Pathogens may exit a host in respiratory secretions from the nose and mouth, or be shed on dead skin or in feces, urine, saliva or tears. Pathogens transmitted through the environment may survive from hours to years outside the host, depending on the organism and the environment. This is because their survival time outside the host is very brief. Examples include the herpes virus, Neisseria gonorrhoeae (gonorrhea) and Treponema pallidum Many human pathogens can only be transmitted by direct or close contact with an infected person or animal. Pathogenic microorganisms usually originate from an infected host (either human or other animal), or directly from the environment. The concentration of enteric viruses during gastroenteritis may be as high as 10 10–10 12 per gram of feces.įood or water ingestion, direct and indirect contactįood/ingestion, direct or indirect contactįood or water ingestion, direct and indirect contact, aerosol?įood ingestion, direct and indirect contact The concentration of organisms released into the environment varies with the type of organism and the route of transmission ( Table 22.2 In the case of hepatitis A virus, the maximum excretion in the feces occurs before the onset of signs of clinical illness. Although the maximum release may occur at the height of the disease, it can also precede the first signs of clinical illness. At any time during infection the pathogen may be released into the environment by the host in feces, urine or respiratory secretions. This may range from as short as 6 to 12 hours in the case of norovirus diarrhea, or up to 30 to 60 days for hepatitis A virus, which causes liver disease. The time between infection and the appearance of clinical signs and symptoms (diarrhea, fever, rash, etc.) is the incubation time ( Table 22.1 To cause illness, the pathogen must usually first grow within or on the host. Opportunistic pathogens are common in the environment, and may be present in the human gut or skin without causing disease. Opportunistic pathogens are usually capable of causing infections only in immunocompromised individuals such as burn patients, patients taking antibiotics, those with impaired immune systems or elderly patients with diabetes. A frank pathogen is a microorganism capable of producing disease in both normal healthy and immunocompromised persons. In the case of enteric infections (i.e., diarrhea) caused by Salmonella, only half of the individuals infected develop clinical signs of illness. Infection does not necessarily result in disease, since it is possible for the organism to grow in or on the host without producing an illness (see Chapter 23). Infection is the process in which the microorganism multiplies or grows in or on the host. Disease-causing microorganisms are called pathogens. 22.1. Environmentally Transmitted PathogensĪlthough humans are continually exposed to a vast array of microorganisms in the environment, only a small proportion of these microbes are capable of interacting with the host in such a manner that infection and disease will result.
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