The microfilarial load in the anterior segment of the eye. A parameter of intensity of onchocerciasis*

Bulletin of the World Health Organization, 55 (6): 731-737 (1977) The microfilarial load in the anterior segment of the eye. A parameter of intensity of onchocerciasis* B. THYLEFORS 1 & U. K. BRINKMANN 2 The presence of microfilariae of Onchocerca volvulus in the eye is associated with an increased risk ofdeterioration of existing eye lesions. An opthalmological andparasitologi- cal examination of630 persons was carried out in a hyperendemic focus ofonchocerciasis in northern Togo. The prevalence of microfilariae increased in the cornea as well as the anterior chamber up to the age of 40-50 years, then decreased. The prevalence of onchocercal punctate keratitis, on the other hand, showed a peak for the age group 10-20 years. In two-thirds of the cases microfilariae were present in the anterior chamber as well as in the cornea. The relative distribution of microfilariae between the anterior chamber and the cornea did not change with the development of severe anterior lesions but in cases with severe posterior lesions relatively more microfilariae were found in the anterior chamber than in the cornea. In all cases of severe ocular lesions the numbers of microfilariae both in the anterior chamber and in the cornea were increased. The average number ofmicrofilariae in the eye can be used as a parameter to enumerate the severity of ocular onchocerciasis. The presence of microfilariae in the anterior seg- ment of the eye is the most important diagnostic sign of ocular onchocerciasis. Microfilariae in the anterior chamber are easily seen with a slitlamp or with an ophthalmoscope (16) but their quantitative assessment necessitates either positioning the patient's head for 1-2 minutes before the examination (2) or an ocular massage (8). In the cornea, dead and living microfilariae can be counted using a slitlamp. Examination for living microfilariae requires a x 25 magnification and a retro-illumina- tion technique (1). The diagnostic value of corneal opacities due to microfilariae has been much discus- sed but, in general, the typical snowflake opacity can be distinguished from non-onchocercal opacities. Onchocercal punctate keratitis is considered to be a sign of early or light infections (3, 4, 7). The presence of microfilariae in the anterior cham- ber of the eye is known to be associated with an increased risk of blindness (6,15). Anderson et * From the WHO Onchocerciasis Control Programme in the Volta River Basin Area, B.P. 549, Ouagadougou, Upper Volta. 1 Ophthalmologist. 2Epidemiologist. al. (5) showed that visual acuity and onchocercal eye lesions were more likely to deteriorate over a period of four years in patients who showed initially high microfilarial concentrations in the cornea or anterior chamber than in those who did not. It appears, therefore, that the total microfilarial load in the anterior segment of the eye could be used as a means of measuring the level of intensity and severity of ocular onchocerciasis and the risk of blindness. A simple enumeration system for the ocular parasite load would also be useful for com- parison with other clinical parameters, such as skin density of microfilariae and microfilaruria. The present study deals only with the distribution of microfilariae in the eye. The relationship with other signs of onchocerciasis will be reported later. The results of epidemiological field surveys are usually recorded on forms suitable for subsequent computer analysis. For practical reasons the number of microfilariae observed in different parts of the eye are therefore recorded in numerical groups (e.g. 0, 1-4, 5-19, etc.) each of which has a separate code. The exact number of microfilariae in the anterior segment of the eye can subsequently only be esti- mated from such coded data. The present study was 3658 731 - B. THYLEFORS & U. K. BRINKMANN Table 1. The population examined ophthalmologically, persons with microfilariae in the anterior segment of the eye (MFAS), and the prevalence of MFAS (%) by sex and age Age group No. examined MFAS present Prevalence of MFAS (years) M F T M F T M F T 0-4 1 1 2 - 1 1 0 100 50.0 5-9 71 78 149 30 28 58 42.3 35.9 38.9 10-19 81 74 155 61 47 108 75.3 63.5 69.7 20-29 35 55 90 28 39 67 80.0 70.9 74.4 30-39 42 67 109 38 53 91 90.5 79.1 83.5 40-49 29 34 63 21 25 46 72.4 73.5 73.0 > 50 22 40 62 20 21 41 90.9 52.5 66.1 281 349 630 198 214 412 70.5 61.3 65.4 undertaken to determine whether these coded results could be used to assess the ocular parasite load with reasonable accuracy. MATERIALS AND METHODS An ophthalmological examination for onchocer- ciasis was performed on a total of 630 persons in three villages in northern Togo. The patients were examined in a dark room with a slitlamp (Haag- Streit 900, magnification x 16 and x 25). Before examination, each patient had his head bent down for at least 1 minute. The number of microfilariae in the anterior chamber was then counted, first in the right and then in the left eye. Dead and living microfilariae in the cornea and onchocercal opacities were also counted. Counts above 50 were rounded to the nearest ten; in the four cases where numbers were exceedingly high it was only possible to make estimations. The observed counts were recorded both individually and grouped as follows: 1-4; 5-19; 20-49; 50-99; 100-499; 500-1000. RESULTS Table 1 shows the age and sex distribution of the population examined in the three villages and the number of persons with signs of ocular onchocercia- sis. This population showed a homogeneous endemi- city level of onchocerciasis, with prevalences ranging from 79.9% to 88.9 %. The rates of ocular onchocer- ciasis by age and sex are typical of hyperendemic villages. Fig. 1 shows age-specific rates for four parameters of onchocerciasis in the anterior segment of the eye: onchocercal opacities, living and dead microfilariae in the cornea, and microfilariae in the anterior chamber. Onchocercal opacities begin to appear early in life and show a peak of prevalence in the age group 10-20 years. The presence of dead microfilariae in the cornea is the second most important diagnostic sign of ocular onchocerciasis in younger age-groups, and their prevalence from the age of 30 years is very close to that of microfilariae in the anterior chamber. Fig. 1. The age-specific prevalence (%) in the study population of microfilariae in the cornea (LMFC = living microfilariae in the cornea, DMFC = dead micro- filariae in the cornea), microfilariae in the anterior chamber of the eye (MFAC) and onchocercal opacities (OCOP)-ages in years. 732 OCULAR ONCHOCERCIASIS Table 2. Comparison of the microfilaria counts in the right and left eye for three para- meters of ocular onchocerciasis No. of Mean no. of microfilariae Mean difference-right and left observations eye (standard error of theRight eye Left eye difference) MFACa 306 13.4 12.7 + 0.62 (+0.56) DMFC b 326 7.5 7.3 + 0.22 (+0.34) LMFC c 156 11.7 10.9 + 0.76 (+0.64) a MFAC = microfilariae in the anterior chamber. b DMFC = dead microfilariae in the cornea. c LMFC = living microfilariae in the cornea. Fig. 2. The prevalence of microfilariae in the cornea and in the anterior chamber of the eye. Isolated and joint occurrence are depicted by overlapping circles re- presenting: all cases with microfilariae in the anterior chamber of the eye (MFAC), all cases with living micro- filariae in the cornea (LMFC) and all cases with dead microfilariae in the cornea (DMFC). The area of the circles is proportional to the number of cases. The presence of living microfilariae in the cornea shows a prevalence steadily increasing with age up to 40-50 years, after which there is a decrease as with the other parameters. No significant differences, at the level ofP <0.01, were found when the mean microfilariae-counts in the corneas and the anterior chambers of the right and left eyes were compared (Table 2). Fig. 2 shows the frequencies with which microfila- riae were found in the cornea and in the anterior chamber in 412 patients with ocular onchocerciasis. In almost two-thirds (61.2 %) microfilariae were present in the anterior chamber as well as in the cornea. The most prevalent isolated sign was the presence of dead microfilariae in the cornea (66 cases, or 16%). Isolated onchocercal opacities were found in only 28 cases (6.8 %). In Table 3, the individual counts of living micro- filariae in the cornea, dead microfilariae in the cornea, and microfilariae in the anterior chamber for both eyes of each patient have been added together and divided by two to give the mean load of microfilariae in the anterior segment. The figures for microfilariae in the anterior segment for the whole population have then been placed in the appropriate numerical groups for analysis of the arithmetic and geometric means. The frequency distributions of microfilariae in the anterior chamber, dead micro- filariae in the cornea, and living microfilariae in the cornea were found to be positively skewed, and the observed arithmetic and geometric means cor- respond more closely to the geometric group mid- point. To estimate the number of microfilariae from the coded groups we have therefore used, in accor- dance with Anderson et al. (5), the rounded geo- metric midpoints of each group. (Total 412 cases) 733 B. THYLEFORS & U. K. BRINKMANN Table 3. Theoretical and observed arithmetic and geometric group midpoints in the distribution of microfilariae in the anterior segment RoundedNumerical No. of Observed arithmetic Group arithmetic Observed geometric Geometric midpoint geometric groupgroup observations mean (sx) midpoint mean (sxi) midpoint 1-4 203 1.95 2.5 1.69 2.00 2 (0.07) (1.1 1) 5-19 104 10.86 12.0 9.92 9.76 10 (0.44) (1.25) 20-49 59 30.44 34.5 29.45 31.30 30 (1.06) (1.31) 50-99 27 67.67 75.5 66.14 70.36 70 (2.88) (1.29) 100-499 15 172.93 299.5 161.02 223.61 220 (19.91) Table 4 compares the geometric mean numbers of microfilariae in the anterior chamber and cornea in patients with and without signs of severe ocular onchocerciasis. This was defined as the presence of one or more of the following lesions: iritis or sclerosing keratitis or onchocercal choroidoretinitis, associated or not with optic atrophy. The ratios between the mean numbers of microfilariae in the cornea and the mean number of microfilariae in the anterior chamber remain the same regardless of the severity of the ocular lesions. In Table 5, the cases of ocular onchocerciasis have been separated into three groups: (i) non-severe, (ii) severe anterior lesions (i.e., sclerosing keratitis or iritis), (iii) severe posterior lesions (i.e., onchocercal choroidoretinitis or optic atrophy). In eyes with anterior segment lesions, the distribution of micro- filariae between the cornea and the anterior chamber remained the same regardless of the severity of the lesions although the mean counts were always higher in severe cases. In eyes with posterior lesions, the microfilariae counts in absolute terms were again generally high as compared with non-severe lesions but the distribution of microfilariae within the ante- rior segment differed from that seen in cases with anterior lesions or in non-severe cases. Relatively more microfilariae were found in the anterior cham- ber than in the cornea. Table 4. The mean numbers (geometric mean) of microfilariae counted in different parts of the anterior segment of the eye: comparison of cases with and without signs of severe ocular onchocerciasis MFACa DMFC b LMFCC No signs of Severe ocular No signs of Severe ocula No signs of Severe ocular severe ocular ular nhocerciasis severe ocular ocoecaionhcrissonchocerciasis onchocerciasis onchocerciasis onchocerciasis No. of observations (eyes) 360 197 403 172 151 107 Geometric mean 4.03 11.75 2.37 6.78 2.96 9.33 Geometric means as propor- tions of the geometric mean MFAC a 1 1 0.59 0.58 0.73 0.79 a MFAC = microfilariae in the anterior chamber. b DMFC = dead microfilariae in the cornea. c LMFC = living microfilariae in the cornea. 734 OCULAR ONCHOCERCIASIS Table 5. The mean numbers (geometric mean) of microfilariae counted in different parts of the anterior segment of the eye: comparison of cases with severe posterior or anterior and without severe lesions MFACa DMFC b LMFC C Non-severe Severe Severe No-eee Severe Severe Non -severe Severe Severeanterior posterior Non-severe anterior posterior anterior posterior No. of observations 360 126 52 403 104 51 151 67 29 Geometric mean 4.03 11.82 12.18 2.37 7.16 5.01 2.96 9.97 6.89 Geometric mean as proportions of MFAC a 1.00 1.00 1.00 0.59 0.61 0.39 0.73 0.84 0.54 a MFAC = microfilariae in the anterior chamber. b DMFC = dead microfilariae in the cornea. c LMFC = living microfilariae in the cornea. DISCUSSION The slitlamp examination of the anterior segment of the eye, used in this study, is subject to some sources of error. The number of living microfilariae counted in the anterior chamber is influenced by the position of the patient's head immediately before the examination (2) and the recommended procedure of bending the head downwards is difficult to stan- darize accurately. The microfilariae also tend to sink down into the lower angle of the chamber and this may give a tendency for higher counts to be recorded in the right eye, which is the first to be examined. This tendency, noted by Hamilton et al. (12), was observed in the present study but the difference was not large enough to be statistically significant. The quantitative assessment of microfilariae in the cornea is more accurate (5) but requires a coopera- tive patient. Onchocercal punctate keratitis, as an isolated lesion, may be relatively more common in popula- tions with lower intensities of infection than the present sample, however the snowflake opacity is not pathognomonic of the disease and the diagnosis may be difficult in patients with other coexisting ocular disorders. Then again, two or more snowflake opacities may coalesce to form a single opacity, which may lead to a bias in the resulting count. The importance of onchocercal punctate keratitis alone as a sign of intensity of infection is limited, as it is mainly found in young age groups with early infec- tions. Onchocercal opacities have not therefore been included in the calculations of the ocular parasite load in this study. The correlation between the number of microfila- riae in the anterior segment and the severity of ocular onchocerciasis is complex. The relative distri- bution of microfilariae between the anterior chamber and the cornea does not change with the develop- ment of severe anterior lesions, defined as sclerosing keratitis or iritis. However, in patients with severe anterior lesions, the numbers of microfilariae both in the chamber and in the cornea are generally in- creased. Sclerosing keratitis can be considered to be the result of a massive microfilarial invasion of the cornea during a certain period, and it is then logical to find an increased number of microfilariae in the anterior segment. The patients with posterior lesions, on the other hand, show a relative increase in the proportion of microfilariae in the anterior chamber. This could be explained by the route of entry into the eye. The microfilariae may pass along the sheaths of the ciliary vessels and nerves (13) into the eyeball, possibly from an orbital nodule (9). They may enter via the bloodstream (11) or they may perhaps pass via the cerebrospinal fluid along the sheath of the optic nerve (10, 14). Furthermore, whatever the route followed, posterior lesions may well exist in the presence of low numbers of micro- filariae in the anterior segment. Possibly such patients present merely the end result of a previous severe invasion of relatively short duration. Microfilariae in the anterior chamber have been considered as a more important risk factor for blindness than microfilariae in the cornea (6,15). Recent developments in examination techniques (1) have, however, resulted in a more accurate evalua- tion of the microfilarial load in the cornea and of its 735 736 B. THYLEFORS & U. K. BRINKMANN relation to the intensity of infection (5). In the present study, a slight relative predominance of microfilariae in the anterior chamber was found in patients with posterior lesions as compared with the group showing sclerosing keratitis. This may indi- cate that microfilariae inside the eye, as evidenced by their presence in the chamber, predispose to the development of iritis or a lesion of the fundus; it should not be forgotten, however, that microfilariae are also present in the cornea in the vast majority of these patients. Long-standing and severe cases of ocular oncho- cerciasis tend to have both anterior and posterior lesions, even if one or the other dominates the clinical picture (6). It is therefore reasonable to assume that the intensity of infection and the risk of blindness depend on the total number of microfila- riae visible in the eye. The mean number of micro- filariae in the anterior segment of both eyes gives a good positive correlation with severe anterior lesions and thus with the risk of blindness, especially in savanna areas (3). The number of microfilariae in the anterior segment also correlates with posterior lesions but the relative importance of microfilariae in the anterior chamber as opposed to the cornea, as well as the influence of the time factor, needs further investigation. For epidemiological field studies, it is useful to have a figure for the ocular parasite load that can be calculated easily, instead of operating with two or more parameters. Estimation of the numbers of microfilariae from quantitative coded groups, using their rounded geometric group midpoint, gives an acceptable level of accuracy. ACKNOWLEDGEMENTS We wish to express our gratitude to Dr B. 0. L. Duke for helpful criticism and for revising the manuscript. RtSUMt LA CHARGE MICROFILARIENNE DANS LE SEGMENT ANTERIEUR DE L'CEIL-- PARAMETRE DE L'INTENSITE' DE L'ONCHOCERCOSE On sait que la presence de microfilaires d'Onchocerca volvulus dans l'oeil s'accompagne d'un risque accru d'aggravation des lesions oculaires existantes et donc de cecite. On a visite trois villages dans un foyer d'oncho- cercose hyper-endemique dans le nord du Togo, et 630 personnes ont ete examinees au point de vue ophtal- mologique et parasitologique. La numeration des microfilaires dans le segment anterieur de l'ceil a ete pratiquee chez chaque sujet. On a observe que la fre- quence des microfilaires augmente dans la cornee de meme que dans la chambre anterieure jusqu'a l'age de 40 a 50 ans et diminue ensuite. La frequence de la keratite ponctuee onchocerquienne, elle, presente un maximum dans le groupe d'age 10-20 ans. II y avait des microfilaires dans la chambre anterieure de meme que dans la cornee chez les deux tiers des sujets (61,2 %'). La distribution relative des microfilaires entre la chambre anterieure et la cornee ne changeait pas lors du developpement de graves Iesions anterieures, a savoir keratite scirosante et (ou) iritis. Cependant, chez des sujets presentant de graves lesions posterieures, c'est-a-dire chorior6tinite et (ou) atrophie optique, on a trouv6 relativement plus de filaires dans la chambre anterieure que dans la cornee. Dans tous les cas de lesions oculaires graves, les nombres de microfilaires tant dans la chambre anterieure que dans la cornee dtaient en general eleves. Le nombre moyen de microfilaires par ceil (qui etait calcule en divisant par deux le nombre total obtenu pour les deux yeux) peut etre utilise comme parametre pour evaluer la gravite de l'onchocercose oculaire. REFERENCES 1. ANDERSON, J. & FUGLSANG, H. Living microfilariae of Onchocerca volvulus in the cornea. Br. J. Ophthal- mol., 57: 712 (1973). 2. ANDERSON, J. & FUGLSANG, H. 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