Ecology Theory

Zoonotic diseases

This article was researched and written by a student at Texas Tech University participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.


Zoonoses are diseases and infections which are naturally transmitted between vertebrate animals and humans. The pathogens that cause zoonotic diseases are bacteria, fungi, viruses, parasites or protozoa. Disease transfer occurs via direct or indirect exposure to the pathogen. A pathogen is contracted directly through skin to skin contact of an infected animal or through ingestion of diseased animal products.These products can be infected feces, urine, saliva, blood, milk or other bodily fluids. A bite or sting from a diseased animal or through an infected invertebrate host directly transfers the pathogen to humans as well. Indirect exposure occurs through air, water or soil in which the pathogen can survive for a limited amount of time without a host.

Historically Significant Zoonoses

caption Human Population Growth. (Source: CDC)

Many historical epidemics have had a zoonotic origin. Once a disease is transferred to a human from an animal, the disease has the ability to be passed very rapidly from person-to-person. The following are a few significant zoonotic diseases in history.

Bubonic Plague

The bubonic plague, or Black Death, epidemic seems to have begun in China in the early 1330s. An Italian merchant ship that had traded with Chinese merchants carried infected rats and crewmembers dying of the plague back to Sicily. From this start, the disease spread rapidly throughout Europe. It is believed the plague killed 137 million people in a 400-year period. Presently, the World Health Organization reports that 1,000-3,000 cases of plague are reported each year.

Bubonic plague is caused by the bacterium Yersinia pestisi. The cycle through which the plague is perpetuated starts when a flea bites an infected rat. The bacteria multiples within the flea’s gut and then the flea bites a human who becomes infected. The most common form of infection in a human is a swollen and painful lymph gland that forms buboes, from which the disease is named.


Malaria is a very old disease, thought to have originated in Africa about 10,000 years ago. By the 19th century, malaria had spread globally. Over one-half of the world’s population was at significant risk, and 1 in 10 people affected were expected to die from it. Today, 350-500 million cases of malaria occur worldwide, with over one million people who die from it each year.

Malaria is a mosquito-borne disease that can use non-human primates as reservoirs. It is caused by the parasite Plasmodium. Some of the different malaria parasites include Plasmodium vivax, P. malariae, P. falciparum and P. ovale. Infected people often experience fever, chills and flu-like illness. If left untreated, severe complications may ensue and death is likely.


References to measles can be found dating back to the 7th century A.D. By some people it was described as being more dreaded than smallpox. By the 16th century, most people grown in Europe, Asia and North America had already had measles and so were immune to it. As such, it became a disease of mainly children in these areas. The worst measles plague was when European traders and explorers gave measles, along with smallpox, to the people of North and South America in the 1500s. These people had no resistance to the disease. About nine out of ten people in North and South America were killed from measles and small pox.

Measles is a viral disease in the family Paramyxoviridae and the genus Morbillivirus. This disease can spread from monkey to man, man to man, man to monkey and monkey to monkey. It is highly contagious. Signs of measles include rash in the mouth and on the cheeks, neck, chest and body. Other complications may occur such as middle ear infection, bronchopneumonia and encephalitis.

Factors Influencing Disease Transfer

caption Ringworm. (Source: CDC)

The length of time an animal is infected with the pathogen determines how potent the infection is within the animal. The more time the pathogen spends in an animal, the more time there is for the pathogen to reproduce or multiple. Incubation time may be important for some diseases. Animals may be studied or euthanatized before they become infective for humans if the disease has a long incubation period.

The stability of the pathogenic agent influences the risk of disease transmission. This is more important in cases where the agent may be exposed to environmental changes. As such, climate and geography are major factors in zoonotic diseases. There are certain conditions under which a pathogen, such as a bacterium, parasite or virus, can survive. Temperature and precipitation tend to affect more agents, but wind, soil type, sea level elevation and daylight duration can also play a role. For example, ringworm is a zoonotic fungal infection that affects many mammals including cattle, dogs and cats. Most cases of ringworm occur in the fall and winter and are less prevalent during the spring and summer.

Population density of an animal within an area affects the spreading of a disease. A larger population means there is a greater chance that more individuals are infected, and it gives the pathogen more hosts with which to survive and infect other animals and humans. Other factors in disease transfer include the virulence of the agent, the route of transmission, and the maintenance and control of insects and wild rodents which can harbor many different pathogens.

Classification of Zoonoses

The World Health Organization Expert Committee on Zoonoses came up with the following classification system of zoonoses based on the type of life cycle of the pathogen.

Direct Zoonoses

Direct zoonoses are transmitted from an infected vertebrate host to another host by direct contact, fomite or mechanical vector. The pathogen does not undergo developmental change or propagation during the transmission. An example is rabies.


A cyclozoonosis requires more than one vertebrate host but no invertebrate host is needed.


A metazoonosis does require an invertebrate host where the pathogen multiples or develops before it can infect a vertebrate host. An example of a metazoonotic disease is bubonic plague.


Saprozoonoses are diseases transferred through a non-animal reservoir, such as a plant, or through the abiotic environment, such as through water or soil.

An Increasing World Problem

Zoonotic diseases are increasingly becoming more of a world problem due to several factors. As the human population continues to swell, more bodies are made available to become reservoirs for disease. The chance of disease transmission from person-to-person will increase because of the greater human density within an area. In some places around the world, economic instability has caused breakdowns in public health. This can be attributed to the increased population where not enough resources exist, such as food, clean water and shelter. A bigger population also means that there are greater numbers of older people and immuno-compromised people who are more easily infected with disease. Animal populations are increasing as well, so that there are larger numbers of people and animals in close contact.

Methods of travel are much more advanced than they were in the past. People can cross whole oceans or continents in a matter of hours using a ship, airplane, automobile or train. The ease with which it is to travel has caused the more people to travel meaning diseases can be introduced to new areas. Infectious agents can also be unknowingly transported, such as infected animals or diseased microbes carried on clothes or flesh.The movement of people into previously uninhabited areas can introduce new infectious strains into the human population as well.

There is a lot of worry about the use of zoonoses in bioterrorism where diseased animals may be intentionally introduced to a population. The CDC’s Bioterrorism Preparedness and Response Office developed a classification scheme for diseases. Category A contains the highest priority cases such as plague, anthrax, botulism, smallpox and tularemia. Category B is the second highest priority diseases such as typhus fever, viral encephalitis, toxins, food safety threats and water safety threats. Category C is the list of diseases with the least priority. This list includes nipah virus and hantavirus.

Some Known Zoonoses

Bacterial Diseases

Rickettsial diseases

  • Boutonneuse fever
  • Ehrlichiosis, Sennetse fever
  • Murine typhus
  • North Asia tick-borne rickettsiosis
  • Q fever
  • Queensland tick typhus
  • Rickettsial pox
  • Rocky Mountain spotted fever
  • Scrub typhus
  • Typhus

Fungal Diseases

  • Aspergillosis, Allergic bronchopulmonary aspergillosis
  • Blastomycosis
  • Candidiasis
  • Coccidioidomycosis
  • Cryptococcosis
  • Dermatophilosis
  • Histoplasmosis
  • Nocardiosis
  • Ringworm
  • Babesiosis
  • Balantidiasis
  • Chagas’ disease
  • Cutaneous and mucosal Malaria
  • Pneumosystis pneumonia
  • Sarcocystosis
  • Toxoplasmosis
  • Trypanosomiasis

Parasitic and Protozoan diseases

  • Clonorchiasis
  • Dicrocoeliasis
  • Echinostomiasis
  • Fascioliasis
  • Fasciolopsiasis
  • Gastrodiscoidiasis
  • Heterophyiasis
  • Metagonimiasis
  • Nanophyetiasis
  • Opisthorchiasis
  • Paragonimiasis
  • Swimmer’s itch
  • Bertielliasis
  • Coenuriasis
  • Diphyllobothriasis (Fish tapeworm infection)
  • Dipylidiasis (Dog tapeworm infection)
  • Echinococcosis
  • Hymenolepiasis (Dwarf tapeworm infection)
  • Inermicapsifer infection
  • Mouse or rate tapeworm
  • Pork tapeworm disease
  • Asian taeniasis
  • Raillietine infection
  • Sparganosis
  • Taeniasis (beef tapeworm disease)
  • Cysticercosis
  • Angiostrongyliasis (visceral larva migrans)
  • Anisakiasis (visceral larva migrans)
  • Hepatic capillariasis
  • Intestinal capillariasis
  • Pulmonary capillariasis
  • Dioctophymosis (Giant kidney worm infection)
  • Dracunculiasis (Guinea worm infection)
  • Dirofilariasis
  • Malayan filariasis
  • Tropical eosinophilia
  • Gnathostomiasis
  • Gongylonemiasis
  • Larva migrans, cutaneous
  • Larva migrans, visceral
  • Oesophagostomiasis ternidensiasis
  • Strongyloidiasis
  • Thelaziasis
  • Trichinosis
  • Trichostrongyliasis
  • Trichuriasis (whipworm infection)
  • Macracanthorhynchosis
  • Hirudiniasis
  • Acariasis (mange)
  • Myiasis
  • Pentastomid infections
  • Tick paralysis
  • Tunga infections

Viral Diseases

  • African hemorrhagic fever
  • Filovirus infections
  • Argentinean hemorrhagic fever
  • Bolivian hemorrhagic fever
  • Brazilian hemorrhagic fever
  • California group infections
  • LaCross encephalitis
  • Tahyna fever
  • Central European tick-born encephalitis
  • Colorado tick fever
  • Contagious ecthyma (Orf)
  • Cowpox
  • Crimean-Congo hemorrhagic fever
  • Eastern equine encephalomyelitis
  • Encephalomyocarditis
  • Far eastern tick-borne encephalitis (Russian spring-summer encephalitis)
  • Foot-and-mouth disease
  • Hantaviral disease
  • Hantaviral pulmonary syndrome
  • Hemorrhagic fever with renal syndrome (Korean hemorrhagic fever)
  • Other hantaviral diseases
  • Simian herpes B virus disease
  • Influenza
  • Japanese B encephalitis
  • Kyasanur forest disease
  • Lassa fever
  • Louping ill
  • Lymphocytic choriomeningitis
  • Milker’s nodules (Pseudocowpox)
  • Monkey pox
  • Murray Valley encephalitis
  • Newcastle disease
  • Omsk hemorrhagic fever
  • Rabies and rabies-related infection
  • Lyssaencephalitis
  • Rift Valley fever
  • St. Louis encephalitis
  • Sindbis virus disease
  • Ross River fever
  • Ranapox
  • Venezuelan hemorrhagic fever
  • Venezuelan equine encephalomyelitis
  • Vesicular stomatitis
  • Wesselsbron fever
  • West Nile fever
  • Western equine encephalomyelitis
  • Yabapox

Further Reading



Battershell, L. (2011). Zoonotic diseases. Retrieved from


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