4. Biological Agents Biological agents range in size from microscopic to large complex organisms. Biological agents include viruses, bacteria, protozoa, ectoparasites and nematodes. have two mechanisms by which they cause cellular injury. The first is cytologic. In this case the virus inserts its own genetic information into the nucleus of the host cell, thereby using the cell’s organelles to replicate viral components and produce more viruses. The second mechanism is oncogenic. In this case the virus triggers the host cell to become cancerous and replicate rapidly and uncontrollably, spreading the virus as the tumour grows. Some are harmful while others are beneficial to normal body function. For example, the flora of the gastrointestinal tract aid in the breakdown of food molecules, produce some vitamins and even help to protect the gastrointestinal tract from harmful agents. Other bacteria harm the body through the production of . Certain bacteria produce toxic molecules and either coat themselves in these molecules or else secrete these molecules into their environment. Toxins found on the surface of bacteria are called . Other bacteria only release their toxic molecules when they die and are broken down. These molecules are called . Both exotoxins and endotoxins usually have an effect on the cell by disrupting receptors on the cell membrane or interfering with the activity of mediator molecules. Occasionally there will be an explosion in the population of normal bacteria and bacterial colonies may spread to regions of the body that are not designed to support bacteria. In these cases, the bacteria damage cells and necrosis occurs. We see this as pus when the normal bacteria living on the outside of skin enters a wound. When there is a bacterial population in the blood we call this . Like bacteria, are unicellular organisms. Protozoa damage tissue as they consume organic materials as food. Some protozoa perform phagocytosis while others have pores that act like mouths. In horses the major impact protozoa have on tissues is when they enter a cyst phase and remain in tissues such as muscle and nerves. Protozoal cysts release chemicals that irritate the surrounding tissues and trigger an immune reaction. live on the skin surface and compete with the host organism for nutrients. For example, fleas bite the skin and compete with the host tissues for blood. Other ectoparasites inject venoms or chemicals into the host tissues that cause lysis of particular cells, or block certain receptors. are a class of internal parasites that are commonly called . Like ectoparasites, many nematodes compete with the host for nutritional resources. Other nematodes damage tissues as they consume them, or during migration. In the horse, blood vessels and even the heart are damaged by migrating nematodes. Still other nematodes produce toxic waste products that chemically interfere with normal cell function.
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