versão impressa ISSN 0042-9686
Bull World Health Organ vol.80 no.11 Genebra Nov. 2002
Rapid assessment method for prevalence and intensity of Loa loa infection
OBJECTIVE: To assess the validity of observations on eye worm and Calabar swellings for the rapid assessment of the prevalence and intensity of loiasis at the community level.
METHOD: A total of 12 895 individuals over the age of 15 years living in 102 communities in Cameroon and Nigeria took part in the study. A standardized questionnaire was administered to participants from whom finger-prick blood samples were collected and examined for Loa loa microfilariae. Rapid assessments of the prevalence and intensity of loiasis were made on the basis of a history of eye worm or Calabar swellings.
FINDINGS: There was a strong correlation between the indices of the rapid assessment procedures and the parasitological indices of L. loa endemicity. The rapid assessment indices were effective in diagnosing high-risk communities (sensitivity 94-100%; specificity 66-92%). The highest sensitivity (100%) and specificity (92%) were obtained with a rapid assessment procedure based on a history of eye worm lasting 1-7 days together with confirmation by the guided recognition of a photograph of adult L. loa in the eye.
CONCLUSION: Rapid assessment of the prevalence and intensity of loiasis at the community level can be achieved using a procedure based on the history of eye worm lasting 1-7 days together with confirmation by the guided recognition of a photograph of an adult L. loa in the eye.
Keywords Loiasis/diagnosis/parasitology/epidemiology; Loa/growth and development; Eye foreign bodies/parasitology; Inflammation/parasitology; Endemic diseases/epidemiology; Ivermectin/adverse effects; Sensitivity and specificity; Reproducibility of results; Cameroon; Nigeria (source: MeSH, NLM).
Mots clés Loase/diagnostic/parasitologie/épidémiologie; Loa/croissance et développement; Corps étranger oculaire/parasitologie; Inflammation/parasitologie; Maladie endémique/épidémiologie; Ivermectine/effets indésirables; Sensibilité et spécificité (Epidémiologie); Reproductibilité des résultats; Cameroun; Nigéria (source: MeSH, INSERM).
Palabras clave Loiasis/diagnóstico/parasitología/epidemiología; Loa/crecimiento y desarrollo; Cuerpos extraños en el ojo/ parasitología; Inflamación/parasitología; Enfermedades endémicas/epidemiología; Ivermectina/efectos adversos; Sensibilidad y especificidad; Reproducibilidad de resultados; Camerún; Nigeria (fuente: DeCS, BIREME).
Onchocerciasis is a debilitating disease affecting 20-40 million people in 34 countries of sub-Saharan Africa, the Arabian Peninsula and South America, causing blindness and disfiguring skin changes. In recent years, the prospects for its control have been improved with the development of community-directed treatment with ivermectin, a safe, effective, and affordable drug. However, encephalopathies with occasional fatal outcomes have been reported following its use to treat persons coinfected with Onchocerca volvulus and Loa loa (1-4). These adverse outcomes are an obstacle to the sustainability of large-scale ivermectin treatment for the control of onchocerciasis and lymphatic filariasis in areas of Africa where there is a potential for coendemicity with L. loa. The incidence of severe adverse events, notably of a neuropathological nature, is related to the intensity of infection with L. loa (1).
A linear relationship between the prevalence and intensity of L. loa infection in communities has been reported (5); thus a high level of L. loa prevalence indicates a high intensity of infection, with a concomitant high risk of severe adverse events. An operational threshold for L. loa endemicity has been established above which the risk of severe reactions is too high for routine community-directed treatment with ivermectin to be carried out, i.e. a prevalence of microfilaraemiae in thick blood film >20%, corresponding to a prevalence of high intensity of infection (>8000 microfilariae/ml) above 5% (3) or a prevalence of very high intensity of infection (>30 000 microfilariae/ml) above 2% (2).
The collection, processing and examination of blood samples using the thick blood smear method for the determination of individuals and communities with high and very high intensities of infection is costly, time-consuming, cumbersome, unsustainable in communities where the disease is endemic and not feasible for the implementation of community-directed treatment with ivermectin. A rapid assessment procedure (RAP) whereby a threshold for the safe implementation of such treatment could be identified would be a valuable tool for onchocerciasis control programmes in areas where O. volvulus and L. loa coexist. A model using satellite mapping of key environmental factors affecting the development of the vector (Chrysops sp.) has been proposed (6). Still undergoing development, the model may prove useful for rapidly identifying potential high-risk zones.
Other investigators have suggested the use of clinical signs and symptoms of L. loa infection as indicators of the prevalence of infection with this parasite at the individual (7) and community level (8), i.e. migratory angioedemas known as Calabar swellings and worm migration through the conjunctiva. Conflicting results were obtained when attempts were made to apply this approach (9) perhaps because of non- standardized interview methodologies, small sample sizes, and the inclusion of only a limited number of communities.
The present study was designed to resolve this uncertainty by using a carefully designed, standardized methodology and involving a large sample of communities covering a wide range of L. loa endemicities and different ethnic groups. The objective was to develop a RAP to be used in the identification of villages at high risk for severe adverse events, i.e. villages with a prevalence of L. loa microfilaraemia >20% and a prevalence of high intensity of L. loa infection above 5% or a prevalence of very high intensity of L. loa infection above 2%.
The investigation was conducted by three research teams based in Buea and Yaoundé, both in Cameroon, and in Calabar, Nigeria. Field investigations were carried out from April to June 2001. The processing and microscopic analysis of blood samples took place between July and August 2001. The three research teams comprised epidemiologists, parasitologists, social scientists and clinicians. Clearance for the protocol was obtained from institutional and national ethical committees.
The Buea study sites were in South-West and North-West Provinces of Cameroon. Study sites for the Yaoundé team were located in the East Province of Cameroon and those of the Calabar team in south-eastern Nigeria (Cross River State).
The annual rainfall ranged from 1500 mm to 3000 mm at the Yaoundé and Calabar study sites and from 2000 mm to 4000 mm at the Buea sites. The vegetation ranged from tropical rain forest to savannah.
Selection of villages
At each of the study sites, 30 or more villages were selected in areas where, on the basis of epidemiological and biogeographical data, it was thought that L. loa infection ranged from non-endemic to hyperendemic. Villages receiving mass ivermectin treatment were excluded. Altogether, 102 villages were selected. Collective informed consent for participation was obtained from traditional authorities and opinion leaders in each participating village.
Selection of households and subjects
A minimum of 96 participants were required in each village in order that the prevalence of loiasis could be estimated with a precision of 10% at the 95% confidence level ; however, we aimed to include125 participants per village to compensate for the cluster effect. Households were selected at random and numbered.
In each household, all individuals aged >15 years who had resided in the village for at least 5 years were selected for inclusion. Individuals who had received ivermectin treatment were excluded. All selected individuals were informed of the objective of the study and of their right to refuse to participate. Each person who consented to participate was assigned a code number and interviewed. A blood sample for parasitological analysis was collected from each participant.
Rapid assessment methods
During a workshop attended by the research teams, questionnaires on histories of eye worm and Calabar swellings were developed for application at the community and individual levels. Subsequently they were pretested in the three study areas. Staff were trained how to create a rapport with participants, conduct interviews and record data. The interview data were checked by field supervisors during the process of data collection and at the central field station.
The leaders in each participating village were briefed on the objectives and expected outcomes of the study. The local names for eye worm and Calabar swellings were ascertained by conducting interviews with key informants, e.g. village heads, headmasters, schoolteachers, health workers, patent medicine dealers, traditional healers and leaders of women's groups. Where required, interpreters from the communities assisted in the interviewing process.
Individual questionnaires were designed to elicit responses on experience of eye worm and Calabar swellings. The conditions were described to each respondent and local names were used in order to aid understanding.
The key question on experience of eye worm was: "Have you ever experienced or noticed worms moving along the white of the lower part of your eye?". The interviewer then presented a photograph of an adult L. loa worm under the conjunctiva, guided the respondent to recognize the worm in the eye and asked: "Have you ever had the condition in this picture?". If a positive response was given, the following question was asked: "How many days passed before the worm disappeared?".
The questions on Calabar swellings concerned the duration of the symptom and its tendency to cause itching. The key question was: "Have you ever experienced swellings under the skin which changed position or disappeared?". If the answer was in the affirmative, the respondent was asked: "How many days passed before the swellings disappeared?" and "Did the swellings itch?".
Qualified and trained health personnel collected a 50-ml sample of finger-prick blood from each study participant between 10:00 and 16:00 using a 75-ml non-heparinized capillary tube. Safety measures for the prevention of the transmission of human immunodeficiency virus and hepatitis B virus included the use of one sterile lancet per participant and the decontamination of used capillary tubes and lancets.
Standard procedures were used for the processing and analysis of the blood samples. Slides were examined under an optical microscope with a x10 objective and L. loa microfilariae were identified and counted (10). The results were expressed as microfilariae per ml of blood.
The data were entered in EpiInfo (version 6) and exported into SPSS (version 10) for analysis. The intensity and prevalence of L. loa microfilaraemia were calculated for each community using the following parasitological indices: prevalence of microfilaraemia in thick blood film; prevalence of high intensity of infection (>8000 microfilariae/ml); prevalence of very high intensity of infection (>30 000 microfilariae/ml); and community microfilarial load, i.e. the geometric mean microfilarial load in persons aged >15 years, including microfilarial negatives (11).
The following RAP indices were calculated for each community: a) percentage of people who reported past experience of eye worm; b) percentage of people who were positive for a restricted definition of eye worm, i.e. those who reported past experience of eye worm, confirmed by means of a photograph of adult L. loa in the eye and that it lasted 1-7 days; c) percentage of people who reported past experience of Calabar swellings; d) percentage of people who reported past experience of Calabar swellings with itching that lasted 1-7 days; e) percentage of people with past experience of eye worm and Calabar swellings; and f) percentage of people with past experience of eye worm or Calabar swellings.
Spearman's rank order correlation coefficient was used to determine the relationship between each of the RAP indices and the parasitological indices. Scatter plots were used to define cut-off points and determine thresholds. Determinations were made of the sensitivity, specificity and predictive value of the RAP for the diagnosis of communities at risk of severe adverse events following community-directed treatment with ivermectin.
Local names for eye worm and Calabar swellings
The study involved 102 villages, of which 28 were in the Calabar study sites, 42 were in the Buea study sites and 32 were in the Yaoundé study sites (12). The study sites covered a wide range of prevalence and intensity of L. loa infection. Of the 12 895 participants, 55% were female and all were aged >15 years. The average age of those examined was 36.5 years.
Local terms for eye worm were reported from all the villages in the Yaoundé study sites. For the Baya tribe, "peng" = worm, "li" = eye, "peng li" = worm of the eye, "yolo" = worm and "yolo li" = worm of the eye. A few communities used the term "filère", a corruption of the French word "filaire". Other ethnic groups used non-descriptive terminology.
Local terms for eye worm were reported from 33 of the localities visited in the Buea study sites. One of the descriptive terms for eye worm was "damole nyi" ("damole" = worm, "nyi" = eye; "damole nyi" = worm of the eye). Other terms used in two villages were not descriptive of the condition. No local terms were identified for eye worm in seven village communities.
The majority of the localities visited by the Calabar team had local terms for both eye worm and Calabar swellings. The term "etung enyin" used by the Efik and Ibibio ethnic groups translated into eye worm and was used consistently to describe the condition.
Experience of eye worm lasting 1-7 days was reported by 45% of the respondents. Calabar swellings had been experienced by 51% of the respondents, most of whom reported a duration of 1-7 days. Of the respondents who reported experiencing Calabar swellings, 91% indicated that the swellings itched. The locations most commonly mentioned for the swellings were the hands, arms and legs. Reports of Calabar swellings were less consistent than those of eye worm.
Prevalence and intensity of Loa loa infection
The prevalence of microfilaraemia varied considerably between the study sites. Cross River State in Nigeria had the lowest prevalence (range = 0-18%, median = 2%) . In Cameroon's East Province it was much higher (range = 8- 48%, median = 30%). The Buea study sites showed the widest range of endemicity: most villages had a prevalence of less than 10% but there were seven villages with very high prevalences.
The prevalence of high intensity of L. loa infection (>8000 microfilariae/ml) increased with the prevalence of L. loa microfilaraemia at the community level. The 20% threshold prevalence of L. loa in thick smears at the community level corresponded to a 5% prevalence of high intensity of infection. There was a very clear relationship between the two indices and the pattern was similar at all study sites (Fig. 1).
The community microfilarial load increased with prevalence (Fig. 2), slowly at first but accelerating rapidly when a prevalence of 20% was attained. The relationship between the community microfilarial load and prevalence appeared to have a hyperbolic form, suggesting an upper limit for the prevalence of microfilaraemia of around 60%.
Correlation between rapid assessment procedure indices based on eye worm and prevalence of Loa loa microfilaraemia
All the RAP indices showed a statistically significant relationship with the parasitological indices (Table 1). There was a better correlation for RAP indices based on eye worm than for those based on Calabar swellings. The correlation coefficients for indices that combined eye worm and Calabar swellings were lower than the correlation coefficients for the restricted definition of eye worm. The prevalence of eye worm according to the restricted definition increased with the prevalence of microfilaraemia. The cut-off point of a 40% prevalence of eye worm according to the restricted definition corresponded to a 20% prevalence of microfilaraemia at the community level.
There was a non-linear relationship between the prevalence of high microfilarial loads (>8000 microfilariae/ ml) and the RAP based on the restricted definition of eye worm. The prevalence of high loads rose sharply when more than 40% of interviewees reported a history of eye worm according to the restricted definition. Below the 40% threshold, microfilarial loads of more than 8000 microfilariae/ml were relatively rare (Fig. 3).
There was a non-linear relationship between the prevalence of very high microfilarial loads (>30 000 microfilariae/ml) and the RAP based on the restricted definition of eye worm. The threshold of 40% for a history of eye worm provided a good division between high-risk and low-risk communities. Above this threshold the prevalence of very high microfilarial loads increased rapidly. Very high microfilarial loads were rare below the 40% threshold and even completely absent where fewer than 25% of respondents reported a history of eye worm.
Sensitivity and specificity of the rapid assessment procedures
All the RAPs had a high sensitivity for the diagnosis of high- risk communities (prevalence of microfilariae >20%, prevalence of high microfiliarial loads >5%, prevalence of very high loads >2%). The specificity ranged from 48% to 92% (Table 2), being relatively low for the simple definitions of eye worm and Calabar swellings. For both the simple and restricted definitions, the specificity for eye worm was much better than that for Calabar swellings. The best performance was obtained with the restricted definition of eye worm, which had a sensitivity of 100% and a specificity in the range 79-92%.
All communities where L. loa was highly endemic had local names for eye worm and Calabar swellings, indicating a general awareness of the condition. In villages of very low endemicity, where the prevalence of L. loa infection was <10%, no local terms for eye worm were identified. The clinical signs reported for Calabar swellings were less consistent than those of eye worm.
The prevalence and intensity of L. loa infection varied considerably between the Buea, Calabar and Yaoundé study sites. Endemicity was low in Cross River State, Nigeria, very high in Cameroon's East Province and ranged from low to high in the South-West and North-West Provinces of Cameroon. The reported range of prevalences of L. loa microfilaraemia in the present study confirms results obtained previously in eastern Nigeria (13-15) and south-western Cameroon (1).
Intensity of Loa loa infection
High intensity L. loa infection is the principal risk factor for severe adverse reactions to ivermectin treatment. There has been some controversy about where highly infected individuals might be found, i.e. whether a high intensity of infection could occur in any community with loiasis or whether such infection would be found predominantly in communities of high endemicity. The clear relationship between the prevalence of microfilaraemia and the intensity of L. loa infection reported in the present study confirms earlier investigations carried out in the Centre Province of Cameroon (5). The 20% threshold prevalence of microfilaraemia corresponded to about a 5% prevalence of high microfilarial loads, which is the cut-off point above which there is a risk of severe adverse reactions involving functional impairment after ivermectin treatment (3). Thus there is now abundant evidence that the prevalence and intensity of L. loa infection are directly related.
The rapid increase in the community microfilarial load from a prevalence of 20% towards an upper limit of around 60% confirms the threshold value for severe adverse events. Above a prevalence of 20%, the intensity of infection increases rapidly, as does the risk for severe adverse events. It was previously reported that a significant proportion of the population in a community with L. loa endemicity was symptomatic but amicrofilaraemic, probably because of an efficient immune response (7, 16, 17). Indeed, individuals with occult loiasis have high levels of IgG antibodies that bind to a microfilarial surface antigen of molecular mass 23 kDa and can mediate cellular adherence. It is worth noting that there was a tenfold increase in the community microfilarial load over the rather narrow prevalence range of 30-50%. This suggested major increases in adult worm loads, possibly related to a significant rise in the intensity of transmission, notwithstanding the limited variation in the prevalence of patent infection.
The high correlations between the parasitological indices and the RAP indices reflected the reliability of the latter for predicting the prevalence and intensity of L. loa infection. Although Calabar swellings have been described as a pathognomonic sign of loiasis, their use in the present study was limited and obscured by other causes of swellings that were non-specific. The migration of adult L. loa through the conjunctiva is more striking and its occurrence is preceded by itching and pain. Eye worm is readily identifiable by persons in close proximity to each other or by individuals harbouring the worm. In the Calabar study sites a high prevalence of a history of eye worm was reported in conjunction with a low prevalence of L. loa microfilaraemia. This underlined the necessity of confirming the condition by using photographs to which people could relate, as well as by considering the duration of recent episodes.
The best indicator of L. loa infection consisted of a restricted definition of a history of eye worm, confirmation by means of a photograph and a duration of 1-7 days for the most recent episode (RAPLOA). With a threshold of 40% this was a good predictor of high-risk communities, with a sensitivity of 100% and a specificity in the range 76-92%. RAPLOA should be used to identify communities with a high prevalence and intensity of L. loa infection. Where the procedure indicates a prevalence of >40%, there is a high risk of adverse reactions during community-directed treatment with ivermectin. Under such circumstances, ivermectin delivery for the control of onchocerciasis should only be undertaken under close medical supervision.
The fact that the present study covered a wide range of prevalences and intensities of infection and a wide range of bioecological zones strengthens the reliability of the model.
We thank Dr M. Ntep, Coordinator of the National Programme for Onchocerciasis Control, Cameroon, Dr M. Boussinesq and Dr J. Enyegue Oyé for their contributions and comments. We are grateful to the following persons for field and laboratory assistance: Nicholas Tendongfor, Mathias Esum, Sali Ndindeng, Tatah Peter, Patience Ntumi, Richard Mbentengam, Dr Dennis Nkanga, Bam Ogar and Asuquo Bassey. We are also grateful to the community members who participated in the surveys. The present investigation received financial support from the African Programme for Onchocerciasis Control and the UNDP/ World Bank/WHO Special Programme for Research and Training in Tropical Diseases.
Conflicts of interest: none declared.
Méthode d'évaluation rapide de la prévalence et de l'intensité des infections à Loa loa
OBJECTIF: Examiner la validité des observations portant sur la présence de macrofilaires de Loa loa dans l'il et d'dèmes de Calabar pour l'évaluation rapide de la prévalence et de l'intensité de la loase au niveau communautaire.
MÉTHODES: Au total, 12 895 personnes de plus de 15 ans vivant dans 102 communautés villageoises du Cameroun et du Nigéria ont participé à l'étude. Un questionnaire standardisé a été administré aux participants, chez qui on a réalisé un prélèvement de sang par piqûre au doigt à la recherche de microfilaires de L. loa. Une évaluation rapide de la prévalence et de l'intensité de la loase a été réalisée sur la base des antécédents de présence de L. loa dans l'il et d'dèmes de Calabar.
RÉSULTATS: L'étude a montré une forte corrélation entre les indices tirés de la méthode d'évaluation rapide et les indices parasitologiques d'endémicité de L. loa. Les indices tirés de l'évaluation rapide permettent d'identifier les communautés à haut risque (sensibilité : 94-100 % ; spécificité : 66-92 %). Les plus fortes valeurs de la sensibilité (100 %) et de la spécificité (79-92 %) ont été obtenues avec la méthode d'évaluation rapide reposant sur la présence observée de L. loa dans l'il pendant 1 à 7 jours avec confirmation par la reconnaissance, avec l'aide de l'enquêteur, d'une photographie montrant une filaire adulte de L. loa dans l'il.
CONCLUSION: Une évaluation rapide de la prévalence et de l'intensité de la loase au niveau communautaire peut être réalisée selon la méthode reposant sur la présence observée de L. loa dans l'il pendant 1 à 7 jours avec confirmation par la reconnaissance, avec l'aide de l'enquêteur, d'une photographie montrant une filaire adulte de L. loa dans l'il.
Método de evaluación rápida de la prevalencia e intensidad de la infección por Loa loa
OBJETIVO: Evaluar la validez de las observaciones de Loa loa a nivel ocular y de la tumefacción de Calabar como criterios de evaluación rápida de la prevalencia e intensidad de la loasis a nivel de la comunidad.
MÉTODO: Participaron en el estudio 12 895 individuos mayores de 15 años de 102 comunidades de aldeas del Camerún y Nigeria. Se distribuyó entre los participantes un cuestionario normalizado y se les extrajeron muestras de sangre por punción digital para determinar la presencia de microfilarias de Loa loa. La evaluación rápida de la prevalencia e intensidad de la loasis se hizo a partir de los antecedentes de afectación ocular o de tumefacción de Calabar.
RESULTADOS: Se observó una estrecha correlación entre los índices deparados por los procedimientos de evaluación rápida y los índices parasitológicos de endemicidad por L. loa. Los índices de evaluación rápida permiten identificar las comunidades de alto riesgo (sensibilidad: 94%-100%; especificidad: 66%-92%). La sensibilidad y especificidad más altas (100% y 79%-92%, respectivamente) se obtuvieron con el procedimiento de evaluación rápida basado en el criterio de unos antecedentes de afectación ocular de 1-7 días de duración, confirmados por el reconocimiento guiado, en una fotografía, de una filaria adulta de L. loa en el ojo.
CONCLUSIÓN: Es posible efectuar una evaluación rápida de la prevalencia y la intensidad de la loasis a nivel comunitario utilizando como criterio la observación del nematodo en el ojo por espacio de uno a siete días, confirmada por el reconocimiento guiado, en una fotografía, de un ejemplar adulto de L. loa en el ojo.
1. Boussinesq M, Gardon J. Prevalences of Loa loa microfilaraemia throughout the area endemic for the infection. Annals of Tropical Medicine and Parasitology 1997;91:573-89. [ Links ]
2. Chippaux JP, Boussinesq M, Gardon J, Gardon-Wendel N, Ernould J-C. Severe adverse reaction risks during mass treatment with ivermectin in loiasis endemic areas. Parasitology Today 1996;12:448-50. [ Links ]
3. Gardon J, Gardon-Wendel N, Demanga-Ngangue, Kamgno J, Chippaux JP, Boussinesq M. Serious reactions after mass treatment of onchocerciasis with ivermectin in an area endemic for Loa loa infection. Lancet 1997;350:18- 22. [ Links ]
4. Nzenze JR, Kombila MY, Boguikouma JB. Encéphalopathie mortelle au cours d'une loase hypermicrofilarémique traitée par Ivermectine : Première description au Gabon. [Fatal encephalopathy in a case of hypermicrofilaremic loiasis treated with ivermectin: first description in Gabon]. Médecine d'Afrique Noire 2001;48:375-7. In French. [ Links ]
5. Boussinesq M, Gardon J, Kamgno J, Pion SD, Gardon-Wendel N, Chippaux JP. Relationship between the prevalence and intensity of Loa loa in the Central Province of Cameroon. Annals of Tropical Medicine and Parasitology 2001;95:495-507. [ Links ]
6. Thomson MC, Obsomer V, Dunne M, Connor SJ, Molyneux DH. Satellite mapping of Loa loa prevalence in relation to ivermectin use in West and Central Africa. Lancet 2000;56:1077-8. [ Links ]
7. Kershaw WE. Studies on the epidemiology of filariasis in West Africa, with special reference to the British Cameroon and the Niger Delta. I. methods of survey for infections with Loa loa and Acanthocheinema perstans. Annals of Tropical Medicine and Parasitology 1950;44:361-78. [ Links ]
8. Noireau F, Apembet JD, Nzoulani A, Carme B. Clinical manifestations of loiasis in an endemic area in the Congo. Tropical Medicine and Parasitology 1990;41:37-9. [ Links ]
9. Onomo PE. Identification des signes cliniques prédictifs de la loase hypermicrofilaremique [Doctoral thesis]. [Identification of clinical signs of hypermicrofilaraemic loasis]. Yaoundé, University of Yaounde I; 1999. [ Links ]In French.
10. WHO. Microfilariae In: Basic Laboratory methods in Medical Parasitology. Geneva: World Health Organization; 1991, p. 89-92. [ Links ]
11. Remme J, Ba O, Dadzie KY, Karam M. A force-of-infection model for onchocerciasis and its applications in the epidemiological evaluation of the onchocerciasis control programme in the Volta River basin area. Bulletin of the World Health Organization 1986;64:667-81. [ Links ]
12. Wanji S, editor. Rapid assessment procedures for loiasis: report of a multi-centre study. Geneva: UNDP/World Bank/Special Programme for Research Training and Research in Tropical Diseases; 2001.p.38. [ Links ]
13. Ogunba EO. Loa loa microfilaraemia in the Nigerian population. Tropical and Geographical Medicine 1977;29:51-5. [ Links ]
14. Udonsi JK. Filariasis in Igwun River Basin, Nigeria: an epidemiological and clinical study with a note on the vectors. Annals of Tropical Medicine and Parasitology 1988;82:75-82 [ Links ]
15. Ufomadu GO. The occurrence of loiasis, mansonellosis and wuchereriasis in the Jawara River Valley of Nigeria. Acta Tropica 1990;48:137-47. [ Links ]
16. Garcia A, Abel L.Genetic epidemiology of host predisposition microfilaraemia in human loiasis. Tropical Medicine and International Health 1999;4:565-74. [ Links ]
17. Pinder M, Dupont A, Egwang TG. Identification of a surface antigen on Loa loa microfilariae the recognition of which correlates with the amicrofilaraemic state in man. Journal of Immunology 1988;14:2480-6. [ Links ]
1 Department of Public Health, Faculty of Medicine and Biomedical Sciences, University of Yaoundé I, PO Box 1364 Yaoundé, Cameroon (email: firstname.lastname@example.org). Correspondence should be sent to Dr Takougang at this address.
2 Department of Paediatrics, College of Medical Sciences, University of Calabar, Calabar, Nigeria.
3 Department of Life Sciences, University of Buea, Buea, Cameroon.
4 Department of Sociology and Anthropology, University of Buea, Buea, Cameroon.
5 Akada Konsults, Lagos, Nigeria.
6 Institute of Demographic Training and Research, University of Yaoundé II, Yaoundé, Cameroon.
7 Department of Biological Sciences, University of Calabar, Calabar, Nigeria.
8 Tropical Medicine Research Station, Kumba, Cameroon.
9 Department of Social and Preventive Medicine, University of Ibadan, Ibadan, Nigeria.
10 UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases, World Health Organization, Geneva, Switzerland.
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