Major diseases of the respiratory system
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Viral infections of the respiratory system
A wide variety of viruses are responsible for acute respiratory disease. The common cold—frequently of viral origin—can cause inflammation of the trachea and laryngitis, and such inflammation may extend to involve the lower bronchial tree. After such episodes the ciliary lining of the bronchial tree may be damaged, but the repair process is usually rapid.
Infections with rhinoviruses and adenoviruses are especially important in children, in whom they cause a febrile (fever-associated) illness, occasionally with severe bronchiolar involvement. Although recovery is usually rapid, in some infections with respiratory syncytial virus an extensive bronchiolitis develops that may be severe enough to threaten life. In epidemics of these diseases, occasional cases occur in which the course is complicated by inflammation of the pericardium—the membrane enclosing the heart—or by a pleural effusion.
Influenza and parainfluenza viruses are capable of causing severe illness. The influenza virus attacks many systems of the body simultaneously, but the primary site of viral replication appears to be the alveolar cells of the lung. There the virus multiplies many times over within a 24-hour period, and the pulmonary involvement may begin in the parenchyma and cause considerable consolidation and inflammation of lung tissue. Severe tracheitis, bronchitis, and bronchiolitis often occur at the same time. Another form of the disorder is that described as viral pneumonia, in which a distinguishing feature is the presence of patchy areas of atelectasis, or partial collapse of lung tissue, without extensive involvement of the bronchial tree. All these conditions are more dangerous in small children and in the elderly, and the lung that is the seat of a severe attack of influenza may quickly become secondarily infected.
It was secondary bacterial infection that accounted for the high mortality in the influenza epidemic of 1918–19. Today this epidemic is more precisely called a pandemic because it affected populations around the world. It has been estimated that at least 25 million (and possibly as many as 40–50 million) people worldwide died during the outbreak. It was a characteristic of this pandemic that young people were severely affected. The high mortality resulted from the lack of antibiotics for treating the secondary bacterial infection; widespread malnutrition probably also contributed to the death rate.
There are three immunologically distinct types of influenza virus, designated A, B, and C; parainfluenza viruses are designated by the letter D. Types A, B, and D cause epidemic disease. Within type A there are distinct strains. The “Asian” strain of type A was responsible for the 1957 influenza epidemic. Epidemic influenza tends to occur in two- or three-year cycles; careful study has allowed predictions to be made of their future occurrence. Although infected individuals develop lasting immunity to a particular strain following an attack of influenza, the immunity is highly specific as to type, and no protection is afforded against even closely related strains. Artificial immunization with high- potency vaccines is of value in protecting against previous strains, and the vaccines have been shown to ameliorate the infection in the general population. Their use is particularly indicated in elderly people whose cardiac or lung function is already compromised.
Psittacosis and ornithosis, primarily infections of birds and particularly common among parakeets and parrots, are transmitted to humans by inhalation of dust particles from the droppings of infected birds. The onset of psittacosis may be quite severe, with headache, insomnia, and even delirium. Gastrointestinal symptoms such as vomiting and pain are frequent, and a cough productive of clear sputum usually develops after a few days. Mild attacks are often unrecognized and dismissed as due to influenza. Recovery is usually complete, but convalescence may be slow. A pandemic of this disease in 1929 was caused by the shipment of 5,000 parrots into Argentina from Brazil for auction. Many of the birds died, and there was considerable human mortality. Mandatory isolation of imported birds for observation has largely controlled this disease in many countries around the world.
Chickenpox (varicella), particularly when it occurs in adults, may affect the lung. Acute lesions may occur in the lung parenchyma, leading to a transient but significant fall in arterial oxygen tension (hypoxemia), occasionally necessitating oxygen therapy. Recovery may be slow but is usually complete, although shadows may remain on a chest radiograph as a result of it.
Whooping cough occurs in epidemic form among children and appears to be linked to the later development of the chronic infective process known as bronchiectasis, which occurs as a result of bronchial damage. In Western countries, both whooping cough and measles (which causes an acute bronchiolitis) have been largely controlled by effective vaccines, although whooping cough sometimes occurs in adults many years after vaccination. In some developing countries, where these vaccines are not consistently available, whooping cough and measles can still be major causes of mortality in children. Mortality is worsened by malnutrition, which reduces resistance to acute respiratory diseases and is present in many children of developing countries.
The reparative processes in the lung after any viral attack may be quite slow. Apparent clinical recovery may occur relatively quickly and radiographs may show no remaining shadows, yet repair and restitution of the alveolar wall may take several additional weeks. Sometimes a cough persists for two or three months after systemic symptoms have resolved, reflecting continued healing of the bronchi. However, the occurrence of a severe viral infection in childhood may impair subsequent development of the lung or even set the stage for chronic respiratory disease in later life.
Bacterial infections of the respiratory system
Pneumonia
Before effective antibiotics became available, pneumonia was the respiratory disease responsible for the greatest mortality and consequently was one of the most-feared diseases. Because it frequently led to the death of severely disabled elderly people, it was also known as the “old man’s friend.” The most common form of the disease, streptococcal pneumonia (sometimes called pneumococcal pneumonia), is caused by a streptococcus called Streptococcus pneumoniae. This form of pneumonia begins abruptly with a high fever and severe malaise followed by natural resolution in survivors after several days or longer. In some cases, infection is followed by complications, such as a lung abscess, pleurisy, or heart failure. Prompt antibiotic therapy controls the acute process within 24 hours in most instances; however, some deaths occur despite appropriate antibiotic therapy. Emergence of antibiotic-resistant S. pneumoniae strains is a growing concern. Streptococcal pneumonia may also occur as a complication of an acute attack of influenza because susceptibility to infection by streptococci is increased. In addition, the much-lowered mortality of influenza can be explained by antibiotics effective against streptococci. Staphylococcal pneumonia occurs as an acute illness in small children and may lead to rapid destruction of lung tissue with abscess formation; however, if the acute state is survived, as it usually is with chemotherapy, the lung recovers fully. This type of pneumonia may also occur as a complication of preexisting lung disease of any kind and may follow aspiration of stomach contents into the lung. The development of antibiotic-resistant staphylococci, such as methicillin-resistant Staphylococcus aureus (MRSA), has meant that this form of pneumonia may be a problem in the hospital environment, complicating other lung diseases or occurring postoperatively.
Pneumonia due to infection with Klebsiella pneumoniae may be difficult to treat and characteristically may occur as a repetitive series of episodes of pneumonia, each running a rather long course with slow resolution.
The organism Hemophilus influenzae is commonly isolated from the sputum of patients with chronic bronchitis during acute exacerbations of infection and is an important cause of pneumonia in adults.
Mycoplasma, identified in 1944 as responsible for a group of pneumonias previously thought to be of viral origin, is a member of a group of organisms known as the pleuropneumonia-like organisms and has also been called the “Eaton agent” after the scientist who first described it. Mycoplasma pneumoniae is the single most common cause of pneumonia in school-age children and young adults. The infection produces soft patchy shadows on the chest radiograph and relatively few signs on physical examination. A nonproductive cough and a fever occur for a few days. Familial spread is common, and disease occurs in epidemic form in young healthy people brought together in clusters, as in military recruit camps and colleges, where a number of outbreaks have been documented. It is not usually a life-threatening disease, but in rare cases it may progress to cause acute respiratory distress syndrome.
In all these bacterial pneumonias, the diagnosis may be made from the characteristic radiographic pattern, together with isolation from the sputum of the bacterium primarily responsible.