Routes of transmission of infectious diseases agents
The mode of introduction of an exotic disease of animals will depend on the route of transmission for a given agent. Thus, the mode of introduction of a given pathogen will depend on whether it is transmitted horizontally from animal to animal or vertically from parent to offspring, if an intermediate host or vector is required for transmission, how persistent the pathogen is in the environment, and whether the disease agent is immediately infectious or requires time in the environment to develop to the infectious stage.
Horizontal transmission is the transfer of a pathogen from an infected animal to a na�ve animal, independent of the parental relationship of those individuals. Vertical transmission is the transfer of a pathogen from a parent, usually the dam, to the offspring through reproduction.
Horizontal transmission
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Horizontal transmission may occur by either direct or indirect contact. Disease agents may be transferred by direct contact through actions such as licking, rubbing, biting, and coitus. An example of horizontal transmission by direct contact is hog cholera virus, the causative agent of Classical Swine Fever (CSF). This virus is transmitted from pig to pig primarily by oral contact with blood, tissue, and secretions. The most common method of introduction of this disease is by mixing infected pigs with noninfected pigs, thus livestock shows and auction sales are two high-risk places for infection. Feeding of raw or undercooked garbage to pigs may also contribute to the spread of the virus.
Airborne transmission is considered another form of direct horizontal transmission because pathogenic agents contained in aerosols usually don’t survive extended periods of time within the aerosolized particles and thus close proximity of infected and non-infected susceptible individuals is required for transmission. A classic example of a disease that uses airborne transmission is Avian Influenza. The virus that causes Avian Influenza is transmitted through the air when birds are in close contact.
Direct contact may be with an infected animal in the same group or with an animal from a reservoir population. Reservoir hosts serve as habitat for the pathogen to survive and may or may not become ill from the infection.
Indirect contact via vectors or fomites also allows transmission of disease agents. The term vector is sometimes used in a broad sense to signify anything that allows the transport and/or transmission of a pathogen. However, according to a strict, ecological definition, vector-borne transmission occurs when a living creature, because of its ecological relationship to others, acquires a pathogen from one living host and transmits it to another. Thus vector-borne transmission is a form of indirect horizontal transmission in which a biological intermediary, often an arthropod, carries a disease agent between animals.
Vectors may be either biological or mechanical. A biological vector is a vector that supports replication of the pathogen. The disease agent and the biological vector are considered to have a long-standing ecological relationship. Biological vectors are usually persistently infected with the disease agent and may even be a required part of that organism’s life cycle. A mechanical vector, on the other hand, is a vector that carries the pathogen but the pathogen is not altered while on the vector. Infection in mechanical vectors tends to be short-lived and a mechanical vector is considered little more than a flying fomite.
| Flies feeding on cow may transmit the agent of pink-eye mechanically between animals. |  |
Fomites are inanimate objects that can carry infectious agents from one animal to another. Examples of fomites include used needles, dirty clippers, contaminated clothing or vehicles, and contaminated food and water supplies. Iatrogenic transmission is a specific form of horizontal transmission by fomites in which the veterinarian or physician accidentally furthers the spread of a disease agent via routes such as contaminated instruments or vaccines.
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In the western United States, anaplasmosis caused by Anaplasma marginale in cattle is transmitted by members of the hard tick genera Dermacentor and Boophilus and thus is a vector borne disease. However, in the southeastern and midwestern US mechanical transmission by biting flies and iatrogenic transmission with contaminated equipment and needles appears to be more important in maintaining this disease in cattle. Routine procedures such as bleeding, tagging, dehorning, and vaccinating can create an opportunity for iatrogenic transmission through contaminated pieces of medical equipment, which serve as fomites for the disease agent.
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