Chikungunya re-emergence a continuous possibility
The chikungunya virus originated from Africa but has spread to different continents, causing outbreaks in numerous countries. There are three genotypes of chikungunya virus: East Central South African (ECSA), Asian and Indian Ocean.
Speaking at the National Dengue and Arboviruses Infection Conference 2017, Professor Dr Jamal I-ching Sam said human factors such as movement, population immunity and transmission risk of asymptomatic patients; and mosquito factors such as difficulties in vector control, distribution of species and globalization play a role in the re-emergence of the viral disease.
Locally, the first documented chikungunya presence was seroprevalence evidence that showed low seroprevalence levels in the states bordering Thailand in the 1960s. In 1998, the first confirmed case of chikungunya occurred in Port Klang, caused by the Asian genotype of the virus. In 2006, there were outbreaks in Bagan Panchor with the Asian genotype and later in Ipoh with the ECSA genotype. The ECSA genotype was suspected to be introduced by Indian nationals working in Ipoh as there were chikungunya outbreaks in India with the same genotype around that time. A year later, the Asian genotype was discovered in macaques in Kuala Lipis and this suggests that the Asian genotype has been endemic in the region, he said.
Between 2008 and 2010, there was a big nationwide chikungunya outbreak with the ECSA genotype. There was evidence suggesting a link between the virus and people with recent travel history to India. The outbreak started in Johor and spread nationwide, to Singapore and Thailand. This year, there was a small outbreak reported in the Baling district and the virus is similar to the strain that originated in India.
Both Aedes albopictus and Ae. aegypti carries chikungunya virus. Generally, Ae. albopictus is more commonly found in rural areas, and Ae. aegypti in urban and suburban areas. Studies have shown rural areas are more prone to chikungunya outbreaks, he said. There is no published data on which mosquito was the main vector during the 2008 to 2010 outbreak.
However, an entomological study in rural Kelantan during the chikungunya outbreak in 2009 found that 58 percent of mosquitoes collected were Ae. albopictus and no Ae. aegypti. [Indian J Med Res 2011;133:670–673] Even though no chikungunya virus was found in the vectors, their findings suggest that Ae. albopictus played a role in the outbreak. Moreover, Ae. albopictus was the main vector in many countries between 2005 and 2010, said Jamal. This suggests a link between Ae. albopictus and the ECSA genotype, and this association is supported by researches showing the ECSA virus has a glycoprotein mutation that enables it to adapt to Ae. albopictus. [PLoS Pathog 2007;3(12):e201]
On the other hand, the Asian genotype has been found to infect Ae. aegypti more frequently than the ECSA genotype. This vector was identified as the main vector during the outbreak in Bagan Panchor. Thus, there are now two main genotypes circulating in the country, he said.
Global re-emergence of chikungunya is characterized by the spread of different genotypes and lineages. Reintroductions are a continuing possibility, Jamal summarized.