Zoonotic Diseases Shared Between Animals

• Zoonotic influenza
• Salmonellosis
• West Nile virus
• Plague
• Emerging coronaviruses (e.g., severe acute respiratory syndrome and Middle East respiratory syndrome)
• Rabies
• Brucellosis
• Lyme disease

A zoonosis is any disease or infection that is naturally transmissible from vertebrate animals to humans, There are over 200 known types of zoonoses, Zoonoses comprise a large percentage of new and existing diseases in humans, Some zoonoses, such as rabies, are 100% preventable through vaccination and other methods. 

A zoonosis is an infectious disease that has jumped from a non-human animal to humans. Zoonotic pathogens may be bacterial, viral or parasitic, or may involve unconventional agents and can spread to humans through direct contact or through food, water or the environment. They represent a major public health problem around the world due to our close relationship with animals in agriculture, as companions and in the natural environment. Zoonoses can also cause disruptions in the production and trade of animal products for food and other uses.

Zoonoses comprise a large percentage of all newly identified infectious diseases as well as many existing ones. Some diseases, such as HIV, begin as a zoonosis but later mutate into human-only strains. Other zoonoses can cause recurring disease outbreaks, such as Ebola virus disease and salmonellosis. Still others, such as the novel coronavirus that causes COVID-19, have the potential to cause global pandemics. 

Prevention and control

Prevention methods for zoonotic diseases differ for each pathogen; however, several practices are recognized as effective in reducing risk at the community and personal levels. Safe and appropriate guidelines for animal care in the agricultural sector help to reduce the potential for foodborne zoonotic disease outbreaks through foods such as meat, eggs, dairy or even some vegetables. Standards for clean drinking water and waste removal, as well as protections for surface water in the natural environment, are also important and effective. Education campaigns to promote handwashing after contact with animals and other behavioural adjustments can reduce the community spread of zoonotic diseases when they occur.

Antimicrobial resistance is a complicating factor in the control and prevention of zoonoses. The use of antibiotics in animals raised for food is widespread and increases the potential for drug-resistant strains of zoonotic pathogens capable of spreading quickly in animal and human populations.

Who is at risk?

Zoonotic pathogens can spread to humans through any contact point with domestic, agricultural or wild animals. Markets selling the meat or by-products of wild animals are particularly high risk due to the large number of new or undocumented pathogens known to exist in some wild animal populations. Agricultural workers in areas with high use of antibiotics for farm animals may be at increased risk of pathogens resistant to current antimicrobial drugs. People living adjacent to wilderness areas or in semi-urban areas with higher numbers of wild animals are at risk of disease from animals such as rats, foxes or raccoons. Urbanization and the destruction of natural habitats increase the risk of zoonotic diseases by increasing contact between humans and wild animals.

WHO Response

WHO works with national governments, academia, non-governmental and philanthropic organizations, and regional and international partners to prevent and manage zoonotic threats and their public health, social and economic impacts. These efforts include fostering cross-sectoral collaboration at the human-animal-environment interface among the different relevant sectors at regional, national and international levels.

WHO also works to develop capacity and promote practical, evidence-based and cost-effective tools and mechanisms for zoonose prevention, surveillance and detection through reporting, epidemiological and laboratory investigation, risk assessment and control, and assisting countries in their implementation.

As part of the One Health approach, the World Health Organization collaborates with the Food and Agriculture Organization of the United Nations (FAO) and the World Organisation for Animal Health (OIE) on the Global Early Warning System for Major Animal Diseases (GLEWS). This joint system builds on the added value of combining and coordinating alert mechanisms of the three agencies to assist in early warning, prevention and control of animal disease threats, including zoonoses, through data sharing and risk assessment.

Zoonoses - an emerging problem Over the last two decades, there has been considerable change in the importance of certain zoonotic diseases in many parts of the world, resulting from ecological changes such as urbanization, industrialization and diminishing proportion of persons working in the so-called primary sector. We do not know with what challenge nature will confront us in the world of constant interference with ecology.

Most of the infections of man that have been discovered in the last twenty years are shared with lower animals and a number of other diseases previously thought to be limited to man have now been found to be zoonoses. Reference may be made to various types of encephalitis, eosinophilic meningitis, capillariasis, anisakiasis, Lyme disease, monkeypox diseases in humans, lassa fever, Marburg disease and Ebola for all of which an animal link has been established.

Among those zoonoses recognized today as particularly important are anthrax, plague, brucellosis, Bovine tuberculosis, leptospirosis, salmonellosis, spotted fever caused by Rickettsiae, rabies, several common arthropodae bore viral infections (arboviral infection), certain parasitic diseases, especially cysticercosis, hydatid disease, trypanosomiasis and toxoplasmosis.

Classification With the advanced laboratory techniques and increased awareness among medical and veterinary scientists, ecologist and biologists, more than 300 zoonoses of diverse aetiology are now recognised. Thus, a very large number of zoonoses calls for classification, especially for easy understanding. These are classified as follows:

According to the etiological agent's

Bacterial zoonoses:- e.g. anthrax, brucellosis, plague, leptospirosis, salmonellosis, Lyme disease Viral zoonoses:- e.g. rabies, arbovirus infections, KFD, yellow fever, influenza, CCHF

Rickettsial zoonoses:- e.g. murine typhus, tick typhus, scrub typhus, Q-fever Protozoal zoonoses:- e.g. toxoplasmosis, trypanosomiasis, leishmaniasis Helminthic zoonoses:- e.g. echinococcosis (hydatid disease), taeniasis, schistosomiasis, dracunculiasis

Fungal zoonoses:- e.g. deep mycosis - histoplasmosis, cryptococcosis, superficial dermatophytes Ectoparasites:- e.g. scabies, myiasis According to the mode of transmission Direct zoonoses - These are transmitted from an infected vertebrate host to a susceptible host (man) by direct contact, by contact with a fomite or by a mechanical vector. The agent itself undergoes little or no propagative or developmental changes during transmission, e.g. rabies, anthrax, brucellosis, leptospirosis, toxoplasmosis.

Cyclozoonoses - These require more than one vertebrate host species, but no invertebrate host for the completion of the life cycle of the agent, e.g. echinococcosis, taeniasis Metazoonoses - These are transmitted biologically by invertebrate vectors, in which the agent multiplies and/or develops and there is always an extrinsic incubation (prepatent) period before transmission to another vertebrate host e.g., plague, arbovirus infections, schistosomiasis, leishmaniasis. Saprozoonoses - These require a vertebrate host and a non-animal developmental site like soil, plant material, pigeon dropping etc. for the development of the infectious agent e.g. aspergillosis, coccidioidomycosis, cryptococcosis, histoplasmosis, zygomycosis.

According to the reservoir host Anthropozoonoses - Infections transmitted to man from lower vertebrate animals e.g. rabies, leptospirosis, plague, arboviral infections, brucellosis and Q-fever. Zooanthroponoses - Infections transmitted from man to lower vertebrate animals e.g. streptococci, staphylococci, diphtheria, Enterobacteriaceae, human tuberculosis in cattle and parrots. Amphixenoses - Infections maintained in both man and lower vertebrate animals and transmitted in either direction e.g. salmonellosis, staphylococcus is

Factors Influencing Relevance of Zoonoses: Ecological changes in man's environment With the expansion of the human population, man is forced to exploit the virgin territories and natural resources like harnessing the power of rivers, constructing roads and pipelines through virgin or thinly populated areas, clearing, irrigating and cultivating new land, deforestation. All this would lead to the entering of humans into the unaccustomed ecosystem in which potential pathogens form part of the biotic community (natural focus). Large scale expansion of agricultural and engineering resources, construction of dams, artificial lakes, irrigation schemes, clearing of forests -all these lead to changing of the biting habits of the blood-sucking vectors and alteration in the population of reservoir animals which has led to the spread of leptospira, tularaemia, helminthic infections etc.

Handling animal by-products and wastes (occupational hazards) - There are significantly higher attack rates in workers during the course of their occupation than the rest of the population, e.g. anthrax in carpet weavers, livestock raisers and workers with animal hair in the textile industry, leptospirosis in rice field workers, listeriosis in agricultural workers, erysipeloid in butchers and fish merchants, tularemia and trypanosomiasis in hunters, creeping eruptions in plumbers, trench diggers etc. Other examples of zoonoses as occupational hazards are Q-fever in abattoir and rendering plant workers, jungle yellow fever and tick-borne diseases in woodcutters, salmonellosis in food processors, bovine tuberculosis in farmers etc.

Increased movements of man - Land development, engineering project work, pilgrimages, tourism, etc. expose the people to contaminated food and water leading to diseases like amoebiasis, colibacillosis, giardiasis, salmonellosis, shigellosis, etc. 4. Increased trade in animal products - Countries that import hides, wool, bone meal, meat, etc. from an area where some of the zoonoses are endemic, are likely to introduce the disease into their territories, e.g. salmonellosis, foot and mouth disease, anthrax, Newcastle disease etc. 5. Increased density of animal population - Animals may carry the potential risk of increased frequency of zoonotic agents in man e.g. dermatophytosis, tuberculosis, brucellosis etc.

. Transportation of virus infected mosquitoes - aircraft, ship, train, motor and other vehicles bring the viruses in to a new area, e.g. yellow fever Chikungunya fever, dengue fever etc.

Cultural anthropological norms - In Kenya, people allow dogs and hyenas to eat human dead bodies infected with hydatidosis. This helps to perpetuate the transmission cycle of the disease. Zoonoses is a Public Health Problem Although poorly documented, zoonotic diseases are a major public health problem in India. The plague has killed nearly 120 lakhs people since 1898.

Rabies continues to be a serious health problem in the country. Approximately 20,000 deaths due to rabies are estimated to occur every year while more than 17 lakhs persons bitten by suspected rabid animals seek antirabies vaccination at rabies treatment centres. Typhus killed many people during World War-I. Brucellosis alone is estimated to cause an annual loss of approximately 300 lakhs man-days in addition to an annual economic loss of Rs.2400 lakhs through brucellosis in cattle and buffaloes. Japanese encephalitis is another emerging zoonotic disease in India causing several outbreaks and considerable morbidity and mortality.

Studies on the reservoir of this disease are yet inconclusive, Kala-azar although proved zoonotic all over the world continues to be non-zoonotic in India in spite of the epidemiological evidence suggesting it to be zoonotic. Cutaneous leishmaniasis which was hitherto considered an anthroponosis in India has been proved to be a zoonosis recently with the Indian desert gerbil Meriones hurricane as the animal reservoir.

It is not surprising, that in India, where approximately 80%of population lives-in rural areas in close contact with a large domestic animal population (5120 lakhs approximately, 7290 lakhs poultry and equally large populations of wild and semi-wild animals) abundance of vectors because of suitable climate, low socio-economic conditions and lack of proper medical care, zoonotic diseases assume great public health significance. However, because of inadequate diagnostic facilities, the unfamiliarity of physicians with these diseases and lack of coordination between physicians, veterinarians, and epidemiologist, the extent of their existence is obscured.