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ABSTRACT |
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Chikungunya virus illness is associated with fever, severe arthralgias, rash, headache, and malaise. Other symptoms include muscle aches and retro-orbital pains. Arthralgia can be debilitating and prolonged.4,6,7 Chikungunya disease is rarely fatal but is associated with significant morbidity. Chikungunya illness has an approximate incubation period of 1–2 weeks.
The vectors principally responsible for transmission of the virus are Aedes mosquitoes.6,8 In Africa, CHIKV apparently is maintained in a sylvatic transmission cycle involving primates and forest-dwelling Aedes mosquitoes.9 Sylvatic vectors that have been implicated in transmission include Ae. africanus in East Africa,10 Ae. furcifer, Ae. taylori, Ae. delzieli, and Ae. luteocephalus in West Africa,6,11 and Ae. taylori and Ae. codellieri in South Africa.12 In contrast, transmission of CHIKV in Asia has been documented to occur mainly in urban areas where Ae. aegypti and Ae. albopictus are the identified vectors.13–15
Chikungunya is a re-emerging disease of public health importance in both African and Southeast Asian countries, causing major outbreaks. Outbreaks in Democratic Republic of Congo,1 Malaysia,16 Indonesia,17 and Senegal18 were reported earlier. An outbreak of Chikungunya occurred on Lamu Island beginning May 2004 and peaked in July 2004. Illness caused by Chikungunya had not been recognized previously on Lamu Island. After that outbreak, other associated outbreaks occurred in Mombasa, Kenya, between November and December 2004, Comoros Islands from January to May 2005 (unpublished data), Reunion Island,19 other islands in the Indian Ocean,20 and in India21 in 2006.
The Ministry of Health, Lamu district, documented at least 1,300 patients meeting a clinical case definition of Chikungunya illness with no deaths. Of 130 specimens collected and sent to KEMRI for testing, IgM antibodies against CHIKV were detected in 60, and CHIKV was isolated by viral culture or detected by polymerase chain reaction in an additional 22 specimens. The extent and scope of CHIKV infection on Lamu Island was unknown. A seroprevalence study was carried out to define the magnitude of the outbreak and make recommendations for prevention and control interventions. This paper describes the findings of the serosurvey.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Standardized questionnaires were administered by field workers to collect data on demographics, symptoms, and treatment of members of each household. The questionnaires were translated into Kiswahili, the local dialect. With informed consent, questionnaires were administered and blood specimens were collected. The Kenya Ministry of Health determined that this study was part of its public health response to the outbreak and that review by an Ethical Review Committee was not required.
Study site.
Lamu Island covers an area of 102.4 km2 along the Kenyan coast in the Indian Ocean (Figure 1
). The projected population for 2004 was 18,000 based on the 1999 census. The Lamu population is predominantly urban and is mostly composed of Swahili-speaking people who are Muslims of Arabic origin.
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Laboratory diagnosis.
Sera were separated from whole blood specimens and tested at Kenya Medical Research Institute (KEMRI) for IgM and IgG antibodies to CHIKV using antibody capture enzyme-linked immunosorbent assay (MAC ELISA). All sera were heat inactivated at 56°C for 30 minutes before testing for presence of CHIKV IgM using either indirect ELISA22 or IgG using direct ELISA.17 For both tests, we used a positive control serum specimen obtained from a previous Chikungunya virus outbreak in East Africa; a negative control serum specimen was from a person from a non-endemic region who tested negative for arbovirus infection. For CHIKV-specific IgM detection, 96-well poly-styrene ELISA plates were coated with 1:1,000 dilution of anti-human IgM (Kirkegaard and Perry Laboratories, Gaithersburg, MD) overnight at 4°C. After five washes with phosphate-buffered saline (PBS) with 0.05% Tween-20 (PBS-T), non-specific binding was blocked by adding 5% non-fat dry milk in PBS with 0.5% Tween-20 and incubated for 30 minutes at room temperature. Test serum was added at 1:400 dilution and incubated for 60 minutes at 37°C. Each diluted test serum sample was added in quadruplicate, with two wells serving as positive control (with CHIKV antigen) and two wells serving as negative control. After adding a 1:40 dilution of CHIKV antigen (S-27; Centers for Disease Control and Prevention, Fort Collins, CO), plates were incubated overnight at 4°C before adding a horseradish peroxidase–conjugated alphavirus-specific monoclonal antibody 2A2C-3 (Centers for Disease Control and Prevention) at 1:6,000 dilution and further incubated for 60 minutes at 37°C. Presence of CHIKV antibodies was detected by adding ABTS [2,2' amino-bis(3-ethylbenthiazoline-6-sulfonic acid)] substrate (Kirkegaard and Perry Laboratories), and the absorbance was read at 405 nm. Positive samples required a mean optical density (OD) value 
0.2 above that of the negative control for each sample. The range of OD values for positive serum specimens in this study was 0.3–1.5 above that of the negative control.
For virus-specific IgG detection, plates were coated with 1:2,000 dilution of CHIKV antigen (United States’ Naval Medical Research Unit, Bethesda, MD) overnight at 4°C. Heat-inactivated serum was added at 1:100 dilution and incubated for 60 minutes at 37°C. Each diluted test serum sample was added in quadruplicate, with two wells serving as a positive control (with CHIKV antigen) and two wells serving as a negative control. After adding a 1:3,000 dilution of horseradish peroxidase–conjugated anti-human IgG (Kirkegaard Perry Laboratories), presence of CHIV-specific IgGs was detected by adding ABTS substrate, and the absorbance was read at 405 nm. Positive samples required a mean OD value 0.2 above that of the negative control for each sample.
Data analysis.
Data collected from the questionnaires were entered and analyzed using Epi Info 2002 software (Centers for Disease Control and Prevention). Age, sex, and locality distribution of the seropositive and seronegative participants were compared. Locality was defined as urban (living within Lamu town) or rural (living away from Lamu town). 
2 was used for statistical testing.
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RESULTS |
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Among 288 serum specimens tested, CHIKV IgG or IgM antibodies were detected in 215 (75%); 206 (72%) had IgG antibodies and 53 (18%) had IgM antibodies. Nine serum specimens with antibodies detected had only IgM antibodies (no IgG detected). For patients with detectable IgG or IgM antibodies, levels of IgG antibodies were higher, as indicated by higher OD values compared with IgM antibodies (Figure 2). Given a population of 18,000 and an attack rate of 75%, we estimate that 13,500 people were infected on the island during the outbreak (95% CI, 12,458–14,328 persons).
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). The highest seropositivity ratio was in the 1- to 4-year-old group (86%) (Table 2), although participants 15 years old were more likely to be seropositive than those < 15 years old (P = 0.04; Table 2). The proportion of participants who were IgG seropositive was higher than the proportion of participants who were IgM seropositive for all age groups. Among participants 15 years old, 192 of 257 had CHIKV IgG antibodies detected, which was significantly higher than the proportion of participants < 15 years of age with IgG antibodies (P = 0.002). However, there was no statistical difference in the presence of IgM antibodies among persons 15 years of age and the younger participants (19% versus 16%; P = 1.00). Although there were seropositive persons throughout 30 localities widely dispersed on Lamu Island, there was a higher (although not statistically significant) prevalence of seropositivity in urban areas than within rural areas on the island (77% versus 52%, P = 0.075; Table 3).
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). Most of the 118 febrile seropositive cases (86%) reported being hospitalized or confined to bed at home for a mean of 7 days (median, 7 days), with a range of 1–90 days. Among symptomatic seropositives, 84% (97/115) reported missing work or school for a mean of 7 days (range, 1–90 days; median, 7 days).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We cannot be certain that all seropositives were infected during the course of this outbreak; however, the disproportionately high IgG, but not IgM, seropositivity of persons 15 years old compared with younger participants suggested that previous outbreaks of CHIKV or other alphaviruses (because antibodies to other alphaviruses may cross-react with CHIKV antibodies) or sporadic infections might have occurred on Lamu Island. However, our survey was conducted after the peak of the outbreak; thus, it is possible that IgM antibodies had already waned in many participants exposed during the outbreak peak. This hypothesis is supported by data showing higher levels of IgG than IgM in most of the participants with detectable antibodies (Figure 2
). In addition, it is possible that persons 15 years old were more likely to have behaviors that increased the risk of exposure to CHIKV-infected Aedes mosquitoes. It is possible that older residents may have been previously infected with CHIKV or other alphaviruses and boosted their IgG antibodies on exposure to CHIKV during this outbreak. At the moment, little is known about the lifeline of circulatory CHIKV specific antibodies, beyond what is generally known about the kinetics of IgG and IgM antibodies with other immune correlates.
Although persons with IgG or IgM antibodies to CHIKV (seropositives) were distributed throughout Lamu Island, infection seemed to occur more commonly in urban areas. As with dengue, CHIKV vector is most frequently transmitted by the Ae. aegypti mosquito,13–15 which tend to thrive in urban areas where more favorable conditions for breeding and transmission exist, including the presence of water storage containers, discarded water holding containers, and other debris in which stagnant water can accumulate.
Symptoms associated with CHIKV infection such as fever, joint pains, and myalgias are non-specific and could be mistakenly identified with a variety of other diseases including dengue, malaria, Rift Valley fever, and influenza. However, pronounced persistent severe joint pains that affect wrists, elbows, fingers, and knees in some patients should raise the suspicion of alphavirus infection, especially chikungunya disease or O’nyong nyong fever, which also occurred in epidemic form in East Africa in the late 1990s.4,7,23
Among those who were symptomatic, a large proportion (84%) reported absence from work or school because of the illness for prolonged periods. An epidemic of O’nyong-nyong virus infection in East Africa in 1959–1962 caused at least one quarter of employed adults to miss at least 5 days of work.24 Although not commonly associated with mortality, epidemics of Chikungunya disease present public health threats because of substantial morbidity, associated suffering, and loss of economic productivity.
Participants were asked about exposures, which likely occurred months earlier, so there may have been problems with recall, resulting in the loss of important information about illnesses. The refusal rate (29.4%) for blood collection could have resulted in an overestimation of the magnitude of infection, if persons who had been ill or lived in close proximity to people who were ill (and were more aware and concerned about the outbreak) were more likely to agree to participate.
In summary, Chikungunya virus infection during the outbreak on Lamu Island was widespread with a high attack rate, affecting an estimated 75% of the Lamu population. In addition to substantial illness-associated discomfort and suffering, the outbreak most likely had a significant economic impact.
Received September 2, 2006. Accepted for publication July 2, 2007.
Acknowledgments: We thank the people of Lamu Island for their participation in the study and the Lamu District Health Office for its tremendous support. We appreciate the many helpful suggestions that we received during the course of this study and manuscript review from Drs. Roy Campbell, Ned Hayes, Barry Miller, and Ann Powers and other staff within the Division of Vector-Borne Infectious Diseases, CDC, Ft. Collins, CO.
* Address correspondence to Robert F. Breiman, International Emerging Infections Program, CDC-KEMRI, Nairobi, Kenya. E-mail: RFB2{at}cdc.gov 
Authors’ addresses: Kibet Sergon, Field Epidemiology and Laboratory Training Program, Nairobi, Kenya. Charles Njugana and Rosalia Kalani, Disease Outbreak Management Unit, Ministry of Health, Nairobi, Kenya. Victor Ofula, Clayton Onyango, Limbaso S. Konongoi, and Rosemary Sang, Kenya Medical Research Institute, Nairobi, Kenya. Sheryl Bedno, US Army Medical Research Unit, Nairobi, Kenya. Athman M. Dumilla, Ministry of Health, Lamu, Kenya. Joseph Konde, Central Bureau of Statistics, Mombasa, Kenya. Heather Burke, M. Kariuki Njenga, and Robert F. Breiman, International Emerging Infections Program, Nairobi, Kenya, Telephone: 254-20-271-3008, x166, Fax: 254-20-271-4745, E-mail: RFB2{at}cdc.gov.
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REFERENCES |
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
