Zoonotic diseases constitute the single biggest threat to human health in the 21st century, writes Saket Tiwari
The increasing incidence of bird and swine flu outbreaks around the world have raised public awareness about zoonosis, or naturally transmissible diseases from vertebrate animals to humans. While such transmissions from domestic livestock to humans are becoming better understood, transgenic diseases from wild-animal species remain something of a mystery. The Kyasanur Forest Disease (KFD), first discovered in 1957 in Shimoga district of central Karnataka and which infects some 500 people a year, is a case in point.
The disease came to light when local authorities found high mortality rates among red-faced bonnet macaques and langurs, two monkey species that are predominant in these forests. Scientists from the National Institute of Virology, Pune researching the outbreak initially thought it was a yellow-fever virus, but then discovereda tick-borne flavivirus that was the cause.
The incubation period for KFD is 3-8 days after which the patient develops a fever, chills, and headache. In extreme cases, it causes internal haemorrhaging and neurological imbalances. Diagnostic techniques, including enzyme linked immunosorbent assay (Elisa) and polymerase chain reaction (PCR), are effective because the family to which the KFD virus belongs carries the gene NS-5. But there is no specific treatment for the disease.
“All our funds and resources are being used to tackle existing diseases,” says Dr Sarman Singh, professor & head of clinical microbiology & molecular medicine at the All India Institute of Medical Sciences (AIIMS), New Delhi in an interview with Down to Earth. “We do not have the time and resource to anticipate danger or create awareness as other countries do.”
Human activities are the cause of changes in the genome of micro-organisms that cause diseases. Six out of ten infectious diseases in India are transgenic and wildlife constitutes a major source of infection. Major causes include increased deforestation, timber harvesting and the practice of slash-and-burn cultivation. These practices disturb the forest ecosystem and force wild animals to move beyond their own territories or habitats.
In the last few decades, India’s forest cover shrunk by a third, which has led to more animal-human interaction. Human activities have made pathogens adapt and survive in hoststhatthey weren’t able to previously. Domestic animals, birds and stray dogs also acts as reservoirs for zoonotic infections. Migratory birds play a major role in transmitting avian influenza. The threat from zoonotic diseases has not been taken very seriously, despite the fact that India accounts for a third of all rabies deaths in the world.
People working in agriculture or tending cattle, on poultry farms or in fisheries, especially those who live in close proximity to animals, are more likely to get exposed to infections. In India, a human population of 1.34 billion shares limited space with 512 million livestock and 729 million poultry, in many case sharing even their dwelling spaces. About 70% of the human community is dependent on farming either directly or indirectly.
During the late 1980s and 90s, the risk of infectious diseases from animal populations became apparent after incidents of major outbreaks, like H1N1 in Asia and Ebola in Africa, started surfacing. More recently, the spread of the Zika virus from central Africa to south America and Mexico and to south and east Asia underscored and highlighted how extensive zoonotic infections could spread, with serious implications for human health.
As Joanne Webster and her co-authors observed in a recent issue of Evolutionary Applications, “In recent years, the emergence or re-emergence of animal and human infectious diseases has been increasingly documented around the world, with an average of three new human infectious diseases being reported approximately every two years, and a new infecting organism described every week.”
“The ‘megacities’ of the world constitute obvious melting pots for the mixing of human and animal parasites and their potential rapid spread, both locally and internationally,” they write. “The World Bank has estimated that zoonoses have cost global economies more than $20 billion in direct, and $200 billion in indirect, costs between 2000 and 2010.”