Causes and Types of Diseases

Farm workers in a barn with a herd of pigsBiosecurity is a set of preventative measures designed to reduce the risks of infectious disease transmission to and among livestock. It means doing everything you can to reduce the chances of an infectious disease being carried onto your farm/property by people, animals, equipment or vehicles.

What Causes Diseases?

Three factors are needed to cause any disease:

  • Host

    Hosts for diseases can be animals, plants, people or microorganisms. The host's suseptibility (likeliness of being infected) to a disease can be influenced by age, strength of immunity, nutritional status, genetic makeup or hybrid vigor, vaccinations and breeding.

  • Agent

    An agent causes a disease or illness, and can be biological, chemical or physical in nature. Biological agents include bacteria, viruses, insects, prions, and fungi. Chemical agents are poisonous substances (produced by many types of organisms such as bacteria, plants and fungi), substances that cause allergies, and agricultural chemicals.

  • Environment

    Factors in the environment that can contribute to getting and spreading a disease include weather, geographic area, animal housing, animal health practices, sanitation, biosecurity procedures, and the presence of "vectors" (an organism, typically a biting insect or tick, that transmits a disease or parasite from one animal or plant to another). Many of these factors can be controlled to either prevent disease or minimize the impacts of them.

Disease Spread

Livestock diseases can be spread in many different ways. These routes are important to consider when creating biosecurity procedures and practices to protect animal health, so you can anticipate and manage any gaps and vulnerabilities. 

How diseases are spread in livestock:

  • Direct Contact

    When animals are close enough to touch. There is an immediate transfer of a disease agent to a host through open wounds, mucous membranes, or the skin. It may occur by contact with blood, saliva, nose-to-nose contact, rubbing, or biting from an infected animal. Reproductive contact includes diseases spread through mating or to the fetus during pregnancy.

  • Fomites/Indirect Contact

    Transfer of disease agent is indirect by contact with an inanimate object such as equipment, vehicles, clothing and footwear. A fomite passively transfers or carries a disease agent. Traffic is a subtype of fomite transmission in which a vehicle, trailer, or human spreads organic material to another location.

  • Aerosols

    The disease agent is contained in suspended particles or droplets passed through the air from one animal to another.

  • Ingestion

    Consumption of a disease agent in contaminated feed or water or by licking or chewing contaminated objects.

  • Vectors

    A disease agent spread by blood-feeding insects such as mosquitoes, ticks, biting midges and flies. Understanding the life cycle of vectors is a factor in their control, such as alternate hosts, favored breeding locations, and time of year of emergence.

Types of Diseases

In general, there are four major categories of diseases that can affect livestock. Some are considered high consequence, meaning they spread rapidly from animal to animal/herd to herd, and are expensive and difficult to eradicate. The response to high consequense diseases involves various state and federal agencies in the efforts to control the further spread.

  • Zoonotic

    A disease that can be passed directly or indirectly between animals and humans. Domestic animals, wild animals and insect species are the common link as either the origin, a reservoir (animal or insect that carries an infectious agent but is not harmed by it) or vector (disease acquired from blood-feeding insects). Some examples of zoonotic diseases include rabies, anthrax, West Nile virus, Lyme disease and salmonellosis.

  • Reportable Diseases/Foreign Animal Diseases

    A reportable animal disease is one that, by law, must be reported to state and/or federal animal or public health officials, typically by a livestock producer's veterinarian. Reportable means that the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) has a control or eradication program in place for the disease. Reporting helps identify disease outbreaks, limits their spread and minimizes the economic and health impacts on animals and people. These diseases are high risk with severe economic, animal health and often public health consequences. Examples of reportable diseases include brucellosis, scrapie, bovine tuberculosis, pseudorabies, New World screwworm and vesicular stomatitis.

    Among reportable diseases are foreign animal diseases (FAD), also called transboundary animal diseases. These are also high risk animal diseases that are not normally present in the United States or have been previously eradicated, such as foot and mouth disease, highly pathogenic avian influenza, African swine fever, classical swine fever and virulent Newcastle disease.

  • Endemic Diseases

    Endemic is the constant presence and/or commonness of a disease or infectious agent of animals within a geographic area. Anthrax is endemic in limited areas of the western and midwestern United States, for example. Other levels of disease such as epidemic refers to an often sudden increase in the number of cases of a disease above what is normally expected in a population in a location. Outbreak carries the same definition as epidemic, but is often used for a more limited geographic area. Sporadic refers to a disease that occurs infrequently and irregularly.

  • Emerging Diseases

    A disease can be considered "emerging" if it is newly identified or previously unknown, causes disease, infection or infestation in animals, and has the potential to result in significant animal or public health impacts. It might also be a previously known disease that has changed in some way, either by an increased ability to cause disease, an expanded host range, a change in geography, and/or causing unexpected sickness and death. West Nile virus, avian influenza and porcine epidemic diarrhea virus are some examples of emerging diseases.

How do you manage risks?

Are you risk averse or risk tolerant? What does it mean for you, and for biosecurity planning?