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Onchocerca volvulus (gr. όγκος „Widerhaken“, κέρκος „Schwanz“; lat. volvulus „kleines Knäuel“) ist der Name eines tropischen, zu den Filarien gehörenden Fadenwurms. Er ist ein Parasit des Menschen und der Erreger der Flussblindheit.
Onchocerca volvulus kommt in weiten Gebieten des tropischen Afrikas und in Zentral- sowie im nördlichen Südamerika vor. Ein isolierter Herd liegt im Jemen.
Sein Verbreitungsgebiet beschränkt sich auf Feuchtregionen entlang schnellfließender Flüsse.
Die Adultwürmer sind sehr schmal (fadenförmig) mit einem Durchmesser von weniger als einem Millimeter. Weibchen können bis zu 70 Zentimeter lang werden, das Männchen erreicht maximal Längen um 40 Millimeter. Die Mikrofilarien (Larvenform) werden 220 bis 280 µm lang.
Onchoceren haben wie alle parasitären Filarien einen Lebenszyklus mit Wirtswechsel. Der Mensch ist der einzige Endwirt. In Endemiegebieten sind nahezu 100 % der Bevölkerung infiziert. Als Zwischenwirt dient die weibliche Kriebelmücke (Simulium damnosum), die die Mikrofilarien beim Stich aufnimmt. In der Mücke entwickeln sich die Larven durch Häutung zum infektiösen Stadium L3, in dem sie bei erneutem Stich auf den Menschen übertragen werden. Sie wandern ein bis zwei Jahre durch das Bindegewebe und gelegentlich durch die Augen. Ausgewachsene (adulte) Würmer überleben jahrelang eingekapselt in subkutanen Knoten (Onchozerkom). Oft finden sich mehrere Filarien in einem solchen Knoten zusammen. Die Weibchen produzieren täglich etwa tausend Mikrofilarien, die durch die Lymphspalten des Bindegewebes wandern und schließlich in den Blutkreislauf eingeschwemmt werden. Auch die Mikrofilarien befallen das Auge, wodurch die hauptsächliche Schadwirkung (Erblindung) zustande kommt.
Kriebelmücke bei der Blutmahlzeit
Kriebelmücke mit am linken Fühler austretender Onchocerca-Filarie
Onchocerca volvulus (gr. όγκος „Widerhaken“, κέρκος „Schwanz“; lat. volvulus „kleines Knäuel“) ist der Name eines tropischen, zu den Filarien gehörenden Fadenwurms. Er ist ein Parasit des Menschen und der Erreger der Flussblindheit.
Onchocerca volvulus is a filarial (arthropod-borne) nematode (roundworm) that causes onchocerciasis (river blindness), and is the second-leading cause of blindness due to infection worldwide after trachoma. It is one of the 20 neglected tropical diseases listed by the World Health Organization, with elimination from certain countries expected by 2025.[2]
John O’Neill, an Irish surgeon, first described Onchocerca volvulus in 1874, when he found it to be the causative agent of ‘craw-craw’, a skin disease found in West Africa.[3] A Guatemalan doctor, Rodolfo Robles, first linked it to visual impairment in 1917.[4]
O. volvulus is primarily found in sub-Saharan Africa, and humans are the only known definitive host; there is also disease transmission in some South American nations, as well as Yemen (see global map bottom right). It is spread from person to person via female biting blackflies of the genus Simulium.[5]
O. volvulus parasites obtain nutrients from the human host by ingesting blood or by diffusion through their cuticle. They may be able to trigger blood-vessel formation because dense vascular networks are often found surrounding the worms.[6] They are distinguished from other human-infecting filarial nematodes by the presence of deep transverse striations.[7]
It is a dioecious species, containing distinct males and females, which form nodules under the skin in humans. Mature female worms permanently reside in these fibrous nodules, while male worms are free to move around the subcutaneous tissue. The males are smaller than females, with male worms measuring 23 mm in length compared to 230–700 mm in females.[7]
The release of oocytes (eggs) in female worms does not depend upon the presence of a male worm, although they may attract male worms using unidentified pheromones.[8] The first larval stage, microfilariae, are 300 μm in length and unsheathed, meaning that when they mature into microfilariae, they exit from the envelope of the egg.[9]
The average adult worm lifespan is 15 years, and mature females can produce between 500 and 1,500 microfilariae per day. The normal microfilarial lifespan is 1.0 to 1.5 years; however, their presence in the bloodstream causes little to no immune response until death or degradation of the microfilariae or adult worms.[10]
O. volvulus causes onchocerciasis, which causes severe itching. Long-term infection can cause keratitis, an inflammation of the cornea in the eye, and ultimately leads to blindness.[9] Symptoms are caused by the microfilariae and the immune response to infection, rather than the adults themselves. The most effective treatment involves using ivermectin, although resistance to this drug has been reported as developing.[12] Ivermectin prevents female worms from releasing microfilariae for several months, thus relieving symptoms and temporarily preventing transmission. However, this does not kill adult worms, so it must be taken once annually as long as adult worms are present.[13]
O. volvulus has been proposed as one of the causative agents of nodding syndrome, a condition that affects children aged 5 to 15 and is currently only observed in South Sudan, Tanzania, and northern Uganda. Although the cause of the disease is unknown, O. volvulus is being increasingly studied as a possible cause due to its ubiquity in areas where the disease is found.[14]
An estimated 187 million people are at risk of O. volvulus infection, with 17–25 million people infected and 0.8 million showing some impairment of vision. O. volvulus has not directly caused a single death, but has cost 1.1 million disability adjusted life years, which measure the number of years of healthy life lost due to a specific disease and show the burden of a disease.[16]
Simulium blackfly adults require moving water to breed and eggs remain in water until they exit from the pupa and enter the adult stage of their lifecycle. Due to this restriction, O. volvulus is only found around streams or rivers. Artificial water systems, such as hydroelectric power plants, built in Africa, provide ideal conditions all year for blackfly development and make controlling its spread difficult.[17]
About 99% of cases of onchocerciasis are found in 31 countries in sub-Saharan Africa, although areas of limited transmission occur in Brazil, Venezuela, and Yemen.[15] The disease is thought to have been imported into Latin America through the slave trade.[18] Onchocerciasis was eliminated from Colombia in 2013, Ecuador in 2014, Mexico in 2015, and Guatemala in 2016[16] due to control programs that used mass drug administration with ivermectin.[15]
The total genome size of O. volvulus is 1.5x108 base pairs and contains around 4,000 genes, with genes for collagen and cuticular proteins being highly expressed in mature adults.[19] O. volvulus has four chromosome pairs, which include a single pair of sex chromosomes. A large X sex chromosome and a smaller Y sex chromosome determine male worms, while two X chromosomes determine female worms.[20]
One of the three nonsex chromosomes is thought to have formed by a fusion event between two smaller chromosomes.[19]
(Simplified phylogenetic tree of the genus Onchocerca.[21])
Onchocerca volvulus
O. volvulus has low genetic variation between individuals. This suggests a population bottleneck occurred in the past that caused a rapid decrease in the population size.[19] It also shows high haplotype diversity, which is a measure of how unique a group of linked genes is. This pattern of low genetic variation and high haplotype diversity suggests fast population expansion after a bottleneck and has led to the theory that a host shift event from cattle allowed O. volvulus to infect humans.[22] This is also supported by genetic data that place O. ochengi (a cattle-infecting strain) as the sister group to O. volvulus.[21]
Adult worms are found in nodules and are hidden from most components of the human immune system. Microfilariae are more vulnerable to attack by immune cells because they exit nodules to complete their lifecycle. O. volvulus can be detected by the immune system through the release of soluble antigens and antigens found on the surface of microfilariae and infective J3 juveniles. These antigens allow the immune system to detect the presence of a foreign organism in the body and trigger an immune response to clear infection.[23]
The immune response involves raising antibodies (IgG, IgM and IgE type) that can react with soluble antigens released by Onchocerca volvulus.[24] Opsonising antibodies that tag cells for destruction are also found against the infective J3 stage and microfilariae, but there is not enough evidence at the moment to say whether this is protective.[25]
The antigens of O. volvulus are highly complex and show cross-reactivity with several other filarial worms. Little evidence indicates that antibodies made are specific to O. volvulus. However, after the age of 40, the number of parasites carried (the intensity of infection) decreases, suggesting that over time, some sort of protective immune response develops.[23]
Microfilariae can also modulate the immune system to avoid destruction. The complement system is used to enhance the effect of antibodies and phagocytic cells, which engulf and destroy other cells. Microfilariae block this pathway by cleaving C3b—an important protein in this process—to form iC3b. iC3b cannot go on to activate the next step in the pathway and allows microfilariae to remain in the body with little to no attack by the immune system.[26]
O. volvulus, along with most filarial nematodes, share an endosymbiotic relationship with the bacterium Wolbachia. In the absence of Wolbachia, larval development of O. volvulus is disrupted or ceased.[27] These bacteria have been proposed to enhance the symptoms and severity of onchocerciasis by triggering inflammatory responses in the host.[28]
Onchocerca volvulus is a filarial (arthropod-borne) nematode (roundworm) that causes onchocerciasis (river blindness), and is the second-leading cause of blindness due to infection worldwide after trachoma. It is one of the 20 neglected tropical diseases listed by the World Health Organization, with elimination from certain countries expected by 2025.
John O’Neill, an Irish surgeon, first described Onchocerca volvulus in 1874, when he found it to be the causative agent of ‘craw-craw’, a skin disease found in West Africa. A Guatemalan doctor, Rodolfo Robles, first linked it to visual impairment in 1917.
O. volvulus is primarily found in sub-Saharan Africa, and humans are the only known definitive host; there is also disease transmission in some South American nations, as well as Yemen (see global map bottom right). It is spread from person to person via female biting blackflies of the genus Simulium.
Onchocerca volvulus es una especie de nemátodo que en un estado adulto habita en el tejido conjuntivo y subcutáneo de la piel. La hembra puede medir hasta 50 cm, mientras que el macho llega a 5 cm; generalmente forman ovillos encapsulado, donde puede haber más de una pareja de parásitos. Las microfilarias no tienen membrana envolvente y las masas nucleares no llegan hasta el extremo superior, su longitud varía entre 150 y 350 micras. Es responsable de la oncocercosis.
Los parásitos adultos dan origen a las microfilarias, las cuales se movilizan por la dermis sin periodicidad especial; pueden invadir los ojos y ocasionalmente la sangre, ganglios linfáíicos o vísceras. De la piel son tomadas por la hembra del género Simulium más comúnmente por "Simulium damnosum", díptero hematófago que para alimentarse lesiona la piel y forma una pequeña laguna de sangre, que se observa como un punto rojizo. Las microfilarias que están en la dermis son succionadas con esta sangre y dentro del vector sufren transformaciones hasta llegar a larvas infectantes que se localizan en el aparato picador del insecto. En este caso, el vector insecto es la mosca negra, mientras que el huésped definitivo es el ser humano.
La patología producida por los parásitos adultos consiste en nódulos subcutáneos llamados oncocercomas, que están formados por tres partes, una cápsula fibrosa periférica, otra intermedia fibrosa y celular vascularizada y en el centro los parásitos enrollados que pueden vivir 10 años o más. Después de muerto el parásito, el nódulo se vuelve más fibroso. La localización de las nodulaciones varía en las distintas zonas endémicas; en América predominan en la cabeza y el tronco, mientras que en África tienen cierta predilección por la parte pélvica, muslos y brazos, aunque se encuentran en cualquier parte de la piel. Por acción de la microfilarias y posiblemente por procesos alérgicos, se producen dermatitis, alteración de la pigmentación, hiperqueratosis, paquidermia, eczema, atrofia cutánea y fibrosis. Alrededor de las microfilarias muertas se forma un granuloma y un infiltrado de eosinófilos. En sangre circulante se pueden encontrar entre 15 a 50% de eosinófilos. Las microfilarias invaden los ganglios linfáticos que se vuelven fibrosos y la obstrucción linfática, con linfadenitis, pueden causar hipertrofia de lo tejidos y raramente elefantiasis. En la región inguinal se observa, en algunos casos, un crecimiento colgante.
Las microfilarias tienen una tendencia a invadir el globo ocular y producen patología oftálmica, con compromiso de la cámara anterior y el tracto uveal. La patogenia de dichas lesiones se atribuye a los productos tóxicos liberados al morir éstas y a reacciones de hipersensibilidad.
La oncocerciosis clínica se caracteriza por la afectación de la piel, el tejido subcutáneo, los ganglios linfáticos y los ojos. Las manifestaciones clínicas de la infección se deben a la reacción inflamatoria aguda y crónica frente a los antígenos liberados por la microfilaria conforme emigra a través de los tejidos. El período de incubación desde las larvas infecciosas hasta los gusanos adultos varía entre algunos meses y 1 año.
La parasitosis debuta con fiebre, eosinofilia y urticaria. Cuando los gusanos maduran, copulan y producen microfilarias, comienzan a aparecen nódulos subcutáneos que pueden encontrarse en cualquier parte del cuerpo. Esos nódulos son más peligrosos cuando aparecen en la cabeza y el cuello debido a las microfilarias pueden migrar hacia los ojos y causar daños graves con riesgo de ceguera. Se cree que la enfermedad ocular se debe a una combinación de la invasión directa por microfilarias y al depósito de complejos antígeno-anticuerpo en el seno de los tejidos oculares. El cuadro clínico evoluciona desde la conjuntivitis con fotofobia hasta la queratitis puntiforme y esclerosante. También es posible la enfermedad ocular interna, con uveítis anterior, coriorretinitis y neuritis óptica.
En la piel, el proceso inflamatorio conduce a pérdida de elasticidad y áreas de despigmentación, engrosamiento y atrofia. Diversas alteraciones cutáneas guardan relación con la presencia del parásito, entre las que cabe citar el prurito, hiperqueratosis, engrosamiento mixedermatoso y una forma de elefantiasis conocida como ingle colgante, que aparece cuando los nóduos que albergan al parásito se localizan en la proximidad de los genitales.
El 30 de julio de 2013, tras una campaña de 17 años en Colombia, la Organización Mundial de la Salud notificó la eliminación de esta enfermedad y de su transmisión.[1]
Onchocerca volvulus es una especie de nemátodo que en un estado adulto habita en el tejido conjuntivo y subcutáneo de la piel. La hembra puede medir hasta 50 cm, mientras que el macho llega a 5 cm; generalmente forman ovillos encapsulado, donde puede haber más de una pareja de parásitos. Las microfilarias no tienen membrana envolvente y las masas nucleares no llegan hasta el extremo superior, su longitud varía entre 150 y 350 micras. Es responsable de la oncocercosis.
Onchocerca est un genre de nématodes (les nématodes sont un embranchement de vers non segmentés, recouverts d'une épaisse cuticule et mènant une vie libre ou parasitaire).
Onchocerca volvulus est une filaire responsable de l'onchocercose chez l'homme.
Onchocerca volvulus è un nematode responsabile della oncocercosi, detta anche "cecità fluviale", che tramite un'infiammazione a lungo termine della cornea o cheratite porta ad un ispessimento dello stroma della cornea che dopo un po' di tempo porta alla cecità. Gli esseri umani sono gli unici ospiti per questo nematode, e i vettori dell'infestazione sono i simuliidae, le cui femmine si cibano di sangue. Il nematode è inoltre sospettato di causare un'altra malattia, la nodding disease, sebbene la correlazione tra questa malattia e l'infestazione del parassita sia solo un'ipotesi. L'Onchocerca volvulus, come molti nematodi filariali, condivide una relazione endosimbiotica con il batterio Wolbachia, e in assenza di questo lo sviluppo larvale è incompleto e cessa.
Il ciclo vitale di questo nematode inizia quando una femmina di moscerino del genere simulium si ciba di sangue da una vittima infetta, con l'ingestione del volvulus nella forma microfilaria. Sul moscerino il volvulus migra dall'intestino ai muscoli del volo, ed entrano nel primo stadio larvale (J1). Allo stadio larvale J2 migrano verso la proboscide raggiungendo la saliva, dove raggiungono il terzo stadio larvale (J3S) Le larve passano dunque nell'ospite quando il moscerino si ciba del suo sangue e migra nei tessuti sottocutanei, dove formano dei noduli in cui maturano fino a diventare vermi adulti in un periodo che va dai 6 ai 12 mesi. Dopo essere maturati gli adulti maschi migrano verso i noduli in cui risiedono le femmine, dove si riproducono dando alla luce tra i 1.000 e i 3.000 microfilaria al giorno. Il normale ciclo vitale di un singolo verme adulto può durare fino a 15 anni. Le uova degli Onchocerca volvulvus maturano fino a diventare microfilaria dentro il corpo delle femmine, che successivamente li rilascia uno alla volta dove rimangono nei tessuti sottocutanei. In questo stadio i microfilaria sono poi prelevati dai moscerini durante il pasto di sangue, per poi infettare un altro ospite.
Onchocerca volvulus è un nematode responsabile della oncocercosi, detta anche "cecità fluviale", che tramite un'infiammazione a lungo termine della cornea o cheratite porta ad un ispessimento dello stroma della cornea che dopo un po' di tempo porta alla cecità. Gli esseri umani sono gli unici ospiti per questo nematode, e i vettori dell'infestazione sono i simuliidae, le cui femmine si cibano di sangue. Il nematode è inoltre sospettato di causare un'altra malattia, la nodding disease, sebbene la correlazione tra questa malattia e l'infestazione del parassita sia solo un'ipotesi. L'Onchocerca volvulus, come molti nematodi filariali, condivide una relazione endosimbiotica con il batterio Wolbachia, e in assenza di questo lo sviluppo larvale è incompleto e cessa.
Il ciclo vitale di questo nematode inizia quando una femmina di moscerino del genere simulium si ciba di sangue da una vittima infetta, con l'ingestione del volvulus nella forma microfilaria. Sul moscerino il volvulus migra dall'intestino ai muscoli del volo, ed entrano nel primo stadio larvale (J1). Allo stadio larvale J2 migrano verso la proboscide raggiungendo la saliva, dove raggiungono il terzo stadio larvale (J3S) Le larve passano dunque nell'ospite quando il moscerino si ciba del suo sangue e migra nei tessuti sottocutanei, dove formano dei noduli in cui maturano fino a diventare vermi adulti in un periodo che va dai 6 ai 12 mesi. Dopo essere maturati gli adulti maschi migrano verso i noduli in cui risiedono le femmine, dove si riproducono dando alla luce tra i 1.000 e i 3.000 microfilaria al giorno. Il normale ciclo vitale di un singolo verme adulto può durare fino a 15 anni. Le uova degli Onchocerca volvulvus maturano fino a diventare microfilaria dentro il corpo delle femmine, che successivamente li rilascia uno alla volta dove rimangono nei tessuti sottocutanei. In questo stadio i microfilaria sono poi prelevati dai moscerini durante il pasto di sangue, per poi infettare un altro ospite.
Onchocerca volvulus é uma espécie de nematóide parasita, cuja forma adulta apresenta secção circular e reprodução sexuada, podendo viver até 14 anos dentro do hospedeiro humano. É o causador da oncocercose, também chamada "cegueira dos rios" ou "mal do garimpeiro", raramente fatal, mas a segunda maior causa infecciosa de cegueira. É transmitida por mosquitos do gênero Simulium, conhecidos no Brasil por piúm na região norte ou por borrachudo nas outras regiões.
