In the 2018 outbreak in Kerala, only the index case was infected in the community. All the others got the infection via nosocomial transmission in 3 different hospitals.
Since all the environmental samples collected from the surroundings of the residence and the potential workplace of the index case of the 2018 outbreak tested negative for NiV, experts have held direct transmission from fruit bats as the most plausible explanation.
“Though it is difficult to verify, we came to understand that the index case had the habit of keeping pets and was a nature explorer. It is possible that he could have come in contact with an NiV-infected bat probably a pup,” explains Prof Arunkumar Govindakarnavar, Director of Manipal Institute of Virology Manipal Academy of Higher Education (Deemed University).
Fruit bats belonging to the species Pteropus giganteus (Indian flying fox), is quite abundant in the area. It is likely that he got infected in April, the breeding season of bats particularly the parturition (birthing) time. The shedding of the virus would be increased during the breeding season as the bats’ metabolic activities are high and secrete more body fluids, he points out.
Further, the absence of the viral RNA in the bat-bitten fruit does not rule out the zoonotic transmission from the bat.
The source of transmission in the lone case of NiV infection in June this year is yet to be identified.
“We are now doing general surveillance on pigs and bats in certain regions around Kochi. There is no ‘hotspot’ where we can focus our investigation on. It makes an epidemiological investigation through collecting samples difficult, potentially avoiding any sort of bias at this point,” said an animal health expert who is part of the team sent by the central health authorities. Collection of samples on a random basis could lead to bias. Moreover, such an exercise may create unnecessary panic among the public in the location, he added, preferring anonymity as only the head of the institution and the spokesperson are entitled to comment on the current situation.
Compared to the Malaysian strain (NiV-M), the rate of human-to-human transmission was found to be faster in the sub-continental outbreaks. Strong evidence indicative of human-to-human transmission of NiV was found in Siliguri, West Bengal in 2001 and in Bangladesh in 2004. The presence of respiratory symptoms likely increased human-to-human transmission.
All the infected people got the virus when the index case had persistent cough. Although several persons came into contact with the infected, including contact with dead bodies or bodily fluids, only those with direct exposure to the patient’s coughing appear to have contracted the disease. This underscores the prevalence of human-to-human transmission via droplets, the study reports.
The high proportion of patients with respiratory system involvement in the Kozhikode outbreak was similar to that in Bangladesh, but was significantly different from the incident in Malaysia. Corpse-to-human transmission was also reported from Bangladesh.