Zoonotic Spillover

Zoonotic spillover is the transmission of a pathogen from a vertebrate animal into a human, resulting in human infection. It occurs when a pathogen maintained in an animal reservoir crosses the species barrier, either directly or through an intermediate host or vector.

How spillover occurs

Spillover requires both an opportunity for exposure and the pathogen's ability to survive, enter human cells and overcome host defences.

• Exposure may occur through direct contact, respiratory secretions, animal products, contaminated environments or vectors.
• An intermediate host may facilitate transmission or pathogen adaptation, but it is not required in every spillover event.
• A spillover infection may remain a single case or limited cluster. A larger epidemic requires sufficiently effective human-to-human transmission.

Factors increasing spillover risk

Spillover risk rises when ecological or social changes increase contact among wildlife, domestic animals and humans.

• Land-use change and habitat disturbance can create new human-animal interfaces.
• Wildlife handling and trade, poorly regulated animal markets and inadequate farm biosecurity can increase exposure.
• Intensive animal production and dense populations may amplify pathogens after introduction.
• Climate and ecological changes can alter the distribution of animal reservoirs and disease vectors.
• Travel and trade primarily accelerate geographical spread after emergence.

Prevention and surveillance

The One Health approach coordinates human, animal and environmental health systems because spillover develops across these connected domains.

• Integrated surveillance can detect unusual illness or pathogens in animals and exposed human populations.
• Farm biosecurity, veterinary vaccination where available, safe food practices and vector control reduce transmission opportunities.
• Occupational protection and regulation of high-risk animal handling reduce exposure at the source.
• Rapid diagnosis, reporting, contact tracing and infection control help prevent a spillover event from becoming sustained transmission.

How UPSC asks this