ONCHOCERCA VOLVULUS: CHARACTERISTICS, MORPHOLOGY, DISEASES - BIOLOGY - 2025

-Species: Onchocerca volvulus.

Morphology

These nematode worms are cylindrical in shape and elongated in body. They have a kind of cuticle that covers their entire body. With regard to color, they are generally white.

They are dioecious, which means that the sexes are separated, that is, there are female individuals and male individuals.

Likewise, they present sexual dimorphism, which implies that there are certain aspects that allow differentiating the females from the males.

Because they present an indirect development, when they are born they do so in the form of larvae known as microfilariae. These are very small, barely reaching 300 microns and have a tail that is pointed.

Males

They are much smaller than females. They are generally 5 cm in length. The terminal end of the body is curved. They also have two structures known as spicules that, in most cases, have different lengths.

In addition to this, compared to the female, the cuticle has a greater number of layers, in addition to having a wrinkled appearance.

Female

Females are considerably larger than males. They can reach up to more than 50 cm in length. Its rear end ends in a point, it is not curved like that of the males. Regarding the cuticle, the female has fewer layers and is not wrinkled in appearance, but rather has certain protrusions.

Biological cycle

These insects are blood-sucking, that is, they feed on human blood through stinging.

Guest

The definitive host of this parasite is the human being. Inside, microfilariae larvae (L1) are found mainly in the dermis. When the insect bites an infected individual, to the extent that it feeds on blood, it also ingests microfilariae found on the skin.

Fly interior

Inside the body of the animal, the parasite loses its protective covering (cuticle) and migrates from the stomach to the thoracic muscles. There it undergoes a metamorphosis, going from state L1 to L3. These L3 larvae migrate back to the insect's oral cavity, specifically to the proboscis.

Interior of the human being

When an infected fly bites a healthy human being, it transmits the L3 larvae, which enter the body through the wounds caused by the bite. In humans, they lodge in the subcutaneous tissue, where they undergo metamorphosis again, from L3 to L4 and L5, until finally reaching the adult state.

Life cycle of Onchocerca volvulus. Source: See page for author

The adult worms remain in the subcutaneous tissue, usually in some connective tissue nodule. There they reproduce and the females begin to release microfilariae (L1 larvae). These larvae can be found on the skin for an average of 10-12 months after the parasite has entered humans.

Diseases

The disease caused by the parasite Onchocerca volvulus is known as onchocerciasis. It is also known by other names such as river blindness, onchocerciasis, and Robles disease, among other names.

Symptoms and signs

The adult parasites cause a series of signs and symptoms to be triggered in the infected human being, at the level of various systems.

Manifestations on the skin

Some symptoms appear on the skin that are mainly related to irritation caused by the parasite.

The main symptom is pruritus (itching), edema (swelling), as well as hyperthermia (increased temperature). Eventually, the itching leads to irritation of the skin from excessive scratching.

Vector insect of Onchocerca volvulus. Source: Otis Historical Archives of “National Museum of Health & Medicine” (OTIS Archive 1)

Later, with the progress of the infection, hyperpigmented areas or areas that lose their pigmentation appear on the skin, as well as lesions called lichenified plaques.

If the infection is not treated, the skin loses its elasticity and a condition known as pachydermitis develops.

The presence of skin nodules, known as onchocercomas, is also common. These are located mainly at the level of the scalp.

Ocular manifestations

One of the favorite tissues of these parasites in humans is the ocular conjunctiva. Due to this, infected people can present various symptoms at the ocular level.

Among the manifestations at eye level we can mention:

- This is sensitivity to light.

- Choroiditis: chronic inflammation of the choroid and retina.

- Uveitis: inflammation of the middle layer of the eye. Several structures such as the iris, choroid, and ciliary body are affected here.

- Sclerosing keratitis: it is an inflammation of the cornea. Here a permanent opacity of the cornea occurs.

- Atrophy of the optic nerve.

All these alterations greatly compromise the view. Eventually, a person with this infection gradually loses vision to total blindness.

Other manifestations

The progression of the infection can lead to neurological and kidney alterations. In addition to this, manifestations have been described at the level of the lymphatic system, such as obstruction of the lymphatic ducts. This obstruction leads to exaggerated inflammation. The hanging groin is a representative example of this.

Diagnosis

The diagnosis of the disease is based both on the clinical observation of the symptoms and signs, as well as some tests that include a skin biopsy and a specialized eye examination.

If a doctor suspects that his patient may have the disease, he will proceed to take a skin sample (biopsy), which will be placed in saline for 24 hours and then proceed to observe it under a microscope. If microfilariae are seen, then it is positive for Onchocerca volvulus infection.

Geographic distribution of Onchocerca volvulus. Source: BlankMap-World6.svg: Canuckguy (talk) and many others (see File history) Derivative work: Tardigrade95 (User Talk)

Likewise, if the patient is suspected of having ocular involvement, he should undergo an examination using an instrument known as a slit lamp. This allows the doctor to visualize the eye in a magnified way and detect whether or not microfilariae or the adult parasite are present.

Blood tests are not reliable to accurately diagnose Onchocerca volvulus infection, as there are other filarial-type parasites that can cause similar blood disorders.

Treatment

Treatment of onchocerciasis is long-lasting. The medication currently prescribed to treat this infection is an anthelmintic known as ivermectin. The form of administration is a single dose every six months. The duration depends on the persistence of the symptoms.

The mechanism of action of this medicine is based on the fact that it destroys microfilariae and, although it does not kill adult worms, it does greatly reduce their fertility, such that they cannot produce microfilariae.

If the patient has onchocercomas, the doctor may make the decision to surgically remove them. Of course, the treatment is determined by the doctor's criteria, taking into account the severity and evolution of each particular case.

References

1. Botero, D. (2012). Human parasitosis. 5th edition. Corporation for Biological Research.
2. Carvajal, J., Zambrano, J., Suárez, J., Duque, D. (2016). Onchocerciasis: from basic to clinical. UPB 35 Medicine (2)
3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Neafie, R. (1972). Morphology of Onchocerca volvulus. American Journal of Clinical Pathology. 57 (5).
6. Noguera, J. (2003). Onchocerciasis. Archives of the Spanish Society of Ophthalmology. 78 (4)