chapter  18
66 Pages

Viral Pathogens

Viruses are the simplest type ofmicroorganism consisting of a protein outer coat containing genetic material (i.e., RNA, DNA). They are much smaller than bacteria and vary in size from0.01 to 0.27µm. They are anobligate (single set of conditions) intracellular parasite and lack a system for their own metabolism. They replicate by taking over metabolic processes of the invaded host cell and redirecting them toward virus production. Approximately 60% of infectious disease in man is caused by a virus. Most treatment for viral infections is supportive because of limited antiviral medications. Viruses are very costly, time consuming, and difficult to grow in large quantities. They

only replicate in living cells and cannot be grown in a growth media like bacteria (Chapter 17). In some cases, viruses can be grown on the chorioallantoicmembrane of fertilized eggs. Pathogens employed as biologicalweapons can beused for both lethal and incapacitating

purposes. They cause disease by invading tissue or by producing toxins (Chapter 16) that are detrimental to the infected individual. Pathogens can be selected to target a specific host (e.g., humans, cows, pigs) or they may pose a broad threat to both animals and to people. Pathogens deployed as antianimal biological weapons are generally used to produce

lethal effects in an agriculturally significant species such as cows, pigs, or chickens. Although these pathogens are selected to target a specific animal species, there is the possibility that the disease may migrate to humans. The diseases produced by these crossover pathogens may be difficult for medical personnel not trained in exotic pathology to diagnose. Other pathogens are selected to produce lethal effects in an agriculturally significant crop

species such as wheat, corn, or rice. Symptoms in plants vary greatly and include mottling of leaves, stunting, lesions, leaf rolling, and death. Natural transmission fromplant to plant is often by insects. There is little potential for migration of these pathogens to humans or animals. A final group of biological warfare pathogens are those used as simulants to model

the release of other, more hazardous agents. Pathogens employed as biological warfare simulants do not generally pose a significant risk to people, animals, or plants. However, individuals with respiratory illness or suppressed immune systems may be at risk should they be exposed to an infectious dose of the agent. Some viruses can be crystallized, similar to chemical compounds, while others can be

stored as freeze-dried powders. In these forms, viruses are easy to disperse. However,

limitations to themethods that can be used. They can also be stored and dispersed via infected vectors (e.g., mosquitoes, fleas). In most cases, large-scale attacks will be clandestine and only detected through epidemiological analysis of resulting disease patterns. Localized or small-scale attacks may take the form of “anthrax” letters. Even in these cases, without the inclusion of a threat the attack may go unnoticed until the disease appears in exposed individuals (e.g., the initial 2001 anthrax attack at American Media Inc., which claimed the life of Robert Stevens). In general, unless a local reservoir (i.e., intermediate host thatmay ormay not be affected

by the virus) is established, pathogens are easily killed by unfavorable environmental factors such as fluctuations in temperature, humidity, food sources, or ultraviolet light. For this reason, their persistency is generally limited to days. However, freeze-dried pathogens can remain in a preserved state almost indefinitely and are reactivated when exposed to moisture. It is possible that local insects can become both a reservoir and a vector for the pathogen.