WILDLIFE AS A SOURCE OF EMERGING AND RE-EMERGING ZOONOSES
Dr Prabha Sharma
MVSc (Surgery & Radiology)
DUVASU, Mathura (UP), 281001
Corresponding author– prabhasharmadr@gmail.com
Abstract
Mankind is currently facing many different challenges, which requires global solutions. One of these challenges is the spread of zoonotic diseases that emerge or re-emerge at the interfaces between animals, humans and the ecosystems. Approximately 60% of emerging human pathogens are zoonotic. Among zoonoses, infectious pathogens originating in wild animals have become increasingly important in recent time, as they have substantial impacts on human health, agricultural production and wild life. The complex nature of the human-animal interface is constantly influenced by the effects of climate change, anthropogenic and natural factors. Zoonotic diseases transmitted by arthropod vectors are most markedly affected by geo-climatic change. Travel, tourism and trade are the major human factors impacting the epidemiology of zoonotic diseases. The re-emergence of zoonotic diseases is also driven by pathogen adaptation and animal migration.
Keywords: Epidemiology, re-emergence, zoonotic, wildlife
Wildlife plays an important role in balancing our environment by maintaining the ecological stability. There are huge numbers and immense diversity of wild animals distributed throughout the world. Also, wildlife is the reservoir of a number of zoonotic infections. From the very beginning, wildlife has been an important source of infectious diseases transmissible to humans.
Humans coexist in a complex, interdependent relationship with animals as well as with the environments .The interface between humans, animals, and the environments acts as a source of diseases. Such diseases, transmissible from animals to humans through direct contact or indirectly through food, water, and the environment are commonly referred to as “zoonoses.” A zoonotic disease can emerge as the result of increased human contact with the animal hosts, animal tissues, vectors, or environmental sources of the pathogens. Zoonoses with a wildlife reservoir constitute a major public health problem. The majority of human diseases are zoonotic and many have their origins in wildlife. An estimated 60 percent of emerging human pathogens are zoonotic and out of these over 70 percent have wildlife as their origin.
HISTORICAL ASPECTS
- Rabies was first described in Mesopotamia, in hunting dogs, as early as 2,300 BC. Bats are the most frequently reported rabid wildlife species followed by raccoons skunks and foxes .
- Ancient accounts and modern hypotheses suggest that Alexander the Great, who died in Babylon in 323 BC, died of encephalitis caused by West Nile virus a virus that has wild bird as its reservoir.
- Bubonic plague, a bacterial disease for which rats and fleas play a central role in transmission, has caused substantial illness and death around the world since ancient times.
- Measles is thought to have derived from spillover of Rinderpest or Canine distemper virus infection in Mesopotamia following urbanization and increase in human population allowed its persistence
MODES OF TRANSMISSION
Broadly, two different patterns of transmission of pathogens from wild animals to humans are evident-
- Actual transmission of the pathogen to humans
It is a rare event but, once it occurs, human-to-human transmission maintains the infection for some period of time or permanently.
E.g. HIV /Acquired Immune Deficiency Syndrome, Influenza A, Ebola virus and severe acute respiratory syndrome.
- Direct or vector-mediated animal-to-human transmission
It is the usual source of human infection. Wild animal populations are the principal reservoirs of the pathogen and human-to-human disease transmission is rare.
E.g. Rabies and other Lyssavirus, Nipah virus, West Nile virus, Hantavirus, and the agents of Lyme borreliosis, Plague, Tularemia, Leptospirosis and Ehrlichiosis.
FACTORS AFFECTING EPIDEMIOLOGY OF WILDLIFE ZOONOSES
Phenomena such as species loss, habitat degradation, pollution, invasive alien species, and global climate change are fundamentally altering life on our planet .The rise of emerging and re-emerging infectious diseases threatens not only humans but also the fauna and flora comprising the critically needed biodiversity that supports the living infrastructure of our world.
- Climate change and global warming
Environmental change including deforestation, habitat fragmentation or climate change can create new opportunities for pathogens that were previously circulating only in wildlife or environmental reservoirs to spill over into people or livestock hosts. It is estimated that the global warming is currently increasing at 0.2°C per decade. The reason mostly attributed to the anthropogenic emission of greenhouse gases. As a result of global warming, a rise in sea temperatures causes melting of polar ice caps and glaciers leading to coastal flooding and risks of water-borne zoonoses. Increased precipitation creates more potential breeding sites for mosquitoes. Arthropod vectors are the most sensitive to variability in climatic temperature. Mosquitoes, ticks and sandflies are ectothermic and have life cycles dependent on ambient temperatures.
Increased water temperature cause mosquito larvae to develop faster also increasing overall vector capacity. High temperature causes increased reproduction, increased frequency of blood meals and also pathogens harboured by mosquitoes also mature faster.
- g. Mosquitoes- Chikunguniya, Dengue, Rift valley fever, West Nile virus
- Expansion of tick populations causing Lyme’s disease
- Sand flies – Leishmaniasis
Rodents are the main reservoir of Hantavirus infections. With warmer climate, the protective environment provided by snow is removed and rodents seek shelter in human habitats, increasing transmission of Hantavirus.
Francisella tularensis, the causative agent of tularemia, can be transmitted by skin contact with an infested, diseased, or dead hare or rodent.
- Changes in human demography, behavior and practices
The human population explosion creates an increased demand for food and land by deforestation for agriculture and livestock farming which disrupts natural ecosystems. Deforestation leads to a direct loss of wildlife habitat. It results in more interaction between wildlife and humans. Close contact with animals and their environments provides more opportunities for diseases to pass between animals and people.
- g. Agriculture and urbanization destroy bat habitats, forcing them into human dwellings.
- Ebola virus in west-central Africa and the Nipah virus in east Asia.
AIDS represents a disease in which demographic factors and human behavior have contributed to its development into a global public health problem. The origin of HIV, the virus causing AIDS, is still a matter of controversy, but HIV thought to be spread to humans by nonhuman primates in West Africa.
- Food borne Zoonoses
Eating habits can also play a role in transmission of zoonoses.
- g. The Corona virus outbreak in 2019 and SARS outbreak of 2003 in China both shared history of consuming bats and civet cats respectively in ‘Wet market’.
- Enterohaemorrhagic Coli O157:H7 outbreaks are associated with undercooked meat .
- Eating meat from exotic animals such as bear increases the risk of acquiring Trichinellosis.
- Travel & tourism
- Increased frequency and speed of local and international travel over long distances alleviate the possibility of rapid global transport of infected person, exotic arthropod vectors and infectious agents. E.g. Ebola virus infection (migration of monkeys from Uganda and Africa).
- Efficient air and land travel links make disease containment difficult as illustrated by the SARS Corona virus outbreak.
- Tourism to more exotic locations and eco-adventure travel exposes travelers with weak immunity to endemic diseases.
- Hiking, camping, and hunting are activities that may represent risk factors. Water sports E.g. canoeing, river rafting has been related to Leptospirosis outbreaks as animals shed the organism in their urine.
- Trade
- Chikungunya is considered as a paradigm of emergence and globalization of vector-borne diseases.
- Globalization allowed the rapid invasion of the Asian tiger mosquito and thus a shift from the historical sylvatic transmission of Chikungunya to widespread human epidemics.
- Increasing international trade of live animals and animal food products favored the spread of zoonotic infections. E.g. coli O157:H7, Listeriosis, Campylobacteriosis.
- Breakdown of public health measures and infrastructure
- Plague in Maharastra, Gujrat in 1994.
- Leptospirosis in Kerala, Tamil Nadu and Andaman islands during last two decades.
- Microbial adaptations and changes
Genetic shift in Influenza virus resulted in new variants causing worldwide epidemics H5N1. For the human population as a whole, the main danger appears to be simultaneous infection with an avian and a human influenza virus. Severe acute respiratory syndrome (SARS) is the current example of microbial adaption.
- Human susceptibility
Increased human susceptibility has contributed to the emergence or recognition of some opportunistic pathogens. This has been increased by factors such as the AIDS epidemic results in increased number of immune-compromised people making them more susceptible for zoonotic diseases.
PREVENTION AND CONTROL
The fundamental concept in prevention, control and eradication of zoonotic diseases is focused upon “breaking the weakest chain of transmission at its epidemiologically weakest link” in the infection i.e. controlling the reservoirs (animals), breaking the routes of transmission and immunization of susceptible host (human beings).
| SOURCE OF RESERVOIR |
| SUSCEPTIBLE HOST |
| MODE OF TRANSMISSION |
EPIDEMIOLOGICALLY WEAKEST LINK IN THE TRANSMISSION PATHWAY
- Targeting reservoir hosts: moving beyond culling towards alternative non-lethal approaches
- An ecological intervention to reduce transmission from bats to pigs was devised to manage spillover of Nipah virus. The fruit trees, which attract bats were planted at a maximum distance from pig farms prevented further outbreaks of Nipah virus in Malaysia since 1998.
- Oral vaccination of vampire bats has been proposed to reduce rabies spillover.
- Targeting the environment: habitat, vector control and ecosystem management
- g. Spillover transmission of avian influenza can be managed in live-bird market systems by providing ‘rest days’ (during which no birds are brought to market) and lessening stay-time in markets.
- For Hendra virus spillover, blocking horses overnight access to trees in pastures to prevent viral transmission from bats to horses as this would delay horses access to grass contaminated by bat urine thereby reducing the probability that a horse would come into contact with bat urine.
- For vector-borne zoonoses, vector control should be an integral part of any intervention strategy.
- Recent policy reform in India and Pakistan, banning diclofenac, may allow wild vulture restoration to decrease volume of uneaten carcasses , which act as environmental breeding grounds for diverse zoonotic spillover of pathogens including anthrax, brucellosis and bovine tuberculosis .
- Targeting the interface between reservoir and spillover hosts
- g. The use of bed nets to curb malaria is a classic example of controlling the interface between mosquitoes and people.
- Biosecurity efforts to reduce rates of contact for avian influenza virus transmission between wild birds and poultry have been an important component of avian influenza risk management.
- Foodborne zoonoses can often be interrupted by using good sanitation and hygiene during food preparation, eliminating cross-contamination of foods, cooking all foods of animal origin (including invertebrates such as mollucks and snails) to safe temperatures, and thoroughly washing vegetables shortly before eating.
In addition to above, all diseases for which wildlife act as a reservoir and have an impact on animal and human population need an extra attention from the international community.
- Public education and behavioral change are also important factors for successful intervention and implementing restrictions on anthropogenic animal movement.
- Improvement in national surveillance systems for humans and animals to increase the ability of recognition of zoonoses , as well as better international integration and sharing of information from such systems.
- Interdisciplinary and international collaboration, such as WHO and OIE is necessary for the rapid identification and effective management of outbreaks of zoonoses.
Habitat degradation, fragmentation, and pollution are aspects of habitat destruction caused by humans that leads to spillover of most of the zoonotic diseases. In the present crisis of Corona virus pandemic lockdown, empty streets and skies let the birds be heard and left animals free to roam as well and compel us to examine how humans changed the earth biodiversity.
Recent outbreaks of West Nile Virus, Ebola Hemorrhagic Fever, SARS, Monkey pox, Mad Cow Disease , Avian Influenza and COVID -19 remind us that human and animal health are intimately connected. A broader understanding of health and disease demands a unity of approach achievable only through a compliance of human, domestic animal and wildlife health as “ONE HEALTH”.
Humans, animals and the environment are inextricably linked—a fact that needs to be remembered and exploited in our modern approach to health. The interaction between living beings, including men, animals and pathogens, which share the same environment, should be considered as a ‘holistic and coherent’ system. A multidisciplinary and integrated approach is more effective to assure animal (domestic and wild) and human health, as well as the environment protection.
