• View in gallery

    Map of study area: Location of surveyed villages and their historical Rapid Assessment Procedure for Loa loa (RAPLOA) prevalence. LGA = local government area. This figure appears in color at www.ajtmh.org.

  • View in gallery

    Village Rapid Assessment Procedure for Loa loa (RAPLOA) prevalence vs. L. loa microfilaria (mf) prevalence (N = 110). This figure appears in color at www.ajtmh.org.

  • View in gallery

    Village Rapid Assessment Procedure for Loa loa (RAPLOA) prevalence vs. maximum observed L. loa microfilaremia. This figure appears in color at www.ajtmh.org.

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In Southern Nigeria Loa loa Blood Microfilaria Density is Very Low Even in Areas with High Prevalence of Loiasis: Results of a Survey Using the New LoaScope Technology

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  • 1 The Carter Center, Owerri, Nigeria;
  • 2 The Carter Center, Atlanta, Georgia;
  • 3 The Carter Center, Jos, Nigeria;
  • 4 Parasitology Department, Imo State University, Owerri, Nigeria;
  • 5 Federal Ministry of Health, Abuja, Nigeria;
  • 6 University of California Berkeley, Berkeley, California;
  • 7 National Institute of Health, Bethesda, Maryland;
  • 8 Centre for Research on Filariasis, Yaoundé, Cameroon;
  • 9 Faculty of Medicine and Biomedical Sciences, University of Yaounde I, Yaoundé, Cameroon

Ivermectin treatment can cause central nervous system adverse events (CNS-AEs) in persons with very high-density Loa loa microfilaremia (≥ 30,000 mf/mL blood). Hypoendemic onchocerciasis areas where L. loa is endemic have been excluded from ivermectin mass drug administration programs (MDA) because of the concern for CNS AEs. The rapid assessment procedure for L. loa (RAPLOA) is a questionnaire survey to assess history of eye worm. If ≥ 40% of respondents report eye worm, this correlates with ≥ 2% prevalence of very high-density loiasis microfilaremia, posing an unacceptable risk of CNS-AEs after MDA. In 2016, we conducted a L. loa study in 110 ivermectin-naïve, suspected onchocerciasis hypoendemic villages in southern Nigeria. In previous RAPLOA surveys these villages had prevalences between 10% and 67%. We examined 10,605 residents using the LoaScope, a cell phone–based imaging device for rapidly determining the microfilaria (mf) density of L. loa infections. The mean L. loa village mf prevalence was 6.3% (range 0–29%) and the mean individual mf count among positives was 326 mf/mL. The maximum individual mf count was only 11,429 mf/mL, and among 2,748 persons sampled from the 28 villages with ≥ 40% RAPLOA, the ≥ 2% threshold of very high Loa mf density could be excluded with high statistical confidence (P < 0.01). These findings indicate that ivermectin MDA can be delivered in this area with extremely low risk of L. loa–related CNS-AEs. We also concluded that in Nigeria the RAPLOA survey methodology is not predictive of ≥ 2% prevalence of very high-density L. loa microfilaremia.

INTRODUCTION

Onchocerciasis, commonly known as river blindness, is a filarial nematode infection caused by Onchocerca volvulus, transmitted by certain insect vector species of the genus Simulium.1 This disease is of public health importance because of its associated visual impairment, blindness, stigmatizing skin disease, and debilitating itching. Human disease results from inflammation around microfilaria (mf) released from fertilized adult female worms residing in fibrous subcutaneous “nodules.” Disease is more severe in individuals who have high numbers (“intensities”) of mf. The Simulium black fly vectors breed in rapidly flowing rivers and streams and become infected when they ingest mf during a blood meal; mf develop into third stage larvae that can infect humans when the vector takes subsequent blood meals. The World Health Organization (WHO) estimates that about 198.2 million people are at risk of infection and that 99% of them reside in Africa.13 Nigeria is among the countries with the highest burden of onchocerciasis.4

Mass drug administration (MDA) with ivermectin (Mectizan®, an oral medication donated by Merck, Kenilworth, NJ) kills the mf stage of the parasite and prevents onchocerciasis-associated eye and skin disease. A global partnership against river blindness, which includes the endemic countries, nongovernmental organizations, the Mectizan® Donation Program, and the WHO, has given more than 1 billion treatments for onchocerciasis since 1987.5 If the coverage and duration of ivermectin MDA programs are sufficient, mf densities can be kept sufficiently low to prevent the vectors from transmitting the infection. Because there are no important animal reservoirs of O. volvulus to maintain the transmission cycle independent of the human population, permanent elimination of transmission of onchocerciasis can be achieved, such as in four countries in the Americas and in some parts of Africa.6,7 In 2014 the African Program for Onchocerciasis Control called for a new goal of onchocerciasis transmission elimination for Africa. As part of that policy, an expansion of ivermectin MDA into previously untreated areas was proposed. These areas (the so-called “hypoendemic” areas) are those with sufficient O. volvulus transmission to maintain the adult parasite population but very little morbidity due to the near absence of high mf density infections. Untreated areas bordering ivermectin MDA programs are those most likely to be hypoendemic and therefore newly targeted for MDA.8

Loa loa, another filarial parasite prevalent in central Africa, is complicating the MDA expansion plan under the new onchocerciasis elimination paradigm. Loa loa is transmitted by deerflies (Chrysops species) that breed in high canopy-forested areas in Africa. Adult L. loa worms may migrate under the eye’s conjunctiva and be recognized by the infected individual.911 Adult female L. loa worms produce mf that (unlike in onchocerciasis) enter the blood stream; circulating L. loa mf can reach extremely high densities in the blood. The abrupt death of mf after the administration of a microfilaricidal agent such as ivermectin can rarely result in central nervous system adverse events (CNS-AEs) shortly after treatment that include changes in consciousness and, rarely, coma. Deaths have resulted from complications arising from prolonged coma events.12 Only individuals with very high L. loa mf densities (≥ 30,000/mL of blood) are at risk of these CNS-AEs.1315

A technique called the Rapid Assessment Procedure for L. loa (RAPLOA) was developed over a decade ago to quickly and noninvasively assess an area for the risk of L. loa–related CNS-AEs after ivermectin MDA. A sample of 80 residents aged 15 years and older are individually asked if they at some point in the past experienced a worm moving across the surface of their eye. During the interview the respondents are shown a photograph of a L. loa worm in the eye. A multi-country study showed a strong correlation with ≥ 40% of residents answering “yes” (e.g., a RAPLOA prevalence of ≥ 40%), a village prevalence of L. loa microfilaremia ≥ 20%, and the village prevalence of very high-density L. loa ≥ 2%.1620 These critical and correlated thresholds (RAPLOA ≥ 40%, L. loa microfilaremia prevalence ≥ 20% and very high-density L. loa ≥ 2%) define an area at high risk for L. loa CNS-AEs. The magnitude of this risk is poorly defined.14

High RAPLOA determinations in onchocerciasis hypoendemic areas are roadblocks to the onchocerciasis elimination agenda in L. loa–endemic countries such as Nigeria. Expansion of MDA into these hypoendemic areas is difficult to justify because the benefit from MDA in reducing morbidity from onchocerciasis is low compared with the risk of CNS-AEs from L. loa treatment. We report a survey in just such an area in Nigeria where there is presumed hypoendemic onchocerciasis and hyperendemic L. loa. Our purpose was to reevaluate the relationships among RAPLOA, L. loa microfilaremia prevalence, and most importantly, very high-density L. loa. We also assessed for onchocerciasis endemicity using a rapid diagnostic test for OV16 IgG4 antibodies; the results of that study will be reported elsewhere.

MATERIALS AND METHODS

Study area.

The survey was conducted in five Nigerian states assisted by The Carter Center in Abia, Anambra, Delta, Ebonyi, and Imo states, which are located in the forested South-South and Southeast geopolitical zones of Nigeria. The economic activities in these states are mainly agricultural subsistence farming and fishing. The climate is characterized by a rainy season from March to October and a dry season from November to February.

Study design.

The survey was built in a way to purposefully identify villages likely to have both (hypoendemic) onchocerciasis and individuals with very high-density L. loa infections. First, twenty ivermectin untreated districts (in Nigeria called local government areas, LGAs) were selected that bordered LGAs already under ivermectin MDA for meso- or hyperendemic onchocerciasis. It was assumed that due to this proximity the selected LGAs would likely have hypoendemic onchocerciasis. Second, villages within those LGAs were selected for survey that had the highest historical RAPLOA prevalence in studies conducted in 2012–2013 by The Carter Center or in 2014–2015 by the Federal Ministry of Health (FMOH) (Figure 1, Table 1). Villages were sorted by RAPLOA prevalence in descending order. All villages having ≥ 20% RAPLOA were selected for survey, followed by a portion of villages with 10–19.9%, until the target of 110 villages was met. Five villages were excluded due to insecurity in their LGA and were replaced by five villages with the next highest RAPLOA prevalence. In surveyed villages, we determined L. loa mf prevalence and density using a revised version of the original LoaScope.21 This is an innovative cell phone–based microscope reader that gives a reading of L. loa mf density within 3 minutes of placing a special cubical capillary tube containing uncoagulated blood (obtained by finger stick) under the lens. From the same finger stick another 10 μL of blood was obtained for a thin smear and10 μL for an OV16 RDT test for onchocerciasis IgG4 antibodies.

Figure 1.
Figure 1.

Map of study area: Location of surveyed villages and their historical Rapid Assessment Procedure for Loa loa (RAPLOA) prevalence. LGA = local government area. This figure appears in color at www.ajtmh.org.

Citation: The American Journal of Tropical Medicine and Hygiene 99, 1; 10.4269/ajtmh.18-0163

Table 1

Village sample size, Rapid Assessment Procedure for Loa loa (RAPLOA) information, max and average cellScope counts, and LoaScope prevalence

StateLGAVillageNumber surveyedMax of RAPLOA (%)Source of max RAPLOA valueYear of RAPLOA surveyMax of LoaScope mf/mLAverage LoaScope mf/mL among positivesPrevalence of LoaScope positives (%)
AbiaOsisiomaAmapu Ife9615FMOH20154392077
Umuakpara9914FMOH20155922299
Umule10119FMOH20152821797
Umumba7723FMOH20155262096
UgwunagboOwerri Aba10014FMOH20151,0496022
Umule Osoamadi10011FMOH201592921
Umuode10013TCC20134613704
Umuodo8724FMOH20159212718
AnambraAnambra eastAgbudu Nando9865TCC201292126414
Nneyi Umueri9867TCC201287726326
Ogwari Nsugbe10062TCC201268023721
Otuocha10049TCC20122631972
Ubaru Ugwuoji10455TCC20121,25931319
Anambra westMmiata Anam10146TCC20121,25429211
Nzam Assa9956TCC20121971215
Umuenwelum10047TCC201285617911
Umueze Anam10059TCC201246119718
Umuoba Abegbu10147TCC20121,93030115
OgbaruAtani10020FMOH201548616618
Isiolu Ugalo9761TCC201230714310
Odekpe10214FMOH20152,63229715
Ohita9913FMOH20156142986
Okpoko9918FMOH20151,29020822
Onyili/Ibelenta10167TCC20121285912
Umudashi/Esielle10065TCC201232912810
Umuezegbo10050TCC20121,10018220
Umunankwo10026FMOH20151,29018421
Umuokoloigbo10149TCC201235112013
Onitsha northAmerican Quarters10016FMOH201561611
Onitsha southFegge10021FMOH20151541373
DeltaEthiope eastEkrejeta6928TCC20131,7915916
Eku (Emure)9943TCC20122,6105697
Igun10048TCC20125632169
Okpara Inland10011TCC201311,4294,0473
Okurekpo9935TCC20121,1774056
Orhoakpo10025TCC20124612173
Oria Abraka10029TCC20139213683
Otorho Abraka10029TCC20133071655
Samagidi10034TCC201231311
Urhuovie Igun9913FMOH20155225221
Isoko northOfagbe10040TCC20123383381
Okpe Isoko10060TCC20121841075
Otor-Igho/Emevor10013TCC20137,5681,2818
Owhelogbo10020TCC20123381485
Ozoro10040TCC20127062319
Isoko southEmede10115TCC20123291426
Emore9920TCC20127461727
Irri10021TCC20129983635
Olomoro10021TCC20125262874
Uzere10029TCC20123071494
PataniAbari9736TCC2012000
Bolu Angiama9716TCC20134654651
Bulu Aperebiri10039TCC2012154922
Odorubu10035TCC20123691384
Patani II10034TCC2012000
Uduophri10035TCC2012000
Ugheli northOdovie10013TCC201344441
Oghara Agharha10120TCC20133972292
Onidjor Uwheru10025TCC201392513
Orogun9721TCC2013000
Otovwodo10218TCC2013000
EbonyiAbakalikiAbofia (Unagbo Oke)10033TCC20124391265
Amachi Unuhu10025FMOH20154913212
Amagu Onicha10028TCC20125263965
Ametta Amachi10025FMOH20154832285
Azugwu9821FMOH20157682578
Azuiyiokwu10028FMOH2015230994
Egwudinagu10039FMOH2015000
Enyigba10040FMOH20152631752
Igbegu Unuhu10014TCC20122761464
Ndiaja ndiagu10044FMOH20154832853
Ndigboke10024FMOH20151971373
Nkaliki10025FMOH20153073071
Onuebonyi10044TCC2012000
Orizor ndiuruku10024FMOH2015000
Uburu Amachi10038TCC20125055051
Ugwuachara10026FMOH2015000
Unagboke10031FMOH201551511
Agodo10043FMOH20152,8744879
Enyimagu10036TCC20121,2593347
Ezzamgbo10025FMOH20152461274
Iniki ri8733FMOH20157,8751,5598
Ishi-izhia8935FMOH20158298291
OhaukwuNdi agu obu okposhi ehaku8940FMOH20155055051
Ndiaguonwe8238TCC20121541534
Ndiakparata Ugwuagba8431FMOH20156143996
Obodo Ogbani8839TCC20125051885
Omocha7648FMOH20151,7507219
Otaku Umuagara8440TCC20121,2285957
Ufoboto7733TCC20122151385
ImoNkwerreEzemeneha10030TCC20133953951
Umuakuma10018TCC20137062635
Umunaga10019TCC20137902464
OgutaAbosi Izombe10020TCC2013276887
Ejemekwuru9719TCC20134862933
Obujuju Akabor9910TCC201392921
Ugbele Izombe10028TCC20134912047
Umuoba Agwa10030TCC20131021021
Umuofeke Agwa9625TCC2013000
Ohaji/EgbemaObitti10041TCC20132,8151,0764
Umuogologo10019TCC201331311
Oru eastAbia Omuma6330TCC20131,1066145
Amagu Awo5415TCC20134351827
Awo-omama7113FMOH201559216727
Eziawo8114TCC20135,04295321
Ihitte Akata12014TCC20134,2127956
Ubachima Awo9229TCC20121501502
Oru westIhite Oha9115TCC20131501502
Nempi6621TCC20134,93664029
Umuduru Ezigbidi10120TCC20139729721
Total/Max/Average/Overall10,605NANANA11,4293266

FMOH = federal ministry of health surveys; LGA = local government area; TCC = the Carter Center surveys.

Procedures.

In each village, we aimed to test 50 adults (more than 18 years of age) and 50 children (≥ 5 and < 10 years of age). We excluded anyone who was ill or who might not tolerate finger-stick blood collection. Village leaders met with the research team who explained the study and then obtained verbal permission for the study to take place. Village leaders selected one of their common meeting areas as the survey venue for adults. For children, the venue was the village primary school. On the day of the study, written consent was obtained from each individual (assent from children) and a unique ID number was assigned. The ID and general information (name, age, gender, occupation, and duration of time living in the village) was recorded in tablets and on paper forms.

Sampling took place from 10:00 am to 4:00 pm when the maximum number of L. loa mf circulate.22 Whole blood was obtained using standard finger-lancing procedures by gloved technicians working in an enclosed private area. After cleaning the puncture site with isopropyl alcohol, a sterile single-use lancet was used to obtain blood from the index finger of the subject. A glass capillary tube with a rectangular cross section (Vitrocom Inc., Mountain Lakes, NJ) was filled first and immediately passed on to another technician who operated the LoaScope and read the number of L. loa mf in 15 μL of whole blood. A 10-μL capillary tube was used to make a thin smear blood slide. A final sample of 10 μL of blood was obtained by capillary tube and immediately placed on a Bioline Ov16 rapid test card (Standard Diagnostics, Inc., Gyeonggi-do, Republic of Korea) and then read per directions. LoaScope and RDT results were recorded in the tablets and on paper. The LoaScope automatically recorded in its memory the mf results along with the participant’s ID number; the team was later able to verify the LoaScope results uploaded by the tablets using the stored data uploaded directly from the LoaScopes. The entire registration, blood draw and testing procedure took approximately 10–15 minutes for each participant; the LoaScope portion of this process took about 5 minutes. It should be noted that the LoaScope is calibrated to capture high-density infections accurately, and as a result, a single capillary per person in the standard settings for number of fields of view per capillary is accurate to within 150 mf/mL.

The 10 μL blood smears were labeled with the participant’s ID, air-dried and returned to The Carter Center laboratory in Owerri. All individuals with LoaScope readings ≥ 20,000 mf/mL and a random selection of 10% of slides from those with readings < 20,000 (“negatives”) were stained with Giemsa and examined under light microscopy by an experienced technician. All L. loa mf on the slide were counted and recorded with their corresponding ID on paper forms.

Data analysis.

Paper data forms with personal identifiers were kept locked in a file cabinet at The Carter Center Owerri office. Data were uploaded from the tablets and LoaScopes using only ID number identifiers, combined in a Microsoft Excel spreadsheet, cleaned, and then analyzed using IBM SPSS 24, Excel pivot tables, Epinfo7, and OpenEpi statistical packages. Crude (age unadjusted) prevalence of L. loa mf, overall and by village, were calculated by dividing the number of mf positive persons by the number of persons examined. Density among LoaScope-positive readings was expressed as arithmetic means and medians. Very high-density L. loa infection was defined as ≥ 20,000 mf/mL because the lower 95% confidence intervals from LoaScope readings of ≥ 30,000 included 20,000 mf/mL.23 Prevalence of very high-density infection was calculated as the number of persons with ≥ 20,000 mf/mL divided by the total number of persons examined overall, by age group (adult or children), and in the subset of villages having RAPLOA prevalence ≥ 40%. Two percent very high-density infection was assumed to be at a CNS-AE risk threshold at or above which ivermectin MDA should not be given. The categorical χ2 test was used to determine if the observed percent positive with high-density infection differed from the expected result of 2%. We used the Spearman’s rho to test the correlation between RAPLOA versus L. loa village mf prevalence, and RAPLOA versus peak village L. loa mf density.

The 10 μL-thin blood films from all persons with LoaScope readings of ≥ 20,000 mf/mL were examined by light microscopy to confirm those results. In addition, 10% of negatives (e.g., < 20,000 mf/mL) were also examined to determine if high-density carriers might have been missed by LoaScope. Results however were not used to check the range of LoaScope density readings because the volume of blood used in the thin smear was 50% less than that examined by the LoaScope, and so less sensitive.

RESULTS

One of the twenty selected ivermectin-naive LGAs could not be visited due to insecurity. In the remaining 19 LGAs, 110 villages were surveyed: 82 of these (75%) had RAPLOA prevalence of ≥ 20% and 28 villages (25%) were ≥ 40% (range 40–67%). A total of 10,605 persons were tested by LoaScope, an average of 96 persons per village; 5,776 (54.5%) were females and 5,282 (49.8%) were children. The sample included 2,748 residents of the highest risk villages (RAPLOA ≥ 40%).

Only 661 (6.2%) persons were positive for mf by LoaScope. The mean village L. loa prevalence was 6.3% (median 4.9%, range 0–28.8%). The mean mf count among positive individuals was 326 mf/mL (median 150 mf/mL) and the maximum individual mf count was 11,429 mf/mL. Results for villages with RAPLOA ≥ 40% showed a mean village prevalence of 7.1% (median 5.0%, range 0.0–25.5%), a mean mf count among positives of 267 mf/mL (median 150 mf/mL), and the highest individual mf count 2,874 mf/mL. There was no correlation between L. loa mf prevalence and RAPLOA results (Figure 2). Peak village microfilaremia also did not correlate with RAPLOA results (Figure 3).

Figure 2.
Figure 2.

Village Rapid Assessment Procedure for Loa loa (RAPLOA) prevalence vs. L. loa microfilaria (mf) prevalence (N = 110). This figure appears in color at www.ajtmh.org.

Citation: The American Journal of Tropical Medicine and Hygiene 99, 1; 10.4269/ajtmh.18-0163

Figure 3.
Figure 3.

Village Rapid Assessment Procedure for Loa loa (RAPLOA) prevalence vs. maximum observed L. loa microfilaremia. This figure appears in color at www.ajtmh.org.

Citation: The American Journal of Tropical Medicine and Hygiene 99, 1; 10.4269/ajtmh.18-0163

No participants were detected with high-density L. loa microfilaremia. The highest count in the study (11,429 mf/mL) was in a resident of the low risk RAPLOA village of Okpara Inland (RAPLOA 11%) of Ethiope East LGA in Delta State. The second (7,875 mf/mL) and third highest (7,568 mf/mL) mf densities were in residents of villages with RAPLOAs of 33% and 13%, respectively.

The 2% prevalence of very high-density microfilaremia did not occur in the overall sample (P < 0.01) and the subsample of 2,748 persons resident in ≥ 40% RAPLOA villages (P < 0.01). Similarly, the 2% threshold was not exceeded when only adults were considered in the analysis for all villages (N = 5,323, P < 0.01) or villages with RAPLOA ≥ 40% (N = 1,380, P < 0.05).

A random selection of 1,342 (12.7%) of the 10-μL blood slides were stained and read by light microscopy. The maximum L. loa density was 11,800 mf/mL, from the same individual with a LoaScope reading of 11,429. No very high-density L. loa infections were observed in the blood slide reading.

DISCUSSION

In 2013 the FMOH of Nigeria released a master plan for neglected tropical diseases that included a new policy for using ivermectin MDA to interrupt onchocerciasis transmission throughout the country. Soon thereafter the minister of health launched the Nigeria Onchocerciasis Elimination Committee (NOEC) to provide technical advice to the FMOH on how to accomplish this goal by 2025. In 2016 the NOEC completed a “National Guideline for Onchocerciasis Elimination in Nigeria” that called for the MDA program to expand treatment into hypoendemic areas.24 That expansion had to address concerns about possible CNS-AEs in L. loa endemic states, especially where there were villages with RAPLOA survey results ≥ 40%. The purpose of our study was to access whether MDA expansion into hypoendemic areas in L. loa endemic states assisted by The Carter Center could be done with minimal risk. Using the new LoaScope technology we sampled residents from all villages in these states with > 20% RAPLOA prevalence located in ivermectin-naïve and likely onchocerciasis hypoendemic LGAs. Our purpose was to determine the prevalence of very high-density L. loa microfilaremia. A key outcome was to identify how many of these villages had ≥ 2% residents with microfilaremia ≥ 30,000 mf/mL.

In 110 villages sampled we found no LoaScope readings more than 20,000 mf/mL in 10,605 LoaScope examinations. In residents of villages with RAPLOA ≥ 40%, where the prevalence of very high-density microfilaremia should equal or exceed 2%, we could exclude the 2% threshold with > 99% statistical confidence. We concluded that ivermectin MDA for treatment of hypoendemic onchocerciasis could be deployed in these areas with very low risk of CNS-AEs.

In our study L. loa prevalence was lower than expected based on the RAPLOA findings. We found only 661 participants (6.2%) to be positive for mf and a mean village prevalence of 6.3% (median 4.9%, 0–28.8% range). Only eight villages had a L. loa prevalence ≥ 20%, compared with 28 villages having RAPLOA > 40%. Loa loa prevalence did not increase with RAPLOA as other studies has shown.25 We also were unable to demonstrate a correlation between RAPLOA and L. loa mf density, in contrast to Takougang et al.’s study that showed this relationship in Cameroon’s Southwest and Northwest provinces. Although Takougang et al.25 detected no persons with very high-density microfilaremia in Nigeria’s Cross River State, the RAPLOA prevalence of all villages in his study was below 40%. An important contribution of our study is that we were able to assess residents of 28 Nigerian villages where RAPLOA results were above the 40% threshold, and found no very high-density infections there. Our results are consistent with a report by Hassan et al.,26 who studied a village in southwest Nigeria (Ikpakodo) with 54% eye worm history where the highest L. loa mf count was only 420 mf/mL. Our village sample included an expanded range of RAPLOA prevalence (10–67%) that mitigated our risk of missing very high-density infections if they occurred in villages with RAPLOA < 40% that are not believed to be at risk of CNS-AEs.

The lack of reported CNS-AEs after more than 70 million ivermectin MDA treatments in known L. loa areas in Nigeria is further evidence that very high-density L. loa infections are exceedingly rare in the country. Of 758 CNS-AEs reported to the Mectizan Donation Program between 1990 and 2014, 99% of these were from Democratic Republic of Congo and Cameroon (Y. Sodahlon, personal email communication). Only one case (in 1991) in this series (0.1%) was reported from Nigeria.15 The NOEC has concluded that the lack of CNS-AEs despite millions of ivermectin treatments is evidence that the risk of these events in Nigeria is essentially nonexistent.27

The reasons that L. loa mf densities rarely, if ever, reach very high levels in Nigeria are unknown but could include the following: 1) The Nigerian strain of the L. loa parasite is not prone to produce very high-density microfilaremia; 2) the Nigerian human population may have genetic or other host factors that limit mf circulation; 3) deforestation or other recent environmental changes may have reduced Chrysops breeding and biting rates, leading to fewer adult L. loa adult parasites resulting in a decreasing mf prevalence and density; 4) unrecognized ivermectin treatment in the study population that has lowered mf prevalence and density. This last possibility is unlikely considering that in a questionnaire survey administered to 5,910 (56%) of our study participants, 98% stated that they had not taken ivermectin.

In Cameroon, the LoaScope has been successfully used to determine each individual’s treatment eligibility based on his/her L. loa density, excluding those with microfilaremia exceeding 20,000 mf/mL.21,23 This strategy, called “Test and not Treat,” is proposed as a possible way forward for onchocerciasis programs in L. loa–endemic areas. In this report we used the LoaScope as a survey technology to judge village (rather than individual) risk for CNS-AEs. We demonstrated that the LoaScope can be used to rapidly delineate areas where high-density L. loa infections do not occur and so allow ivermectin MDA using approaches that can be implemented more quickly and cheaply than “Test and not Treat.” This same survey approach could be used for lymphatic filariasis (LF) elimination programs in areas in Africa coendemic for L. loa. In this case, LF MDA programs could use a strategy of ivermectin and albendazole once per year rather than the costlier WHO-recommended strategy of albendazole alone every 6 months.28 Our recommendation, therefore, is that the LoaScope should be the primary tool in surveys needed to rapid expand ivermectin use in onchocerciasis and LF elimination programs operating in Africa where RAPLOA findings suggest that L. Loa microfilaremia densities could be dangerously high.

This study had a number of limitations: 1) We selected study villages based on historical RAPLOA results from surveys conducted between 2012 and 2015. This was justified because these villages were not receiving ivermectin treatment so that the prevalence and density of L. loa infections were unlikely to change dramatically in the 1–4 years between the RAPLOA assessment and the LoaScope survey; 2) We assumed that a convenience sample of 50 adults (aged ≥ 18 years) and 50 children (aged ≥ 5 and < 10 years) accurately reflected village L. loa prevalence. The sample was skewed toward younger children to assess OV16 antibody in an age group where positive results would indicate recent transmission of O. volvulus in the area. It is likely that having 50% of our overall sample consisting of children under 10 years of age resulted in an underestimate of the village L. loa prevalence and density, assuming adults are more likely than children to have very high-density L. loa infections. However, in a subanalysis of our data we could still statistically rule out the ≥ 2% very high-density prevalence threshold among all adults in the study, and among adults resident in high risk (RAPLOA ≥ 40%) villages.

CONCLUSION

We conducted a LoaScope-based blood survey in 10,605 residents of 110 ivermectin-naïve villages located in L. loa–endemic areas in southern Nigeria. The villages likely have hypoendemic onchocerciasis transmission and need to be treated with ivermectin MDA. Twenty-five percent of the villages surveyed had RAPLOA values of ≥ 40%. We determined that there is minimum risk for CNS-AEs because no very high-density L. loa infections could be detected, and the LoaScope prevalence of very high-density infections was significantly below 2%. The LoaScope is a useful tool for use in village surveys designed to assess risk of ivermectin MDA in L. loa–endemic areas.

Acknowledgments:

We would like to thank the State and Local Government Area Ministries of Health, including the field teams who made this study possible. We would also like to thank the village members who assisted us in village mobilization and organization, and the village members who participated in the study. We express our gratitude to Josephine Obiezu for her excellent and tireless data management. The study was funded by The Carter Center and the Sir Emeka Offor Foundation. The Bill and Melinda Gates Foundation and USAID supported LoaScope development.

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Author Notes

Address correspondence to Lindsay J. Rakers, The Carter Center, 453 Freedom Parkway, One Copenhill Ave., Atlanta, GA 30307. E-mail: lindsay.rakers@cartercenter.org

Financial support: D. A. F. reports grants from Bill & Melinda Gates Foundation, during the conduct of the study; In addition, Fletcher has a patent High Numerical Aperture Telemicroscopy Apparatus (US Patents 8,743,194 and 8,786,695) licensed to Thermo Fisher Scientific; CellScope, Inc., and a patent Automated Hardware and Software for Mobile Microscopy (US Patent pending). M. D. reports grants from Bill & Melinda Gates Foundation, during the conduct of the study; In addition, D’Ambrosio has a patent Automated Hardware and Software for Mobile Microscopy (US Patent pending).

Authors’ addresses: Emmanuel Emukah, Carter Center, Owerri, Nigeria, E-mail: emmanuel.emukah@cartercenter.org. Lindsay J. Rakers, Emily Griswold, and Frank O. Richards, Carter Center, Atlanta, GA, E-mails: lindsay.rakers@cartercenter.org, emily.griswold@cartercenter.org, and frank.richards@cartercenter.org. Barminas Kahansim, Emmanuel S. Miri, and Cephas Ityonzughul, Carter Center, Jos, Nigeria, E-mails: barminas.kahansim@cartercenter.org, emmanuel.miri@cartercenter.org, and cephas.Ityonzughul@cartercenter.org. Bertram E. B. Nwoke, Parasitology Department, Imo State University, Owerri, Nigeria, E-mail: bebndie@yahoo.com. Yisa Saka, Ifeoma Anagbogu, and Emmanuel Davies, Federal Ministry of Health, Abuja, Nigeria, E-mails: yisaasaka@yahoo.com, ifechuba@yahoo.co.uk, and enimed2003@yahoo.com. Michael D’Ambrosio, Matthew Bakalar, and Daniel A. Fletcher, University of Berkeley, Berkeley, CA, E-mails: mdambrosio@berkeley.edu, matthew.bakalar@gmail.com, and fletch@berkeley.edu. Thomas Nutman, National Institute of Health, Bethesda, MD, E-mail: tnutman@niaid.nih.gov. Joseph Kamgno, Centre for Research on Filariasis, Yaoundé, Cameroon and Faculty of Medicine and Biomedical Sciences, University of Yaounde I, Yaoundé, Cameroon, E-mail: kamgno@crfilmt.org.

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