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Nodules
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Adult female O. volvulus measure 20 to 50 cm in length and about 0.4 mm in diameter.
They ball preferably in the subdermis and form a so-called nodule, or onchocercome,
which is encapsulated by connective tissue of the host.
Nodules can also be found in deeper tissue layers and rarely are even attached to the bone tissue.
Several female O. volvulus can aggregate in a nodule which then may increase
to the size of a walnut and can be diagnosed by palpation.
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The process of nodule formation can be described stochastically by a branching process.
Fitting the resulting model to nodule ectomy data suggested that adult female O. volvulus
prefer to invade existing nodules, rather than forming a new one.
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On average, about on third of the nodules can be found by palpation.
Diagnosis by palpation, however, is difficult because the probability to palpate an existing nodule
increases with age, because of large variability between individuals and between the skills of the investigating physician.
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The diagnostic value of nodule palpation is too low to be capable of supporting epidemiological decisions.
Sensitivity is poor because of the low probability to find a nodule by palpation,
and specificity is poor because of frequently occurring false-positive diagnoses.
Furthermore, the predictive values
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