Preventing the dengue threat

By Dr Debapriya Mukherjee-December 9, 2018, 8:18 pm

Former Senior Scientist Central Pollution Control Board, India

Mosquitoes are the world's deadliest creatures, killing thousands of people and causing various illnesses each year. Among them, dengue is one of those insidious infections that cause few or no symptoms in the majority of cases -- and can therefore spread silently among populations and cause crucial public health problems with an enormous burden to the health system.

Globally, the severity of incidence has increased 30-fold in the past 50 years. A study done on global distribution and burden of dengue by Oxford university estimates that India has the highest number of dengue cases with about 33 million apparent cases and 100 million asymptomatic cases occurring annually. National Vector Borne Disease Control Program (NVBDCP) clearly shows an increasing number of reported cases as well as death due to dengue in India over the last 5 years. At present, dengue is endemic throughout India including all states and union territories.

According to the WHO (World Health Organization), 40% of the world population is at risk of contracting dengue leading to a great impact on the global economy. Cases of the mosquito-borne disease generally peak in October after the monsoon season because rainfall creates favorable breeding conditions for the Aides aegyptimosquito, the primary vector of the disease. India is a tropical country with high temperatures, abundant rainfall patterns and significant humidity with variations between dry and wet weather, which are favorable to the proliferation of dengue vectors.

Dengue is caused by virus belonging to family ﬂaviviridae and having four serotypes numbered 1 to 4. A ﬁfth serotype (DENV-5) was discovered in 2013 but no case has been reported from India. In India, the ﬁrst case was reported in 1946 followed by several outbreaks in different parts of the country which involved all four serotypes. Dengue epidemiology is always together with vector ecology with distinct pattern of transmission generally depending on season (within a year) and cycle (with outbreaks in certain years), interactions between the climate (rainfall and temperature), the mosquito, the circulating virus, and population immunity.

The primary vector for dengue, Aedesaegypti, is highly adapted to the urban environment and avidly attracted to human blood, entering homes and breeding in small collections of water, such as those found in discarded plastic cups and bottles, ﬂower pots, drains, used tyres and any type of container that stores water. Behaviors of dengue viral infection is quite critical not like other viral infections such as measles. If any person is infected with measles, that person develops lifelong immunity, but dengue is different.

If any person is infected by one of the dengue viruses, that person becomes immune to that virus but not to the other three. The person who is infected for a second time by another form of the dengue virus is more likely to develop a severe form of the disease. When individuals who have been infected once are infected for a second time by another form of the dengue virus, their antibody levels increase, but not enough to protect them, it is as if the antibodies clung to the virus but were unable to neutralise it.

These individualsare at high risk of developing hemorrhagic dengue. Fever cases with suspicion of dengue are routinely conﬁrmed by testing for IgM antibodies or NS1 antigen using ELISA technique with NS1 showing bettersensitivity and speciﬁcity as comparedto IgM ELISA for early detection of dengue/ According to World Report, India’s poor surveillancenetwork possibly leads to huge under-reporting of cases—one study estimates thatdengue could be 282 times higher than oﬃcially reported. Under reporting of cases also seemsto be politically manipulated to forge effectiveness of control programs. This typeof practices and lack of wide-spread effective sentinel surveillance, theproblem cannot be fully evaluated and controlled. India’s response to theseacute a public health emergency point to a health-care system is chronicallyunderfunded, unregulated, and has inadequate infrastructure.

Successivegovernments have promised health reform but at the same time funding for many health ministry programs isnot adequate. Despite some gains in health care, India only spends little over1% of its gross domestic product on health, which is among the lowest in theworld.

Recent years have also witnessed increased awarenessand reporting of dengue cases. In 2011, a Lancet Series called for the implementation of a universalIndian health service. The recent debate arguing for greater dependence on theprivate sector for health care is deeply troubling and reﬂects an ideologicalsplit within the government. Differences in laboratory diagnostic methods and test results have further aggravated the problem of under reporting in India. Many laboratories in India do not have stringent quality control regulations to ensure the reliability of these tests as done in the WHO-approved laboratories under the global network of dengue laboratories program. Thereby all laboratories should strictly follow the CDC (Centre for Disease Control and Prevention) testing algorithm for diagnosis and reporting of dengue cases in India at specialized referral laboratories.

For addressing these problems, the Indian government should strictly follow the recommendations of dengue surveillance experts. The recommendations are: (i) reporting of dengue cases to the government should be made mandatory in all dengue endemic countries; (ii) electronic reporting systems should be developed and used at all areas; (iii)the government dengue surveillance data should include age-stratiﬁed data of incidence, hospitalization rates and deaths; (iv) additional system sensitivity checking studies should be performed;

(v) diagnostic laboratories should share expertise and data; (vi) dengue antigen tests should be used in patients with Fever for four days or less, whereas antibody tests should be used after day 4 to diagnose dengue; and (vii) the national surveillance systems should aim for early detection and prediction of dengue outbreaks.

Part of the purpose of a surveillance system is to indicate how the situation in one year compares with that in other years. Therefore, this goal requires consistency and stability in the system. In addition to these recommendations, the most important step for control of any arboviral infection is the control of its vector for preventing the disease.

Though inspection protocols have been defined and placed in different areas in India but the re-emergence of the Aedes mosquito has raised concern mainly due to a history of incursion and the extreme survival capacity of the Aedes eggs. Still emergent need is to improve disease surveillance at dengue risk areas that may prevent the growing threat of dengue infection spreading across the country.

The number of dengue incidences and their correlation to rainfall data have been critically evaluated to support and guide health policies of the decision makers. Though climatic factors has been identified as potential risk factors linked to dengue outbreaks but social factors, interstate and overseas travelers, rainwater tanks and economic status are also responsible for dengue outbreak. It is also shown that continuous entomological surveillance during on-transmission period and appropriate intervention can decrease vector density and subsequent dengue cases in transmission season. Ant larval like temephos, fenthion, malathion is increasingly used but resistance has developed in many parts of India which is a cause of major concern . Newer and safer alternatives are being investigated which include pheromone 21 and calotropis procera. In this context it is pertinent to mention that the mosquitoes first attain optimal body size and nutritional status to become active for reproduction and effective disease carriers. Recently researchers,have succeeded in using CRISPR-Cas9, a powerful tool for altering DNA sequencesand modifying gene function, to decrease mosquito body size to eliminatingmosquitoes that carry dengue fever and Zika virus. The researchers succeeded inpostponing mosquito development, shortening the animal's lifespan, retarding egg development, and diminishing fat accumulation. Further research is required to answer one important question how CRISPR-Cas9 gene modification could be introduced into the wild mosquito population. For preventing the dengue threat, Japan has adopted a pragmatic approach to ensure that participants and spectators of international sporting events during the Olympic and Paralympic games in 2020 are not at risk of acquiring imported and locally endemic infectious diseases. They have realized considering the nature of infectious disease that it is virtually impossible to prevent any pathogen from entering a country just by enhancing border control during that period. This approach is to have a preparedness plan so that health professionals along with others know how to recognize symptoms and how to respond. In this context, failure mode and effects analysis (FMEA) is being implemented to test the vulnerability and resiliency of the current preparedness plans and to strengthen these current plans in order to prevent the failures. FMEA is a procedure for the analysis of potential failure modes within a system in order to classify and quantify risks by their occurrence, severity and detection controls already in place. This is high time to realize that the health of humans is connected to the health of animals and the environment. Due to exploitation of nature, many factors have changed the interactions among humans, animals, and the environment, which have caused the emergence and reemergence of many diseases, including dengue. Successful public health interventions for disease control require the cooperation of the human health, veterinary health, and environmental health communities.