Monday, October 05, 2015

Outbreak Watch: Dengue in India


The current local news on "Dengue" in India is gaining fast pace. With Delhi on spotlight, dengue cases have become one of the most talked about topic, and has left most people searching for information. Sadly, the news and material is quite scattered and there is no clear picture. So let me put the current picture in perspective.

I have earlier written about dengue in my earlier posts (Link). Here is a quick summary. Dengue is a mosquito-borne infection caused by a flavivirus member. The icosahedral structured virus contains a genome of single positive-stranded RNA, about 11 Kbp in length coding for 3 structural (capsid protein C, membrane protein M, envelope protein E) and 7 non structural proteins (NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5). The vectors commonly known to be involved in transmission are Aedes aegypti and Aedes albopictus. They are reportedly prevalent in the north-east region of India. The virus has four closely related types DEN-1 to DEN-4.

Fig 1: Dengue Cases reported in India (2009- 2014).
The epidemiological data on number reported cases of dengue in India is documented, updated and published by NVBDCP. It has been stated that the dengue surveillance is not good in the country and the number of cases reported is far less than actually is the case. Yet from the data, I made a graph, to give an idea on the number of cases. It is evident that in recent history, 2013 represents the worst with a total of 75808 cases. However, the number of deaths was proportionately less. Relying on the same data, I have  plotted a second graph to get you the idea of deaths.

Fig 2: Dengue deaths reported in India (2009- 2014)
So, why all this sudden media attention? As per government's Press Information Bureau, “It was noted that at national level, there has been decrease in the number of dengue cases detected with 75 808 cases in 2013, 40 571 cases in 2014, and nearly 21 000 cases in 2015 up to second week of September and the recovery has been in more than 99.9% of cases”. As of on 5th Oct, 2015 Delhi alone has recorded 6500 confirmed cases. In nearly 15 days, the total number of cases has nearly doubled. The number of case was 3791 as on 20th Sept, 2015. That's why it is a news.

Fig 3: Reported dengue cases in India,
top 10 states shown. Data as of on
20, Sept 2015, for year 2015.
This prompted me to look into the statistics, once again. Is Delhi the only affected? I don't have latest data. So I will once again rely on the NVBDCP, statistics, published as of on 20th Sept, 2015. I selected the top 10 states reporting Dengue cases. Of course, by the time I'm posting this the data is almost obsolete, since the numbers have drastically changed. Bu the following graph will still throw you the idea. It has been speculated that the sudden spurge in number of cases is possibly due to monsoons and increased mosquito breeding. According to the report inefficiency in removing stagnant water from premises and in undertaking fumigation to control mosquitoes, and the emergence of a rare strain (type 4) of the dengue virus, everything contributes.Vector control programs have thus been intensified to control the problem.

Diagnosis is usually through commercially available ELISA kits, testing for IgM antibodies. PCR is the best option but expensive. Currently, a lot of reports is based on rapid diagnostic tests, with good probability of false positive reactions. The government is considering banning such tests so as to get a better diagnostic report.

Dengue Facts
A second issue of concern that has been widely circulating in the news is the factor of under-reporting. Let me clarify the issue. Dengue is not clinically evident in a proportion of population and hence that percentage is lost in reporting. A study by INCLEN Study Group had made a claim that annual number of dengue fever cases in India could be 282-times higher than the number. There are some serious limitation in the original study. First, the data was generalized for the country based on data from Madurai, only on tested dengue cases. Second there was a inability to collect a proportion of data due to lack of documentation.

In case you are still curious, the global statistics are not that great. The latest estimates is about 390 million dengue infections per year of which 96 million (about 24.6%) manifest clinically. Also various reports have indicated that the numbers reported are increasing in several places. However, it is not entirely clear if it is because of increased number of cases or there is simply better reporting.

On a final note, as of yet this is not the worst case scenario as reported by most of the media. The cases is still way below what it was in 2013. But I agree that we need better surveillance and preventive measures.
Bagcchi, S. (2015). Dengue surveillance poor in India The Lancet, 386 (10000) DOI: 10.1016/S0140-6736(15)00315-3

Bhatt S, Gething PW, Brady OJ, Messina JP, Farlow AW, Moyes CL, Drake JM, Brownstein JS, Hoen AG, Sankoh O, Myers MF, George DB, Jaenisch T, Wint GR, Simmons CP, Scott TW, Farrar JJ, & Hay SI (2013). The global distribution and burden of dengue. Nature, 496 (7446), 504-7 PMID: 23563266

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