ONCHOCERCIASIS
IN AFRICA
OVERVIEW
Onchocerciasis is an eye and skin disease caused by the parasitic worm Onchocerca volvulus (WHO, 2016a). This disease is the 2nd leading infectious cause of blindness according to WHO (USAID’s, 2014). However, this parasite does not always cause blindness. Most cases are characterized by intense itching and disfigured skin. Skin disfiguration usually occurs due to uncontrollable itching to the point where the skin becomes raw and bloody (Silver, 2016). This itching is due to the death of microfilariae in the skin, which releases toxins. Repeated exposure to the parasite can cause irreversible blindness and leopard or lizard skin (WHO,2016).
“This video aired on Animal Planet as part of the Monsters Inside Me series. It describes the life cycle of Onchocerca volvulus, and the negative immune responses it triggers in the human host.”
LIFE CYCLE
The lifecycle begins when an infected Simulium species of black fly infected with 3rd stage filarial larvae feeds on the human host, transmitting the parasite. The larvae develop into adult worms that reside in subcutaneous nodules that can contain more than 50 worms each. Adult females spend up to 15 years living and reproducing in these nodules. Each adult female can produce approximately 1,600 microfilariae per day to increase their density in the blood. A new fly feeds on the infected human and ingests the microfilariae, which then migrate from the midgut to the thoracic muscles. Here the flies develop into the 3rd stage larvae. The larvae then migrate to the proboscis where they can be transmitted during the fly’s next blood meal (USAID’s, 2014).
SYMPTOMS
The major symptoms of onchocerciasis include itchy skin, rashes, nodules under the skin, and vision changes. The skin reactions range from mild and local to generalized and intense itching that can lead to open sores and infections. This can lead to chronic rough dermatitis often accompanied by hyper-pigmentation if untreated. In the later stages of the disease, the skin becomes thin and loses pigmentation in patches producing “leopard skin” (USAID’s, 2014).
Eye lesions take many years of severe infection to develop. As a result, it is normally only found in people over 30 years of age (USAID’s, 2014). As the larvae die in the eye, they release toxins that elicit an inflammatory response from the immune system. This inflammation causes reversible small opaque spots on the cornea and redness of the conjunctiva. If untreated, this cornea becomes permanently cloudy. At this point the toxins and inflammation have caused damage to all of the anatomical parts of the eyes including the optic nerve (USAID’s, 2014).
DIAGNOSIS
The most common ways used to diagnose onchocerciasis in Africa are physical examination to detect dermatitis and nodules (USAID’s, 2014). However, this method is very inaccurate. It only detects the disease once it has progressed to a higher level. The other common diagnosis is the skin snip test. This method is highly invasive. It involves cutting off a sliver of skip from the hip or shoulder blade. This skin is then placed in salt water and observed for the emergence of worms as they try to escape the unfavorable conditions. In order for this test to detect onchocerciasis, the infection has to be present for 3-15 months (Silver, 2016).
Other less invasive methods of diagnosis have been explored in order to find an acceptable alternative to the skin snip test. The polymerase chain reaction can diagnose low levels for the infection. However, this test requires an expensive machine, so it is not widely used (USAID’s, 2014). Worms can be detected in the eye using a slit-lamp microscope, but this requires a more severe infection in order to detect the parasite. An anti-body test using blood, plama, or serum has also been developed. This test is quick, relatively easy to perform, and non-invasive (PATH, 2015). However, this test can only indicate whether the person has been exposed to onchocerciasis in the past rather than determining if they are currently infected. This test is useful for people that visit areas with onchocerciasis, but have not had previous exposure to the disease (USAID’s, 2014).
VECTOR
Onchocerca volvulus is spread by Simulium species of black flies (WHO, 2016a). These flies breed in flast-flowing waters, such as streams and rivers. The fast flow of the water allows for better aeration of the water to increase oxygen concentrations. The flies are found widespread in the world. They are mostly a nuisance in Western countries, but pose serious risks for sub-tropical regions such as Africa, Latin America, etc., where it acts as a vector for various parasites including O. volvulus (Valent, 2014).
