THE ARTHROPOD-BORNE ONCHOCERCIASIS: IS IT DESERVED to BE NEGLECTED? by MAMDOUH M
Total Page：16
File Type：pdf, Size：1020Kb
Journal of the Egyptian Society of Parasitology, Vol.45, No.3, December 2015 J. Egypt. Soc. Parasitol. (JESP), 45(3), 2015: 639 - 654 THE ARTHROPOD-BORNE ONCHOCERCIASIS: IS IT DESERVED TO BE NEGLECTED? By MAMDOUH M. M. EL-BAHNASAWY1, AYMAN T. A. MORSY2 AND TOSSON A. MORSY3 Military Medical Academy1, Consultant of Tropical Medicine, The Ministry of Interior 2 3 Hospitals , and Faculty of Medicine, Ain Shams University, Cairo 11566 , Egypt. Abstract Onchocerciasis a filarial parasitic nematode, also known as river blindness and Robles dis- ease, is a neglected tropical disease infecting more than 18 million people mainly in sub- Saharan of Africa, the Middle East, South and Central America and many other countries. Dis- ease infectivity initiates from Onchocerca volvulus (Filarioidea: Onchocercidae) transmitted by the blackfly, Simulium sp. which introduces the infective stage larva with its saliva into the skin. Within human body, adult females (macrofilaria) produce thousands of larvae (microfilar- iae) which migrate in skin and eye. Infection results in severe visual impairment or blindness for about 2 million, as being the world’s second-leading cause of blindness after trachoma, as well as skin onchocercomata. Key words: Egypt, Onchocerciasis, blackfly, General review, Recommendations Introduction fection rates may be as high as 80 to 100% Onchocerciasis is caused by the filarial by the age of 20 in certain areas. Clinical nematode, Onchocerca volvulus. It is a ma- manifestations become apparent later, with jor cause of both skin disease and blindness blindness peaking at 40 to 50 years of age. in endemic areas, known as "river blindness Hyperendemic regions were frequently de- and Robles disease" because the blackfly populated because of high rates of ophthal- vector for this infection breeds in areas near mic complications. Neto et al. (2009) in fast-flowing streams and rivers. Onchocerci- Brazil reported that the prevalence of ocular asis can be a major cause of morbidity for lesions strongly correlated with the cutane- individuals in endemic areas and can have ous nodules and eosinophilia, suggested that major socioeconomic consequences (Taylor, skin nodules might be an indication for an 1990). eye examination, since significant infection Epidemiology: Onchocerciasis is transmit- and onchocerciasis eye disease persisted in ted in limited areas of tropical Africa, Yem- certain regions of Northern South America. en, and Latin America. Humans are the only There are two different strains of O. volvulus definitive host for O. volvulus, although in Africa, one strain is transmitted by black there are a number of other species that in- flies that tend to breed in savannas; this fect only animals tell now. Approximately strain is more commonly associated with 18 million people are infected worldwide. ocular disease, even with only a moderate More than 99% of symptomatic cases occur parasite burden and second strain is trans- in sub-Saharan Africa, particularly in Nige- mitted by vectors that breed in forests and ria and Zaire, It where 3 to 4 million infect- generally was not associated with blindness ed individuals with skin disease, and 1 to 2 (Higazi et al, 2005). Black flies are not effi- million with blindness or visually impaired. cient vectors; as 12 months in endemic areas The disease tended to be found in rural areas is generally necessary to acquire infection, (WHO, 1995). The visual impairment in- and as most helminthes, the worms do not crease, in part, as the prevalence and intensi- replicate in humans, so an increase in adults ty of infection in a community rises. In Afri- burden requires additional exposure to a bite can endemic areas, the prevalence of infec- of an infective vector (Nguyen et al, 2005). tion can vary from village to village, but in- 639 Life cycle: Humans are infected with O. hind most of its morbidity. Dying microfilar- volvulus by bite of the black fly; Simulium iae release Wolbachia surface protein that genus. The black fly transmits infective third activatedTLR2 & TLR4, triggering innate stage larvae to humans, and these larvae are immune responses and producing the in- deposited into the host skin. Over a period of flammation and its associated morbidity 6 to 12 months, the larvae mature into adult (Baldo et al, 2010). The severity of illness is worms. Females measure 20 to 80 cm in directly variable and depends upon the cir- length, live in subcutaneous or deeper intra- culating microfiliaria number and the host muscular tissues surrounded by a fibrous immune response (Burnham, 1998). Some capsule, whereas males measure 3 to 5cm patients have a minimal immune response to and migrate between nodules to fertilize fe- parasite antigens, permitting microfilariae males. The prepatent period ranges from 10 proliferation in the absence of symptoms to 12 months, the females begin to produce (Prost et al, 1979). Others have a relatively microfilariae. The Simulium female takes a robust immune response against circulating blood meal from an infected human host, microfilariae (King and Nutman, 1991). Du- and ingests microfilariae. In the Simulium, ration of infection may also play a role in the microfilaria enter the gut and thoracic degree of inflammatory response; it has been flight muscles, progressing into the first lar- suggested that individuals with early O. vol- val stage, larvae mature into the second lar- vulus infection mount a substantial cell-med- val stage, and move to proboscis and into the iated immune response, while those with the saliva in its third larval stage. Maturation chronic infection tend to have a blunted cel- takes about seven days. When the black fly lular immunity (Cooper et al, 2001). Patients takes another blood meal, it passes the lar- with a significant cellular immune response, vae into the next human’s blood. In man, polyclonal B cell activation and hypergam- larvae migrate to the subcutaneous tissue maglobulinemia are common, as are non- and undergo two more molts, and form nod- specific elevations in IgG & IgE. However, ules as they mature into adults over 6 to 12 there was minimal evidence for the acquired months. After maturing, males mate with immunity to O. volvulus in endemic areas females in subcutaneous tissue to produce (Kruppa and Burchard, 1999). Also, release from 1000 to 3000 microfilariae/day. The of Wolbachia-derived antigens from dying microfilariae migrate to the skin at daytime, adults and microfilariae can activate innate and the vectors only feed in the day, so the immune responses and may elicit inflamma- parasite is in a prime position for the female tion in the skin and eye (Johnson et al, fly to ingest it. Black flies take blood meals 2005). The heightened propensity for ocular to ingest the microfilariae to restart the cy- disease due to the Savanna strain of O. vol- cle. Adults can survive for up to 15 years, vulus relative to the milder Forest strain cor- and microfilariae can survive for one to two related with higher quantities of the Wol- years (Brattig, 2004). bachia DNA by PCR in the Savanna strain. Pathogenesis: O. volvulus adults reside in Both Wolbachia and Onchocerca antigens fibrous skin nodules where they are protect- appear to influence corneal inflammation, ed from the host immune response. The mi- although dermatologic manifestations were crofilariae, in contrast, are capable of mov- attributable to Onchocerca antigens alone ing through subcutaneous and ocular tissues. (Udall, 2007) Although they provoke a minimal immune Clinical manifestations: The onchocercia- response while alive, on dyeing incite a clin- sis manifestations include dermatologic ones ical inflammatory response. Wolbachia sp. (subcutaneous nodules and pruritic dermati- is endosymbiosis of adults and microfilariae, tis) and ocular manifestations. and are thought to be the driving force be- I- Subcutaneous nodules (onchocercomata) 640 are typically 0.5 to 3 cm in diameter; they chenified dermatitis affecting one or multi- usually contain one or two adult male and ple limbs; characterized by the dramatic skin two or three adult female worms. Nodules disease and a brisk T-helper 2 (Th2)-type are not associated with inflammation and do immune response with high IgE levels not usually cause significant symptoms. (Hoerauf et al, 2002). So, patients with sow- They typically appear over bony prominenc- da have a low microfilarial burden (microfi- es and have an apparent predisposition for lariae are usually absent in skin snips) and different anatomic sites depending upon the ivermectin therapy may be less effective in region in which the infection was acquired such patients. 4- Skin atrophy manifests with (reflecting the differences in vector biting loss of skin elasticity and wrinkling of the habits in different regions). Infections ac- skin; the resulting skin has the appearance of quired in Africa tend to cause nodules over tissue paper. 5- Elephant skin or thickening the iliac crests and lower limbs, while those of human skin associated with onchocercia- acquired in Latin America are more com- sis. 6- Depigmentation manifests with loss monly associated with nodules on the head, of patches of skin with loss of pigment adja- neck, and upper extremities (Little et al, cent to areas of normally pigmented skin can 2004). II- Dermatitis due to onchocerciasis result in a "leopard skin" appearance. is an intensely pruritic inflammatory papulo- Ocular onchocerciasis: Onchocerciasis is a nodular process that may affect up to half of leading cause of blindness in the developing individuals in endemic communities. Exco- world. Individuals with blindness due to on- riation frequently leads to secondary bacteri- chocerciasis have a fourfold increase in mor- al infection. Subsequent lymphadenopathy tality and diminished life expectancy by 7 to can develop, most commonly in the inguinal 12 years. O. volvulus is the second com- region. Atrophic skin over the lymph nodes monest cause of blindness, after trachoma can become loose, leading to hanging folds (WHO, 2014).