filarial worms: Wuchereria bancrofti, Loa loa, Onchocerca volvulus

Dracunculus medinensis. Filarial worms: Wuchereria bancrofti, Loa loa, Onchocerca volvulus

Roundworms of the tissues, not the gut. The Guinea worm female, about a metre long, comes out through a blister on the leg, and people catch it from drinking water; it is nearly eradicated. The filariae, Wuchereria, Loa and Onchocerca, are passed on by biting insects and cause elephantiasis, the eye worm and river blindness.

Dracunculus medinensis, the Guinea worm
The Guinea worm, drawn as a parasitology textbook draws it. On the left, the adults to one scale. The gravid female, about 60 to 120 cm long and 1.5 mm wide, is drawn coiled so that she fits on the page: an even, ivory cylinder, rounded at her head on the outside of the coil and pointed at her tail in its middle, with a pink uterus running almost her whole length. Below the coil, the male, a thin thread of no more than 4 cm. At the top right, the female's front end enlarged and cut off behind: the rounded head, the short coral oesophagus, the thin olive intestine pressed against her back, and the broad pink uterus filling the rest of her body, packed with coiled larvae. Below it, a first-stage larva enlarged, as it is released into the water: a slender body with a plain oesophagus and intestine and a long tail drawn out to a point.

Point at a name or a part to light it.

The Guinea worm female is one of the longest roundworms of people, about a metre long, and when she is ready to give birth she is little more than a tube full of larvae. The male is tiny by comparison and dies after mating.

Simplified: the adults drawn to one scale, the female coiled; the front end and the larva each enlarged by a different amount; the gut and uterus drawn as simple tubes and the larvae in the uterus as short threads; the head's papillae, the nerve ring and the male's tail left out; the larva's inner organs drawn simply.

6 sources
  1. Centers for Disease Control and Prevention. DPDx: Dracunculiasis. Last reviewed 3 June 2024.
  2. World Health Organization. Dracunculiasis (Guinea-worm disease), fact sheet. 30 January 2025.
  3. Marie C, Petri WA Jr. Dracunculiasis. MSD Manual Professional Edition.
  4. Pellegrino C, Patti G, Camporeale M, Belati A, Novara R, and others. Guinea worm disease: a neglected disease on the verge of eradication. Tropical Medicine and Infectious Disease 2022;7(11):366.
  5. Natalini MB, Notarnicola J, Monje LD, Vallejos AB, Gavier FS, Kowalewski MM, and others. A new Dracunculus species (Nematoda: Dracunculoidea) in neotropical otters (Lontra longicaudis) from Argentina. International Journal for Parasitology: Parasites and Wildlife 2023;22:234.
  6. Wikipedia. Dracunculiasis; Dracunculus medinensis.

At a glance

  1. The Guinea worm, Dracunculus medinensis, is caught by drinking water holding copepods that carry its larvae. A year later the female, about a metre long, comes out through a burning blister, usually on the lower leg, and releases her larvae into water.
  2. No drug kills it: the worm is wound out slowly on a stick. Filtering drinking water and keeping people with an emerging worm out of it have brought it close to eradication, with 10 human cases in 2025.
  3. The filariae are thread-like roundworms whose females give birth to microfilariae. A biting insect, their vector and intermediate host, takes these up; inside it they grow into infective larvae, which its next bite brings to a new person.
  4. Wuchereria bancrofti, carried by mosquitoes, lives in the lymphatic vessels and causes lymphoedema, elephantiasis and hydrocele; its sheathed microfilariae are in the blood at night. Diethylcarbamazine treats it.
  5. Loa loa, carried by deer flies, wanders under the skin and across the eye. Onchocerca volvulus, carried by black flies, lives in nodules under the skin; its microfilariae in the skin and eye cause itching and river blindness, and ivermectin treats it. Check for Loa loa before giving ivermectin or diethylcarbamazine.

Dracunculus medinensis, the Guinea worm

Dracunculus medinensis, the Guinea worm, is a roundworm of the tissues that people catch from drinking water. In the mid-1980s it infected about 3.5 million people a year in 20 countries of Africa and Asia; in 2024, 15 human cases were reported, in Chad and South Sudan, and in 2025 only 10. As of mid-2025 it was still passed on in six countries: Angola, Cameroon, Chad, Ethiopia, Mali and South Sudan. It infects dogs and cats too, and these animal infections, mostly in Cameroon and Chad, now slow its eradication more than the human ones.

The worms

  • The female is one of the longest roundworms of people: 60 to 100 cm long, by some counts up to 120 cm, and about 1.5 mm wide. White, smooth and cylindrical, when she is ready to give birth she is little more than a tube, her uterus filling most of her body with larvae.
  • The male is tiny by comparison, at most about 4 cm long, and dies after mating.
  • The first-stage larva, about 0.6 mm long, has a long, pointed tail, with which it swims in the water.

The Guinea worm's life cycle

Dracunculus medinensis: the life cycle
A map of the life cycle of the Guinea worm. Across the top, the patient's pale panel: the patient is the definitive host. On the right, larvae freed in the stomach from a swallowed copepod; they go through the gut wall. An arrow runs left: they grow into adults in the abdomen and mate, and the males die. Another arrow runs left: the gravid female moves under the skin to the leg and comes out 10 to 14 months after infection. From her an arrow runs down out of the panel onto the white page, the water: the blister bursts when the leg is put in water, and she releases her first-stage larvae, which swim in the water. Down again, into the copepod's panel at the lower left: a copepod eats them, and in 2 weeks they grow infective inside it; the copepod is the intermediate host. From the copepod two arrows leave. One climbs back to the larvae in the patient: the patient drinks the copepod in unfiltered water. The other runs right into the panel of the frogs and fish, which eat copepods and carry the infective larvae unchanged, paratenic or transport hosts; from them an arrow climbs back to the patient: eaten raw or undercooked.

Point at a name to light its pathway.

The Guinea worm needs water twice: the female puts her larvae into it, and people drink the copepods that carry them. Filtering or boiling drinking water, and keeping people with an emerging worm out of it, breaks the cycle.

Simplified: a map, not anatomy: the hosts drawn as icons and the parasite drawn small, nothing to scale; dogs and cats, which are definitive hosts too and now carry most infections, left out; one fish drawn for the frogs and fish; the larvae's way from the gut wall to the abdomen, and the female's way under the skin, each drawn as one arrow.

5 sources
  1. Centers for Disease Control and Prevention. DPDx: Dracunculiasis. Last reviewed 3 June 2024.
  2. World Health Organization. Dracunculiasis (Guinea-worm disease), fact sheet. 30 January 2025.
  3. Pellegrino C, Patti G, Camporeale M, Belati A, Novara R, and others. Guinea worm disease: a neglected disease on the verge of eradication. Tropical Medicine and Infectious Disease 2022;7(11):366.
  4. Delea MG, Sack A, Eneanya OA, Thiele E, Roy SL, and others. Slaying the serpent: a research agenda to expand intervention development and accelerate Guinea worm eradication efforts. American Journal of Tropical Medicine and Hygiene 2024;111(3 Suppl):12.
  5. Wikipedia. Dracunculus medinensis.
  1. Into the patient. People drink unfiltered water holding copepods, tiny crustaceans such as Cyclops (often called water fleas), that carry infective larvae. The copepods die in the stomach, and the freed larvae go through the wall of the stomach and intestine into the abdomen.
  2. In the abdomen. There, within two to three months, they grow up and mate, and the males die. The female grows on, filling with larvae, over the rest of the year.
  3. Under the skin. The gravid female moves through the tissues under the skin, usually to the lower leg, and 10 to 14 months after the infection makes a blister there, which bursts.
  4. Into the water. The blister burns, and the patient cools it in water. When the wound touches water, a loop of the female's uterus pushes out and releases larvae; over the days the worm comes out she releases hundreds of thousands of them.
  5. In the copepod. A copepod eats them, and in 2 weeks, after two moults, they become infective larvae.

Frogs and fish that eat infected copepods may carry the larvae without their growing, as paratenic or transport hosts; eaten raw or undercooked, they pass them on to people and dogs.

Dracunculiasis

For about a year there are no symptoms.

  • The blister: as the worm comes out there is intense itching and a burning pain, often with an allergic reaction to the worm: hives, redness, breathlessness and vomiting. The wound heals once the worm is out.
  • Infection of the track: in about half of cases, bacteria infect the wound along the worm's track.
  • A broken worm: if the worm breaks while it is coming out or being pulled, a severe inflammation follows, with disabling pain.
  • Joints: chronic infection brings pain and swelling of the joints, and can leave them stiff with contracted tendons. Many patients are laid up for weeks or months.
  • Worms that never reach the skin die, and some calcify; they can be seen on a radiograph.

Diagnosis, treatment and prevention

  • Diagnosis is made by looking: the white, thread-like worm in a skin ulcer. There is no blood test.
  • Treatment: no drug kills the worm, and there is no vaccine. It is wound slowly out on a small stick, a few centimetres a day, usually within a month, with care of the wound; it can also be removed surgically, though this is seldom possible where it is found. Antibiotics treat a bacterial infection of the wound.
  • Prevention: filtering drinking water through fine cloth or a pipe filter, boiling or chlorinating it; protected wells; keeping anyone with an emerging worm out of the drinking water; and treating ponds with an insecticide, temephos, that kills the copepods. Where the risk is high, dogs and cats are kept tied up in the season of transmission so they cannot reach the water, fish entrails are buried, and fish and frogs are cooked well.

The filariae

The filariae are thread-like roundworms of the tissues. Their adults live in the lymphatic vessels or under the skin, and the females give birth not to eggs but to microfilariae, early larvae that move into the blood or the skin. A biting insect, a mosquito or a fly, takes them up as it feeds; inside it they grow into infective third-stage larvae, which its next bite brings to a new person. The insect is the vector and intermediate host; people are the definitive host. Wuchereria and Onchocerca carry bacteria of their own, Wolbachia, which they need, so an antibiotic can harm the worms.

Microfilariae of Wuchereria, Loa and Onchocerca
Three microfilariae, the young of the filarial worms, drawn to one scale and lying across the page, head to the left, with a few red cells on the right for size. Each is a slender ivory body, blunt at the head and tapering to the tail, about as wide as a red cell and filled with a column of blue nuclei. At the top, Wuchereria bancrofti, 240 to 300 µm long, gently curved, inside a clear sheath; its nuclei are loosely spaced and stop short of the tip of the tail. In the middle, Loa loa, 230 to 250 µm long, also in a sheath; its nuclei run to the very tip of the tail. At the bottom, Onchocerca volvulus, 300 to 315 µm long, with no sheath; its tail is sharply bent and its nuclei stop short of the tip.

Point at a name or a part to light it.

Filariae are roundworms passed on by biting insects. Their females give birth to microfilariae, which an insect takes up as it feeds: Wuchereria's and Loa's from the blood, Onchocerca's from the skin. Under the microscope they are told apart by the sheath, the nuclei and the tail.

Simplified: the three microfilariae and the red cells drawn to one scale, at the lengths the CDC gives for stained films (for Onchocerca, from its description of the microfilaria); the adult worms, which are centimetres long, not drawn; the sheath drawn clear and even; the nuclei drawn as round dots in DNA's blue, not in a stain's colours; the microfilariae's few other structures left out.

4 sources
  1. Centers for Disease Control and Prevention. DPDx: Lymphatic filariasis.
  2. Centers for Disease Control and Prevention. DPDx: Loiasis.
  3. Centers for Disease Control and Prevention. DPDx: Onchocerciasis.
  4. Wikipedia. Microfilaria; Red blood cell.
Wuchereria bancroftiLoa loaOnchocerca volvulus
Adults livein the lymphatic vesselsunder the skin, wanderingin nodules under the skin
Adult female80 to 100 mm40 to 70 mm33 to 50 cm
Microfilariaesheathed; nuclei stop short of the tip of the tailsheathed; nuclei run to the tip of the tailno sheath; tail often sharply bent
Found inthe blood, mostly at nightthe blood, by daythe skin
Vectormosquitoes: Culex, Anopheles, Aedes, Mansoniadeer flies, Chrysopsblack flies, Simulium
Diseaselymphatic filariasis, elephantiasisloiasis, the eye wormonchocerciasis, river blindness

The sheath is the envelope of the egg, which the microfilaria keeps in some species and sheds in the insect's gut.

The filariae's life cycles

Wuchereria, Loa and Onchocerca: the life cycles
A map of the life cycles of the three filariae, one column each, each in its own colour. Across the top, the patient's pale panel: the patient is the definitive host of all three. In each column, the adult worms, and below them an arrow down to the microfilariae the females give birth to. Wuchereria bancrofti's adults live in the lymphatic vessels and its sheathed microfilariae are in the blood, mostly at night. Loa loa's adults live under the skin and may cross the eye; its sheathed microfilariae are in the blood by day. Onchocerca volvulus's adults live in nodules under the skin for about 15 years; its microfilariae, without a sheath, are in the skin. From the microfilariae an arrow crosses down into each column's insect, the vector and intermediate host, in a panel of its own across the foot: mosquitoes of the genera Culex, Anopheles, Aedes and Mansonia for Wuchereria, the deer fly Chrysops for Loa, the black fly Simulium for Onchocerca. The insect takes the microfilariae up as it feeds; they shed their sheath if they have one and go to its thoracic muscles, where they grow into infective third-stage larvae, which move to the proboscis. From the larvae an arrow climbs the right edge of the column back to the adults: at the next bite the larvae go in through the wound and grow into adults.

Point at a name to light its pathway.

Each filaria needs its insect as well as a person: the microfilariae grow into infective larvae only inside the insect, and its bite brings them back.

Simplified: a map, not anatomy: the hosts drawn as icons and the parasites drawn small, nothing to scale; one Anopheles mosquito drawn for the several kinds of mosquito that carry Wuchereria; the larvae's growth in the insect, through the first and second stages to the third, drawn as one step; Brugia, which also causes lymphatic filariasis, left out.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Lymphatic filariasis.
  2. Centers for Disease Control and Prevention. DPDx: Loiasis.
  3. Centers for Disease Control and Prevention. DPDx: Onchocerciasis.
  1. The bite. As an infected insect feeds, it leaves infective larvae on the skin, and they go in through the bite wound.
  2. In the patient. They grow into adults, in the lymphatic vessels or under the skin, over months to more than a year. The adults mate and the females give birth to microfilariae.
  3. Into the insect. An insect that feeds on the patient takes up microfilariae. They shed their sheath, if they have one, and go through the wall of its gut to its thoracic muscles.
  4. In the insect. There they grow into infective larvae, which move to its mouthparts, ready for its next bite.

Filariae do not multiply in the body: every adult comes from a larva the insect brought, so a heavy infection takes many bites.

Lymphatic filariasis

Wuchereria bancrofti causes about 90% of lymphatic filariasis; Brugia malayi and B. timori, found in South and South-East Asia, cause the rest. It is found in the tropics and subtropics of Africa, Asia, the Pacific and the Americas. Its mosquitoes vary from place to place: Culex in towns and cities, Anopheles in the countryside, Aedes on the Pacific islands. In 2018 about 51 million people were infected, and in 2023, 657 million in 39 countries lived where everyone must be treated each year.

  • The adults, the females 80 to 100 mm long and the males about 40 mm, live in the lymphatic vessels for about 6 to 8 years. Larvae grow into adults in 6 to 12 months.
  • The microfilariae are in the blood mostly at night, when the mosquitoes bite; in many Pacific islands, by day.

The disease

Infection is usually caught in childhood, and most infections cause no symptoms, though they quietly damage the lymphatic vessels and the kidneys. The disease comes from the adults and from bacteria that infect the damaged tissues.

  • Acute attacks: bouts of fever with inflamed lymph nodes and vessels, acute adenolymphangitis, lasting days to weeks, or inflammation of the epididymis and spermatic cord. They come back again and again.
  • Chronic disease, years later: blocked lymphatic vessels cause lymphoedema of a limb, which can progress to elephantiasis, thick, hard, folded skin; and hydrocele, fluid round the testis. The breasts and genitals can swell too. Lymph leaking into the urine makes it milky, chyluria. The disfigurement brings stigma and poverty.
  • Tropical pulmonary eosinophilia: an uncommon allergic reaction to microfilariae in the lungs, with cough, wheeze, shadows on the chest radiograph and a very high eosinophil count.

Diagnosis, treatment and prevention

  • Blood taken at night (or by day, where the microfilariae appear then), filtered or spun down to concentrate them, shows sheathed microfilariae.
  • A rapid antigen test for W. bancrofti, used in elimination programmes, works at any time of day. Antibody tests cross-react with other worms and cannot tell a past infection from a present one.
  • Ultrasound can show live adults moving in dilated lymphatic vessels, the "filarial dance".
  • Treatment: diethylcarbamazine (DEC) kills the microfilariae and some of the adults, but only after Loa loa and Onchocerca have been ruled out. Doxycycline, for 4 to 6 weeks, kills the Wolbachia bacteria and with them many adults, and can ease lymphoedema. Lymphoedema needs lifelong care of the skin, antibiotics for its infections, raising and bandaging the limb; a hydrocele can be operated on.
  • Prevention: mass drug administration, a dose every year to everyone where it is found, which lowers the microfilariae in the blood so mosquitoes stop passing it on: albendazole with ivermectin where onchocerciasis is also found, albendazole with DEC where it is not, and albendazole alone, twice a year, where Loa loa is found. Bed nets and repellents prevent bites.

Loa loa, the African eye worm

Loa loa lives in the rain forests of western and central Africa, and people are its only known natural reservoir. It is carried by deer flies, Chrysops, which bite by day, when its microfilariae are in the blood.

  • The adults, the females 40 to 70 mm long, the males 30 to 34 mm, wander through the tissues under the skin and can live for more than a decade.
  • Most infected people have no symptoms, and eosinophilia is common.
  • Calabar swellings: itchy swellings, mostly on the arms and legs, where an adult moves through the tissues. They last a few days and come back; visitors get them more often and worse than people who live there.
  • The eye worm: an adult can be seen crossing the eye under the conjunctiva. It is alarming but rarely harms the eye.
  • Diagnosis: blood taken between 10 am and 2 pm, when most microfilariae are in it, and their number counted; or an adult seen crossing the eye, or taken out of it.
  • Treatment: DEC is the only drug that kills both the adults and the microfilariae, given with expert advice. With more than about 8000 microfilariae in a millilitre of blood, killing them quickly can cause an encephalopathy that can be fatal, so a heavy infection is first brought down with albendazole or apheresis. Onchocerciasis must be ruled out before DEC.
  • Prevention: repellents and long sleeves. Because the flies bite by day, bed nets do not help.

Onchocerciasis, river blindness

Onchocerca volvulus infected about 21 million people in 2017, more than 99% of them in Africa and Yemen, with small foci on the border of Brazil and Venezuela. Its black flies, Simulium, breed in fast-flowing rivers and streams, so those who live and work near them are infected: hence river blindness. Many bites are needed before disease appears. It is the second infectious cause of blindness in the world, after trachoma.

  • The adults, the females 33 to 50 cm long and the males 19 to 42 mm, live coiled in nodules under the skin, onchocercomas, for up to about 15 years. Larvae grow into adults in 12 to 18 months.
  • The microfilariae, with no sheath, move through the skin and into the eye, and the disease comes mostly from the inflammation round them as they die.

The disease

  • Nodules: lumps under the skin, or deeper, holding the adults; they cause no symptoms of their own.
  • The skin: intense itching, which may be the only symptom, then a rash, scratching, thickened skin, patchy loss of pigment, early wrinkling and loss of elasticity; in severe cases folds of loose skin in the groin, the "hanging groin".
  • The eye: microfilariae dying in the cornea cause a punctate keratitis, which clears; years of it scar the cornea, sclerosing keratitis, which blinds. The iris, retina and optic nerve can be damaged too.
  • Epilepsy: infection early in life is linked to epilepsy in children.

Diagnosis, treatment and control

  • Skin snips: thin shavings of skin, taken from several places and put in saline, release the microfilariae, which come out of them and can be seen under the microscope. A slit-lamp examination can show them in the cornea and the front of the eye. A nodule can be cut out to find the adults.
  • Treatment: ivermectin, a single dose every 6 to 12 months, clears the microfilariae from the skin and eyes and stops new ones for months, but does not kill the adults, so it is repeated for years. Moxidectin clears them better still. Doxycycline for 6 weeks kills the Wolbachia bacteria and with them more than half of the adult females. DEC is not used: it causes a severe reaction to the dying microfilariae, the Mazzotti reaction, that can damage the skin and eyes further. Check for Loa loa first in anyone from central Africa.
  • Control: between 1974 and 2002, spraying insecticide on the rivers from aircraft to kill the blackfly larvae brought it under control in West Africa. Today everyone eligible where it is found takes ivermectin, donated by its maker, at least once a year for 10 to 15 years, the life of the adults. Colombia, Ecuador, Mexico, Guatemala and, in 2025, Niger have been verified free of it.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1How do people catch the Guinea worm?

2A Guinea worm is coming out of a blister on a woman's ankle. What is the treatment?

3A blood film taken at midday from a traveller back from Cameroon shows a sheathed microfilaria whose nuclei run right to the tip of its tail. Which worm is it?

4A man from a coastal village in India has a swollen leg. When should blood be taken to look for Wuchereria bancrofti?

5Why must a patient from central Africa be checked for Loa loa before they are given ivermectin or diethylcarbamazine?