Introduction

Loa loa , also known as the African eye worm, is a filarial nematode endemic in the rain forests of West and Central Africa.  It is transmitted by Chrysops species also known as mango flies or horse flies and humans are the only known reservoir.  The microfilariae exhibit diurnal periodicity, the highest numbers being detected in blood between 10am and 2 pm.

Life cycle

The adult worms live in the suncutaneous and deep connective tissues and the microfilariae are found in the peripheral blood , where they can be in ingested by the Crysops fly. 

Morphology

Adult males of Loa loa are 2 – 3.5cm long and the females from 5 – 7cm.  The microfilariae of Loa loa are 250 - 300.  They possess a sheath which stains blue-grey with Delafield's haematoxylin.  The sheath does not stain with Giemsa.  The tail gradually tapers to a rounded end, the densely packed nuclei extending to the tip.

Microfilaria of Loa loa stained with Delafield's haematoxylin

The tail of a microfilaria stained with Delafield's haematoxylin exhibiting the nuclei extending to the tip.

Clinical disease

Many patients infected with Loa loa appear to be asymptomatic and the migration of the adult worm through the subcutaneous tissues often goes unnoticed, unless passing beneath the conjunctiva of the eye. Hypereosinophilia and increased antibody levels, especially IgE are also noted.

The most common pathology associated with Loa loa infections are Calabar swellings, which are inflammatory swellings resulting in a localised subcutaneous oedema.  These swellings are due the host's response to the worm or its metabolic products and can be found anywhere in the body but most commonly in the extremities.  These swellings last from 1 – 3 days. They develop rapidly and last one to three days, usually accompanied by localised pain, urticaria and pruritis.

Serious complications such as cardiomyopathy, encephalopathy, nephropathy and pleural effusion have been recorded.

Laboratory diagnosis.