Zoonotic diseases, which are transmitted between vertebrate animals and humans, have a long history of adapting to new reservoirs as human populations expanded across continents. The advancement of air transportation and globalization has further facilitated the rapid spread of infectious agents worldwide. This article explores the changing spectrum of infectious diseases, the emergence of new zoonotic pathogens, the impact of urbanization on disease transmission, and the significance of zoonoses for public health.
Emerging Zoonotic Diseases
Recent studies have shown that the majority of Emerging Infectious Diseases (EIDs) are zoonoses, accounting for 60.3% of all EIDs. Among these, 71.8% originate in wildlife, such as the severe acute respiratory syndrome (SARS) virus and Ebola virus. Over the last four decades, approximately 50 new pathogens have been identified, including equine morbillivirus, Nipah virus, South American hantaviruses, and new strains of Ehrlichia and Babesia, which can cause severe diseases or fatalities in humans.
Urbanization and Disease Transmission
The process of urbanization has led to mass movement of workers from rural areas to cities, resulting in the potential spread of zoonoses. Outbreaks of diseases like brucellosis in urban communities, such as Lima, Peru, and conflict areas like the Gaza Strip and West Bank, highlight the consequences of uncontrolled urbanization. The expansion of human populations into new areas also contributes to the emergence of zoonotic diseases. For instance, the settlement of new agricultural communities in the remote jungles of Peru and Brazil led to major outbreaks of vampire-bat rabies.
In Asia, Japanese encephalitis is a significant zoonotic disease that affects thousands of people each year. The disease is closely associated with rice field irrigation practices, which create vast areas of stagnant water, providing an ideal breeding ground for the mosquitoes that transmit the virus. Mosquitoes breed in these stagnant water bodies, and the proximity of human populations to these areas increases the risk of transmission. The incidence of Japanese encephalitis in Asia is particularly high, with almost 30,000 human cases and 7,000 deaths reported annually.
Public Health Importance
Zoonoses are of great importance to public health due to their high number, frequency, and severity in relation to human health. It is estimated that 40,000 to 70,000 people worldwide die each year from rabies, primarily through exposure to dog bites. The World Health Organization (WHO) Zoonoses Expert Committee has identified over 250 zoonotic diseases. Infections commonly occur when humans come into contact with infected animals, as seen with diseases like rabies, brucellosis, and tuberculosis. Zoonoses not only result in loss of human life and suffering but also impose a significant economic burden due to the loss of diseased animals, decreased agricultural production, prevention and treatment costs, and reduced human productivity.
Prevention and Control
The prevention and control of zoonotic diseases involve a three-tier approach: direct protection of humans, reduction or elimination of infection in animal reservoirs, and anti-vector measures. Specific vaccination programs may be necessary, such as leptospirosis vaccination for sewage workers or rabies vaccination for high-risk individuals like veterinarians and taxidermists. Preventing rabies in dogs requires vaccination, confinement of animals, quarantine of biting dogs, and removal of stray animals. Disease control in animals focuses on disease eradication through culling of sick and infected animals, along with quarantine, testing, and segregation of infected flocks or herds. Hygienic management practices on farms and in slaughterhouses play a crucial role. Vector and vehicle control measures are essential to prevent the spread of infectious agents, and the destruction of infected materials is necessary, especially within the food chain. International organizations like the Food and Agricultural Organization, the World Animal Health Organization (Office International des Epizooties), and WHO facilitate the harmonization of regulations and interagency cooperation to prevent and control the spread of zoonoses.
With the increasing interconnectedness of our world, zoonotic diseases pose significant threats to public health. The emergence of new zoonotic pathogens, urbanization, and environmental changes amplify the risk of disease transmission. Preventing and controlling zoonoses requires a comprehensive approach involving human and animal health sectors, vector control, vaccination, and effective surveillance systems. By prioritizing the prevention and control of zoonotic diseases, we can mitigate their impact on human and animal populations, improve public health outcomes, and reduce the economic burden associated with these diseases.
(Dr. Abdus Sabur Sheikh, Community Veterinary Expert with over 20 years of experience in livestock-based livelihood development across Rural India)