Onchocerciasis
| Onchocerca volvulus | ||||||||||||||
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| File:O.volvulus 20u.jpg O. vovulus, the causative agent of river blindness. | ||||||||||||||
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Onchocerciasis , also known as river blindness , Roble’s disease or sowda , is the world's second leading cause of blindness due to infection,with trachoma being number one. It is a chronic non fatal disease caused by a parasitic worm , Onchocerca volvulus , that enters the body through the bite of a black fly. It is estimated that 37 million people are infected at present and that among these more than 250,00 are blind and 500,00 have some degree of visual impairment (T.D.R. 2005).
Distribution of Vectors
insert map of disease distribution here
The distribution and spread of onchocerciasis is a result of blackflies that carry the infectious nematode,Onchocerca volvulus. The primary agent for transmission (the vector) of this parasite is the genus Simulium. In Africa , where 99% of the cases occur ,covering 30 countries in the sub-Saharan region, the most widespread vector is Simulium damnosum(sensu stricto). There are some regional species that are also important such as S.ethiopiense in Ethiopia , S. woodi in Malawi and Tanzania and S.neavei in east Africa.
In Latin America ,S.ochraceum,S.metallicum and S.callidum are important vectors in Mexico , Guatemala, and Venezuela. In Colombia and Ecuador , S. exiguum is the primary vector with S. amazonicum being predominant in Brazil (W.H.O. 1987).
The most likely vector in Yemen is S.damnosum , but the most important transmitter in Saudi Arabia is not certain (Awadzi 1989).
Blackfly Life Cycle
Blackflies ,of the Simulium damnosum complex, lay their eggs in swiftly moving water where the eggs attach to submerged objects , such as rocks and plants. When the eggs hatch, within 36 to 48 hours,they become black larvae that remain attached to stationary objects in the fast flowing water. The larvae feed and molt over a period of 8 to 10 days and pupate for 2 to 5 days without losing their anchorage. After pupation the adult fly emerges. The adults of S.damnosum may live up to one month and some simulium species may live up to three months.
The eggs of the female blackfly become fertilized when she mates with an adult male blackfly. After mating she will seek a blood meal from a warm-blooded mammal. This blood nourishes the eggs and allows them to reach maturity. The female fly then seeks flowing water to lay her eggs and begin the cycle again.
Blackflies will feed on both livestock and people , and some species of onchocerca, other than O.volvulus, will develop within cattle. Although there are a few reported cases of O.volvulus developing naturally in gorillas and spider monkeys, human beings are the most important host (Awadzi 1989).
The emergence of blackflies from rapidly moving rivers and streams makes any human beings living or working nearby a prime target for being bitten. Those who spend more time near the river are more likely to become bitten and infected by blackflies. The infection often develops into blindness and thus came to be known as ‘river blindness’. People came to realize that they should move their villages away from the river and avoid working near it if they wanted to avoid the blindness. However, adult flies have been known to travel up to 150 kilometers. It was not until 1926 that Breadablane Blacklock, a scottish parasitologist, discovered the connection between flies and the blindness of onchocerciasis while working in Sierra Leone (Blacklock 1926).
O.volvulus life cycle
The female fly bites the subject and causes a pool of blood to form on the skin surface. When she drinks this blood from an infected person, she will also receive the first stage microfilariae (L1) of the parasite, which will pass through the mouth (proboscis) and into the stomach. Most of the parasites will be digested by the stomach of the fly but the few that survive will migrate into the cells of the flight muscles. They will molt twice,within seven days, to form stage three infective microfilariae (L3), which will then migrate back to the proboscis of the mouth.
When the fly has another blood meal , the third stage microfilariae(L3) will be carried into the human host when the fly injects its anticoagulant saliva. They will enter the subcutaneous tissues where, within 3 months, they will mature into male and female adults. They will then mate and produce offspring that are stage one microfilaria. The formation of these microfilariae requires usually more than one year after the initial infection. It has been estimated that a single fertilized adult female can release up to 1,000 to 2,000 L1 microfilariae per day , and can live for up to 10 to 15 years. These infective larvae can then live from 6 to 30 months. They stay near subepidermal capillaries, which makes them available to the invading proboscis of the next blackfly vector.
O.volvulus Morphology
The adult worms are visible to the naked eye. They are white or cream colored with a threadlike appearance and tapered at both ends. The female worm is much larger than the male , measuring 230 millimeters to 500 millimeters (mm) long and 0.25 to 0.50 mm in width. The male is only 19 to 42 mm long with a width of about 0.14 mm.
The microfilariae require a microscope for examination. They have rounded heads,pointed tails and measure 300 to 360 microns in length and 5 to 9 microns in width. They may be distinguished from other genera of microfilariae (Mansonella) by the absence of nuclei in the tip of the pointed tail (Awadzi 1989).
Disease Symptoms and Pathology
The areas most affected by O.volvulus are the skin ,lymph nodes, and eyes. Onchocerciasis presents itself with varying degrees of involvement of each of these tissues depending on the geographic area and individual susceptibility. Some may show severe skin lesions without the blindness and others may show blindness with only minimal skin and lymph node involvement.
skin
The time from a bite by an infected blackfly to the first presence of microfilariae in the skin ranges from 7 to 34 months. The first skin symptom,excluding a reaction to the bite, is usually itching (pruritis) (Eezzuduemhoi et al. 2005).
When the microfilariae migrate to the skin they are a target for the immune system. White blood cells release various cytokines that have the effect of damaging the surrounding tissue and causing inflammation. This kills some microfilariae but is the cause of several symptoms associated with this disease.
In the skin this can cause intense itching that leads to the skin becoming swollen and chronically thickened, a condition often called lizard skin or elephant skin. The skin may also become lax as a result of the loss of elastic fibres. Over time the skin may lose some of its melanin pigment in some areas and gain melanin in other areas this gives rise to a condition known as leopard skin.
The adult worms become encased in a fibrous capsule that lies in the dermis and usually protrudes above bony bulges such as the head , shoulders, wrists , hips , knees , and ankles. These fibrous subepidermal protrusions are called skin nodules ,onchocercal nodules or onchocercomas. Skin nodules may contain two or three adult females plus an occasional adult male which may migrate between nodules to fertilze the females. The much smaller microfilariae may be present as they are being released by the fertilized females. As the adults age they become calcified within the nodules.
lymph nodes
The lymph nodes are visibly affected in this disease. There is scarring of the outer capsule. In some cases the lymph nodes are swollen ,especially in the groin area. The lymphphoid follicles show signs of atrophy. The germinal centers are fewer than normal. The central medulla is filled with eosinophils ,mast cells, and plasma cells among others. Microfilariae are usually present in the fibrous capsule and may also be found in smaller numbers in the blood and lymph vessels of the nodes (Awadzi 1989)
eyes
The infection of the eyes tends to be progressive and happens very quickly in those with head nodules, which serve as a source of microfilariae. The microfilariae migrate to the surface of the cornea where they are attacked by the immune system. The cornea becomes opaque in small regions ( punctate keratitis ) as the microfilaria die. This condition can be reversed as the inflammation subsides. However, if the infection is chronic ,the entire cornea can become irreversibly opaque, resulting in blindness.
With specialized microscopy, the microfilariae can be observed within the anterior chamber,the cornea and the vitreous humor of the eye. The live microfilariae appear to cause minimal damage but upon their death , a severe inflammatory response ensues.
The iris and ciliary body of the eye can also be invaded by microfilariae producing anterior uveitis. This can cause a distortion of the pupil, atrophy of the iris,glaucoma and cataracts.
The choroid and retina of the eye can also be affected in a condition called chorioretinitis. The choroid becomes inflammed and shows a loss of pigment from the vessels. The retinal pigment epithelium shows decreased pigmentation in some areas and increased pigmentation in others. The photoreceptors begin to disappear and then the inner layers of neuronal connections to the optic nerve begin to degenerate. Surprisingly, the macular region, responsible for visual acuity ,is spared until the latter stages of onchocerciasis. The mechanism of how chorioretinitis is accomplished (pathogenesis) is not understood (Eezzuduemhoi et al. 2005).
Treatment and control
The treatment for onchocerciasis is ivermectin,trade name mectizan. It is a semisynthetic lactone which was first produced in 1982 by Merck ,Sharp and Dohme. It is obtained as an isolate from the fermentation products of the bacterium Streptomyces avermitilis. The recommended dosage is 150 micrograms/kilogram of body weight. A single dose will kill 90% of the microfilariae for up to one year. However, for people living in heavily infested areas ,treatment is recommended every six months (Eezzuduemhoi 2005). While the drug does not kill the adult worms, it does prevent them from producing additional offspring , which alleviates some of the symptoms and decreases transmission to other people.
Since 1988, ivermectin has been provided free of charge by Merck & Co. through the Mectizan Donation Program . This program works together with ministries of health and non-governmental development organizations such as the World Health Organization to provide free mectizan to those who need it.
Precautions for ivermectin,include pregnant women, children under age ??? and people who are infected by another filarial worm known as Loa loa, who have adverse reactions to ivermectin.
There are various control programs that aim to stop the spread of onchocerciasis. The first was the Onchocerciasis Control Program (O.C.P.)which was launched in West Africa in 1975 by the United Nations and at its peak covered 30 million people in eleven countries. Through the use of larvicide spraying of rivers to control black fly populations and the use of ivermectin to treat infected people, the O.C.P. eliminated most of the onchocerciasis in its targeted areas and was discontinued in 2002.
In 1992 the Onchocerciasis Elimination Programme for the Americas (O.E.P.A.) was launched. The OEPA also relies on ivermectin.
In 1995 the African Programme for Onchocerciasis Control (A.P.O.C.) began supplying another nineteen countries with the ivermectin from Merck and Co., but without the larviciding or rivers.
Recent research suggests that the Wolbachia bacteria carried by O. volvulus may actually provoke the damaging inflammatory response rather than the worm itself,opening the possibility for antibiotic treatment.
The research on developing macrofilaricides, drugs that can kill or sterilize the adult worms, continues. Moxidectin, a compound similar in structure to ivermectin, has shown some promise in killing adult Onchocerca in animals (T.D.R. 2005)
Another line of continuing research is to control the population of blackflies. One approach is through spraying larvicides into the water to prevent hatching. Most popular is Temephos ,trade name abate. Another is a bacterium known as Bacillus thuringiensis .
ReferencesISBN links support NWE through referral fees
- Awadzi,K. in Goldsmith,R. and Heyneman,D. (eds.) 1989. Tropical Medicine and Parasitology. Norwalk ,Conn :Appleton & Lange.
- Blacklock, B. 1926. The development of Onchocerca volvulus in Simulium damnosum. Ann. Trop. Med. Parasitol. 20:1-48
- Carter Center River Blindness Program
- Eezzuduemhoi,D.,Wilson,D.,Sheppard,Jr.,J.,Fong,D.S.,Walton,R.C.,Brown,L.L.,and Roy, Sr.,H.R. 2005 . Onchocerciasis [1]
*Kenney,M. 1973. Scope Monograph on Pathoparasitology, A color atlas of parasites in tissue sections. Upjohn Co.,Kalamazoo,Michigan. P.47.
- O'Neill, John. "On The Presence of a Filaria in “Craw-Craw." The Lancet. 20 Feb. 1874: 265-266.
- T.D.R. (Tropical Disease Research) 2005. Seventeenth Programme Report, Progress 2003-2004.[2]
- W.H.O. 1987. WHO Expert Committee on Onchocerciasis,Third report.Geneva,Switzerland: World Health Organization.
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