Pathogen transmission (Forensic Science)
Bacteria, protozoa, fungi, and viruses can all act as natural pathogens. Transmission occurs either directly, through contact with a host, or indirectly, through contact with a contaminated object (a fomite) or with a vehicle carrying the pathogen, such as air, water, or food. Living vectors, such as insects or animals, can also transmit various pathogens. Each pathogen has a specific way it gains entry to a host to cause disease and a specific manner by which it is released from the initial host to find a new host.
Criminals employing pathogens as weapons can use any of the natural transfer mechanisms to inflict damage or death, or they can artificially cause transfer, introducing pathogens or their parts (such as spores or toxins) by routes that the natural organisms would not or could not use. For example, consider the toxin produced by the bacterium Clostridium botulinum. This organism is normally found in the soil and infects improperly prepared foods. A malefactor, however, might affect a single individual by putting the bacterium in the target’s food or injecting it into the person; alternatively, terrorists might contaminate a city’s water supply with C. botulinum, potentially affecting millions of people.
Investigators working to solve cases involving pathogen transmission typically use techniques of epidemiology, the science of understanding how diseases behave in populations. These techniques include...
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Further Reading (Forensic Science)
Henderson, Donald A., and Thomas V. Inglesby. Bioterrorism: Guidelines for Medical and Public Health Management. Chicago: American Medical Association, 2002.
Loue, Sana. Case Studies in forensic Epidemiology. New York: Springer, 2002.
Wheelis, Mark, Lajos Rózsa, and Malcolm Dando, eds. Deadly Cultures: Biological Weapons Since 1945. Cambridge, Mass.: Harvard University Press, 2006.
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Pathogen Transmission (World of Forensic Science)
Forensic investigation of an illness, outbreak, or a death can be concerned with disease causing (pathogenic) microorganisms and, more specifically, with their route of transmission. Unearthing how an organism infected the victim(s) can be crucial when the organism is capable of spreading through a population quickly, or is a threat to public health.
Pathogens are microorganisms such as viruses, bacteria, protozoa, and fungi that cause disease in humans and other species. Pathogen transmission involves three steps: escape from the host, travel, and infection of the new host. Pathogen transmission occurs in several ways, usually dependent on the ecology of the organism. For example, respiratory pathogens are usually airborne, while pathogens of the digestive tract tend to be food- or waterborne. Epidemiologists group pathogen transmission into two general typesirect and indirect contactithin which there are several mechanisms.
Pathogen transmission by direct contact takes place when an infected host transmits a disease directly to another host. The pathogens that travel this way are extremely sensitive to the environment and cannot be outside of the host for any length of time. For example, pathogens that cause sexually transmitted diseases (STDs) are transmitted via blood, semen, or saliva. Some pathogens responsible for STDs include Tremonema palidum (syphilis), Neisseria gohorrhoeae (gonorrhea) and human immunodeficiency virus (HIV) (acquired immunodeficiency syndrome or AIDS). The viruses responsible for hemorrhagic fever, such as Ebola, are also transmitted by direct contact via the blood.
Indirect transmission occurs when an agent is required to transfer the pathogen from an infected host to a susceptible host. The agent may be either animate or inanimate. Inanimate forms of transmission include air, water, and food, which are referred to as disease vehicles. Inanimate agents also include fomites, which are objects on which the pathogen has been deposited. Examples of fomites are toys, clothes, bedding, or surgical instruments. Animate, or living, agents of disease transmission are most often insects, mites, fleas, and rodents. Living agents of transmission are referred to as vectors. Diseases that are spread via indirect contact in hospitals are specifically referred to as nosocomial infections.
Many respiratory viruses and bacterial spores are light enough to be lifted by the wind. These agents can subsequently be inhaled, where they cause lung infections. A particularly important example of an airborne bacterial pathogen is the spore form of the anthrax-causing bacterium Bacillus anthracis. This bacterium forms spores that can spread through the air and causes a severe respiratory disease when inhaled.
A common route of indirect pathogen transmission is via water. The ingestion of contaminated water introduces the microbes into the digestive system, where they can attack the gastrointestinal tract. Some pathogenic organisms use the cells that line the digestive tract in order to gain entry to the bloodstream. From there, an infection can become systemic. A common waterborne pathogen is Vibrio cholerae, the bacterium that causes cholera. The contamination of drinking water by this bacterium still causes cholera epidemics in some areas of the world.
Foodborne pathogens are grouped in two categories. Those that produce toxins that poison the host and those that infect the host and then grow there. Food poisoning is most often caused by the bacterium Staphylococcus aureus, which produces enterotoxins that result in vomiting and diarrhea. The bacterium Clostridium botulinum is responsible for the disease botulism, which is an extremely severe and sometimes fatal food poisoning.
Vectors harbor the microorganisms that cause disease and transfer them to humans via a bite or by other contact. Coxiella burnetti, the bacterium that causes Q fever, is transmitted to humans from the handling of animals such as sheep. Insects are common vectors of disease. Mosquitoes spread the protozoan Plasmodium vivax that causes malaria. Deer ticks are responsible for infection by the spirochete Borrelia burgdorferi that causes Lyme disease. The bacterium that causes plague, Yersina pestis, is transmitted by the rat flea.
SEE ALSO Anthrax; Bacterial biology; Biosensor technologies; Bioterrorism; Escherichia coli; Spores; Toxins.