Viral upper respiratory tract diseases

Respiratory disease may take several forms, and the predominant etiologies are different in different age groups. Incidence statistics also vary depending on the geographic area and the population selected. Of the known viruses, rhinoviruses are predominantly associated with acute upper respiratory tract disease (including the common cold) in adults, whereas in children, rhinovirus, adenovirus, parainfluenza virus, and the enteroviruses are important. Acute bronchitis in children is most often due to respiratory syncytial virus and parainfluenza virus. In croup, parainfluenza is said to be the most important virus.

Viral pneumonia

Respiratory syncytial virus is the predominant cause of pneumonia in infants and young children, beginning at age 1 month with a peak incidence at about age 6 months, followed by adenovirus or parainfluenza virus. In older children or adults, bacterial pneumonia (most often due to Pneumococcus or Mycoplasma pneumoniae) is more common than viral pneumonia. Among viral agents known to cause pneumonia in adults, the most common is probably influenza. In any study, a large minority of cases do not yield a specific etiologic agent.

Viral meningitis

Viruses are an important cause of meningitis, especially in children. They typically produce the laboratory picture of aseptic meningitis: the classic cerebrospinal fluid (CSF) findings are variable, but often include mildly increased protein levels, increased cell counts with mononuclear cells predominating, normal glucose levels, and no organisms found on culture. It should be remembered, however, that tuberculous meningitis gives similar findings, except for a decreased CSF glucose level, and likewise shows a sterile culture on ordinary bacterial culture media. Some patients with mumps meningoencephalitis may have decreased CSF glucose levels in addition to CSF lymphocytosis. Enteroviruses are the largest etiologic group causing aseptic meningitis. Among the enteric viruses, poliomyelitis used to be the most common organism, but with widespread polio vaccination programs, echovirus and coxsackievirus have replaced polio in terms of frequency.

After the enteroviruses, mumps is the most important. A small but significant number of patients with mumps develop clinical signs of meningitis, and a large number show CSF changes without demonstrating enough clinical symptoms to warrant a diagnosis and workup for meningitis. Changes in CSF or the clinical picture of meningitis may occur in patients without parotid swelling or other evidence of mumps. Lymphocytic choriomeningitis and leptospirosis are uncommon etiologies for aseptic meningitis.

Encephalitis is a syndrome that frequently has CSF alterations similar to those of meningitis. The two cannot always be separated, but the main difference is clinical; encephalitis features depression of consciousness (lethargy, coma) over a prolonged period, whereas meningitis usually is a more acute illness with manifestations including fever, headache, vomiting, lethargy, stiff neck, and possibly convulsions. In severe bacterial infection, encephalitis may follow meningitis. Encephalitis is most often caused by viruses, of which the more common are mumps, herpes simplex type 1 (HSV-1), measles, and the arboviruses. Sometimes encephalitis is a complication of vaccination.

Viral gastroenteritis

Viruses are likely to be blamed for diarrhea that cannot be explained otherwise. In most cases, definitive evidence is lacking because enteric virus is present in a significant number of apparently healthy children. Bacterial infection should always be carefully ruled out. Two clinical types of viral gastroenteritis have been described. One type usually occurs in epidemics, more often in older children and in adults, with clinical signs of an acute self-limited gastroenteritis of 1-2 days’ duration. The most commonly associated etiology is the Norwalk-type group of viruses. The other type of illness is sporadic and affects mostly infants and younger children. There is severe diarrhea, usually accompanied by fever and vomiting, which lasts for 5-8 days. Rotavirus is the most frequently isolated virus in these patients. About 5%-10% of gastroenteritis in infants less than 2 years old is said to be caused by adenovirus types 40 and 41

Viral infections in pregnancy

By far the most dangerous viral disease during pregnancy is rubella. Statistics are variable, but they suggest about a 15%-25% risk of fetal malformation when rubella infection occurs in the first trimester (literature range, 10%-90%). The earlier in pregnancy that maternal infection occurs, the greater the risk that the fetus will be infected. However, not all infected fetuses develop congenital malformation. When the fetus is infected early in the first trimester, besides risk of congenital malformation, as many as 5%-15% of fetuses may die in utero. Risk of fetal malformation in second trimester infections is about 5%. After the fourth month of pregnancy, there is no longer any danger to the fetus. Cytomegalovirus (CMV) infection is more frequent than rubella, but CMV has a lower malformation rate. Cytomegalovirus damage is more severe in the first two trimesters. Other viruses may cause congenital malformations, but evidence is somewhat inconclusive as to exact incidence and effects. Herpes simplex and the hepatitis viruses are in this group.