Why Did Chikungunya Come Back with a Vengeance?

Kaushik Bharati, PhD

What is Chikungunya?

Chikungunya (CHIK) is a viral disease that is caused by the chikungunya virus (CHIKV), which is an arthropod-borne virus (Arbovirus) spread by the bite of Aedes mosquitoes. Although Aedes aegypti is the most common vector, Aedes albopictus has also been identified as a vector for the transmission of CHIKV. The virus belongs to the genus Alphavirus in the family Togaviridae. It is a small (60-70 nm diameter), spherical, enveloped, single-stranded, positive-sense RNA virus. The viral genome is approximately 11,000 nucleotides long and encodes two polyproteins – the non-structural and structural polyproteins consisting of 4 and 5 proteins respectively. The non-structural proteins take care of internal biochemical functions of the virus, while the structural proteins are responsible for imparting structural integrity to the virus, as exhibited by the envelope and capsid proteins, and responsible for such properties as infectivity, virulence, and tropism (affinity for particular tissues or organs).

What are the symptoms of CHIK?

The most common symptoms of CHIK infection are fever, joint pain, and rash, although other symptoms like severe headaches, joint swelling, and muscle pain can also occur. The symptoms generally begin within a week of the bite of an infected mosquito. Although death is generally not reported for CHIK disease, the symptoms are often very severe and debilitating. Although most patients recover within a couple of weeks, the joint pain can last for several months or even up to 2 years! The most susceptible target groups include the very young (newborn babies), the old (generally >65 years of age), and people suffering from co-morbidities such as diabetes, heart disease, hypertension and Acquired Immune Deficiency Syndrome (AIDS).

How is CHIK infection diagnosed?

CHIK infections can be clinically diagnosed in the context of an outbreak situation, where the triad of symptoms i.e. fever, joint pain, and rash are the major presentations in a patient. However, definitive diagnosis can be made only based on lab confirmation, since the symptoms can overlap with that for dengue. Laboratory diagnosis is generally accomplished by testing serum samples for detection of virus, viral RNA, or virus-specific antibodies (IgM) and neutralizing antibodies. The specific tests are indicated below:

• Virus isolation: This is done by growing the virus (which could presumably be present in the serum of CHIK patients) in cell culture. This is regarded as the “Gold Standard”, but requires a Biosafety Level III (BSL-III) Lab Facility, which is found only in high-profile research institutes. Therefore, virus isolation is usually not carried out in routine clinical lab testing.
• Detection of viral RNA: This is done by employing the Polymerase Chain Reaction (PCR), which amplifies the viral RNA present in the serum of infected CHIK patients. This is also a high-end test, requiring elaborate lab facilities. Therefore, this test is only carried out in tertiary healthcare settings.
• Detection of IgM antibodies: This is the most common test and is carried out by IgM Capture Enzyme-linked Immunosorbent Assay (ELISA).
• Detection of neutralizing antibodies: This is carried out by employing the Plaque Reduction Neutralization Test having a 50% end-point (PRNT₅₀). This is generally carried out in research lab settings.

What is the treatment for CHIK infections?

There is no specific treatment for CHIK infections. No drugs or vaccines are so far available for the disease. Therefore, management involves symptomatic treatment, using pain-killers, antipyretics, and non-steroidal anti-inflammatory drugs (NSAIDs).

Is prevention better than cure?

Indeed, this can’t be more true than in case of CHIK infections. The most effective way to prevent the disease is to avoid mosquito bites. Since Aedes mosquitoes are daytime biters, it is important to wear long-sleeved clothing while outdoors, use mosquito repellents while indoors, and use mosquito repellent creams on the exposed skin. The other major preventive strategy involves the destruction of all possible breeding places for these mosquitoes. It is most important to empty cooler tanks, flower vases, flower pots etc. on a regular basis. Overhead water tanks should be checked regularly for any signs of mosquito larvae. In general, the house and its surroundings should be kept clean and free of litter and stagnant water.

Is the virulence of CHIKV increasing?

In 2016, CHIK infections have been reported from Delhi, Mumbai, and Nashik. The CHIK outbreak in Delhi last year saw a total of 12,221 reported cases, of which 9,749 had been confirmed by lab tests till December 24, 2016.

CHIKV rarely affects newborns. But this time, reports coming from Mumbai, have indicated that CHIKV infected at least 9 newborns in the city. Particularly worrisome is the fact that the severity of the disease is much more in babies than in adults.

CHIK infections have also been reported from tribal areas in and around Nashik in Maharashtra. Blood samples from 9 patients that were sent to the National Institute of Virology (NIV), tested positive for the virus.

Conclusion

CHIK used to visit us every 30 years. However, the latest outbreaks have occurred within a span of a few years only, and the frequency is steadily increasing. Importantly, the virus seems to have come back with a vengeance and its virulence and distribution have increased appreciably. As already indicated above, last year, CHIK outbreaks have occurred in far-flung places like Delhi, Mumbai, and Nashik. This indicates that the distribution of the virus has increased appreciably and the disease no longer occurs in specific areas. Moreover, the nature of the symptoms indicates that the virulence of CHIKV is also increasing.

Importantly, the virus is infecting hitherto unaffected populations such as newborn babies. Could this mean that the virus is being transmitted transplacentally from infected pregnant mothers? This is indeed the case, as the mothers tested positive for CHIKV by the PCR test. This evidence suggests that CHIKV is behaving much like the Zika virus (ZIKV), which is also transmitted across the placenta. Importantly, ZIKV has been causing havoc in Latin America and the Caribbean for over a year now.

Since Dengue virus, CHIKV, and ZIKV, all share the same vector (Aedes mosquitoes), it is high time that an integrative approach to vector control be adopted to tackle these diseases on a war footing!