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Dengue yo-yo: Why virus blows alternately hot, cold in Delhi

While the number of patients has increased sharply, the symptoms, seemingly paradoxically, have become less virulent.

Written by Pritha Chatterjee |
September 1, 2015 1:08:38 am
dengue, dengue delhi, delhi dengue, dengue cases in Delhi, Delhi dengue cases, dengue deaths in delhi, dengue, mosquitoes, dengue-transmitting mosquitoes, Sex-switching mosquito gene, mosquito gene, mosquito, deadly dengue, delhi news, india news, indian express, #Explained In Delhi, there have been 778 cases in August alone, against only 11 last year; 831 cases since January 1, in comparison to 995 cases for the whole of 2014.

The dengue virus has been especially active in Delhi this year. The numbers indicate an outbreak that is likely to get worse as the season progresses — there have been 778 cases in August alone, against only 11 last year; 831 cases since January 1, in comparison to 995 cases for the whole of 2014.

And yet, doctors say, the symptoms have been very mild compared to earlier years — fever, bodyaches, nausea and, in a small number of cases, rashes. Very few patients have required admission to hospital. The fall in platelet count, leading to shock syndrome and haemorrhagic fever, danger signs that were worryingly common in the last two years, have hardly been seen this year.

So, while the number of patients has increased sharply, the symptoms, seemingly paradoxically, have become less virulent.

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Microbiologists who have studied dengue patterns in Delhi say the trends, both in numbers and symptoms, are along expected lines. The behaviour of the virus, medically known as its epidemiology, has changed with every outbreak. The dengue virus has four serotypes, depending on the proteins — called antigens — that make it. They are related, but have minor differences in DNA. Each type has its characteristic symptoms

Type 1, the classic dengue fever, and Type 3, which causes high grade fever without shock, are identified as relatively mild serotypes. The severe strains are Type 4, which leads to fever with shock, and Type 2, which causes thrombocytopenia or a drop in platelets, haemorrhagic fever, organ failure and Dengue Shock Syndrome (DSS).

Globally, Type 2 has been identified as the most common cause of Dengue Haemorrhagic Fever (DHF). In the capital, studies of serological types of dengue by AIIMS and the National Centre for Disease Control (NCDC) have shown the following broad trends.

In 1996, when the first recent outbreak was reported in Delhi with over 10,000 cases, the relatively severe Type 2 virus was identified as the most common type. The virus continued to circulate for next few years, but in significantly reduced numbers.

In 2003, when a sharp rise was reported again with over 2,000 cases, Type 3, a mild strain, emerged as the most common type. In the years that followed, all four types kept emerging, but in controlled numbers.

In 2005, when no spike in patient numbers was seen, isolates of Types 1 and 3 were found. Combined infections — where both strains were present — were seen as well. No dominant strain was seen, but interestingly, Types 2 and 4 were absent altogether.

In 2006, the numbers increased again. This time, Type 3 emerged as the most common type of dengue.

A fall in numbers followed until 2010 — the year that saw another major outbreak with over 6,000 cases. A new strain, Type 1, emerged as the prevalent strain. In the immediately following years, Type 1 remained common, but only a few patients were seen.

The next sharp spike came in 2013 — with over 5,500 cases and the stronger Type 2 as the most common strain. Last year saw fewer cases, but Type 2 continued to be dominant. This year, in keeping with the trend, doctors had been expecting both a spike in numbers and a change in the pattern of symptoms.

In the decades since 1960, Types 1 and 3 have been the most common in Delhi, the studies by AIIMS and NCDC show. Symptoms of every type has changed over time — which is an evolutionary process seen in all micro-organisms. Scientists say the virus probably circulated in areas around the capital for centuries in mild forms, but severe symptoms, which lead to fatal outcomes like haemorrhagic fever, emerged first only in 1988.

So, what does the wave-like pattern for dengue in Delhi mean?

Dr Lalit Dar, additional professor of microbiology at AIIMS, says that for viruses like dengue which have different antigenic types, a population develops immunity to a certain type every few years. “So, if a certain type of the virus circulates for a length of time, a significant portion of the population develops immunity to it. Slowly, fewer cases of that type are seen,” Dr Dar said.

The virus remains in circulation, but affects fewer people. During this time, other types of the virus emerge, but take some time to grow and make their presence felt. After 2-3 years of this trend, a new type becomes stronger, usually helped by environmental factors like intermittent rains.

“Since the population has not been exposed to this new type of the virus, it is able to affect more people, and so the number of patients shoots up, and an outbreak is perceived,” said Dr Dar, who has published several studies on trends of dengue in Delhi.

Effectively then, every spike in dengue usually indicates a change in the dominant serotype — which also means that the symptoms of the disease see a significant shift. While confirmatory tests are still awaited, microbiologists say that the huge rise from 11 to 778 cases in August this year indicates a change in the prevalent strain. And that after this spike, the next few years will likely see similar symptoms but gradually falling numbers of patients, and the slow emergence of new types. Which will inevitably lead to another spike in the coming seasons, with a new type of the virus becoming dominant — and the whole cycle will be repeated.

For viruses that start circulating in an area over time — scientifically known as becoming endemic to a region; such as dengue in Delhi — such cycles are always seen. A similar pattern is observed with influenza as well.

Being endemic to Delhi means the dengue virus circulates in the capital all year round — with the number of cases increasing after the monsoon, in the period between peak summer and winter. When it rains more frequently, or if it starts raining earlier than usual, like it happened this year, the number of cases goes up, irrespective of the dominant virus type, because weather conditions are conducive for the virus to grow.

“The numbers and symptoms are a complex interplay of types of the virus and climatic conditions. But looking at the significantly sharp rise in cases this year, weather alone does not seem to be a factor, the dominant serotype has probably changed,” Dr Dar said.

pritha.chatterjee@expressindia.com

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