Zoonotic diseases: viruses on rampage

Today is World Zoonosis Day which commemorates the first successful vaccination by Louis Pasteur against a zoonotic disease, Rabies, to a boy named Joseph Meister who had been mauled by a rabid dog.

The day is observed with a view to help raise awareness of the growing risk of zoonotic diseases. In the past three decades, over 30 new infectious diseases have emerged across Asia which, unfortunately, is the epicentre of most epidemics. The big question is can the medical-scientific establishment predict incidences of such infections and be prepared to tackle them when they barge into communities?

The May 2018 Nipah episode in the Calicut and Malappuram districts of North Kerala, which claimed 17 lives out of the 19 infected, was the latest threat. Viruses generally lurk in the wild and suddenly unleash themselves upon some unsuspecting host community that results in loss of lives.

These viruses hide in animal “reservoir hosts” (bats for Ebola and Nipah virus, pigs and birds for Influenza virus), till conditions are congenial for them to jump into a new host. Such an infection which humans acquire from animal hosts is called Zoonosis. The major zoonotic diseases include rabies, plague, anthrax, influenza etc. In the past few decades, some viruses like Ebola, Nipah, H1N1, HIV, SARS and MERS have also emerged as potential threats to human health. These diseases are considered dangerous due to the high mortality rates, to the tune of even 70% in the case of Nipah.

Fruit bats are the major reservoir hosts of Nipah virus and hence were the primary suspects in the recent incidence in Kerala too. Though the particular bat from which the sample was collected turned out to be negative this time, that doesn’t remove bats from the shadow of suspicion as reservoir hosts.

Zoonotic diseases cause massive economic loss to a country’s income when large number of animals and birds are killed if they are suspected of being hosts. For instance, the avian influenza which has led to the euthanising of 200 million birds in Asia, resulted in economic losses of over $10 billion.

Zoonotic diseases occurred in earlier centuries too, especially among tribals in forests who lived in small clusters without much inter-mingling with other tribes. Then, even if humans came into contact with wildlife and contracted zoonotic infections, the disease took a toll only among the small clusters and was contained there. Its scope to spread was remote.

However, circumstances have changed dramatically from then to the modern world characterised by globalisation. Encroachment of forest areas, climate change, microbial genetic mutations, changes in “reservoir hosts”, international airline travel, intensive animal farming for milk and meat requirements, have all contributed to greater human-animal contact.

For instance, the use of bush meat as a food source has been linked to the emergence of HIV. Increased temperatures are expanding the host range of disease vectors like mosquito. If the viral infection is new to human hosts, then their immune system will be naïve towards that pathogen. Given that humans lack prior immunity to combat such diseases, they can succumb soon. The 2004 Ebola epidemic in West Africa turned scary because it was not contained at the source and started to spread due to international airline connectivity and globalisation.

Research efforts are progressing in different parts of the world to make vaccines, but the viruses are very smart — they change their genetic profile fast thereby making these vaccines ineffective. One can expect more such zoonotic infections in the world, especially in developing countries, owing to more human-animal interactions like intensive animal farming, encroachment into forest areas for livelihood etc.

In Malaysia, commercial pig farms were installed in bat-inhabited orchards near forests which ultimately led to the first human outbreak of Nipah, via pigs. The problem gets intensified when coupled with the fact that there is not much accountability regarding safety norms in the meat industry sector in developing nations.

The Nipah attack in Kerala was contained, thanks to the timely efforts of doctors, public health officials and the government. If there was any lack of containment measures, events could have taken an ugly turn and more lives could have been at stake. But this is a grave reminder that there should be a concerted effort by all to contain such outbreaks in future.

(The writer is an Associate Professor with the Department of Life Sciences at Christ Deemed to be University, Bengaluru)