Introduction & Overview
Zoonotic diseases (also known as zoonoses) are diseases that are caused by germs that spread between animals and people.
Zoonotic diseases are the hallmark of public health hence a challenge to veterinarians and all professions concerned with public health. Zoonotic diseases are best controlled when veterinarians and public health physicians cooperate in zoonosis control programs.
This enables the eradication of and continued surveillance of zoonotic diseases in animals, humans and the environment. Though some zoonoses have been eradicated in developed countries, they are still a major concern in the developing world, e.g. bovine tuberculosis, bovine and porcine brucellosis, and rabies. Zoonotic diseases also reemerge in areas where they have been eradicated, e.g. Hendra and Nipah viruses are reemerging, while many other zoonoses remain a constant concern.
What are the common causes of Zoonotic diseases?
The causes of zoonotic diseases are bacteria, viruses, fungi, parasites, or prions. Most of these agents are disease-causing organisms of mammals, with most of the diseases by people and nonhuman primates. Birds, reptiles, amphibians, fish, and invertebrates can also be sources of infection, and some agents have both human and animal reservoirs including wildlife. Reverse zoonoses are caused by human pathogens transmitted to animals. In some cases, these agents can later infect people, e.g., Mycobacterium tuberculosis, the agent of human tuberculosis, can colonize the bovine udder and be shed in milk.
The occurrence of a pathogen in both people and animals does not always mean it is a significant zoonosis. Some diseases are acquired from the environment, and transmission between animal or human hosts is either absent or occurs very rarely and under unusual conditions. These are considered infections common to people and animals rather than true zoonoses, e.g. yeasts.
How are Zoonoses Transmitted?
Zoonotic pathogens are acquired through the following routes:-
Close contact with an animal through inhalation, ingestion, or other mechanisms resulting in the contamination of mucous membranes, damaged skin, or in some cases, intact skin. Sources of organisms include body fluids, secretions and excretions, and lesions.
Aerosols are occasionally involved, particularly in confined spaces.
Fomites (inanimate objects) transmit some agents that correlate with the organism’s persistence in the environment.
Ingestion of contaminated food or water infects large numbers of people. Sources of zoonotic pathogens in foodborne disease include:-
Undercooked meat or other animal tissues (including seafood and invertebrates),
Unpasteurized milk and dairy products, and
Contaminated vegetables.
Insect vectors, serving as either biologic or mechanical vectors, are important in transmitting some organisms.
How are the risks of acquiring a zoonosis?
The risk of acquiring a zoonosis can be affected by many factors that include:-
The immune susceptibility of the host
The potential route(s) of transmission,
The number of organisms shed by the animal, and
The ability of the agent to cross species barriers.
Certain occupations or activities can significantly increase the probability of exposure e.g.,
Contact with soil during gardening or childhood play carries a risk of infection with pathogens that reside temporarily or permanently in the soil, such as Toxocara spp or Sporothrix schenckii.
Veterinary practice,
Agricultural activities,
Pet ownership.
Dogs, cats, livestock, or birds may also bring wildlife pathogens into closer proximity to people.
Human activities can also bring people into closer contact with wildlife, e.g. hunting, fishing, and camping, which can result in exposure to organisms carried in wild animals (eg, Francisella tularensis, Yersinia pestis, and Leptospira spp) or transmitted by arthropod vectors (eg, Borrelia burgdorferi and West Nile virus).
Ecotourism also has resulted in human exposure to some exotic wildlife diseases.
Cultural practices such as eating raw fish, gastropods, or molluscs expose humans to zoonotic diseases.
What is the role of Immunosuppression in Zoonoses?
Zoonotic illnesses range from mild, asymptomatic and self-limiting cases to chronic debilitating ones. They run the whole gamut from skin eruptions to life-threatening diseases. These diseases also rely on the strength of the individual's immune system. In a healthy host, the disease can go unnoticed while in an immuno-compromised one the condition can have an unusual presentation including delayed diagnosis.
Immunodeficiencies can either be
Primary immunodeficiencies or
Secondary immunodeficiencies.
Primary immunodeficiencies are mainly genetic defects that may increase susceptibility to certain categories of pathogens or broadly suppress the body defences to certain illnesses.
Secondary immunodeficiencies are caused by conditions that compromise the immune system, e.g. splenectomy (removal of the spleen), diabetes, cancer, and infections such as malaria or AIDS, chronic lung disease, injuries and burns as well as indwelling catheters and implanted medical devices.
The physiologic state can also affect the immune system with it being relatively immature in newborns and young children, and declining in older adults. Pregnancy also interferes with the mother's, the foetus', or both immune systems with about 20% fatalities among pregnant women with pathogens, such as Toxoplasma gondii severely damaging the fetus while remaining mild in the mother.
Why the Emergence and Reemergence of Zoonotic Diseases?
Emerging diseases are mainly zoonotic illnesses that have increased in incidence and are likely to increase in the near future. A zoonotic disease can emerge as the result of:-
Increased human contact with the animal host(s), animal tissues, vectors, or environmental sources of the pathogens.
An increased prevalence of the agent in domesticated or wild animals or in vectors as many currently emerging and reemerging diseases have reservoirs in wildlife and/or are foodborne, e.g. Emerging Corona-viruses.
Breakdowns in public health measures such as sanitation and vaccination.
Changing land-use patterns may alter the number of reservoir hosts, increase the incidence of infection in these animals, encourage genetic changes in the pathogen (eg, recombination with other strains), or bring animal hosts or disease vectors into closer contact with people.
Degradation of natural habitats, as well as the ready availability of food near human dwellings, can encourage wildlife to move into suburban areas.
The growth of the human population also exerts pressures that ultimately result in increased contact with wildlife.
Climate change can be a factor in disease emergence, particularly for tick-borne pathogens such as Rickettsia spp.
Technological and industrial changes in food production can contribute to disease emergence by increasing the concentration, movement, and mixing of animals.
Long-distance transport has been associated with increased shedding of enteric pathogens, including Salmonella.
Decreased genetic diversity may eliminate species, breeds, or individuals with innate resistance to disease.
The development of large-scale farms and food-processing facilities has led to the exposure of greater numbers of people to contaminated food sources.
Increased mobility of people, animals, and goods allows diseases to spread quickly.
Who are at a higher risk of contracting zoonotic diseases?
The people that are more likely than others to get really sick, and even die, from infection with certain zoonotic diseases include:
Children younger than 5 years
Adults older than 65 years
People with weakened immune systems
Pregnant women
How are Zoonotic Diseases Treated?
The treatment of zoonotic and non-zoonotic diseases of animals is the same except that treatments that prolong the shedding of zoonotic organisms are avoided at all costs unless there are overriding considerations. Conversely, animals that carry zoonotic organisms may sometimes be treated to reduce human exposure, even when the infection is sub-clinical or expected to be self-limiting, such as a minor skin lesion caused by dermatophytes. During the treatment of zoonotic diseases, every precaution is taken to prevent human infection, professional judgment is considered to determine whether to keep the animal in its home environment or isolate it in a hospital ward.
Factors to consider include:-
The potential severity of the disease in people,
The susceptibility of individuals in the household, and
The ability of human caregiver(s) to effectively perform barrier nursing, sanitation, and hygiene protocols.
The animal owner is always informed if treatment is not certain to eliminate the pathogen, which could then persist in a latent or chronic, sub-clinical form. Zoonotic concerns may dictate euthanasia of the animal, especially when the disease is likely to be fatal, e.g. rabies.
Public health authorities must always be contacted when a reportable zoonotic disease (eg, rabies) is found in an animal.
How are Zoonoses prevented?
People are protected from some zoonoses by:-
Eliminating the pathogen from its animal reservoir(s).
Vaccination (e.g., rabies),
Treatment of clinical cases,
Periodic testing for parasites or other pathogens,
Good sanitation and hygiene during food preparation.
Water treatment procedures to eliminate most waterborne zoonoses.
Eliminate skin contact with soil, such as wearing gloves when gardening and avoiding dust inhalation, are helpful.
During contact with apparently healthy animals, good hygiene (including hand-washing) is an important preventive measure.
In veterinary hospitals, protective measures include barrier precautions (including gloves, protective outerwear, and other personal protective equipment as appropriate), good hygiene, sanitation and disinfection, appropriate disposal of infectious material, and use of isolation units for animals with known zoonoses.
References
Hugh-Jones, M. E. (2015, October). Overview of Anthrax (Splenic fever, Siberian ulcer, Charbon, Milzbrand). Merck Veterinary Manual.
Rozenbaum, M. (2020, July 6). The increase in zoonotic diseases-the who, the why and the when? Retrieved from Understanding Animal Research: https://www.understandinganimalresearch.org.uk/news/research-medical-benefits/
Spickler, A. R. (2015, May). Pathogens and Host Species in Zoonoses. Merck Veterinary Manual.
Spickler, A. R. (2015, May). Transmission of Zoonoses Between Animals and People. Merck Veterinary Manual.
Website(s):
https://www.merckvetmanual.com/public-health/zoonoses/emergence-and-reemergence-of-zoonotic-diseases
https://www.gov.uk/government/collections/zoonotic-diseases-zoonoses-guidance-data-and-analysis
https://www.avma.org/resources/pet-owners/petcare/zoonotic-diseases-and-pets-faq
https://www.cdc.gov/healthypets/specific-groups/high-risk/children.html
https://www.cdc.gov/healthypets/specific-groups/stay-healthy-animal-exhibits.html