LOA LOA: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2025

Loa loa is a species of roundworm that belongs to the phylum Nematoda. It is a parasite that affects humans, causing a disease known as loasis, which is also known as subcutaneous filariasis.

It is so named because the parasite lives in the host's subcutaneous tissues. Apart from Loa loa there are also other nematodes that cause filariasis, such as Onchocerca volvulus.

Loa seen under the microscope. Source: Michael Wunderli

-Species: Loa loa.

Morphology

Loa loa is a nematode worm and as such has an elongated, cylindrical shape. They are dioecious, which means that the sexes are separate. They also present sexual dimorphism, so that the female and the male have certain differences that allow them to be distinguished.

The body of these worms are divided into three areas or zones: a simple head, a body and a tail that ends in a blunt point. In addition, they are made up of a kind of protective covering that is mainly made up of collagen.

In terms of color, they are mostly pale, whitish in color.

Female

As with many parasites, female Loa loa are larger than males. They can measure up to 8 cm in length. Its tail is straight.

Internally, your reproductive system is made up of two ovaries, from which two oviducts emerge that communicate with the uterus, which empties into the genital pore.

Male

The males are smaller than the females. They measure up to 3 cm. Its tail develops a characteristic ventral curvature. They also have two spicules.

Your reproductive system is made up of a testicle that empties into the seminal vesicle. From this comes the ejaculatory duct, which opens to the outside of the animal in the cloaca.

Biological cycle

As is well known, parasites require certain conditions and elements for their life cycle to develop successfully. Among those elements are of vital importance a host and a vector. Well, Loa loa is no exception.

In the particular case of this parasite, the vector is represented by an insect, a fly belonging to the genus Chysops. This is hematophogenic, that is, it feeds on the blood that it obtains through the bite of individuals such as humans. This constitutes the host par excellence of this parasite.

This parasite has three larval forms: microfilariae (L1), labditiform (L2) and filariform (L3). Microfilariae are produced inside the human being and subsequently undergo their metamorphosis within the fly.

Once this is clarified, the biological cycle of Loa loa deals with the fact that human beings who are infected by the parasite have microfilariae circulating in their bloodstream. When a fly bites you and sucks your blood, it also sucks up those microfilariae.

Loa loa life cycle. Source: CNX OpenStax

In the fly's digestive tract, the microfilaria lose their protective capsule and move towards their thoracic muscles. There it undergoes a process of metamorphosis, passing through stages L2 (labditiform) and L3 (filariform).

The L3 larvae move from the thorax muscles towards the proboscis of the fly. The proboscis is an elongated organ that some invertebrates use to suck. In the case of flies, they use it to suck the blood of the animals they bite.

Once they bite a healthy human being, the filariform larvae (L3) take advantage of the wound made by the insect to enter the host's body.

Inside the human body, the larva travels to the subcutaneous tissues. There they undergo a new metamorphosis and transform into an adult individual.

The adults mate and are capable of producing microfilariae (L1). Microfilariae have been collected from the cerebrospinal fluid, urine, and lung in infected people. They also circulate in the blood during the day.

When a fly bites an infected person, it acquires the L1 larvae, starting the cycle again.

Transmission

As already explained in the description of the biological cycle of Loa loa, this parasite is transmitted through the bite of flies of the genus Chysops. This occurs because when they bite the person, they deposit the larvae of the parasite there and they take advantage of entering the bloodstream.

No cases of direct transmission from one human being to the other have been recorded, so this transmission mechanism is totally ruled out.

Symptoms of infection

The disease that causes Loa loa is known by the name of loasis. This is an infection that mainly affects the subcutaneous tissues of the body, since that is where the parasite reproduces.

The incubation period is approximately three months. After this time, the infected person begins to manifest certain symptoms and signs.

Among the most characteristic signs of this infection is the so-called Calabar edema. This is characterized by being an area in which there is edema (inflammation) without redness. This inflammation is subcutaneous and very extensive, and can measure up to 20 cm.

Also, before edema appears, you may experience pruritus (itching), burning, and pain. Calabar edema develops mainly on the face, wrists and ankles, specifically at the level of the joints. Its duration is variable, from hours to even weeks. When the edema disappears, it is very likely that it reappears but in another location.

At the blood level, the affected person suffers from eosinophilia. This means that eosinophils (a type of blood cell) increase their concentration in the blood. This occurs because these cells have, among one of their many functions, to fight infections by parasites.

Likewise, adult forms of the parasite tend to cause certain local reactions where they are found. One of the favorite tissues of these worms is the ocular conjunctiva. When this occurs, the person experiences tearing, tingling, and a foreign body sensation.

When the infection progresses, complications are possible at the renal, neurological, pulmonary and cardiac levels.

The severity of the infection depends mainly on the state of the immune system of the affected person and the degree of immunity to the parasite. For example, in regions in which the loasis is endemic, it is possible to find microfilariae in the blood of its inhabitants, without these showing symptoms or signs.

Diagnosis

The diagnosis of loasis can be given through several mechanisms:

Direct observation of the worm

The doctor can see the adult form of the worm on the patient's conjunctiva or on the skin.

Blood test

This is the most widely used test to diagnose Loa loa infection. To carry it out, it is necessary to take a blood sample from the patient. This must be done between 10:00 am and 2:00 pm, since this is the time when there is the highest concentration of microfilariae in the blood.

Polymerase chain reaction (PCR)

This is a highly specialized molecular diagnostic test. Thanks to this, it is possible to directly detect the parasite's DNA, as well as to quantify the amount of parasites that there are. This is an extremely expensive exam that must be performed in specialized centers. It is not commonly done to diagnose infection.

Microfilaria in blood. Source: Stefan Walkowski

Treatment

The drug used to treat Loa loa infections is diethylcarbamizine. This is nothing more than an anthelmintic (antiparasitic) that is used to treat infections caused by some nematodes such as Loa loa.

However, the treatment scheme for loasis is somewhat complex, since the behavior to be followed depends on several factors.

The most important factor is the number of parasites per milliliter of blood. In patients whose concentration is higher than 8,000 microfilariae per milliliter, it is not advisable to start treatment with diethylcarbamizine directly.

This is because the parasites, when attacked, release certain substances that can trigger terrible reactions in the patient, such as encephalopathy.

In patients with a high level of microfilariae in the blood, it is common to subject them to albendazole treatment for a period of 21 days in order to reduce the number of parasites.

Once the parasite load has reached more manageable levels, then diethylcarbamizine treatment is applied, always with the required care and monitoring.

References

1. Agbolade O., Akinboye D. and Ogunkolo O. (2005) Loa loa and Mansonella perstans: neglected human infections that need control in Nigeria, Afr. J. Biotechnol. 4
2. Akue, J. (2016). Loa loa Pathogenesis in human. Chapter in book: Human emerging infections: Viral & Parasitic infections. First Edition.
3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Gómez, N., Primelles, R., Gómez, N., Pérez, H. and Tipantasig, W. (2015). Filariasis Journal of Medical Sciences. 19 (1)
5. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
6. Rajeev, J., Chen J., Butcher, A. and Casson, R. (2008). Subconjunctival Loa lloa worm. International Journal of Infectious diseases. 12 (6).