Opportunistic Viral Infections in Immunocompromised Hosts

CMV end-organ disease encompasses a spectrum of manifestations that differ in diagnostic approach, severity, and antiviral management. Understanding the syndrome-specific nuances — particularly the poor sensitivity of blood PCR for gastrointestinal CMV disease — is essential for avoiding misdiagnosis and delayed treatment.

CMV colitis is the most common gastrointestinal CMV manifestation in immunocompromised patients, presenting with profuse watery or bloody diarrhea, tenesmus, and abdominal cramping. A critical diagnostic pitfall is that blood CMV PCR is frequently undetectable or low-level in isolated CMV colitis — the virus replicates locally within colonic mucosa without generating systemic viremia sufficient to be detected in peripheral blood in many patients. Diagnosis therefore requires colonoscopy with biopsy demonstrating characteristic CMV cytopathic effect (enlarged cells with intranuclear and intracytoplasmic inclusions) confirmed by immunohistochemistry or in situ hybridization for CMV DNA. Treatment is intravenous ganciclovir induction (5 mg/kg every 12 hours) for 3–6 weeks followed by oral valganciclovir consolidation; response is monitored clinically and endoscopically, not by blood PCR. CMV esophagitis presents with odynophagia and may be indistinguishable from Candida esophagitis clinically; endoscopy with biopsy is required for diagnosis.

CMV pneumonitis in HSCT recipients is the most life-threatening CMV end-organ manifestation, carrying mortality rates of 30–50% despite antiviral therapy in established disease. It typically presents 1–4 months post-transplant with progressive hypoxia, non-productive cough, and bilateral interstitial infiltrates. The combination of intravenous ganciclovir with intravenous immunoglobulin (IVIG) is standard of care based on retrospective cohort data demonstrating survival benefit over ganciclovir alone, though randomized controlled trial evidence is lacking. Progression from upper respiratory tract CMV infection to lower respiratory tract disease in HSCT recipients — which carries far higher mortality than upper respiratory tract disease — is the target of pre-emptive treatment with valganciclovir at the stage of CMV detection in bronchoalveolar lavage (BAL) samples.

CMV CNS disease encompasses encephalitis, ventriculoencephalitis, and polyradiculomyelopathy, occurring predominantly in patients with CD4 counts below 50 cells/mm³ in HIV disease and in HSCT recipients with severe T-cell depletion. CMV ventriculoencephalitis in advanced HIV presents with cognitive deterioration, confusion, and cranial nerve palsies, with MRI showing periventricular enhancement. CMV polyradiculomyelopathy presents with ascending flaccid paraparesis, bladder dysfunction, and sacral paresthesias — a distinctive syndrome that can be confused with other causes of myelopathy. CSF PCR for CMV DNA is the diagnostic test of choice, with sensitivity above 80% in CNS CMV disease. Treatment of CMV CNS disease uses combination ganciclovir plus foscarnet, exploiting mechanistic complementarity to achieve higher antiviral potency than either agent alone; this combination is used despite greater toxicity burden because of the severity of the syndrome and the importance of maximal viral suppression.

Adenovirus and BK polyomavirus represent two of the most pharmacologically challenging opportunistic viral infections in transplant medicine — both lack robust, well-validated antiviral therapies, and management rests heavily on the balance between antiviral pharmacology and immunological restoration through immunosuppression reduction.

Adenovirus infections in HSCT recipients can cause severe pneumonia, hepatitis, colitis, nephritis, and hemorrhagic cystitis with disseminated disease carrying mortality rates exceeding 50% in severely immunocompromised patients. Cidofovir is the antiviral most widely used for adenovirus disease in transplant recipients — it is active against all adenovirus serotypes through its virus-independent mechanism (does not require viral TK or UL97) and has retrospective evidence of improved survival in severe adenovirus disease in HSCT patients. Brincidofovir (CMX001) — the lipid conjugate of cidofovir — has demonstrated efficacy against adenovirus in HSCT recipients and achieved FDA breakthrough therapy designation; however, gastrointestinal toxicity (primarily diarrhea) and hepatotoxicity have been treatment-limiting in some studies. Cidofovir nephrotoxicity is the primary concern with standard cidofovir in the transplant setting, particularly in kidney transplant recipients or those with pre-existing renal impairment — mandatory probenecid and saline preloading protocol applies as described in Module 4. Reduction of immunosuppression to allow T-cell immune reconstitution is a critical adjunct to antiviral therapy and may be the most important intervention in adenovirus disease when clinically feasible.

BK polyomavirus (BKPyV) nephropathy is a major cause of allograft loss in kidney transplant recipients, occurring in approximately 5–10% of kidney transplant recipients. BKPyV, a ubiquitous human polyomavirus with primary infection in childhood, establishes persistent infection in the urothelium and renal tubular epithelial cells. Under calcineurin inhibitor-based immunosuppression, BKPyV reactivates and can destroy allograft tubular epithelium, causing progressive interstitial fibrosis and tubular atrophy. Screening strategy: plasma BKPyV PCR monthly for the first 6–12 months post-kidney transplant; plasma viral load above 10,000 copies/mL is an indication for immunosuppression reduction to allow immune control of viral replication. The primary management of BKPyV nephropathy is immunosuppression reduction — there is no proven antiviral therapy. Cidofovir, leflunomide, and IVIG have been used anecdotally but without robust clinical trial evidence. The tension in management is between reducing immunosuppression enough to control viral replication and avoiding the rejection risk that comes with reduced immunosuppression in a kidney transplant recipient.

Human herpesvirus 6 (HHV-6) reactivation after allogeneic HSCT is common, occurring in 30–70% of recipients in the first month post-transplant, and can cause encephalitis, bone marrow suppression, and pneumonitis. Chromosomally integrated HHV-6 (ciHHV-6) — a distinct biological phenomenon in which the full HHV-6 genome integrates into the germline — creates a diagnostic trap that must be recognized to avoid inappropriate antiviral treatment.

HHV-6A and HHV-6B are betaherpesviruses closely related to CMV. HHV-6B is the principal cause of HHV-6 disease in HSCT recipients, reactivating from latency in hematopoietic progenitor cells during the engraftment period. HHV-6B encephalitis is the most serious manifestation, presenting with seizures, confusion, and anterograde amnesia with characteristic MRI abnormalities in the bilateral hippocampi and limbic structures — a pattern resembling autoimmune limbic encephalitis that should prompt HHV-6 PCR in CSF in any HSCT recipient with these features. HHV-6 DNA PCR in blood and CSF is the diagnostic test of choice, but interpretation requires knowledge of ciHHV-6 status. Chromosomally integrated HHV-6 (ciHHV-6) is present in approximately 1% of the general population and approximately 1% of HSCT donors and recipients; individuals with ciHHV-6 have stable, very high-level HHV-6 DNA in all cells derived from the integrated germline without evidence of viral replication. Treating ciHHV-6 with antiviral therapy is both futile (integrated viral DNA is not replicated by viral polymerase and cannot be suppressed by antivirals targeting DNA polymerase) and potentially harmful through unnecessary drug toxicity. Distinguishing ciHHV-6 from true reactivation requires specialist evaluation and additional testing beyond standard blood PCR.

Treatment of true HHV-6B reactivation disease uses foscarnet as the preferred agent (90 mg/kg every 12 hours or 60 mg/kg every 8 hours) for HHV-6 encephalitis, with ganciclovir as an alternative. Both drugs are active against HHV-6 in vitro and are used clinically despite the absence of prospective randomized controlled trial data specifically in HHV-6 disease — a recurring reality in transplant virology where disease prevalence is insufficient to power definitive trials. Duration of antiviral therapy for HHV-6 encephalitis is typically 3–6 weeks, guided by clinical response and HHV-6 PCR viral load trajectory in CSF. Immune reconstitution inflammatory syndrome (IRIS) in HSCT recipients differs mechanistically from HIV-associated IRIS but shares the core feature of exaggerated immune responses to previously subclinical pathogens as T-cell immunity reconstitutes. Transplant-associated IRIS typically occurs 1–3 months post-transplant, coinciding with engraftment, and can unmask previously unrecognized infections or paradoxically worsen known infections. The pharmacological approach parallels HIV-IRIS: maintain antimicrobial therapy, consider corticosteroids for severe inflammatory manifestations, and avoid abrupt changes to immunosuppression without hematology consultation.