Human Herpesvirus 6 (HHV-6)
Human herpesvirus 6 (HHV-6) is a double-stranded DNA virus in the subfamily Betaherpesvirinae, genus Roseolovirus. It comes in two genetically and epidemiologically distinct species: HHV-6A, seen mostly in severely immunocompromised people and with a strong affinity for the nervous system, and HHV-6B, the main cause of roseola (exanthem subitum) in young children. HHV-6A enters cells mainly through CD46, whereas HHV-6B uses CD134 (OX40) on CD4+ T cells.
Like other members of the Herpesviridae, HHV-6 stays in the body for life after the initial infection, lying latent in CD4+ T cells and monocytes. It also has an unusual genetic trick: it can insert its genome directly into the telomeres of human chromosomes (chromosomally integrated HHV-6, or ciHHV-6) and be passed on to the next generation through the germline.
Pathogenesis and Chromosomal Integration
Entry Receptors and Cell Tropism
- Surface receptors: HHV-6B uses its gH/gL/gQ1/gQ2 envelope glycoprotein complex to bind CD134 (OX40), a costimulatory molecule found on activated CD4+ T cells. HHV-6A binds CD46, which is present on almost all nucleated cells.
- Target cells: CD4+ T cells, hematopoietic stem cells, glial cells (astrocytes, microglia) and salivary gland epithelium, which is the main source of virus shed in saliva.
Chromosomal Integration (ciHHV-6)
- How it happens: the viral DNA carries repeats that match human telomere sequences (TTAGGG). Through homologous recombination, the HHV-6 genome becomes anchored at the end of a host chromosome.
- Germline transmission: found in about 1% of people worldwide. When integration occurs in an egg or sperm cell, every cell of the resulting child carries a copy of the HHV-6 genome in a chromosome. Viral DNA levels in the blood are therefore always very high (> 10^6 copies/mL of whole blood), without necessarily meaning an acute infection.
Clinical Features
Roseola in Young Children (Roseola Infantum / Exanthem Subitum / Sixth Disease):
- Age: 90% of cases occur between 6 and 24 months of age, as protective maternal antibodies wane.
- Febrile phase: sudden high fever (39–40.5°C) lasting 3–5 days. Most children stay active and alert despite the fever, but about 10–15% may have febrile seizures.
- Nagayama spots: small red macules or papules on the soft palate and uvula on day 3–4 of the illness.
- Defervescence and rash ("fever drops, rash pops"): the most characteristic feature. As soon as the temperature suddenly returns to normal, a rash breaks out within a few hours.
- Rash appearance: rose-pink maculopapular spots, 2–5 mm across, sometimes with a pale halo.
- Distribution: starts on the trunk (chest, abdomen) and spreads quickly to the neck, face and limbs. The spots blanch on pressure, don't itch or scale, and fade on their own within 24–48 hours without scarring.
Reactivation in Immunocompromised Patients:
- Limbic encephalitis: most common in hematopoietic stem cell transplant (HSCT) recipients, 2–6 weeks after transplant. It causes short-term memory loss, confusion and seizures, with medial temporal lobe changes on MRI.
- Bone marrow suppression: neutropenia, thrombocytopenia and graft failure.
Link With DRESS/DIHS: HHV-6 reactivation, typically 2–4 weeks after a drug reaction begins, has been shown to be a key driver of severe, prolonged flares of DRESS (drug reaction with eosinophilia and systemic symptoms), with progressive damage to multiple organs (liver, kidneys, lungs).
Differential Diagnosis of Childhood Rashes
| Roseola (HHV-6) | Measles (Rubeola) | Rubella (German Measles) |
|---|---|---|
| Rash appears AFTER the fever ends | Rash appears at the peak of high fever | Mild fever just before or with the rash |
| Small, separate rose-pink maculopapules | Deep red, raised maculopapules that merge into patches | Small, light pink maculopapules that don't merge |
| Trunk first, then neck, face, limbs | Behind the ears/face, then down to trunk and limbs | Face, then down to the trunk; fades quickly (3 days) |
| Nagayama spots in the throat, febrile seizures | Koplik spots on the inner cheeks, conjunctivitis, cough, runny nose | Swollen, tender postauricular and occipital lymph nodes |
| Clears completely in 1–2 days, no peeling | Fades in the order it appeared, leaving brown staining and fine scaling | Fades quickly, no marks or peeling |
Our Diagnostic Approach at Haute-Derma
Clinical Assessment:
- Recognizing the classic pattern: a young child with 3–4 days of persistent high fever who breaks out in a rose-pink rash on the chest and abdomen as soon as the fever resolves.
- Checking the mouth for Nagayama spots and asking about febrile seizures.
Microbiology and Molecular Testing:
- Quantitative real-time PCR for HHV-6 DNA (in serum/plasma): detects active viral replication.
- Telling acute infection apart from chromosomal integration (ciHHV-6):
- If whole-blood PCR is very high (> 10^6 copies/mL), PCR must also be run on serum/plasma or on hair follicle or nail cells.
- If serum PCR is negative or below 10^3 copies/mL while whole-blood PCR is extremely high → ciHHV-6 is confirmed, and no antiviral treatment is needed.
Management and Treatment at Haute-Derma
Acute Infection in Children (Roseola Infantum, Normal Immunity)
- Supportive care is the mainstay: the illness is self-limiting and antiviral drugs are NOT indicated.
- Fever control and seizure prevention:
- Acetaminophen (paracetamol): 10–15 mg/kg every 4–6 hours (maximum 60 mg/kg/day).
- Never give aspirin to children, because of the risk of Reye syndrome.
- Dress the child in light clothing and sponge with lukewarm water during high fever.
- Fluids and electrolytes: breastfeed more often or give oral rehydration solution.
Immunocompromised Patients or HHV-6 Encephalitis
- Ganciclovir: 5 mg/kg IV every 12 hours for 14–21 days.
- Foscarnet: 60 mg/kg IV every 8 hours (preferred if the patient has significant ganciclovir-related bone marrow suppression).
- Cidofovir: third-line option when the virus is resistant to both ganciclovir and foscarnet.
