Causative Agents

Between 30% and 50% of colds are caused by the 100 or more types of rhinoviruses (rhino means "nose," as in rhinoceros, or "horny nose"), members ofthe picornavirus family (pico is Italian for "small" and rna is ribonucleic acid [RNA]; thus, "small RNA viruses") (figure 23.8). The viruses are non-enveloped and their RNA is single-stranded, positive sense. The rhinoviruses were initially difficult to cultivate in the laboratory, since most failed to infect laboratory animals or cell cultures. In the 1950s, researchers in England accidentally discovered that if cell cultures were incubated at 33°C instead of at body temperature (37°C) and at a slightly acid pH instead of at the alkaline pH of body tissues, many rhinovirus strains would infect them. These conditions of lower temperature and pH normally exist in the upper respiratory tract. Rhinoviruses, however, are killed if the pH drops below 5.3, and therefore, they are usually destroyed in the human stomach. Many other viruses and some bacterial species can also produce the symptoms of a cold.

574 Chapter 23 Respiratory System Infections

100 nm

Figure 23.8 Rhinovirus Transmission electron micrograph.The red spheres are the rhinovirus virions, and the blue object is the cell in which they are replicating.

100 nm

Figure 23.8 Rhinovirus Transmission electron micrograph.The red spheres are the rhinovirus virions, and the blue object is the cell in which they are replicating.

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