The appearance of a pulmonary infiltrate and signs of infection (e.g., fever) are some of the most common and serious complications in individuals with immune systems that are suppressed by disease, immunosuppressive drugs, or therapeutic irradiation. A wide variety of “opportunistic” pathogens, many of which rarely cause infection in normal individuals, can cause infections in these settings, and often more than one agent is involved. The more common pulmonary pathogens include (1) bacteria (P. aeruginosa, Mycobacterium spp., L. pneumophila, and Listeria monocytogenes); (2) viruses (cytomegalovirus and herpesvirus); and (3) fungi (P. jiroveci, Candida spp., Aspergillus spp., and Cryptococcus neoformans).
Infection by cytomegalovirus (CMV), a member of the herpesvirus family, may manifest in various forms, depending partly on the age and the immune status of the host. Cells infected by the virus exhibit gigantism of both the cytoplasm and the nucleus. The nucleus typically contains a large inclusion surrounded by a clear halo (“owl’s eye”), which gives the name to the classic form of symptomatic disease that occurs in neonates—cytomegalic inclusion disease. Although classic cytomegalic inclusion disease involves many organs, CMV infections are discussed here because in immunosuppressed adults, particularly patients with AIDS and recipients of allogeneic hematopoietic stem cell transplants, CMV pneumonitis is a serious problem.
Transmission of CMV can occur by several mechanisms, depending on the age group:
A fetus can be infected transplacentally from a newly acquired or reactivated infection in the mother (congenital CMV infection).
The virus can be transmitted to the infant through cervical or vaginal secretions at birth, or later through the breast milk of a mother with an active infection (perinatal CMV infection).
Preschool children, especially in day care centers, can acquire it through saliva. Toddlers so infected readily transmit the virus to their parents.
In patients older than 15 years of age, the venereal route is the dominant mode of transmission, but spread also may occur through contact with respiratory secretions and by the fecal-oral route.
Iatrogenic transmission can occur at any age through organ transplantation or blood transfusion.
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Fig. 13.40 Cytomegalovirusinfectionofthelung.Adistinctnuclearinclu-sionandmultiplecytoplasmicinclusionsareseeninanenlargedcell.
MORPHOLOGYHistologically,thecharacteristicenlargementofCMV-infectedcellsisreadilyappreciated.Intheglandularorgans,theparenchymalepithelialcellsareaffected;inthebrain,theneurons;inthelungs,thealveolarmacrophagesandepithelialandendothelialcells;andinthekidneys,thetubularepithelialandglomerularendothelialcells.Infected cells are strikingly enlarged, often to a diameter of 40 µm, and exhibit cellular and nuclear pleiomorphism. Prominentintranuclearbasophilicinclusionsspanninghalfthenucleardiameterareusuallysetofffromthenuclearmembranebyaclearhalo(
Fig.13.40).Within thecytoplasmofthesecells,smallerbasophilicinclusionsalsomaybeseen.