rd
PHE National Parasitology Reference Laboratory, Hospital for Tropical Diseases, 3 Floor Mortimer Market,
Centre, Capper Street, London WC1E 6JB, TEL: +44 (0) 207 383 0482, FAX +44 (0) 207 388 8985
Microfilariae of Loa loa
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 mango flies or Chrysops species 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
HUMANS
Adult worms migrate
beneath the conjunctiva or
the subcutaneous tissues
and adult females shed
microfilariae into blood
Chrysops sp. ingests
Microfilariae enter host’s
microfilariae with
blood stream when fly
blood meal
takes blood meal
Microfilariae develop in fly
and when mature,
migrate to mouth parts
Morphology
The microfilariae of Loa loa are 250 - 300. They possess a sheath which stains blue-grey with
Delafield’s haematoxylin. The tail gradually tapers to a rounded end, the densely packed 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.
©Copyright
These teaching sheets are the property of UK NEQAS Parasitology
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. 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 are recorded.
Laboratory diagnosis
When filariasis is suspected, a geographical and clinical history helps to determine the most
appropriate collection time. Thick and thin blood films can be examined. However this is an
insensitive method due to the low microfilaraemia, and larger volumes of blood need to be examined
as in the methods described in pages.
Note the nuclei present at the tip of the tail
Sheath stained with Delafield’s haematoxylin.
©Copyright
These teaching sheets are the property of UK NEQAS Parasitology
