What Are Zoonotic Diseases?

Poacher is selling dead monkeys on the side of the road near Impfondo, Likouala region, Congo-Brazzaville. Editorial credit: Alexandra Tyukavina / Shutterstock.com.
  • Zoonotic diseases are infectious diseases spread between animals and humans
  • 61% of all human diseases are zoonotic diseases
  • Zoonotic diseases can become epidemics

The World Health Organization (WHO) defines zoonotic diseases as diseases and infections spread between vertebrate animals and humans. Zoonotic diseases are also called zoonosis.

Zoonotic diseases can be caused by bacteria, viruses, parasites, and fungi. Common zoonotic diseases include rabies, Ebola virus, salmonellosis, Taenia solium infection and some strains of influenza such as swine and bird flu.

While the origin of most human diseases is from animals, only diseases that regularly include animal to human transmission can be considered as a direct zoonosis.

According to the WHO, 61% of all human diseases are zoonotic diseases. Also, zoonotic infections comprise 75% of all new diseases discovered in the last decade. Some of these zoonotic diseases do not make animals sick but will sicken humans.

Zoonotic diseases range from mild to severe. Others, such as rabies are almost always fatal.

Zoonoses occur when there is contact with or consumption of non-human animals, their products or derivatives.

Modes Of Transmission

Zoonotic diseases have different modes of transmission.

In direct zoonosis, the disease is directly transmitted from animals to humans. A person can come into direct contact with the bodily fluids (blood, urine, saliva, feces) of the infected animal through petting, touching, being bitten or scratched by the animal. Transmission can also be through air, as seen in influenza.

Rabies is an example of a direct zoonotic disease. It is spread when infected animals (usually dogs), bite or scratch a human or when saliva from an infected animal comes into contact with a person’s eyes, mouth, and nose. Rabies is a viral disease. It affects the nervous system, causing inflammation of the brain. Early symptoms of rabies are fever and tingling at the site of exposure. Late symptoms include uncontrolled excitement, violent movements, confusion and loss of consciousness. Rabies almost always results in death.

 Apart from rabies, pets can also transmit diseases such as ringworm and giardia, toxocariasis, and cryptosporidiosis.

Indirect zoonosis involves contacting surfaces or objects that have been contaminated by the infected animal. Such areas can be plants, soil, pet baskets, cages, kennels, chicken coops or aquarium tanks where an infected animal has been. People with an increased risk of contracting zoonotic diseases are veterinarians, pet shop workers, zoo workers, abattoir workers, and farmers.

Contact with animals; be it during farming, ranching, and non-human animal husbandry can cause zoonotic diseases. People who work closely with horses, mules, and donkeys can get glanders, a contagious zoonotic infectious disease caused by the bacterium Burkholderia mallei. Close contact with cattle can lead to cutaneous anthrax. Workers in workers in slaughterhouses wool mills and tanneries are likely to be infected with inhalation anthrax. Contact with sheep that have recently given birth can increase the risk of chlamydiosis, listeriosis, toxoplasmosis, and Q fever. To prevent these diseases, continued clinical veterinarian and occupational risk education is essential.

Vector transmission involves an intermediate species, usually arthropods, carrying the disease without being infected. The vector bites the infected animal and then bites a human, passing on the zoonotic disease. Examples of vectors are ticks, flies, fleas, and mosquitoes. Diseases transmitted by vectors are African sleeping sickness, Zika fever, Western equine encephalitis, West Nile fever, Saint Louis encephalitis, Japanese encephalitis, Eastern equine encephalitis, and Dirofilariasis.

Foodborne zoonotic diseases occur when bacteria, fungi, viruses, and parasites contaminate food and water supply. Examples of zoonotic pathogens causing foodborne disease are Salmonella, Caliciviridae, Campylobacter, and Escherichia coli. Echinococcosis is caused by a tapeworm that can be spread from infected sheep through food or water contaminated with feces or wool. Many foods such as seafood, eggs, dairy, meat, and vegetables can be contaminated compromising food safety.

Reverse zoonosis, also known as anthroponosis occurs when humans infect animals.

History Of Zoonosis

Examples of zoonotic diseases in history include bubonic plague, West Nile Virus, anthrax, rabies, bovine tuberculosis, and tularemia. Many of these diseases were restricted to certain local populations, thanks to the infrequent contact between different population groups.

Many modern diseases started as zoonotic diseases as evidenced by DNA and RNA sequencing. Such diseases include measles, HIV, diphtheria, influenza, smallpox, and some forms of tuberculosis and the common cold. HIV was a zoonosis spread to humans in the early 20th century. Today, HIV has mutated to a separate human-only disease.

Why Zoonotic Diseases Matter?

Zoonoses are of interest because they are often previously unrecognized diseases. New pathogens from animals, especially viruses continue to emerge and spread. These new diseases are unpredictable and have increased virulence in populations that lack immunity. Owing to their epidemic potential, absence of specific treatment and vaccines available control the spread, and high case fatality ratio, zoonotic diseases are much likely to cause much distress.

Increased contact between animals and humans, evidenced by the encroachment of human activity into wilderness areas, and by the movement of wild animals into areas of human activity have contributed significantly to the emergence of new zoonotic pathogens. Examples of these new pathogens include the Nipah virus, avian influenza, and the West Nile virus.

The unparalleled passage of people, animals, and goods across international borders as a result of globalization, has fuelled the spread of zoonotic diseases. An emerging zoonosis in one country is likely to spread and become a global health concern.

Most zoonotic diseases occur in remote areas. This makes it difficult for health services to reach these populations. Therefore detection and diagnosis of these diseases can be delayed. Besides, inadequate transparency regarding timely reporting of emerging zoonotic diseases remains a challenge. Weak surveillance, reporting systems, and limited capacity for laboratory diagnosis of emergent zoonotic diseases are the other challenges faced in preventing zoonotic diseases.

Besides, zoonotic diseases cause morbidity and mortality, plus a high burden on health systems and significant economic losses to the countries. Countries lose trade, tourism and consumer confidence.

Bottom Line

Zoonotic diseases remain a global health concern as current strategies for prevention and control remain patchy with no consistency between the animal and human health sectors.


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