The dengue menace

Many states particularly Uttarakhand, Telangana, Delhi, Tamil Nadu, Maharashtra and Gujarat in this season are witnessing the worst dengue outbreak in recent times. Mosquitoes kill thousands of people and cause various diseases each year. Among them, Dengue is one of those insidious infections that show little to no symptoms in the majority of cases and can, therefore, spread silently among populations causing crucial public health problems in India 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 five years. At present, dengue is endemic throughout India including all states and Union Territories. According to the WHO, 40 per cent of the world population is at risk of contracting dengue leading to a great impact on the global economy.

Primary Vector

Dengue is caused by a 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 of this type from India. In India, the first case of Dengue 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 a distinct pattern of transmission generally depending on the 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, Aedes aegypti (the mosquito) is highly adapted to the urban environment and avidly attracted to human blood. It enters homes and breeds 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.

Proliferation of dengue vector

Cases of the mosquito-borne disease generally peak in October after the monsoon season because rainfall creates favourable breeding conditions for the Aedes aegypti mosquito. India is a tropical country with high temperatures, abundant rainfall patterns and significant humidity with variations between dry and wet weather, all of which collectively is favourable to the proliferation of dengue vectors. The influence of weather variables on the magnitude of dengue distribution involving alterations in infectivity and survival rates of vectors in the incubation period of dengue virus and in the mosquito life-cycle development.

Nature of dengue viral infection

Behaviours of dengue viral infection are quite unlike 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 cling to the virus but are unable to neutralise it. These individuals are at high risk of developing hemorrhagic dengue.

Barriers to control Dengue

According to the World Report, India's poor surveillance network possibly leads to a severe case of outbreaks being unreported. One study estimates that dengue could be 282 times higher than official reports state. Under-reporting of cases also seems to be politically manipulated to forge the effectiveness of control programs. These type of practices and lack of wide-spread effective sentinel surveillance means that the problem cannot be fully evaluated and controlled. India's response to this acute public health emergency is chronically underfunded, unregulated, and has inadequate infrastructure. Successive governments have promised health reform but at the same time funding for many health ministry programs is not adequate. Despite some gains in health care, India only spends a little over one per cent of its GDP on health, which is among the lowest in the world. Recent years have also witnessed increased awareness and reporting of dengue cases. In 2011, a Lancet Series called for the implementation of a universal Indian health service. The recent debate arguing for greater dependence on the private sector for health care is deeply troubling and reflects an ideological split 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 specialised referral laboratories.

Preventive measures

For addressing these problems, the Indian government should strictly follow the recommendations of dengue surveillance experts.

To begin with, reporting of dengue cases to the government should be made mandatory in all dengue-endemic countries. Second, electronic reporting systems should be developed and used in all areas. Third, the government dengue surveillance data should include age-stratified data of incidence, hospitalisation rates and deaths. Fourth, additional system sensitivity checking studies should be performed. Fifth, diagnostic laboratories should share expertise and data. Sixth, dengue antigen tests should be used in patients with fever for 4 days or less, whereas antibody tests should be used after day 4 to diagnose dengue. Finally, 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, 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.

Another 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 has been critically evaluated to support and guide the health policies of the decision-makers. Though climatic factors have been identified as potential risk factors linked to dengue outbreaks but social factors, interstate and overseas travellers, rainwater tanks and economic status are also responsible for dengue outbreak. It is also shown that continuous entomological surveillance during the non-transmission period and appropriate intervention can decrease vector density and subsequent dengue cases in transmission season. Antilarval medication like temephos, fenthion and malathion are increasingly used but resistance to the drugs 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 sequences and modifying gene function, to decrease mosquito body size in order to eliminate mosquitoes that carry dengue fever and Zika virus. The researchers succeeded in postponing 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.

Preventive measures framed by Japan

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 realised that considering the nature of the infectious disease, it is virtually impossible to prevent any pathogen from entering a country just by enhancing border control during that period. Their approach is to have a preparedness plan so that health professionals along with others know how to recognise 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 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.

Interrelation between human health and environment

It is about time to realise 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. Successful public health interventions for disease control require the cooperation of the human health, veterinary health and environmental health communities.

(The author is a former Senior Scientist, Central Pollution Control Board. The views expressed are strictly personal)