April 07, 2020
Camille Joyce O. Cruzada, MD
Dengue is an endemic disease in the Philippines and a challenge to public health1-4. It was prioritized as the most important mosquito-borne viral disease in the world last 2012 but remained to be the largest arboviral threat to humans17,33 and the world’s fastest growing vector disease12. Despite local efforts, it continued to affect an increasing number of individuals yearly with an incidence rate of 22.4 per 100,000 population and an average annual percentage change of 24.4% in the past 3 decades5-10.
Despite global movements, dengue continued to have an upward trend with an estimated 30-fold increase in incidence within the last 50 years, involving territories previously unaffected by the disease4. With urbanization, environmental destruction, travel and climate change further propagating the expansion and disrupting the usual pattern of the disease1,13,33, it is high time that tools for early detection, disease monitoring, and vector control be developed.
Reflection: The Unclear Status of Dengue Worldwide
Dengue is one of the 17 neglected tropical diseases identified by the World Health Organization (WHO) 4,12. It shares the category with Guinea Worm disease and leishmaniasis, to name a few, which are exclusive to poor communities and are endemic to the tropics and subtropics. These diseases also lack public health attention, receive poor research funding, accomplish little progress in research and development, and have high morbidity, low mortality and no specific treatment in comparison to other infectious diseases12.
Unlike other tropical diseases, dengue has spread to developed countries such as France, Croatia, Portugal and the United States14. It also drew more support with the commemoration of June 15 as ASEAN Dengue Day and the birth of international campaigns such as Break Dengue for worldwide awareness15.
Dengue even became the 4th most funded infectious disease following HIV/AIDS, tuberculosis and malaria in 2012 but the gaps in research and increased private interest in vaccine development decreased global funding and kept dengue in this category12,16. The tunnel vision on vaccines has left little insight on the more pressing issues of vector control, diagnosis and most importantly, disease surveillance9, 12.
Disease surveillance is a long-standing problem of dengue control and prevention1,4,10,18. Without a standard protocol, only as little as 13.2% of actual cases are reported in Southeast Asia, where the disease is endemic26. Records of cases come mostly from the symptomatic, younger population seeking consult in public institutions, some records of cases are lost to misdiagnosis and preference for home care4,18,28-31.
Reports lose quality to lack of laboratory confirmation and inconsistent classification2,4,18,29. Aware of the discrepancy, several countries have created prediction models to correct their records, adding as much as 1-65 cases per documented case depending on disease severity2,10,18-27. Available data was also adjusted based on geographic distribution, revealing the burden of disease to be thrice than what WHO reported17.
Though mechanisms can be used to approximate data, these will yield problematic results without uniformed data from active and passive surveillance32. Unless dengue can be properly identified, reported, clinically diagnosed and laboratory confirmed, the true burden of disease cannot be determined.
Vision: The Roadmap to Prevention
Current local clinical practice guidelines rely on signs and symptoms for the diagnosis and immediate management of dengue34 but the role of laboratory confirmation remains crucial4,34-36. Other than to clinch the definitive diagnosis of a disease with diverse clinical presentation to provide properly timed intervention, it is essential for baseline epidemiological studies, existing policy evaluation, and innovative measures for prevention.
There is an array of diagnostic tests available for detecting dengue but none can detecting dengue in various stages of the infection35, comes the search for an easy, rapid test where sensitivity and specificity are not compromised36.
Peeling et al (2010) identified the ideal dengue diagnostic to be rapid, highly sensitive, and serotype specific; with low diagnostic threshold, high potential for real-time surveillance and most importantly, the ability to distinguish dengue from flavivirus infection with similar presentation.
Other things to consider are the cost, ease of use, portability, storage and stability in the field to be applicable in endemic rural communities. Unfortunately, commonly used tools do not fit these perfectly33,37-39. Viral isolation, though capable of definitive diagnosis, fails to deliver results promptly.
Reverse Transcription Polymerase Chain Reaction (RT-PCR), a senstive and specific test, has decreased usability with the operation of specialized equipment, need for thermal cycling protocol training and lack of center-to-center standardization. Nonstructural protein 1 (NS1) antigen test, despite providing rapid results, suffers from inconsistent sensitivity, with decreased sensitivity observed in secondary infections and those caused by DENV-4.
Antibody serology may be able to differentiate primary from secondary infections, however, it violates the most important criteria for an ideal test by yielding false-positive results in the presence of other flavivirus.
Fortunately, alternatives are proving to be closer to ideal. Immune cross-reactivity and decreased antigenemia during secondary infection were shown to be bypassed by antigen-antibody combination kits35,40-42. Not only do these have higher sensitivity and specificity than the individual tests, they also have detection rates comparable to RT-PCR.
Similarly, most diagnostic weaknesses are addressed by the isothermal cycling detection methods— transcription-mediated amplification (TMA), nucleic acid sequence-based amplification (NASBA), reverse transcription loop-mediated isothermal amplification (RT-LAMP) and reverse transcription recombinase polymerase amplification (RT-RPA)43-48.
The close to 100% sensitivity allows RT-LAMP and RT-RPA to turn positive with as little as 100-1000 RNA copies present and enables TMA to detect the disease in even in RT-PCR negative samples. Their high specificity further allows the former to differentiate flaviviruses accurately and prevent cross-reactivity.
The rapid result production of RT-LAMP and RT-RPA in the absence of thermocycling and easy visualization through immunofluorescence use improves disease surveillance and increases their potential as point-of-care diagnostics. Complete evaluation and validation are the only barriers left for their application in current programs 33,41.
In the Philippines, RT-PCR is the confirmatory test of choice19. Though expensive, it is preferred over the rapid diagnostic kits, which miss the disease 65-68% of the time49. Methods involving sandwich enzyme-linked immunosorbent assay (ELISA) and fluorogenic real time RT-PCR were then explored, and revealed the potential of ELISA as basis for laboratory confirmation50-52.
The continued drive to improve diagnostics pushed for nucleic acid amplification test (NAAT)-LAMP technology to be integrated in the national dengue control and prevention program and gave birth to an affordable novel local diagnostic, Biotek-M™Dengue aqua Kit, within the last 2 years53,54. With these, a cost-effective, accurate, and efficient approach for diagnosis is no longer out of reach.
The road to prevention is still long and winding but we must keep treading the detours to cover the lapses we have neglected for so long. Though unsettling, let us continue to implement, assess, and modify existing vector control procedures and be open to new approaches like vaccination to strengthen our defense against dengue4.
Although tedious, let us go beyond disease detection and fuel the budding research on biomarkers for disease severity and noninvasive monitoring to improve outcomes from the infection33. Moreover, let us develop stricter evaluation and regulation schemes for diagnostics to protect developing and endemic areas from advances of private interests 17. When all these are put in place, it is not too late to uncover dengue’s true burden of disease.
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