Parasite morphology
The causative agent of loiasis is the nematode Loa loa. Adult worms (marites) are thread-like with a whitish, translucent body. The helminth cuticle is covered with numerous characteristic rounded protrusions known as tubercles.
The parasite is dioecious, exhibiting pronounced sexual dimorphism:
- Females reach a length of 50 to 70 mm. Their caudal end is bluntly rounded and bears a pair of papillae. The genital pore is located 2.5 mm from the head end. An important feature of females is that they are viviparous.
- Males are significantly smaller, measuring only 30–34 mm in length. The caudal end of the male is curved ventrally and equipped with two unequal spicules.
The parasite larvae, or microfilariae, are microscopic—ranging from 0.25 to 0.30 mm in length (250–300 microns with a width of 6–8.5 microns). They possess an inconspicuous sheath. A key diagnostic feature of Loa loa microfilariae is the compactly arranged nuclei extending all the way to the tip of the pointed tail.
Life cycle
Loiasis is a biohelminthiasis with a vector-borne transmission route. Two hosts are involved in the developmental cycle:
- Definitive host: humans.
- Intermediate host (vector): blood-sucking deer flies belonging to the genus Chrysops.
Development in the fly begins when the insect bites an infected person and ingests microfilariae along with the blood. The larvae penetrate the wall of the fly's stomach and migrate to its thoracic muscles, and subsequently to the salivary glands. In approximately 10–12 days, they reach the infective stage and accumulate in the mouthparts. The insect is capable of transmitting the infection for 5 days.
Human infection occurs through the painful bite of an infected deer fly. Infective larvae enter the bloodstream and subcutaneous tissue. The period required for them to mature into sexually mature adults in the human body takes 1 to 3 years. Adult worms (macrofilariae) actively migrate through the subcutaneous connective tissue. Very frequently they localize beneath the conjunctiva of the eye, which is why Loa loa earned its alternative name, the "African eye worm." They can also be found beneath the serous membranes of various organs. Adult helminths live in the human body for a very long time—from 4 to 17 years.
Reproduction and larval circulation
Sexually mature females release live larvae directly into the tissues, from where they enter the bloodstream. Microfilariae have the ability to accumulate in human lungs, from which they regularly migrate into peripheral blood vessels.
A distinctive feature of Loa loa microfilaria circulation is that they appear in the peripheral blood predominantly during daytime. This evolutionary adaptation is synchronized with the period of peak activity of the disease vectors—deer flies, which feed on blood during the day.
Microscopic diagnostics
Laboratory diagnosis of loiasis is based on detecting the larval stages of the parasite. For this purpose, clinical practice examines a thick blood smear from the patient using standard hematoxylin staining.
In a properly prepared and stained smear, Loa loa microfilariae are reliably identified by two main features:
- Presence of a thin sheath.
- Specific arrangement of nuclei, which compactly fill the entire tail end of the larva up to its very tip (unlike some other filarial species).
Briefly: answers to topical questions
In which geographical regions is loiasis endemic?
Loiasis is endemic only to West and Central Africa.
- Regions — West and Central Africa.
- Latitudinal range — from 8° N to 5° S.
Foci of the infection are confined to tropical forests, swampy riverbanks, and irrigation canals, as these areas serve as breeding sites for deer flies.
Loiasis is an eye worm. Remember the two round letters "o" in the Latin name Loa loa — they visually resemble two eyes, beneath the conjunctiva of which this parasite frequently localizes.
Class and exam questions
Who are the definitive and intermediate hosts of Loa loa?
The definitive host is exclusively human. The intermediate host and vector of the infection are blood-sucking deer flies of the genus Chrysops.
What is the main microscopic feature distinguishing Loa loa larvae?
Microscopy of a stained thick blood smear reveals that the cell nuclei inside the microfilaria are closely apposed and extend to the very tip of the pointed tail.
What is the mechanism of human infection with loiasis?
Infection is vector-borne in nature. Infective larvae penetrate the bloodstream or subcutaneous adipose tissue at the moment of a painful bite from an infected deer fly.
What else to study on this topic
- Features of macrofilariae migration beneath the conjunctiva and serous membranes
- Specifics of larval development in the thoracic muscles and salivary glands of the deer fly
- Causes and mechanisms of daytime microfilariae circulation in peripheral blood
- Comparative characteristics of the cuticle in males and females (presence of tubercles and papillae)
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