Onchocerciasis (Onchocerca volvulus)

From MicrobeWiki, the student-edited microbiology resource
University of Oklahoma Study Abroad Microbiology in Arezzo, Italy[1]
Picture of Adult Onchocerca volvulus worms from the World Health Organization. http://www.who.int/apoc/media/adult_worms.JPG

Etiology/Bacteriology

Taxonomy

| Domain = Animalia | Phylum = Nematoda | Class = Secernentea | Order = Spirurida | Family = Onchocecidae | Genus = Onchocerca | species = O. volvulus |NCBI: [2] Genome: [3]|}

Description

Onchocerciasis, also known as “River blindness” is an eye and skin infection caused by the parasitic worm Onchocerca volvulus. It is considered to be among the list of neglected tropical diseases. According to the CDC a neglected tropical diseases are a group of parasitic and bacterial diseases that attribute to substantial illness for more than one billion people globally [1]. The symptoms of onchocerciasis include nodules under the skin, debilitating itching, and eventually blindness. Onchocerciasis alone is believed to currently affect over 25 million worldwide with 300,000 completely blind and another 800,000 with substantial vision impairment [2]. The filarial worm is transmitted to human hosts through the bite of infected black flies from the genus Simulium. These flies tend to live around wetlands and large river basins, hence the common name “River blindness.” The majority of the population affected by the disease (99%) reside in the World Health Organization Africa Region while cases have been documented around the world including Central and South America [3].

Pathogenesis

Transmission

Life Cycle of O. volvulus.[3]

Transmission of the parasite O. volvulus is via a Simulian black fly vector. When the Simuliid black fly takes a blood meal from a an infected human host, O. volvulus larvae are present in the blood meal. A few days later the larvae has developed in the midgut of the fly and is able to travel in the saliva from the fly into a new host human when the fly feeds again. This period usually lasts about ten days from the first host to the new host. Once in the new host the maturing worm takes up residence in the subcutaneous tissue usually forming nodules or causing joint pain. Here it will mature and become filariae, the mature stage of the parasite. These filariae can live for up to 15 years and grow to be up to 80cm (32 inches) in length[4]. Once mature these filariae will produce anywhere from 700- 1800 microfilariae a day, averaging 1,600[3]. It is these microfilariae (larvae) that will cause most of the damage to the host in their attempt to reach the skin surface to be part of another Simuliid's meal. Microflariae can live in the larval stage in a human host for 3-5 years[5]. .

Epidemiology

Onchocerciasis is considered endemic across the tropical areas of Latin America, Africa and Yemen.

Epidemiology of Onchocerciasis [3]

Usually found near large rivers or wetland areas. The tropical climates in these areas are perfectly suited for the Simuliam fly's mating habits. The Simuliid flies have aquatic larvae. This means that for the flies to complete their lifecycle the larvae require fast-flowing water in which the larvae can attach to submerged rocks or vegetation. Due to this part of the fly’s life cycle, endemic areas usually coincide with the path of rivers and streams. This fact has been confirmed with Onchocerciasis mapping[6]. Approximately 99% of infected people live on the African continent. The other 1% are located in Latin America and parts of the Middle East. It is commonly believed that Onchocerciasis was brought to the Americas by way of the slave trade. The parasite Onchocerca volvulus has become almost completely zoonotic in the areas it is known to exist in Latin America [6].

Infectious Dose, Incubation

The actual infectious dose of O. volvulus required to cause Onchocerciasis is unknown, but it is believed to take hundreds of bites from infected Simuliids to contract the disease[4]. The incubation varies from host to host and by the amount of bites from infected vectors. The generally accepted incubation period ranges from 1-2 years. This is considered the time from initial infection to the time that the mature filariae produce larvae.

Virulent Factors

Clinical Features

Symptoms

Onchocerciasis causes a wide array of symptoms. These can include but are not limited to:

Morbidity/Mortality

Diagnosis

Treatment

Traditional

Merck

Prevention

Children lead men affected by Onchocerciasis. Published in the Atlas of Tropical and Extraordinary Diseases via the National Museum of Health and Medicine. https://www.flickr.com/photos/medicalmuseum/3969963699/in/album-72157622366709345/

Host Immune Response

References

1 Onchocerciasis. CDC.

2 Pearson, Richard D. "Onchocerciasis"

3 Onchocerciasis or River Blindness. USAID.

4 Onchocerca volvulus. Pathogen Safety Data Sheet. Public Health Agency of Canada.

5 Udall, D. "Recent Updates on Onchocerciasis: Diagnosis and Treatment" Clinical and Infectious Diseases. Vol 44 Issue 1. 2007.

6 Goverman, D. Bass, J. Smith, S. "Onchocerciasis (River Blindness)" 2007.

Created by Tyler Potter, student of Tyrrell Conway at the University of Oklahoma.