Loiasis.


The distribution of loiasis is linked to that of its vector (Chrysops) in forests or savannah with gallery forests in West or Central Africa (limits West: Benin; East: Uganda; North: Sudan and South: Angola).

Clinical features

– The subconjunctival migration of an adult worm is pathognomonic of Loa loa infection.

– Localised subcutaneous swellings, allergic in origin, transient (several hours or days), painless, non-pitting, appearing anywhere on the body, frequently the upper extremities and face, often associated with localised or generalised pruritus (« Calabar swellings »).

– Onset of pruritus, in the absence of other signs.

– Subcutaneous migration of an adult worm: pruritic, palpable red cord-like linear lesion, sinuous, advancing (1 cm/hour), disappearing rapidly with no trace1 . Such migration generally arises following treatment with diethylcarbamazine, rarely spontaneously.

Laboratory

– Detection of microfilariae in the peripheral blood (thick film, stained with Giemsa). Blood specimens should be collected between 10 am and 5 pm. Quantify microfilaraemia even if the diagnosis is certain, since treatment is determined by the intensity of the parasite load.

– If the thick film is positive, look for onchocerciasis in regions where onchocerciasis is coendemic (mainly in Central Africa).

Treatment

Antiparasitic treatment

– Diethylcarbamazine (DEC) is the only macrofilaricide available but is contra-indicated in:
• Patients with microfilaraemia > 2000 mf/ml (risk of severe encephalopathy, with poor prognosis).
• Patients co-infected with O. volvulus (risk of severe eye lesions).
• Pregnant women, infants, and patients in poor general condition.

– Ivermectin (and possibly albendazole) is used to reduce microfilaraemia before administration of DEC; however, ivermectin administration may trigger encephalopathy in patients with very high Loa loa microfilaraemia (> 30 000 mf/ml).

– Doxycycline is not indicated since Loa loa does not harbour Wolbachia.

– Management:

1) L. loa microfilaraemia is < 1,000-2,000 mf/ml
A 28-day treatment of DEC may be started using a small dose: 6 mg on D1, i.e. 1/8 of a 50 mg tablet 2 times daily.
Double the dose every day up to 200 mg 2 times daily in adults (1.5 mg/kg 2 times daily in children).
If microfilaraemia or symptoms persist, a second treatment is given 4 weeks later.
If DEC is contra-indicated due to possible or confirmed co-infection with O. volvulusivermectin (150 microgams/kg single dose) treats onchocerciasis, and reduces pruritus and frequency of Calabar swellings.
The treatment may be repeated every month or every 3 months.

2) L. loa microfilaraemia is between 2,000 and 8,000 mf/ml
Reduce microfilaraemia with ivermectin (150 micrograms/kg single dose); repeat the treatment every month if necessary; administer DEC when the microfilaraemia is < 2000 mf/ml.

3) L. loa microfilaraemia is between 8,000 and 30,000 mf/ml
Treatment with ivermectin (150 micrograms/kg single dose) may cause marked functional impairment for several days. Close supervision and support from family member(s) are necessary2 . Prescribe paracetamol as well for 7 days.

4) L. loa microfilaraemia is > 30,000 mf/ml

• If the loiasis is well tolerated, it is preferable to refrain from treatment: the disease is benign and treatment with ivermectin may cause very severe adverse reactions (encephalopathy), albeit rarely.

• If loiasis has a significant clinical impact and/or the patient presents with symptomatic onchocerciasis requiring treatment, ivermectin (150 micrograms/kg single dose) is administered for 5 days under supervision in hospital3 . An attempt to first reduce L. loa microfilaraemia using albendazole (200 mg 2 times daily for 3 weeks) is an option. When L. loa microfilaraemia is < 30 000 mf/ml, treat with ivermectin under close supervision and support, then DEC when the microfilaraemia is < 2000 mf/ml.

Extraction of macrofilariae

Subcutaneous migration of a microfilaria usually results from treatment with DEC; the worm will die beneath the skin and extracting it serves no purpose.

Removal of an adult worm from the conjunctiva: see Loasis, Chapter 5.



Footnotes
Ref Notes
1 For differential diagnosis, see cutaneous larva migrans.
2 Patients may present with various pain syndromes, be unable to move without help or unable to move at all. Monitoring is necessary to determine whether the patient can manage activities of daily living, and provide assistance if necessary. If the patient remains bedridden for several days, ensure pressure sores do not develop (mobilisation, repositioning).
3 A severe reaction may occur on D2-D3. It is usually preceded by haemorrhages of the palpebral conjunctiva on D1-D2. Routinely check for this sign by turning back the eyelids. Symptoms of post ivermectin encephalopathy are reversible and the prognosis favourable, if the patient is correctly managed; the treatment is symptomatic until symptoms resolve. Avoid the use of steroids due to adverse effects.