Loa loa - Introduction, Classification, History, Habitat, Morphology

Introduction to Loa loa

Loa loa is the etiological agent of the disease loiasis or Loa loa filariasis of sub conjunctiva of the eye and subcutaneous layer of the skin. The arthropod-borne filarial nematode, which is commonly referred to as eye worms, is commonly found in Africa. The roundworms cause transient swelling known as Calabar swellings.

Classification of Loa loa

The classification of Loa loa is as follows:

Kingdom: Animalia

Phylum: Nematoda

Class: Chromadorea

Order: Rhabditida

Family: Onchocercidae

Genus: Loa

Species: L. loa

History of Loa loa

Loa loa infection was first noted in the Caribbean in 1770 by Santo Domingo. Mongin, a French surgeon unsuccessfully tried to remove a worm passing across a woman's eye.

In 1778, surgeon François Guyot retrieved the parasite from a man’s eye, from a group of West African slaves on a French ship to America. Ophthalmologist Stephen McKenzie identified the Loa loa microfilariae while Calabar swellings were observed in 1895 by Scottish ophthalmologist Douglas Argyll-Robertson.

The cause-effect between Loa loa and Calabar swellings was linked in 1910 by Patrick Manson while the vector for the parasite was discovered to be Chrysops spp by British parasitologist Robert Thomson Leiper in 1912.

Habitat of Loa loa

Loa loa habitats the subcutaneous tissue of human hosts while unsheathed microfilariae are found in the blood.

Morphology of Loa loa

The body of Loa loa consists of the head, a body, and a blunt tail. The outer body contains a cuticle with 3 main layers which are made up of collagen. These later protect the parasite while they are inside the digestive system of their host.

Both adults and juveniles have similar morphological features and the only difference is size.

Significant morphological forms of Loa loa includes – adult worm, microfilaria, and the infective form (L3).

Adult worm

• adult Loa loa worms are whitish, thun, thread-like

• body surface covered with small knobs

• anterior end tapers to a narrow head

• resides in the subcutaneous tissue

• travels under the host skin

• life-span of 10 to 15 years

Male

• male Loa loa measures 30 to 34mm in length and 0.35–0.42 mm in diameter

• tail end has two spicules of unequal length

Female

• female Loa loa measures 40–70 mm in length and 0.5 mm in diameter

• vulva opens in the cervical region

• deposits microfilariae under the host skin

Microfilariae

• measures 250–300 μm in length, 6–8 μm in diameter

• sheathed

• sheath stains poorly or even remains unstained with Giemsa stain but stains well with iron-hematoxylin

• contain body nuclei that extend to the tip of the tail

• shows diurnal periodicity – appears in the peripheral blood during the day

• in some cases, Loa loa microfilariae are found in the sputum, CSF, urine

• can develop in the host’s blood within 5 to 6 months

• survives upto 17 years

Infective form

The infective form of Loa loa is the stage-three larvae (L3). They develop in female tabanid flies (horseflies, deerflies) of the genus Chrysops silica and Chrysops dimidiata.