Zoonotic Diseases

A rabid dog dripping saliva
Zoonotic Diseases
The word Zoonosis was introduced by Rudolf Virchow in 1880 to include diseases collevtively shared in nature by man and animals. Later, the World Health Organization (WHO) in 1959 defined Zoonoses as diseases and infections which are naturally transmitted between vertebrate animals and humans. It only include those disease where there is a clear evidence of disease transmission between humans and animals and also simply known as the Zoonotic disease.

Historically, Zoonotic diseases have a tremendous effect on the development of modern human. They shaped our culture and society as whole. They are one of the most frequent and deadliest risk to which humans are exposed. Plague which is one of the well known zoonotic disease caused more than 50 million deaths in 14th century Europe alone and still causes 600 deaths on average globally per year. One can say, the effect of zoonotic disease is not restricted to a region and with the rise of interconnected communities and economy, it is more evident. Also, the recent outbreaks of Ebola, Zika Virus and Nipah Virus further embolden this fact.

Classification of Zoonoses

As of now, there are more than 300 zoonotic diseases have identified and with rise of human and animal interactions, this number is going to increase.

On the basis of Etiological Agents
1. Bacterial zoonoses
anthrax, brucellosis, plague, leptospirosis, salmonellosis, lyme disease

2. Viral zoonoses
rabies, arbovirus infections, KFD, yellow fever, influenza, CCHF

3. Rickettsial zoonoses
murine typhus, tick typhus, scrub typhus, Q-fever

4. Protozoal zoonoses
toxoplasmosis, trypanosomiasis, leishmaniasis

5. Helminthic zoonoses
echinococcosis (hydatid disease), taeniasis, schistosomiasis, dracunculiasis

6. Fungal zoonoses
deep mycosis-histoplasmosis, cryptococcosis, superficial dermatophytes

7. Ectoparasites
scabies, myiasis

On the basis of Mode of Transmission
1. Direct zoonoses
A zoonotic disease which gets transmitted by direct contact with a fomite or by a mechanical vector. The prime examples are rabies, anthrax, brucellosis, leptospirosis, toxoplasmosis.

2. Cyclozoonoses
A zoonotic disease that requires two or more species of vertebrates as definitive and intermediate hosts. The prime examples are hydatid disease and trichinosis.

3. Metazoonoses
A zoonotic disease biologically transmitted by an invertebrate vectors, in which agent develops and there is an extrinsic incubation period before actual transmission to another vertebrate host. The prime examples are plague, arbovirus infections and schistosomiasis.

4. Saprozoonoses
A zoonotic disease which can be transmitted to humans by non-animal development sites such as soil, plant material and pigeon dropping. The prime examples of saprozoonoses are cryptococosis, histoplasmosis and zygomycosis

On the basis of Reservoir Hosts
1. Anthropozoonoses
The infection or diseases transmitted  to  man  from  lower  vertebrate  animals. The prime examples are  rabies,  leptospirosis,  plague and Q-fever.

2. Zooanthroponoses
The infections or diseases transmitted  from  man  to  lower  vertebrate  animals. The prime examples are diphtheria, enterobacteriaceae and human tuberculosis.

3. Amphixenoses
The infections or diseases transmitted from either from man to vertebrate or from vertebrate to man. The prime examples are salmonellosis and staphylococcosis.

Factors Behind the Prevalence of Zoonoses

1. Change in Human environment
With rise of urbanization, humans are encroaching on newer territories. They are clearing forests and other lands to make the way for new pasture and cultivation land. This leads to their exposure to a newer variety of pathogens and diseases. These changes are also altering the biting habits of the existing blood sucking vectors and population of reservoir animals and increasing the fatality of earlier defined diseases.

2. Animal Handling (Occupational Hazard)
There is an evident risk of zoonoses among farm workers, zoo workers, animal health workers and other similar animal handling professionals. As these people are directly working with animals. Erysipeloid  in  butchers  and  fish  merchants,  tularemia  and  trypanosomiasis  in  hunters and leptospirosis  in  rice  field  workers are the prime examples of occupational zoonotic diseases.

3. Increased Mobility
Activities such as tourism, pilgrimages and office tours expose people to contaminated food, water and sites, which can lead to zoonotic diseases. Amoebiasis,  colibacilliosis,  giardiasis, salmonellosis are prime examples.

4. Trade
With rise in globalization and international trade, products like wool, meat, hides and even other harmless looking food items increases the chance of introduction of zoonotic infection in new territories. Newcastle disease is such example.

5. Increased Animal Population
The increased animal population also posses the threat of zoonoses in humans.

Zoonotic Diseases in India

Although poorly documented, it is estimated that Plague has killed more than 12 million people since 1898, rabies is the cause of 20000 deaths per year on average, and a major population of is threatened by the outbreak of Kala azar. HIV which leads to AIDS is also the example of zoonotic origin and has affected more than 2.1 million people till now.

The major zoonotic diseases in India are Avian Influenza, Rabies, Nipah virus, Japanese encephalitis, Leptospirosis, Hanta virus, SARS, Cysticercosis, Anthrax, Plague, Echinococcosis and Schistosomiasis, and Kyasanur forest disease (KFD).

1. NCDC (https://ncdc.gov.in/WriteReadData/l892s/File618.pdf)
2. Down to Earth (https://www.downtoearth.org.in/coverage/forests/why-zoonotic-diseases-are-spreading-to-humans-at-a-faster-rate-60598)