About zoonotic diseases

Definition

The word 'Zoonosis' (Pleural: Zoonoses) was introduced by Rudolf Virchow in 1880 to include collectively the diseases shared in nature by man and animals. Later WHO in 1959 defined that Zoonoses are "those diseases and infections which are naturally transmitted between vertebrate animals and man". Zoonoses include only those infections where there is either a proof or a strong circumstantial evidence for transmission between animals and man.

Zoonoses - an international problem

Historically, zoonotic diseases had a tremendous impact on the evolution of man, especially those cultures and societies that domesticated and bred animals for food and clothing. Zoonoses are among the most frequent and dreaded risk to which mankind is exposed. Zoonoses occur throughout the world transcending the natural boundaries. Their important effect on global economy and health is well known, extending from the international movement of animals and importation of diseases to bans on importation of all animal products and restrictions on other international trade practices. So, zoonoses no longer are solely a national problem. For effective control of zoonoses global surveillance is necessary.

With recognition of inter-relationships between countries, the internationalization of control efforts have become more relevant to technical, economic and social fields. The control of zoonoses retains its prominent place among the actions of international agencies according to the health and economic problems specific to each region.

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 athropod borne 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 etiology 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 agents

• 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, cryptococosis, 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, staphylococcosis

Factors Influencing Revelence of Zoonoses

1. Ecological changes in man's environment

With the expansion of 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 entering of humans in 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, tuleraemia, helminthic infections etc.

2. Handling animal by-products and wastes (occupational hazards) - There is significantly higher attack rates in workers during the course of their occupation than the rest of the population, e.g. anthrax in carpet weavers, live stock 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 wood cutters, salmonellosis in food processors, bovine tuberculosis in farmers etc.

3. 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, colibacilliosis, giardiasis, salmonellosis, shigellosis, etc.

4. Increased trade in animal products - Countries which 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 potential risk of increased frequency of zoonotic agents in man e.g. dermatophytosis, tuberculosis, brucellosis etc.

6. 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.

7. Cultural anthropological norms - In Kenya, people allow the dogs and hyenas to eat human dead bodies infected with hydatidosis. This helps to perpetuate the transmission cycle of the disease.

Zoonoses as a Public Health Problem

Although poorly documented, zoonotic diseases are a major public health problem in India. 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 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 reservoir of this disease are yet in conclusive, 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 leishmanias is which was hither to consider an anthroponosis in India has been proved to be a zoonosis recently with the Indian desert gerbil Merriones hurriane as the animal reservoir. It is not surprising, that in India, where approximately 80%of population lives-in rural areas in close contact with 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, unfamiliarity of physicians with these diseases and lack of co-ordination between physicians, veterinarians, and epidemiologist, the extent of their existence is obscured.

Further reading

1. Venkatesan, G., et al. "Viral zoonosis: a comprehensive review." J Vet Anim Adv 5.2 (2010): 77-92.
2. Messenger, Ali M., Amber N. Barnes, and Gregory C. Gray. "Reverse zoonotic disease transmission (zooanthroponosis): a systematic review of seldom-documented human biological threats to animals." PloS one 9.2 (2014): e89055.
3. Shanko, Kebede, Jelalu Kemal, and Dufera Kenea. "A Review on Confronting Zoonoses: The Role of Veterinarian and Physician." Journal of Veterinary Science & Technology (2015).