Viruses are important pathogens associated with significant post-transplant morbidity and mortality. Viral infections result from acquisition of new infection or reactivation of latent viruses. The herpesviruses, in particular, are responsible for common and, sometimes, severe infection syndromes in the transplanted host.
Cytomegalovirus is the most common infection in transplant recipients after the first month post-transplantation. For the transplant recipient, CMV has three major implications: it causes disease associated with substantial morbidity and mortality, it augments immunosuppression that is associated with increased risk of PCP and other infections, and it is associated with allograft rejection. After transplantation, CMV disease is defined as symptomatic viremia or end-organ infection.
Most individuals develop CMV infection at some point during their lifetime, and after the acute phase of the illness, the virus persists in a latent stage within the host. Depending on the recipient's and donor's previous exposure to the virus, transplant recipients are at different risk of developing CMV disease post-transplantation. Thus, three patterns of infection are observed:
1. Primary infection occurs when a CMV-seronegative allograft recipient receives cells latently infected with CMV from a seropositive donor, resulting in viral reactivation (CMV donor+, recipient-).
2. Reactivation infection occurs when endogenous latent virus reactivates in a seropositive recipient (CMV D+ or -, R+).
3. Superinfection (reinfection) occurs when a seropositive recipient receives an organ from a seropositive donor and the virus that reactivates is that of donor origin (CMV D+, R+).
The D+R- patients represent the highest risk group for the development of post-transplant CMV disease, with up to 60% of recipients in this category manifesting this infection. Lower risk groups are the D+R+ and D-R+ in which the incidence of CMV disease ranges from 20-40%. The use of anti-lymphocyte therapies such as OKT3 for induction or rejection increases the risk of any seropositive recipient (R+) for CMV disease, such that these recipients are treated as another high risk group. The D-R- group represents the lowest risk group and may rarely develop primary infection after receipt of unscreened blood products or community exposure. Based on historical data, depending on the organ transplanted, lung and gut recipients are at high risk for CMV disease, whereas liver, pancreas, and heart recipients fall into an intermediate category, and renal transplant recipients represent the lowest risk group.
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