Hermine E. Smikle
The majority of microbes that human encounter on a daily basis do no harm. Some are considered beneficial in most circumstances. Only a minority of these organisms cause diseases when they interact with the human body. These disease- causing bacteria and viruses are called pathogens. Some of these pathogens cause mild diseases while some cause life threatening infections. Some are chronic, and develop over a period of time, while others are rapidly harmful or fatal. Virulence is the term used to describe the degree to which an organism can cause disease.
Diseases that are spread among the community passing from one person to another are called infectious diseases. Diseases can be transmitted by air borne droplets, or through contaminated water, via clothes, bedding, domestic utensils, or other items that have been in contact with an infected host. Some diseases are spread through the involvement with an intermediate agent. This agent is called a vector. These vectors are usually parasites that feed from the human body. Fleas, lice, and mosquitoes are examples of vectors.
If the infection occurs within a population it is said to be sporadic. If the disease spreads continuously in a community it is said to be endemic. An epidemic occurs when the number of cases of a particular infection rises above the endemic level. If the infection spreads throughout the world then it is called a pandemic
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Infections in Humans
Infections spread from a source known as the reservoir of infection. These reservoirs may be human beings or animals, water, or soil. The source of infection is the individual or location from which the infection is acquired. There are five phases in the infection process, the entry into the host; the primary replication; the spread within the host; the exit from the host; and the host's response.
There are three general pathways:
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Entry through mucous membrane, namely the respiratory tract, the urinary tract, the gastro-intestinal tract, and the conjunctiva.
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Entry through the skin, this can be done through insect bites, animal bites, and by skin to skin contact.
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Parenteral transmission, vertical transmission, direct inoculation into the blood or through blood trauma.
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There is a distinction between the ability of an organism to cause disease and its ability to infect. Infection is different from having a disease. A person can be infected, but not have a disease. Infection expressed as an equation is: Infection + signs of symptoms = disease. If an organism infects a host and then become a normal part of the flora then it is called colonization. Infection is measured by the number of organisms on the host, and not the number of hosts that has died. Infectivity is therefore, the ability of an organism to establish itself on a host
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Mode of Transmission
Vertical transmission is a term is used to describe an infection that is transmitted from the mother to the child while in utero. Perinatal infections and post natal infections are not considered vertical infections.
Horizontal infections are infections transmitted from human to humans. There are significant differences in the number of infections that occur between horizontal infections and vertical infections. In general the rate of infection by horizontal infection tends to be greater than the rate of vertical infections. In vertical infection only one person is infected from the host, while in horizontal infection a number of persons can be infected by a single host.
Vector - borne infections: These are infections transmitted by a vehicle or a carrier. These vehicles are living animals such as lice, fleas or mosquitoes. These vectors transit organisms from the bloodstream of infected persons to other persons (susceptible).
Zoonoses: Zoonoses are infections that can be transmitted between animals and humans. The host of the disease is the animal. There are three modes of transmission from animal to human:
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Direct transmission occurs via direct contact with the animal. The organism does not undergo any developmental change between the two hosts.
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The micro organism has an intermediate host in its life cycle. Humans can only be infected with the infective stage of the organism.
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The organism infects a human from a non living site.
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Food - Borne Infections: Food - Borne diseases due to microbes are divided into two categories. These are intestinal infections and intoxications. Intestinal infections are caused by the multiplication of bacteria within the intestine causing diarrhea. Intoxications occur because the organism has multiplied in the food and released entertoxins that are ingested
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Emerging Diseases: These are diseases that have recently appeared in a population, or are diseases that have existed in the past, but are rapidly increasing. A few of these diseases that fall into that category are described in this unit.
Cowpox Virus
The cowpox virus is usually transmitted from pet rats to human. The virus belongs to the family Poxviridae and is closely related to other species, such as the variola virus. Cows are not the only animals infected by the virus. Wild rodents are considered to be the reservoir. Cats, zoo animals, and humans, are incidental hosts. This disease is usually rare. It presents as a localized skin lesion. The virus is transmitted to human by direct contact with the infected animal, usually a rat or cat
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Chronic Wasting Disease Prions in Elk Antler Velvet
This is a contagious fatal prion disease of deer and elk. A prion is an infectious agent that is composed entirely of proteins. Disease is caused by the protein interacting with normal proteins in the host, causing these normal proteins to change shape to match the infectious protein. In the process this can destroy vital tissues such as brain tissue. It is believed that the disease is transmitted by prions in the antler velvet of the affected elk. Humans who eat antler velvet as a nutritional supplement are at risk of exposure to the prions
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Skin and Soft Tissue Infection (Patera Foot) in Immigrants to Spain
An unusual skin and soft tissue infection of the lower limbs has been observed in immigrants from sub- Saharan Africa who cross the Atlantic Ocean crowded on small fishing boats called pateras. These immigrants are trying to reach the Canary Islands. The boats are over crowded with between 40 -50 persons. They have minimal food and water for a journey that lasts several days. During the journey the passengers are exposed to inclement weather, deficient hygienic conditions and are sitting for prolong periods of time in the same position. In instances their feet are immersed in the sea water for long periods. The sea water is contaminated by traces of feces, urine, decaying food and fuel emulsion. These refugees arrive with severe skin and soft tissue infections, which consist of cellulites, deep tissue abscesses and tissue necrosis. Microbiologists suggest that shewanalla algae could be the agent of this infection
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