Definition: Infectious agents range from the submicroscopic viruses to the large tapeworms. In between are bacterias, fungi, and higher forms of parasites.
List of infectious agents
Types
prions
viruses
bacterias
parasites
- helminths
- arthropods
- insects
Detection and identificiation
Some infectious agents or their products can be directly observed in hematoxylin- and eosin-stained sections (e.g., the inclusion bodies formed by CMV and herpesvirus; bacterial clumps, which usually stain blue; Candida and Mucor among the fungi; most protozoans; and all helminths).
Many infectious agents, however, are best visualized by special stains that identify organisms on the basis of particular characteristics of their cell walls or coat-Gram, acid-fast, silver, mucicarmine, and Giemsa stains-or after labeling with specific antibody probes.
Regardless of the staining technique, organisms are usually best visualized at the advancing edge of a lesion rather than at its center, particularly if there is necrosis.
Nucleic acid-based tests have become routine methods for detecting or quantifying several pathogens. Molecular diagnostics have become particularly important in the care of people infected with HIV.40 Quantification of the viral RNA is an important guide to antiretroviral therapy.
The management of hepatitis B and C infections is similarly guided by nucleic acid-based viral quantification or typing to predict resistance to antiviral drugs.
Nucleic acid amplification tests (NAATs), such as polymerase chain reaction (PCR) and transcription-mediated amplification, have become routine for diagnosis of gonorrhea, chlamydia, tuberculosis, and herpes encephalitis. In some cases, molecular assays are much more sensitive than conventional testing.
PCR testing of cerebrospinal fluid (CSF) for herpes simplex virus encephalitis has a sensitivity of about 80%, while viral culture of CSF has a sensitivity of less than 10%.
Similarly, NAATs for genital chlamydia detect 10% to 30% more infections than does conventional chlamydia culture. In other cases, such as gonorrhea, the sensitivity of NAAT testing is similar to that of culture.
See also
etiological agents (pathological agents)