Zoonotic Diseases

By Frank G. Vice, DVM, BS Pharm

Zoonotic DiseasesDorothy H. Crawford wrote in her book, Viruses, A Very Short Introduction, “It must have taken a huge leap of faith for people to accept that tiny, living organisms were responsible for diseases.” The key to this momentous leap in microbe understanding is credited to the Dutch lens-maker Antony Van Leeuwenhoek who, in the 17th century, developed the microscope lens which allowed him to see tiny microbes in water droplets. It was later, during the 1800’s, that Louis Pasteur and Robert Koch finally established the” germ theory” which described how tiny microbes caused infectious disease. Late in the 19th century, Robert Koch isolated the first bacterium, Bacillus anthracis, which is the causative agent for the zoonotic disease of anthrax. Louis Pasteur began to develop an understanding about rabies, which eventually lead to a vaccine. Both of these zoonotic diseases infect humans, a result of exposure to micro-organisms that cause the diseases that have infected the host reservoir animals. These types of infections are referred to as a zoonosis. Most zoonotic infections are due to microscopic organisms – viral, bacterial, fungal and parasitic, which are passed as microbes directly from the infected animal to humans, or easily transmitted by vectors like fleas, mosquitos or ticks.

In 2016, Michael J. Day wrote an article for family physicians which was published by “The Journal of American Family Physicians”. The article describes a short list of zoonotic infections found in domesticated animals. He described dermatophytosis (fungal ringworm) which is easily contracted from cats and dogs, parasitic sarcoptic mange (scabies) infections from pigs and dogs, toxoplasmosis which is acquired from exposure to cat feces and is very dangerous to pregnant women, and a bacterial infection known as bartonellosis (cat scratch fever). People that have reptiles, amphibians and backyard poultry as pets should be aware of the risk of contracting a salmonella infection. Leptospirosis is more often acquired through wild animal exposure. However, infections can occur from unvaccinated dogs. Rats commonly carry and shed leptospirosis organisms through their urine creating Weil’s disease in humans.

People working in agriculture have animal contact throughout their life with exposure to cattle, goats, sheep, poultry and horses. These animals are prime candidates for tuberculosis, Q fever, anthrax, and brucellosis, all of which, are included in the list of dangerous agricultural zoonotic infections. It is probably impossible to calculate the daily interactions humans have with animals. These interactions would include domesticated companions as well as agricultural animals. This does not begin to address the wide variety of wild and exotic species that occupy all parts of the earth. Any of these animal interactions offer opportunities for disease exposure. State, federal and global public health authorities monitor all infectious disease potential on a daily basis. Potential infections develop through vocational responsibilities and domesticated pet companionship. In addition, humans are exposed to zoonotic illness from exotic untamed wildlife reservoir host animals that live throughout the world. The list of all potential zoonotic infections is extensive and beyond the scope of this brief article. However, strict caution should always be observed with any unknown animal interaction due to the possible infection opportunities. Many zoonotic transmission opportunities can occur while hunting, hiking, boating, visiting petting zoos, and living and working on a farm associated with livestock. Some zoonotic microbes are easily transmitted through air, skin or from eating contaminated or fresh uncooked meat. Some of the preventive health measures used to avoid these infections include good hygiene (hand washing), adequate vaccinations for both humans and animals, a competent immune system to provide for our health protection, washing and cooking food appropriately and avoiding fleas and ticks.

According to a 2013 article printed in “Nature Reviews Immunology”, emerging infectious diseases have risen in humans during the past three decades, with 70% of the new infections due to zoonotic viruses or drug-resistant virulent bacterial pathogens. The Center for Disease Control, CDC, aggressively works to provide help in managing emerging zoonotic diseases as they develop around the globe. Many of these new emerging diseases are not found in the United States, however, the impact of their infectious nature is felt in America when international travel is altered due to a potential infected Ebola patient. In addition, bird flu (influenza A) mostly infects poultry but occasionally spreads to humans. All across China, the virus that could spark the next pandemic is already circulating as bird flu (H7N9 or H5N1). This is a serious concern if this type of influenza virus begins to spread from human to human. Currently, this influenza virus spreads only from infected birds, dead or alive.

Emerging zoonotic infectious diseases originating from animal sources have an enormous impact on human health. Many microbes are capable of transmission from animal to humans, with viruses accounting for a significant proportion of emerging infections. The majority have zoonotic origins. It has been suggested that environmental changes, especially in equatorial areas of the world, lead to the emergence of zoonotic diseases. These environmental changes include modernization of farming practices especially in developing areas of the world. Expanded farming leads to habitat destruction and ultimately human encroachment into previous wilderness areas. The resulting change in ecological land balance creates an interaction between wild life and their zoonotic reservoir host. The new changes between wild life and human perpetuates the opportunity for disease exposure by pushing wildlife and domesticated livestock into overlapping environments, all the while significantly affecting vector populations, ticks and mosquito’s, as they increase in numbers. In addition, emerging zoonotic viral infections can occur directly, infecting people from the live reservoir host, posing a serious threat to human and animal health. As an example, there are approximately 1,200 bat species spread around the world. Mammals, such as bats, are environmentally necessary, but human contact should be approached with strict caution due to the wide variety potential pathogenic infections they carry. Pathogenic infections remain in the body of a bat, especially the oral cavity, without symptoms or danger to the bat, until human contact occurs. Many bats shed virulent pathogenic virus through urine, salvia and feces and are known to carry rabies virus. Throughout the world, bats are considered reservoirs for many life threating viral pathogenic infections. These mammals are the natural reservoir for several fatal zoonotic infections. These infected animals can appear healthy and continue to represent a host reservoir source for disease. They will remain an infectious carrier but never get sick or experience the symptoms of the disease even though the same infection in a human could be life threating.

The World Health Organization (WHO) has considered the impact of emerging zoonotic infectious diseases and warned that the source of the next pandemic is likely to be zoonotic and that wild life will be the prime culprit. Although the current list of known emerging infectious diseases is a major concern, it is the unknown infection, with the potential for efficient human to human transmission, that poses the greatest threat. Numerous emerging disease concerns are closely connected to an ever increasing interaction between humans and wildlife. As the escalated need for food production continues, the present and future demands for more land will force more intrusion of agriculture into previously untouched wilderness. In addition, increased globalization and travel have increased the opportunity for rapid disease transmission.

Global health strategies are designed to monitor potential disease outbreaks and part of those strategies include the development of collaborative surveillance system termed “One-Health”. This surveillance approach blends clinical research, along with veterinary medical expertise, in an effort to understand and reduce the impact of zoonotic diseases. This collaborative medical monitoring method will be critically important for preparedness for the next potential zoonotic disaster.

The author wishes to thank the following literary references; Dorothy H. Crawford.” Viruses, a Very Short Introduction.” 2011, Time Magazine, May 15, 2017 Brian Walsh, “The pandemic Panic”, Michael J. Day, “Pet Related Infections” American Family Physician, volume 94 number 10, November15, 2016, CDC/NCID Report, Addressing Emerging Infectious Disease Threats A Prevention Strategy for the United States, U.S Department of Health and Human Services 1994. Center for Disease Control, Emerging Infectious Diseases, Volume 24 Number 2, 2018

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