Fanimal Zoonotic Diseases

Berta Garrido Zabala

We speak of zoonotic diseases, also known as zoonosis, when an animal-specific disease begins to affect humans. There are three classes when describing zoonosis, endemic zoonosis that are present in many locations and affect a wide number of people and animals; epidemic zoonoses that are sporadic and temporary; and emerging  zoonoses that first appear in a population or have existed previously but whose incidence or geographic range is rapidly increasing.

A zoonotic disease can be caused either by bacteria, viruses, parasites or fungi. In fact, zoonosis is the cause of many of the diseases we are familiar with today. One of the particularities of zoonosis is that animals can be carriers of the diseases even if they do not show symptoms, making the identification even harder. Moreover, it is known that the effect they provoke can differ among humans. 

The spread of zoonotic diseases occurs during human-animal interaction through direct or indirect exposure. The ways to get infected can be divided into several subclasses. The main form of transmission is through direct contact with infected animals. Direct contact includes being scratched or bitten by the animal in question, but also being in contact with the saliva, blood, urine or feces of an infected animal. Indirect human-animal contact might occur in different ways. It can happen if the individual goes to a place where the infected animal has been or touches the surface where the affected animal has lain or slept. 

Zoonotic diseases can also be transmitted by vectors; the most common vectors are mosquitoes and ticks. Mosquito-borne diseases are transmitted by females who need protein and iron from the blood to lay the larvae. When an infected animal is bitten by a mosquito, it will carry the pathogen in its salivary glands. Afterward, when the same mosquito bites a human, the pathogen is spread and the human becomes infected. Depending on the type of disease, there are cases where the pathogen develops inside the mosquito. Similarly, tick-borne pathogens can be transferred to humans through the bite of infected ticks. Ticks can be infected with bacteria, viruses, or parasites.

Foodborne and waterborne zoonoses are caused by the consumption of contaminated food or water with disease-causing pathogen microorganisms, such as bacteria, viruses and parasites. They enter the body through the digestive tract, where the initial symptoms often occur. Contamination can occur at any point in the water or food supply chain, either on the farm, at the slaughterhouse, or during processing or preparation. On the farm it may be due to the fact that animal-feed contains bacteria that cause animal infections or by parasites that infect food-producing animals. When processing the food, it can get contaminated if it is in contact with other raw agricultural products or in contact with contaminated surfaces. Waterborne diseases remain a major problem today, faced by both economically developed and undeveloped countries. Waterborne pathogens are mainly pathogens transmitted by the fecal-oral route and by drinking water. Waterborne transmission also includes droplet inhalation and contact exposure. It is interconnected with consumption of seafood and other fishing products and indirect exposure to water in food when water is used for irrigation, in food processing, or as an ingredient.

Examples of Zoonotic diseases

Rabies is a fatal disease of the nervous system that to date remains incurable. Luckily it can be avoided by vaccination. It is generated by the bullet-shape rabies virus, part of the family lyssavirus. This disease is normally transmitted by the bite of a rabid animal. All mammals can get rabies but only a few are really considered reservoirs for the disease such as racoons, bats or foxes. There are 59.000 annual deaths caused by rabies. Despite the efforts to prevent this disease from domestic animals, 99% of the human deaths occurring from rabies are caused by dog bites.

Toxoplasmosis is a disease caused by the parasite Toxoplasma gondii. Most people who have the parasite do not become ill. However, it can be especially severe in immunosuppressed people and babies whose mothers contracted the disease during pregnancy. Symptoms include damage to the brain, eyes and other organs. The most common way of contracting the disease is through cat feces, with sandboxes being the major focus of infection.

Leishmaniasis is an example of a vector-borne disease. In this case it is caused by the bite of a phlebotomine infected mosquito with the protozoan parasite lesihmania. There are several different types of leishmaniasis. The most common are cutaneous and visceral. The cutaneous type causes skin sores and the visceral type affects internal organs. People with this disease usually have a fever, weight loss and an enlarged spleen and liver.

Salmonellosis is the most common foodborne zoonosis. It is caused by a group of bacteria called Salmonella. Salmonella is found in raw poultry, eggs, beef, and sometimes unwashed fruits and vegetables. It can also be acquired after handling pets, especially reptiles such as snakes, turtles and lizards. Symptoms include fever, diarrhea, abdominal cramps, headache and nausea among others.

Other examples include Anthrax, Animal Influenza, Leptospirosis, Lyme disease, Ebola virus disease, yellow fever, and different types of coronavirus, among many more. The main issue of these diseases is that they account for four-fifths of human infections equating to one billion cases of illnesses per year. 

It is our responsibility to prevent the spread of infection and to control the situation. Among the activities that everyone can do, the most important is to wash your hands often, especially if you have been around animals, even if you have not touched them. In addition, try to prevent bites from mosquitoes, ticks and fleas, using environmentally-friendly insect repellent. Learn more about ways to handle food safely, whether it is for you or your family, your pet, or other animals. Avoid animal bites and scratches and always contact your doctor in case of possible infection. 

Needless to say, human intrusion into biodiverse areas increase the risk of spreading new infectious diseases by enabling new contacts between humans and wildlife. Respecting wild animals and their environment, as well as decreasing contact with wildlife is an additional way to curb the spread of zoonotic diseases. 

Sources: Centers of Disease Control and Prevention (CDC), Medline Plus, World Health Organization (WHO)