Onchocerciasis
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Other Names
- The disease is also called River Blindness because the transmission is most intense in remote agricultural villages located near rapidly flowing streams in Africa.
- Filariasis is another name for diseases caused by small parasitic worms and their larvae; Onchocerciasis is a common type of filariasis.
Signs and Symptoms
- Skin rash that is very pruritic (itchy).
- Nodules or bumps under the skin.
- Disease in the eye that can lead to blindness.
As part of the life cycle of O. volvulus (see below), pre-larval forms of the worm migrate through the skin and underlying tissues. This elicits an immune response in the body and the skin rash and itching are due to this phenomenon. Adult worms can live in deeper tissues. The immune response can lead to the formation of a fibrous nodule which contains the living worms. In the eye disease seen in onchocerciasis, the pre-larval forms migrate to the eye and cause an inflammatory response in the eye. This can lead to varying degrees of injury to the cornea and other structures in the eye and can result in blindness.
The normal incubation period (time from infection until start of symptoms) of onchocerciasis ranges from nine to 24 months after the bite of an infected blackfly.
Causes
Adult worms live inside fibrous nodules in humans. The worms produce large numbers of microfilariae, a pre-larval form of the worm. These microfilariae migrate through the skin causing itching and a rash. They sometimes migrate to the forward parts of the eye which can lead to blindness.
The female blackfly can pick up these microfilariae when it bites a person who is infected. Once in the blackfly, the microfilariae migrate to chest muscles and become true larvae. The larvae then migrate to the mouthparts of the blackfly and can then infect another human when the fly next bites. The larvae become adults in the tissues underneath the skin and repeat the life cycle. Adult worms can live for 10-15 years.
Diagnosis
If a person suspects that they might have onchocerciasis, it is a good idea to see a health care provider who may order skin biopsies ('snips'). However, examination of skin snips does not always show the parasites. Blood testing for antibodies is available, but a positive result does not necessarily indicate an active case of onchocerciasis.Nodule palpation (feeling and maniplulating the skin nodules associated with infection) is a major diagnostic tool for identifying communities at risk and selecting them for mass drug administration. While this is inexpensive and easily done, the test results from different examiners can be very different, emphasizing the need for improved diagnostic tests.[1]
Exams and tests
- Skin Biopsy
- Blood tests for antibodies to the parasite.
Treatment
Ivermectin is administered as a single oral dose of 150 micrograms per kilogram (maximum 12 mg) one or two times a year. The drug should probably not be given to pregnant women or children under 5 years. Ivermectin does not kill the adult parasites, but reduces the numbers of microfilariae in skin so the disease does not progress. It may be necessary to repeat the dosage up to three times. A repeat dosage is given every 6-12 months.[2]
Prevention
Blackflies bite during the day. The best prevention is to avoid infective bites of the blackfly by:
- Using insecticides such as DEET, and
- Wearing long sleeve shirts and pants.
There is neither a vaccine nor recommended drug available to prevent onchocerciasis.
Chances of Developing Onchocerciasis
Onchocerciasis is locally transmitted in Mexico, Guatemala, Ecuador, Colombia, Venezuela, Brazil, Yemen and thirty countries in Africa. Onchocerciasis in casual travelers is rare. The infection is transmitted in rural areas, and, unlike malaria, contracting onchocerciasis requires more than one infectious bite. Thus, risk of infection is greater in adventure travelers, missionaries, and Peace Corps volunteers who are likely to have intense and prolonged exposure to blackfly bites (living or working in an endemic region for more than three months). Given the low rate of transmission in the Americas, the likelihood is very low that any travelers in this region (even missionaries and Peace Corps volunteers) would ever get infected.
How Onchocerciasis is Spread
The disease spreads from person to person by the bite of a blackfly. When a blackfly bites a person who has onchocerciasis, microscopic worms (called microfilariae) in the infected person's skin enter and infect the blackfly. The microfilariae develop over 2 weeks into a stage that is infectious to humans. An infectious blackfly will typically inject one or two larvae to transmit the disease to another person. Since the worms can only increase their numbers in the human by first mating and then cycling their microfilariae through the blackfly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual. The more intense the infection, the more severe symptoms a person may have. Blindness almost always occurs in a person with intense infection and is less likely in those with milder infections.
Research
Research into prevention, early detection, and removal of Onchocera infections continues. One promising observation is that Onchocera volvulus requires the presence of a bacteria of the Wolbachia genus to reproduce. Wolbachia belong to the order Rickettsiales and are found in association with all stages of O. volvulus. Importantly, antibiotics that kill Wolbachia result in disruption of the life cycle of female worms. Commonly-available antibiotics such as doxycycline may therefore provide an alternative to ivermectin.[3]
Global impact
Onchocerciasis was a leading cause of preventable blindness and skin disease. However, the global disease burden of this disease has been considerably reduced as the result of very successful disease control programs led by the World Health Organization (WHO). These programs are based on vector control and/or mass administration of ivermectin.[4] As a result of these programs, millions no longer suffer pain, disfigurement, and blindness.
About 500,000 people have some degree of visual impairment from the disease, and about 270,000 are blind.[5]
References
- ↑ Duerr HP, Raddatz G, Eichner M. Diagnostic value of nodule palpation in onchocerciasis.Trans R Soc Trop Med Hyg. 2008 Feb;102(2):148-54.Abstract
- ↑ Brieger WR, Awedoba AK, Eneanya CI, et al. The effects of ivermectin on onchocercal skin disease and severe itching: results of a multicentre trial. Trop Med Int Health. 1998 Dec;3(12):951-61. Abstract | Full Text | PDF
- ↑ Walgate R. Could antibiotics cure river blindness? Bull World Health Organ. 2002;80(4):336. Abstract | Full Text | PDF
- ↑ Tielsch JM, Beeche A. Impact of ivermectin on illness and disability associated with onchocerciasis. Trop Med Int Health. 2004 Apr;9(4):A45-56. Abstract | Full Text | PDF
- ↑ Hoerauf A, Büttner DW, Adjei O, Pearlman E. Onchocerciasis. BMJ. 2003 Jan 25;326(7382):207-10. Abstract | Full Text | PDF
External Links
Centers for Disease Control and Prevention (CDC)
World Health Organization (WHO)
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