Medical Microbiology

594 10 Helminths

Onchocerca

Onchocercavolvulus

Causativeagentofonchocercosis

Thisfilarialspeciescausesonchocercosis,adiseasethatmanifestsmainlyin

theformofskinalterations,lymphadenopathy,andeyedamage,whichlatter

isthereasonforthespecialimportanceofthedisease.

Occurrence.Onchocercavolvulusisendemicin 30 countriesintropical

Africa(fromtheAtlanticcoasttotheRedSea)andsouthernArabia(Yemen)

aswellasinsixcountriesinCentralandSouthAmerica(Mexico,Guatemala,

Columbia,Venezuela,Brazil,Ecuador).About 17 .6millionpersonsarecur-

rentlyinfectedand 267000 areblindduetoonchocercosis(WHO, 199 8,

200 0).TheWHOhasbeencoordinatingsuccessfulcontrolprogramsin 11

Africancountriessince 197 4,andinsixLatinAmericancountriessince

1991 (seebelow).

Lifecycle.TheadultOnchocercaliveintheconnectiveandfattytissues,usual-

lytightlycoiledinconnectivetissuenodulesinthesubcutisordeepertissue

layers(Fig. 1 0. 16 c).Sexuallymatureparasitescanliveaslongas 15 years.

FemaleOnchocercaproducemicrofilariaethatliveinthetissuewithinthe

nodulesorskin.Startingfromthesiteofthefemaleworms,themicrofilariae

migratethroughthedeepcoriumofthedermisintootherskinregionsand

canalsoaffecttheeyes—especiallyifthenoduleislocatedontheheador

upperbody.Throughthelymphatics,themicrofilariaecanpenetrateinto

thebloodstreamandalsoappearinurine,sputum,andcerebrospinalfluid.

Therelativelylargemicrofilariaehavenosheath(Table 1 0. 4 );theirlifespanin

humanhostsisfromsixto 30 months.

Simuliidae(blackflies)aretheintermediatehostsandvectors.Thedevel-

opmentoftheinfectivelarvae,transmittedbyablackflytoahumanhost,

takesmanymonthsbeforethenematodesreachsexualmaturity.Microfilar-

iaeareusuallybedetectedinskinafter 12 – 15 months(sevento 24 months)

(prepatentperiod).

Epidemiology.HumansarethesoleparasitereservoirofO.volvulus.Onch-

ocercosisoccursinendemicfocialongtheriversinwhichthelarvaeandpu-

paeoftheblackfliesdevelop.Therefore,theblindnesscausedbyonchocer-

cosisisdesignatedas“riverblindness.”

Pathogenesisandclinicalmanifestations.Pathologicalreactionsarepro-

vokedbyadultparasitesandbymicrofilariae.Thesereactionsareinfluenced

bytheimmunestatusofinfectedindividuals.

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Kayser, Medical Microbiology © 2005 Thieme