—from Epidemiological Bulletin,
Vol. 24 No. 2, June 2003—
Case Definition:
Onchocerciasis (River Blindness)
Rationale for surveillance
Onchocerciasis is endemic in 34 countries of Africa, the Arabian peninsula and
the Americas. Success at controlling the disease in West Africa was achieved
through the strategy of larviciding for vector control in order to interrupt
transmission; since 1988 this has been combined with treatment by ivermectin,
a safe, effective drug. In Africa, annual distribution of ivermectin is being
used to replace the larvicidal vector control activities of the Onchocerciasis
Control Program in West Africa (OCP). It is distributed annually in community-directed
country programs and is currently the core of the African Program for Onchocerciasis
Control (APOC), which covers all the non-OCP African countries (and the Yemen)
wherein ochocerciasis is endemic. As OCP phases out, it will need to ensure
that detection and control of onchocerciasis recrudescence is integrated within,
and become a routine function of national disease surveillance and control services.
The risk of recrudescence must be kept to a minimum.
While elimination is not a realistic goal in Africa, it is in
the Americas. Elimination of the parasite population from a defined geographical
area means the sustained absence of transmission until the adult parasite population
within that area has died out naturally or has been exterminated by some other
intervention. This should occur within 15 years after interruption of transmission.
Several foci in the Americas are now approaching this goal. WHO Regional elimination
of onchocerciasis will be considered to have been achieved when all countries
in that Region have been certified as having eliminated onchocerciasis. In Latin
America, ivermectin, given 6-monthly, is the basis of the strategic plan for
the elimination of onchocerciasis in all endemic areas.
Recommended case definition
Clinical case definition
In an endemic area, a person with fibrous nodules in subcutaneous tissues.
Laboratory and ophthalmological criteria for confirmation
One or more of the following
– Presence of microfilariae in skin snips.
– Presence of adult worms in excised nodules.
– Presence of typical ocular manifestations, such as punctate keratitis
and/or positive identification of microfilariae (e.g. slit-lamp observations
of microfilariae in the cornea) in the eye.
Case classification
Suspected: A case that meets the clinical case definition.
Probable: Not applicable.
Confirmed: A suspected case that is confirmed by any of the criteria
listed above.
Recommended types of surveillance
In zones of the Americas where onchocerciasis is endemic:
Some of the older programs in the Americas such as those of Mexico and Guatemala
have well characterized foci as a result of thorough assessments conducted over
the last 5 to 6 decades. All other currently known foci (and suspect communities)
in the Americas have been characterized by rapid epidemiological assessments
(REA). REA is based on the prevalence of nodules and/or microfilaremia in a
sample of 30 adult males who have lived in the community for at least 5 years.
Program implementation and impact assessments rely on periodic surveillance
in sentinel communities. Sentinel communities are pre-selected hyperendemic
communities where in-depth epidemiological evaluations take place at regular
intervals; first before treatment starts, then again after two years, and finally
at 4-year intervals thereafter. These evaluations include parasitological (microfilariae
in biopsies and adult worms in nodules), ophthalmological (microfilariae in
anterior chamber and punctuate keratitis), and entomological (by PCR) indicators.
Suppression of infectivity following 4 years of uninterrupted
bi-annual treatment means the absence of infective larvae (L3s) in the Simulium
vector population as determined by polymerase chain reaction (PCR) or any other
valid method, coupled with a 5-year cumulative incidence of <1 new case per
1,000 children under 5 years of age. Even after suppression of infectivity has
been achieved there can still be a population of adult worms capable of reinitiating
transmission if the drug pressure is removed. Interruption of transmission will
occur only by sustaining drug pressure during the entire length of the adult
parasite’s lifespan, this is approximately 15 years after beginning of
treatment activities sustaining a coverage of no less than 85% of the eligible
population.
Entomological evaluation, using PCR to detect parasite larvae
in vector populations, is recommended because of the long prepatent period in
human infection. If positive flies are detected, epidemiological surveys should
be carried out to identify and treat both infected people and the at-risk population.
This post endemic surveillance should be carried out until elimination of onchocerciasis
is declared for the Region.
The International Certification Team is encouraged to use other
villages (extra-sentinel sites) for monitoring, pre-certification or certification
activities.
Migration investigation:
Programs should carry out a systematic investigation to rule out introduction
of the infection in areas with a transmission potential (presence of vector)
and where migration would pose a risk to the spread of a focus.
No active surveillance takes place in non-endemic areas in the
Americas.
Recommended minimum data elements
Individual patient record at peripheral level in the Americas:
Name, age, sex, date and number of current treatment round and number of tablets
received.
Aggregated data for reporting:
Treatment coverage over the eligible population at risk.
Prevalence and incidence rates (microfilaremia, nodules with adult worms, microfilariae
in the eye, and/or punctuate keratitis) in sentinel communities.
Community Microfilarial Load (CMFL) in sentinel communities.
Recommended data analyses, presentation, reports
Graphs: Coverage over eligible population
Tables: Coverage by level of endemicity
Maps: Coverage by geographical area, location of the communities by level
of endemicity, using geographical information system (GIS).
Principal uses of data for decision-making
– Eliminate onchocerciasis as a disease of public health and socio-economic
importance.
– Prevent recrudescence of infection in the onchocerciasis-freed zones.
– Assess effectiveness of intervention.
Special aspects
New diagnostic tests, such as those based on serology (chromatographic-based
antibody detection test) or the DEC (diethylcarbamazine citrate) patch test
may become suitable for use in the field.
Source: Adapted from “WHO Recommended Surveillance
Standards, Second edition, October 1999”, WHO/CDS/CSR/ISR/99.2
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Epidemiological Bulletin, Vol. 24 No. 2, June
2003
