History and Impact of Onchocerciasis Around the World

Onchocerciasis, commonly termed river blindness, is a tropical parasitic disease caused by the nematode Onchocerca volvulus.​ ​ Species of blackfly, Simulium spp.​​, are known vectors of onchocerciasis. Thus, the disease is endemic in areas blackflies call home- usually near rapidly moving rivers and streams. Onchocerciasis occurs mostly in Africa, though a few countries in South America are also affected (WHO, 2015). It is characterized by lesions, disfigured skin, and visual impairments to permanent blindness.

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Genetic testing has confirmed that Onchocerciasis is an old world disease that was brought to the new world, most likely through slade trade (Gustavsen et al., 2011). From sub-saharan Africa, the disease has traveled to Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela. It was even able to reach the isolated populations inhabiting the amazon. Experts have formulated two possible theories as this occurred- either the Amazonian people once lived elsewhere and then moved, or that the movement of other indigenous groups carried the disease closer to the Amazon.
A human is infected when a blackfly carrying O. volvulus​ ​ bites them. The O. volvulus​ larvae enters the skin through the bite wound where they will develop into adult worms. These adult worms reside in nodules that form around them. Though the nodules can be seen as lumps under the skin, they do not provoke an immune response. Each nodule consists of both male and female worms that will reproduce and release microfilariae. The female worms can live for up to 15 years, and release approximately 1300 to 1900 microfilariae a day (Burnham, 1998). The released microfilariae move to the skin and eyes of their host. This process from the bite to the emergence of the microfilariae takes anywhere between 10 and 20 months. Thus, the life cycle of O. volvulus ​makes for a slowly progressing, chronic infection. Microfilariae can be found in an infected person’s blood, urine, organs, and cerebrospinal fluid. Onchocerciasis cannot be transmitted from human to human, it is only passed by the vector, making it a stage 2 disease.
The blackflies as vectors allow for a lot of variation in the disease transmission. Different species of Simulium spp.​ ​ vary in how effectively they are able to transmit O.volvulus​ ​ and in which regions. S. damnosum​ ​ is the most common vector, active in West Africa. S. ochraceum ​ ​is the species active in Guatemala and Mexico, and is said to be less efficient than the other species of blackfly at spreading onchocerciasis (Gustavsen et al., 2011).
Though most infections are asymptomatic, repeated, prolonged infections can result in symptoms, most caused by the immune system’s inflammatory response to dead microfilariae. Infected individuals can have itchy lesions that may clear on their own or be chronic, in addition to musculoskeletal pain and weight loss. When the lesions heal, they may leave behind hyperpigmented skin. An individual that has been infected several times and suffered years of infection is more at risk of complete and permanent blindness and wrinkled, sagging, depigmented skin. However, Researchers have found that the risk of blindness in infected persons is not uniform. (Burnham, 1998) DNA analysis explains this by verifying that different strains of Onchocerciasis inflict different damage. They observed that blindness was a more common outcome in African savannas and woodlands, while skin abnormalities were more common in forested areas. They hypothesize that varying population density, genetics, and nutrition may also affect risk.
Onchocerciasis has difficult socioeconomic implications for those afflicted. Studies have found that agricultural workers who are ill spend more time away from work, are less productive, and thus earn less. This continues a cycle of poverty. The skin abnormalities caused by the disease evoke stigmas amongst affected communities. Those infected face issues getting married and continuing relationships. Mothers have complications with pregnancy and usually breastfeed for shorter periods of time. A cross-sectional study conducted in the rural village of Apana in Nigeria attempted to measure what perception people had of onchocerciasis. The study found that the unaffected held stigmas and tended to avoid and/or discriminate against those who were affected. The affected were socially withdrawn and embarrassed. The study claimed that improving the general attitude towards onchocerciasis would help reduce morbidity. (Wagbatsoma and Okojie, 2004).
For years, strives have been made towards the control and eradication of onchocerciasis. In 1978, Ivermectin was developed as a drug to treat onchocerciasis. Previously the drug was used to treat dog heartworms and parasitic infections of other large animals. In humans, it can rid the body of microfilariae for months at a time by inhibiting the adult worms from releasing them. Unfortunately, it cannot simply kill the adult worms. Ivermectin was mass distributed in endemic areas as a public health initiative called CDTI- community directed treatment with ivermectin. Up till now, programs in west and sub-saharan Africa, have solely focused on the control of onchocerciasis, and have “decreased the burden of disease to such an extent that it is no longer a public health problem in most endemic areas” (Kim et al., 2015). Researchers are now showing that elimination is feasible too in these areas, and many believe that the strategy should be shifted to elimination. In Colombia and Ecuador, elimination has been achieved and endemic South American countries are projected to reach regional elimination in 2020.
Researchers had also found a possible way to combat onchocerciasis with an antibiotic. Wolbachia bacteria share a symbiotic relationship with O.volvulus​ ​; the bacteria is essential for successful reproduction (Enk, 2006). A supposed antibiotic that would kill the bacteria would also prevent the adult worms from releasing microfilariae. With tests, this approach was deemed effective but has not been implemented on the large scale that ivermectin has.
An interesting attribute of the disease and a critique of current public health programs is the unacknowledged ties it has to epilepsy. According to research, there was an observed correlation between onchocerciasis and “onchocerciasis-related epilepsy” (Colebunders et al., 2019) Though there is no concrete evidence of a direct connection between the onchocerciasis and epilepsy, they conclude that it would be beneficial to include epilepsy measures in public health initiatives to keep communities from wavering from control efforts and to ensure that programs continue to be funded.
As the world began to acknowledge the burden that neglected tropical diseases posed on communities, onchocerciasis’ impact has been reduced significantly. However, there is still work left to be done. Onchocerciasis was responsible for 1.1 million DALYs lost in 2015 (WHO). The burden is still present and thus control and elimination efforts must continue to the point of eradication.
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