SARS: harmful effects

The toxic effects of viruses are much more difficult to measure than those of other agents, bacterial as well as chemical. Whereas a chemical will have a finite effect on a cell depending on the amount and concentration of the given chemical, and bacteria are limited by the amount of toxin it can produce, the toxic effects of a virus are only limited by the host cell’s ability to produce more virions. The amount of virus required to induce some toxic effect on the host can theoretically be as low as one virus particle.

HARMFUL EFFECTS OF SARS

SARS-associated Coronavirus (SARS-CoV) harms the host by infecting epithelial cells of the lungs and intestines that come into contact with viral particles. This contact occurs by either the inhalation of virus-containing droplets or the ingestion of fecal droplets. The virus will enter the given epithelial cell and insert its RNA into the host cell’s genetic material. Once embedded into the host’s DNA, the foreign genetic material will direct the host cell to create numerous copies of the original virus. These viruses will fill the cytoplasm of the host cell until there are so many copies that the host cell ruptures from the interior pressure, releasing the multitude of virions to infect any nearby uninfected cells.

Aside from inducing the death of infected cells, SARS-CoV also induces a severe immune reaction in the host. Although at this time the exact mechanism is unknown, the viral infection induces a hypersensitivity reaction in the host. This reaction creates a massive production of mucous and sputum, which leads to hypoxia, or insufficient oxygen in the blood. This, in turn, puts such great stress on the lungs that many of the infected suffer from respiratory failure.

CHARACTERISTICS OF SEVERE ACUTE RESPIRATORY SYNDROME

Incubation period:

Generally 2-7 days, can be up to 10 days
SARS Symptoms:
Begins with a high fever (>100.4ºF, >38.0ºC)
Fever can be accompanied by chills, rigors, headache, malaise, and myalgia
Mild respiratory symptoms may be present at onset of illness
After 3-7 days, onset of lower respiratory symptoms occurs including a dry, nonproductive cough or dyspnea which may be accompanied by or progress to hypoxemia
Most patients develop pneumonia
Diarrhea is typically absent but may occur in some cases
Severity of illness is highly variable ranging from mild illness to death

In many patients the respiratory phase is characterized by early focal interstitial infiltrates progressing to more generalized, patchy, interstitial infiltrates.

Figure 1. Chest radiographs of index patient with severe acute respiratory syndrome (SARS). a, day 5 of symptoms; b, day 10; c, day 13; d, day 15.
http://www.cdc.gov/ncidod/eid/vol9no6/03-0264.htm

www.cdc.gov/mmwr/preview/mmwrhtml/mm5212a5.htm
www.cdc.gov/ncidod/sars/factsheet.htm

SEVERE ACUTE RESPIRATORY SYNDROME (SARS)