Cytomegalovirus Infection

• Viral replication and latency mechanisms: CMV enters cells through fusion of the viral envelope with cellular membranes via glycoprotein B (gB) and the gH/gL complex. Following entry, the viral double-stranded DNA genome is transported to the nucleus where immediate-early (IE) genes are transcribed, encoding proteins that shut down host protein synthesis and interfere with innate immune responses. Early genes encode DNA replication machinery, while late genes encode structural proteins. In immunocompetent hosts, robust CD8+ and CD4+ T-cell responses control viral replication, driving the virus into latency within bone marrow progenitor cells, endothelial cells, and epithelial tissues. Latent CMV can persist indefinitely with minimal gene expression, but the virus maintains mechanisms to evade immune surveillance through expression of viral homologs of MHC molecules and chemokine receptors (US2, US3, US6 gene products). Reactivation occurs when T-cell immunity wanes, allowing explosive viral replication and clinically apparent disease.

• Immune evasion and viral pathogenesis: CMV encodes multiple immunoevasins that directly antagonize T-cell recognition and innate immunity. The UL148 gene product downregulates TRAIL death receptors; UL36 encodes a viral inhibitor of caspases preventing apoptosis; US11 promotes proteasomal degradation of MHC class I molecules, reducing antigen presentation to CD8+ T cells. CMV also produces a viral IL-10 homolog (vIL-10) that suppresses pro-inflammatory cytokines and shifts immune responses toward Th2 responses less effective at controlling viral replication. In severely immunocompromised patients (CD4 <50 cells/μL), these viral escape mechanisms overcome the residual cellular immunity, permitting unchecked replication and direct cytotoxic injury to infected tissues.

• Organ-specific pathology and end-organ disease: CMV causes end-organ disease through both direct cytopathic effects and immune-mediated mechanisms. In CMV retinitis, the virus replicates within retinal epithelial cells and photoreceptors, causing focal areas of retinal necrosis with hemorrhage and edema; the pathognomonic "cottage cheese and ketchup" appearance reflects zones of granular necrosis interspersed with hemorrhage. In CMV colitis, viral replication in colonic mucosa and submucosa triggers mucosal ulceration, bleeding, and potential perforation; histology shows viral inclusion bodies in epithelial cells with associated inflammatory infiltrate. CMV esophagitis results from viral invasion of esophageal mucosa, creating large, deep ulcers often with extensive granulation tissue. In CMV pneumonitis, alveolar epithelial infection triggers interstitial inflammation, alveolar-capillary leak, and diffuse alveolar damage, manifesting as progressive hypoxemia and bilateral infiltrates. The paradoxical immune reconstitution inflammatory syndrome (IRIS) occurs when CD4+ counts recover in HIV patients receiving ART; CMV-specific T-cell recovery triggers exaggerated inflammatory responses to CMV antigens in previously infected tissues, potentially worsening retinitis or colitis.

• Congenital CMV pathogenesis: In utero infection occurs through transplacental transmission, with highest risk of severe congenital disease in mothers with primary infection during pregnancy due to absence of maternal antibodies and high viral load. Fetal CMV infection damages developing neural tissue, causing microcephaly through disruption of neurogenesis, ventriculomegaly through obstruction of CSF flow, and periventricular calcifications from focal necrosis. Sensorineural hearing loss results from CMV replication in the developing cochlea and associated inflammation. Hydrops fetalis may develop from hepatitis and hematopoietic dysfunction.

• Primary CMV infection: Occurs in seronegative individuals through person-to-person transmission via saliva, urine, genital secretions, breast milk, blood products, or tissue transplantation. In immunocompetent individuals, primary infection is usually asymptomatic or causes a mononucleosis-like syndrome (fever, atypical lymphocytosis, hepatitis). Pregnant women with primary CMV infection have ~40% risk of intrauterine transmission and risk of severe congenital disease; primary infection carries higher teratogenic risk than reactivation in pregnancy.

• CMV reactivation disease (most common disease pattern in immunocompromised hosts): Occurs when latent CMV reactivates due to severe immunosuppression. The primary risk factor is CD4+ T-cell count <50 cells/μL in untreated HIV/AIDS; historically, CMV retinitis affected 20-30% of AIDS patients in the pre-ART era. Other reactivation triggers include solid organ or hematopoietic stem cell transplantation (especially in seropositive recipients with seronegative donors, or in heavily immunosuppressed recipients), high-dose corticosteroids, TNF-α inhibitors, and chemotherapy. Post-transplant CMV disease develops in ~25% of solid organ transplant recipients without prophylaxis, with highest incidence in lung and heart transplantation due to technical factors and ischemic injury to the allograft.

• Superinfection with new CMV strain: In transplant recipients and some HIV patients, acquisition of a new CMV strain distinct from latent virus can occur through transfusion or contact with infected individuals; this may cause more severe disease than reactivation alone.

• Congenital CMV infection: Maternal seroprevalence and transmission rates vary geographically; ~1% of all live births have congenital CMV, making it the most common congenital viral infection in developed countries. Risk factors include maternal primary infection during pregnancy (highest risk in first and second trimester), young maternal age, lower socioeconomic status, and non-adherence to hygienic measures. Secondary maternal infection (reinfection with different CMV strain or reactivation) can also transmit to the fetus, though with lower risk of severe disease.

• CMV mononucleosis syndrome (immunocompetent hosts): Presents with fever, malaise, myalgias, and arthalgias lasting weeks to months. Atypical lymphocytosis appears on blood smear, but unlike EBV mononucleosis, pharyngitis and lymphadenopathy are typically absent or mild, making diagnosis challenging. Hepatosplenomegaly and mild hepatitis with elevated transaminases occur in ~75% of cases. Some patients develop severe presentations including interstitial pneumonitis, myocarditis, or thrombocytopenia with bleeding.

• CMV retinitis (hallmark end-organ disease in advanced HIV): Presents with floaters, photopsia (flashing lights), visual field defects, or asymptomatic scotomas detected on screening. Fundoscopic examination reveals the pathognomonic "cottage cheese and ketchup" appearance: areas of granular retinal whitening (necrotic retina) interspersed with hemorrhage. Two clinical patterns exist: hemorrhagic "fulminant" retinitis with rapid progression (often >100 μm per week) involving the macula with risk of rapid vision loss, and indolent "granular" retinitis with slower progression peripherally. Without treatment, all CMV retinitis progresses to blindness; immune recovery with ART can halt progression but cannot restore lost vision.

• CMV colitis: Presents with chronic diarrhea (often >1 month duration), abdominal pain, fever, and weight loss in severely immunocompromised patients. Bloody diarrhea and tenesmus occur in severe cases. Toxic megacolon and colonic perforation can develop, manifesting as acute abdominal pain, peritoneal signs, and sepsis. Colonoscopy reveals multiple large, deep ulcerations (often >5 mm diameter) with granulation tissue, primarily affecting the right colon and cecum, though any colonic segment may be involved.

• CMV esophagitis: Presents with severe odynophagia (pain on swallowing) and dysphagia, distinguishing it from candidal esophagitis which causes dysphagia without severe pain. Large mucosal ulcers with heaped-up borders appear on upper endoscopy. Unlike HSV esophagitis with multiple small vesicular ulcers, CMV causes fewer but larger ulcers. Esophageal perforation and mediastinitis are life-threatening complications.

• CMV pneumonitis: Presents insidiously with progressive dyspnea, nonproductive cough, fever, and hypoxemia. Bilateral interstitial infiltrates appear on chest radiography or CT, often with a miliary pattern. Hypoxemia is characteristically severe relative to minimal radiographic findings, reflecting diffuse alveolar-capillary damage. Rapid progression to respiratory failure occurs if untreated.

• CMV encephalitis and neurologic manifestations: Presents with progressive cognitive decline, memory loss, confusion, and behavioral changes in severely immunocompromised patients. Ventricoencephalitis results from CMV replication in ependymal cells lining the ventricles, causing ventriculitis and hydrocephalus. Polyradiculopathy presents with progressive lower extremity weakness, sacral sensory loss, and urinary retention/incontinence, reflecting viral infection of lumbar and sacral nerve roots; CSF shows pleocytosis with elevated protein and low glucose. CMV-related progressive multifocal leukoencephalopathy (CMV-PMLE) presents with diffuse white matter changes.

• CMV hepatitis: Presents with fever, jaundice, and markedly elevated transaminases (often >1000 IU/L). In immunocompromised hosts, can progress to fulminant hepatic failure. Histology shows viral inclusion bodies and hepatocellular necrosis.

• CMV-associated immune reconstitution inflammatory syndrome (IRIS): Occurs weeks to months after initiating ART in patients with previous CMV disease (especially retinitis). Paradoxically worsens CMV symptoms as recovering CD4+ T cells mount inflammatory responses; CMV-immune recovery uveitis presents with vitritis, anterior chamber inflammation, and pain despite viral suppression by antiviral therapy.

• Congenital CMV disease: Symptomatic congenital infection presents at birth with microcephaly, intracranial calcifications (periventricular), ventriculomegaly, growth restriction, hepatosplenomegaly, jaundice, thrombocytopenia with petechiae and purpura, and direct hyperbilirubinemia. Sensorineural hearing loss (both conductive and sensorineural) occurs in ~50% of symptomatic cases and is the most common sequela in asymptomatic congenital infection (~20-30% incidence). Chorioretinitis may cause visual impairment. Progressive microcephaly and developmental delays manifest over the first years of life.

• Viral culture: Traditional shell vial assay or conventional tissue culture can isolate CMV from blood, urine, saliva, CSF, and tissue samples. Sensitivity varies by specimen type: urine culture is more sensitive than blood culture for detecting CMV viremia. Culture takes 1-4 weeks, limiting utility for acute clinical decision-making. Diagnostic yield is poor in immunocompetent hosts due to lower viral loads. Not recommended as first-line test for acute diagnosis.

• CMV antigenemia (pp65 antigen detection): Detects CMV phosphoprotein 65 in circulating leukocytes using monoclonal antibodies and immunofluorescence. Highly specific (>95%) and sensitive (~80-90%) for active CMV viremia in immunocompromised hosts. Results available within 1-2 days. Quantitative antigenemia (number of positive cells per 50,000 leukocytes examined) correlates with disease risk and response to therapy; antigenemia >50-100 positive cells is associated with high risk of end-organ disease. Increasingly replaced by PCR-based assays but remains clinically useful.

• CMV quantitative PCR (qPCR): Detects and quantifies CMV DNA in plasma, blood cells, CSF, urine, and other body fluids using real-time PCR. Superior sensitivity and specificity (>95% for both) compared to culture and antigenemia. Quantitative viral load correlates with disease risk and treatment response; plasma CMV DNA >100,000 copies/mL is associated with increased risk of end-organ disease in advanced HIV. Results available in 24-48 hours. Now standard of care for CMV diagnosis and viral load monitoring. Assays vary by laboratory; interpretation requires knowledge of assay-specific viral load thresholds.

• CMV IgM and IgG serology: IgM antibody (detected by ELISA or immunofluorescence) indicates recent or acute infection, appearing within 1-2 weeks of primary infection and persisting for 2-4 months. IgM positivity may indicate primary infection, reactivation, or reinfection but cannot reliably differentiate these scenarios in seropositive patients. IgG antibody appears after IgM and persists lifelong, indicating past or current infection. Seroconversion (negative to positive IgG during infection) confirms primary infection. High IgG titers suggest recent infection but are nonspecific. Serology has limited utility in immunocompromised hosts who may not mount adequate antibody responses; qPCR is preferred.

• CMV end-organ disease diagnosis:
• Retinitis: Diagnosed clinically via dilated fundoscopic examination by ophthalmology, revealing characteristic retinal necrosis with hemorrhage (cottage cheese and ketchup appearance). Anterior chamber reaction may be minimal or absent, distinguishing CMV from acute retinal necrosis (ARN) caused by HSV/VZV which presents with more anterior inflammation. Biopsy is contraindicated. Optical coherence tomography (OCT) documents macular involvement and monitors response. CMV qPCR of blood or aqueous humor supports diagnosis but is not required when clinical findings are typical.

• Colitis: Diagnosed by colonoscopy with biopsy showing large mucosal ulcers, granulation tissue, and histologic evidence of CMV inclusion bodies ("owl's eye" intranuclear inclusions). Immunohistochemistry with anti-CMV antibodies increases diagnostic yield. Blood CMV qPCR is usually positive (>1000 copies/mL) at time of colitis diagnosis.

• Esophagitis: Diagnosed by upper endoscopy with biopsy and immunohistochemistry or in situ hybridization for CMV. Large ulcers with minimal surrounding inflammation are characteristic. Biopsy is essential to differentiate CMV from HSV and candidal esophagitis.

• Pneumonitis: Diagnosed by clinical presentation combined with blood CMV qPCR (though sensitivity is lower in pneumonitis than disseminated disease) and bronchoalveolar lavage (BAL) with CMV culture, antigenemia, or qPCR. Lung biopsy (transbronchial or open) showing intranuclear CMV inclusions in alveolar and bronchiolar epithelium with diffuse alveolar damage is gold standard. Diffuse bilateral interstitial infiltrates on imaging support diagnosis but are nonspecific.

• Encephalitis/ventricoencephalitis: Diagnosed by CSF analysis showing pleocytosis (typically 10-100 cells/μL, lymphocytic predominance), elevated protein (often 100-500 mg/dL), and normal to low glucose. CMV qPCR of CSF is highly specific (>95%) and moderately sensitive (75-90%) for CMV encephalitis. CSF must be processed immediately as CMV DNA degrades rapidly. Brain MRI may show ventriculitis, periventricular enhancement, or focal white matter lesions. CS

Immediate priorities

• Sight-threatening retinitis is an emergency: lesions in zone 1 (adjacent to fovea or optic disc) warrant same-day ophthalmology involvement and intravitreal ganciclovir or foscarnet injection in addition to systemic therapy, per the NIH/CDC/IDSA/HIVMA Opportunistic Infections guidelines. Intravitreal therapy alone is inadequate — it does not protect the fellow eye or treat visceral disease.
• Reverse the immune defect: in HIV, start or optimize antiretroviral therapy; in transplant recipients, reduce net immunosuppression where the graft allows (American Society of Transplantation IDCOP / The Transplantation Society consensus).

First-line antiviral therapy

• Nucleoside analogues (ganciclovir/valganciclovir): ganciclovir requires phosphorylation by the viral UL97 kinase, then chain-terminates the UL54 DNA polymerase. Induction is IV ganciclovir 5 mg/kg q12h or oral valganciclovir 900 mg twice daily (valganciclovir is the ganciclovir prodrug with high oral bioavailability), followed by once-daily maintenance. Oral valganciclovir is preferred when the gut is absorbing; severe colitis, pneumonitis, or CNS disease favors IV.
• Maintenance/secondary prophylaxis: continued in HIV retinitis until sustained CD4 recovery on ART, per the OI guidelines.

Escalation and resistance

• Foscarnet or cidofovir: pyrophosphate analogue and nucleotide analogue that bypass UL97, so both retain activity against UL97-mutant (ganciclovir-resistant) virus; UL54 polymerase mutations confer cross-resistance. Send genotypic resistance testing when viral load fails to fall.
• Maribavir: UL97 kinase inhibitor approved for refractory/resistant post-transplant CMV; note it antagonizes ganciclovir and should not be co-administered.
• Letermovir: viral terminase inhibitor used for prophylaxis (allogeneic HSCT, high-risk kidney transplant), not for established end-organ disease.

Prophylaxis and prevention

• Transplant strategy: universal valganciclovir prophylaxis versus preemptive therapy guided by serial quantitative PCR; CMV-seronegative or leukoreduced blood products for seronegative recipients.
• Congenital disease: AAP Red Book supports oral valganciclovir 16 mg/kg/dose twice daily for 6 months in symptomatic congenital CMV to improve hearing and neurodevelopmental outcomes.

Contraindicated/avoid

• Acyclovir/valacyclovir: CMV lacks thymidine kinase — ineffective for treatment.
• Cidofovir with renal impairment or sulfa (probenecid) allergy; ganciclovir with zidovudine (additive marrow suppression); ganciclovir and valganciclovir are teratogenic and avoided in pregnancy.

Ocular (vision-threatening emergencies)

• Retinal detachment: necrotic, atrophic retina becomes full-thickness and tears as lesions heal; signalled by sudden curtain-like field loss or a sharp rise in floaters — urgent vitreoretinal surgery.
• Irreversible blindness: full-thickness retinal necrosis destroys photoreceptors; antivirals halt progression but never restore lost vision.
• Immune recovery uveitis: reconstituted CMV-specific T cells attack residual antigen; vitritis, cystoid macular edema, and vision loss despite undetectable viral load — treated with corticosteroids, not more antiviral.

Gastrointestinal (surgical emergencies)

• Colonic perforation and toxic megacolon: deep transmural ulcers with vasculitic microvascular thrombosis; peritonitis, free air, or a dilated colon on imaging mandates surgical consultation.
• Massive GI hemorrhage; esophageal perforation with mediastinitis: signalled by hematochezia with hemodynamic instability, or chest pain with subcutaneous emphysema.

Systemic and transplant-related

• Respiratory failure from CMV pneumonitis: alveolar-capillary damage causes hypoxemia out of proportion to imaging — high mortality in HSCT recipients.
• Graft dysfunction and rejection: CMV upregulates alloantigen expression and endothelial injury, contributing to bronchiolitis obliterans and allograft vasculopathy.
• "Indirect effects": CMV-induced immunomodulation raises risk of bacterial, fungal (invasive aspergillosis), and PTLD-associated complications.
• Congenital sequelae: progressive sensorineural hearing loss — the most common long-term sequela, including in initially asymptomatic infants — plus developmental delay and chorioretinitis; requires serial audiology.

Treatment-related

• Ganciclovir/valganciclovir myelosuppression: inhibition of marrow progenitor DNA synthesis causes neutropenia, thrombocytopenia, anemia; monitor CBC and consider G-CSF or a switch to foscarnet.
• Foscarnet nephrotoxicity and electrolyte chelation: rising creatinine plus hypocalcemia, hypomagnesemia, hypokalemia, hypophosphatemia — seizures and tetany from ionized hypocalcemia are the emergency; painful genital/oral ulcers are the classic clue.
• Cidofovir proximal tubular injury: Fanconi-type proteinuria and creatinine rise (mitigated by saline hydration plus probenecid), also uveitis and ocular hypotony.
• Letermovir drug interactions: CYP3A/P-gp effects raising calcineurin inhibitor levels — check tacrolimus concentrations.
• Antiviral resistance: persistent or rising viral load on adequate therapy signals UL97 or UL54 mutations.

• "Owl's eye" intranuclear inclusions: the single most tested histologic buzzword — large cells with basophilic intranuclear and granular cytoplasmic inclusions on biopsy of colon, esophagus, lung, or kidney.
• Heterophile-negative mononucleosis: fever, atypical lymphocytosis, and transaminitis with a negative Monospot and minimal pharyngitis/adenopathy is CMV, not EBV. The common distractor is calling it EBV because of atypical lymphocytes — those are reactive CD8+ T cells in both.
• CD4 <50 cells/μL plus floaters or a visual field defect: the single best next step is urgent dilated fundoscopic exam by ophthalmology, not blood PCR, not brain imaging, and never biopsy. Retinitis is a clinical diagnosis ("pizza pie" / cottage cheese and ketchup retina).
• Esophageal ulcer morphology is the classic discriminator: CMV gives few large, deep, linear ulcers; HSV gives multiple small punched-out volcano-like ulcers with multinucleated giant cells and Cowdry A inclusions; Candida gives white plaques with dysphagia but less odynophagia. Biopsy the ulcer base for CMV, the edge for HSV.
• Acyclovir does not treat CMV: CMV lacks thymidine kinase, so ganciclovir (activated by the UL97 kinase) is the answer. Foscarnet is the answer when the patient is neutropenic or ganciclovir-resistant; foscarnet's tells are renal failure with hypocalcemia/seizures and genital ulcers.
• Donor/recipient serostatus: in solid organ transplant, D+/R− carries the highest risk (no pre-existing immunity); in allogeneic HSCT, the seropositive recipient is at risk from reactivation. Letermovir is prophylaxis, not treatment.
• Congenital CMV: periventricular calcifications, microcephaly, "blueberry muffin" petechiae, and sensorineural hearing loss — contrast with toxoplasmosis (diffuse intracranial calcifications, hydrocephalus, chorioretinitis). CMV is the most common congenital infection and the leading non-genetic cause of SNHL; diagnose with urine or saliva PCR within the first 3 weeks of life.
• Worsening retinitis with rising CD4 and undetectable CMV DNA = immune recovery uveitis — treat with steroids, do not escalate antivirals.