A model for a chikungunya outbreak in a rural Cambodian setting: implications for disease control in uninfected areas.

Following almost 30 years of relative silence, chikungunya fever reemerged in Kenya in 2004. It subsequently spread to the islands of the Indian Ocean, reaching Southeast Asia in 2006. The virus was first detected in Cambodia in 2011 and a large outbreak occurred in the village of Trapeang Roka Kampong Speu Province in March 2012, in which 44% of the villagers had a recent infection biologically confirmed. The epidemic curve was constructed from the number of biologically-confirmed CHIKV cases per day determined from the date of fever onset, which was self-reported during a data collection campaign conducted in the village after the outbreak. All individuals participating in the campaign had infections confirmed by laboratory analysis, allowing for the identification of asymptomatic cases and those with an unreported date of fever onset. We develop a stochastic model explicitly including such cases, all of whom do not appear on the epidemic curve. We estimate the basic reproduction number of the outbreak to be 6.46 (95% C.I. [6.24, 6.78]). We show that this estimate is particularly sensitive to changes in the biting rate and mosquito longevity. Our model also indicates that the infection was more widespread within the population on the reported epidemic start date. We show that the exclusion of asymptomatic cases and cases with undocumented onset dates can lead to an underestimation of the reproduction number which, in turn, could negatively impact control strategies implemented by public health authorities. We highlight the need for properly documenting newly emerging pathogens in immunologically naive populations and the importance of identifying the route of disease introduction.

Complex dynamic of dengue virus serotypes 2 and 3 in Cambodia following series of climate disasters.

The Dengue National Control Program was established in Cambodia in 2000 and has reported between 10,000 and 40,000 dengue cases per year with a case fatality rate ranging from 0.7 to 1.7. In this study 39 DENV-2 and 57 DENV-3 viruses isolated from patients between 2000 and 2008 were fully sequenced. Five DENV2 and four DENV3 distinct lineages with different dynamics were identified. Each lineage was characterized by the presence of specific mutations with no evidence of recombination. In both DENV-2 and DENV-3 the lineages present prior to 2003 were replaced after that date by unrelated lineages. After 2003, DENV-2 lineages D2-3 and D2-4 cocirculated until 2007 when they were almost completely replaced by a lineage D2-5 which emerged from D2-3 Conversely, all DENV-3 lineages remained, diversified and cocirculated with novel lineages emerging. Years 2006 and 2007 were marked by a high prevalence of DENV-3 and 2007 with a large dengue outbreak and a high proportion of patients with severe disease. Selective sweeps in DENV-1 and DENV-2 were linked to immunological escape to a predominately DENV-3-driven immunological response. The complex dynamic of dengue in Cambodia in the last ten years has been associated with a combination of stochastic climatic events, cocirculation, coevolution, adaptation to different vector populations, and with the human population immunological landscape.

Reemergence of Chikungunya virus in Cambodia.

Chikungunya virus (CHIKV), probably Asian genotype, was first detected in Cambodia in 1961. Despite no evidence of acute or recent CHIKV infections since 2000, real-time reverse transcription PCR of serum collected in 2011 detected CHIKV, East Central South African genotype. Spatiotemporal patterns and phylogenetic clustering indicate that the virus probably originated in Thailand.

Field evaluation and impact on clinical management of a rapid diagnostic kit that detects dengue NS1, IgM and IgG.

BACKGROUND: Dengue diagnosis is complex and until recently only specialized laboratories were able to definitively confirm dengue infection. Rapid tests are now available commercially making biological diagnosis possible in the field. The aim of this study was to evaluate a combined dengue rapid test for the detection of NS1 and IgM/IgG antibodies. The evaluation was made prospectively in the field conditions and included the study of the impact of its use as a point-of-care test for case management as well as retrospectively against a panel of well-characterized samples in a reference laboratory. METHODOLOGY/PRINCIPAL FINDINGS: During the prospective study, 157 patients hospitalized for a suspicion of dengue were enrolled. In the hospital laboratories, the overall sensitivity, specificity, PPV and NPV of the NS1/IgM/IgG combination tests were 85.7%, 83.9%, 95.6% and 59.1% respectively, whereas they were 94,4%, 90.0%, 97.5% and 77.1% respectively in the national reference laboratory at Institut Pasteur in Cambodia. These results demonstrate that optimal performances require adequate training and quality assurance. The retrospective study showed that the sensitivity of the combined kit did not vary significantly between the serotypes and was not affected by the immune status or by the interval of time between onset of fever and sample collection. The analysis of the medical records indicates that the physicians did not take into consideration the results obtained with the rapid test including for care management and use of antibiotic therapy. CONCLUSIONS: In the context of our prospective field study, we demonstrated that if the SD Bioline Dengue Duo kit is correctly used, a positive result highly suggests a dengue case but a negative result doesn't rule out a dengue infection. Nevertheless, Cambodian pediatricians in their daily practice relied on their clinical diagnosis and thus the false negative results obtained did not directly impact on the clinical management.

Dengue incidence in urban and rural Cambodia: results from population-based active fever surveillance, 2006-2008.

BACKGROUND: Dengue vaccines are now in late-stage development, and evaluation and robust estimates of dengue disease burden are needed to facilitate further development and introduction. In Cambodia, the national dengue case-definition only allows reporting of children less than 16 years of age, and little is known about dengue burden in rural areas and among older persons. To estimate the true burden of dengue in the largest province of Cambodia, Kampong Cham, we conducted community-based active dengue fever surveillance among the 0-to-19-year age group in rural villages and urban areas during 2006-2008. METHODS AND FINDINGS: Active surveillance for febrile illness was conducted in 32 villages and 10 urban areas by mothers trained to use digital thermometers combined with weekly home visits to identify persons with fever. An investigation team visited families with febrile persons to obtain informed consent for participation in the follow-up study, which included collection of personal data and blood specimens. Dengue-related febrile illness was defined using molecular and serological testing of paired acute and convalescent blood samples. Over the three years of surveillance, 6,121 fever episodes were identified with 736 laboratory-confirmed dengue virus (DENV) infections for incidences of 13.4-57.8/1,000 person-seasons. Average incidence was highest among children less than 7 years of age (41.1/1,000 person-seasons) and lowest among the 16-to-19-year age group (11.3/1,000 person-seasons). The distribution of dengue was highly focal, with incidence rates in villages and urban areas ranging from 1.5-211.5/1,000 person-seasons (median 36.5). During a DENV-3 outbreak in 2007, rural areas were affected more than urban areas (incidence 71 vs. 17/1,000 person-seasons, p<0.001). CONCLUSION: The large-scale active surveillance study for dengue fever in Cambodia found a higher disease incidence than reported to the national surveillance system, particularly in preschool children and that disease incidence was high in both rural and urban areas. It also confirmed the previously observed focal nature of dengue virus transmission.

National dengue surveillance in Cambodia 1980-2008: epidemiological and virological trends and the impact of vector control.

OBJECTIVE: Dengue has been reportable in Cambodia since 1980. Virological surveillance began in 2000 and sentinel surveillance was established at six hospitals in 2001. Currently, national surveillance comprises passive and active data collection and reporting on hospitalized children aged 0-15 years. This report summarizes surveillance data collected since 1980. METHODS: Crude data for 1980-2001 are presented, while data from 2002-2008 are used to describe disease trends and the effect of vector control interventions. Trends in dengue incidence were analysed using the Prais-Winsten generalized linear regression model for time series. FINDINGS: During 1980-2001, epidemics occurred in cycles of 3-4 years, with the cycles subsequently becoming less prominent. For 2002-2008 data, linear regression analysis detected no significant trend in the annual reported age-adjusted incidence of dengue (incidence range: 0.7-3.0 per 1000 population). The incidence declined in 2.7% of the 185 districts studied, was unchanged in 86.2% and increased in 9.6%. The age-specific incidence was highest in infants aged < 1 year and children aged 4-6 years. The incidence was higher during rainy seasons. All four dengue virus (DENV) serotypes were permanently in circulation, though the predominant serotype has alternated between DENV-3 and DENV-2 since 2000. Although larvicide has been distributed in 94 districts since 2002, logistic regression analysis showed no association between the intervention and dengue incidence. CONCLUSION: The dengue burden remained high among young children in Cambodia, which reflects intense transmission. The national vector control programme appeared to have little impact on disease incidence.

Cost of dengue and other febrile illnesses to households in rural Cambodia: a prospective community-based case-control study.

BACKGROUND: The average annual reported dengue incidence in Cambodia is 3.3/1,000 among children < 15 years of age (2002-2007). To estimate the economic burden of dengue, accurate cost-of-illness data are essential. We conducted a prospective, community-based, matched case-control study to assess the cost and impact of an episode of dengue fever and other febrile illness on households in rural Cambodia. METHODS: In 2006, active fever surveillance was conducted among a cohort of 6,694 children aged < or = 15 years in 16 villages in Kampong Cham province, Cambodia. Subsequently, a case-control study was performed by individually assigning one non-dengue febrile control from the cohort to each laboratory-confirmed dengue case. Parents of cases and controls were interviewed using a standardized questionnaire to determine household-level, illness-related expenditures for medical and non-medical costs, and estimated income loss (see Additional file 1). The household socio-economic status was determined and its possible association with health seeking behaviour and the ability to pay for the costs of a febrile illness. RESULTS: Between September and November 2006, a total of 60 household heads were interviewed: 30 with dengue-positive and 30 with dengue-negative febrile children. Mean total dengue-related costs did not differ from those of other febrile illnesses (31.5 vs. 27.2 US dollars, p = 0.44). Hospitalization almost tripled the costs of dengue (from 14.3 to 40.1 US dollars) and doubled the costs of other febrile illnesses (from 17.0 to 36.2 US dollars). To finance the cost of a febrile illness, 67% of households incurred an average debt of 23.5 US dollars and higher debt was associated with hospitalization compared to outpatient treatment (23.1 US dollars vs. 4.5 US dollars, p < 0.001). These costs compared to an average one-week expenditure on food of 9.5 US dollars per household (range 2.5-21.3). In multivariate analysis, higher socio-economic status (odds ratio [OR] 4.4; 95% confidence interval [CI] 1.4-13.2), duration of fever (OR 2.1; 95%CI 1.3-3.5), and age (OR 0.8; 95%CI 0.7-0.9) were independently associated with hospitalization. CONCLUSION: In Cambodia, dengue and other febrile illnesses pose a financial burden to households. A possible reason for a lower rate of hospitalization among children from poor households could be the burden of higher illness-related costs and debts.

Improved molecular detection of dengue virus serotype 1 variants.

The dengue virus molecular typing method described by Lanciotti and coworkers (R. S. Lanciotti, C. H. Calisher, D. J. Gubler, G. J. Chang, and A. Vance-Vorndam, J. Clin. Microbiol. 30:545-551, 1992) is used worldwide for diagnosis and surveillance. However, it failed to detect DENV-1 variants in Cambodia due to a point mutation. We describe an improvement of the method that allows the detection of additional DENV-1 strains, including potential variants.