The agent factors which can influence disease occurrence in an environment include:
- Incubation period
- Survivability in the environment
- Infectivity of the agent
- Pathogenicity/virulence of agent
Incubation period is the time lag between the entry of the causative agent into the host and the manifestation of clinical signs and symptoms of the disease by the infected host. This period varies from one agent to another. Some agents have a shorter incubation period while others have a longer incubation period. Example of short incubation period is seen in food poisoning and cholera diseases – which occur immediately after the entry of the agent into the host. Manifestation of disease conditions in the invasion of a causative agent with a long incubation period is not immediate but rather it is delayed; and this usually increases its occurrence and thus the causative agent can be easily transmitted from infected host to non-infected individuals.
SURVIVABILITY OF THE AGENT IN THE ENVIRONMENT
This is the ability of the agent to survive and persist in the environment due to its ability to form spores which can remain in the environment when environmental conditions become unfavourable. Fungi and bacteria are notable in forming spores; and fungal and bacterial spores survive for a very long time in the environment until they find a suitable host or environment where they can thrive or proliferate. But the spores of viruses can easily be killed by environmental forces. Spores of Bacillus anthrax and fungi that survive long in the environment can easily be contracted by a susceptible host and this can increase the spread of disease in the environment.
INFECTIVITY OF THE AGENT
Infectivity is the ability of the agent to cause infection. Infection is the successful entry and multiplication of the agent in the host. Some diseases can lead to high infectivity while others will have a low infectivity rate. Microorganisms that form part of the body’s normal flora such as E. coli and Candida species cause infections with low infectivity rate; and such organisms usually multiply more when the immune system of the host is depressed or compromised. There is a very low disease emergence in occurrence.
Invasiveness is the ability of the agent to move from the point of entry to other parts of the host. The more invasive the agent, the more the organs of the host becomes more affected. But if the agent is less invasive, the more localized the agent become. More invasive agents cause systemic infections that spread all over the body via blood (because they can move beyond their primary portal of entry into the body to other sites or organs of the body) while those that are localized cause localized infections (because they cannot spread beyond the point of their entry into the host’s body). A typical example of a localized infection is ring worm. More invasive agents include Mycobacterium tuberculosis (the causative agent of tuberculosis) and Treponema pallidum which causes syphilis. A localized agent cannot be spread to other parts of the host.
PATHOGENICITY/VIRULENCE OF THE AGENT
Pathogenicity is the ability of the agent to cause disease or harm in the host. The agent does this by destroying the host cell. The causative produces toxins that destroy the host cell by blocking of the flow of fluids in the host. Virulence is only a measure of pathogenicity. It is the severity of pathogenicity. An agent that is more virulent becomes more pathogenic than the other agents that are less virulent. Virulent agents cause more damage to the host’s cells and/or organs than less-virulent agents. The invasion of a host by a more virulent agent leads to the production of clinical signs and symptoms. The spread of the infection here will be reduced because of the awareness of people of the disease. But if the agent is not pathogenic enough, the agent will be spread more easily because no clinical signs and symptoms have been seen, which can deter people from infection.
Salyers A.A and Whitt D.D (2001). Microbiology: diversity, disease, and the environment. Fitzgerald Science Press Inc. Maryland, USA.
Schneider M.J (2011). Introduction to Public Health. Third edition. Jones and Bartlett Publishers, Sudbury, Massachusetts, USA.
Singleton P and Sainsbury D (1995). Dictionary of microbiology and molecular biology, 3d ed. New York: John Wiley and Sons.
Slonczewski J.L, Foster J.W and Gillen K.M (2011). Microbiology: An Evolving Science. Second edition. W.W. Norton and Company, Inc, New York, USA.
Songer T (2005). Study designs in epidemiologic research. Supercourse, (http://www.pitt.edu/~super1/lecture/lec19101/index.htm) (Accesed May 2103).
Stedman’s medical dictionary, 27th edition. Philadelphia: Lippincott, Williams and Wilkins.
Summers W.C (2000). History of microbiology. In Encyclopedia of microbiology, vol. 2, J. Lederberg, editor, 677–97. San Diego: Academic Press.
Taylor LH, Latham SM, Woolhouse ME (2001). Risk factors for disease emergence. Philos Trans R Soc Lond B Biol Sci, 356:983–989.
Willey J.M, Sherwood L.M and Woolverton C.J (2008). Harley and Klein’s Microbiology. 7th ed. McGraw-Hill Higher Education, USA.
Aschengrau A and Seage G.R (2013). Essentials of Epidemiology in Public Health. Third edition. Jones and Bartleh Learning,
Aschengrau, A., & G. R. Seage III. (2009). Essentials of Epidemiology in Public Health. Boston: Jones and Bartlett Publishers.
Biosafety in Microbiological and Biomedical Laboratories. 5th edition. U.S Department of Health and Human Services. Public Health Service. Center for Disease Control and Prevention. National Institute of Health. HHS Publication No. (CDC) 21-1112.2009.
Bonita R., Beaglehole R., Kjellström T (2006). Basic epidemiology. 2nd edition. World Health Organization. Pp. 1-226.