Human Herpesvirus 6 and 7

Classification

General Characteristics and Epidemiology

Basic features of HHV-6 and HHV-7

• Taxonomy:
  • Family: Herpesviridae Herpesviridae A family of enveloped, linear, double-stranded DNA viruses infecting a wide variety of animals. Subfamilies, based on biological characteristics, include: alphaherpesvirinae; betaherpesvirinae; and gammaherpesvirinae. Herpes Simplex Virus 1 and 2
  • Subfamily: Betaherpesvirinae Betaherpesvirinae A subfamily of herpesviridae characterized by a relatively long replication cycle. Genera include: cytomegalovirus; muromegalovirus; and roseolovirus. Cytomegalovirus
  • Genus: Roseolovirus
  • Species:
    • Human betaherpesvirus 6A
    • Human betaherpesvirus 6B
    • Human betaherpesvirus 7
• DNA viruses DNA Viruses Viruses whose nucleic acid is DNA. Virology:
  • Linear
  • Double stranded
• Structure:
  • DNA DNA A deoxyribonucleotide polymer that is the primary genetic material of all cells. Eukaryotic and prokaryotic organisms normally contain DNA in a double-stranded state, yet several important biological processes transiently involve single-stranded regions. DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine). DNA Types and Structure core
  • Icosahedral capsid Capsid The outer protein protective shell of a virus, which protects the viral nucleic acid. Capsids are composed of repeating units (capsomers or capsomeres) of capsid proteins which when assembled together form either an icosahedral or helical shape. Virology
  • Tegument Tegument Herpes Simplex Virus 1 and 2
  • Lipid envelope Envelope Bilayer lipid membrane acquired by viral particles during viral morphogenesis. Although the lipids of the viral envelope are host derived, various virus-encoded integral membrane proteins, i.e. Viral envelope proteins are incorporated there. Virology with glycoprotein spikes

Associated diseases

The clinical relevance of HHV-6A is not entirely known.

• HHV-6B and HHV-7 are associated with:
  • Roseola infantum (also known as exanthem Exanthem Diseases in which skin eruptions or rashes are a prominent manifestation. Classically, six such diseases were described with similar rashes; they were numbered in the order in which they were reported. Only the fourth (Duke’s disease), fifth (erythema infectiosum), and sixth (exanthema subitum) numeric designations survive as occasional synonyms in current terminology. Varicella-Zoster Virus/Chickenpox subitum, roseola, or 6th disease)
  • Acute febrile respiratory disease
  • Febrile seizures Febrile seizures Seizures that occur during a febrile episode. It is a common condition, affecting 2-5% of children aged 3 months to five years. An autosomal dominant pattern of inheritance has been identified in some families. The majority are simple febrile seizures (generally defined as generalized onset, single seizures with a duration of less than 30 minutes). Complex febrile seizures are characterized by focal onset, duration greater than 30 minutes, and/or more than one seizure in a 24 hour period. The likelihood of developing epilepsy (i.e., a nonfebrile seizure disorder) following simple febrile seizures is low. Complex febrile seizures are associated with a moderately increased incidence of epilepsy. Seizures in Children
  • Reactivation Reactivation Herpes Simplex Virus 1 and 2 syndromes in transplant recipients:
    • Viremia Viremia The presence of viruses in the blood. Erythema Infectiosum
    • Encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis
    • Pneumonitis
    • Myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis
    • Hepatitis
  • Possible:
    • Pityriasis rosea Pityriasis rosea Pityriasis rosea is an acute, self-limited skin disease. The etiology is not known, and it commonly occurs in young adults. Patients initially present with a single, ovoid “herald patch.” This is followed by diffuse, pruritic, scaly, oval lesions over the trunk (often in a “Christmas tree” distribution on the back) and extremities. Pityriasis Rosea
    • Lichen planus Lichen planus Lichen planus (LP) is an idiopathic, cell-mediated inflammatory skin disease. It is characterized by pruritic, flat-topped, papular, purple skin lesions commonly found on the flexural surfaces of the extremities. Other areas affected include genitalia, nails, scalp, and mucous membranes. Lichen Planus (HHV-7)

Epidemiology

• HHV-6:
  • > 70% of adults are seropositive.
  • Infection generally occurs by the age of 2 years.
• HHV-7:
  • > 95% of adults are seropositive.
  • Infection often occurs in childhood.

Pathogenesis

Reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli

Humans are the reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli for HHV-6 and HHV-7.

Transmission

HHV-6 and HHV-7 are transmitted through contact with saliva Saliva The clear, viscous fluid secreted by the salivary glands and mucous glands of the mouth. It contains mucins, water, organic salts, and ptyalin. Salivary Glands: Anatomy.

Pathophysiology

• Replication occurs in T lymphocytes T lymphocytes Lymphocytes responsible for cell-mediated immunity. Two types have been identified – cytotoxic (t-lymphocytes, cytotoxic) and helper T-lymphocytes (t-lymphocytes, helper-inducer). They are formed when lymphocytes circulate through the thymus gland and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen. T cells: Types and Functions.
• Viral entry through:
  • HHV-6: CD46 receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors
  • HHV-7: CD4 receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors
• Viral DNA DNA A deoxyribonucleotide polymer that is the primary genetic material of all cells. Eukaryotic and prokaryotic organisms normally contain DNA in a double-stranded state, yet several important biological processes transiently involve single-stranded regions. DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine). DNA Types and Structure is released into the nucleoplasm → production of viral proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis and replication
• Lytic replication → cell death Cell death Injurious stimuli trigger the process of cellular adaptation, whereby cells respond to withstand the harmful changes in their environment. Overwhelmed adaptive mechanisms lead to cell injury. Mild stimuli produce reversible injury. If the stimulus is severe or persistent, injury becomes irreversible. Apoptosis is programmed cell death, a mechanism with both physiologic and pathologic effects. Cell Injury and Death
• Low-level persistent infection occurs in the salivary glands Salivary glands The salivary glands are exocrine glands positioned in and around the oral cavity. These glands are responsible for secreting saliva into the mouth, which aids in digestion. There are 3 major paired salivary glands: the sublingual, submandibular, and parotid glands. Salivary Glands: Anatomy.
• Latency:
  • HHV-6: occurs in monocytes Monocytes Large, phagocytic mononuclear leukocytes produced in the vertebrate bone marrow and released into the blood; contain a large, oval or somewhat indented nucleus surrounded by voluminous cytoplasm and numerous organelles. Innate Immunity: Phagocytes and Antigen Presentation
  • HHV-7: occurs in T lymphocytes T lymphocytes Lymphocytes responsible for cell-mediated immunity. Two types have been identified – cytotoxic (t-lymphocytes, cytotoxic) and helper T-lymphocytes (t-lymphocytes, helper-inducer). They are formed when lymphocytes circulate through the thymus gland and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen. T cells: Types and Functions

Clinical Presentation

Human herpesvirus-6 infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease are usually mild and occur during childhood. Most HHV-7 infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease are asymptomatic.

Roseola infantum

• High fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever:
  • Lasts approximately 3–5 days
  • Resolves abruptly
• Rash Rash Rocky Mountain Spotted Fever:
  • Timing:
    • Follows resolution of fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever
    • Lasts approximately 1–2 days
  • Appearance:
    • Maculopapular Maculopapular Dermatologic Examination or macular
    • Rose-pink or red color
    • Blanching Blanching Dermatologic Examination
  • Nonpruritic
  • Distribution:
    • Starts on neck Neck The part of a human or animal body connecting the head to the rest of the body. Peritonsillar Abscess and trunk
    • Spreads to face and extremities
• Other manifestations:
  • Conjunctivitis Conjunctivitis Conjunctivitis is a common inflammation of the bulbar and/or palpebral conjunctiva. It can be classified into infectious (mostly viral) and noninfectious conjunctivitis, which includes allergic causes. Patients commonly present with red eyes, increased tearing, burning, foreign body sensation, and photophobia. Conjunctivitis
  • Edematous eyelids Eyelids Each of the upper and lower folds of skin which cover the eye when closed. Blepharitis
  • Otitis media
  • Nagayama spots (erythematous uvulopalatoglossal papules)
  • Rhinorrhea Rhinorrhea Excess nasal drainage. Respiratory Syncytial Virus
  • Cough
  • Diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea
  • Vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia
  • Bulting fontanelle Fontanelle Any of six membrane-covered openings between the cranial sutures in the incompletely ossified skull of the fetus or newborn infant. The fontanelles normally close sometime after birth. Skull: Anatomy
  • Lymphadenopathy Lymphadenopathy Lymphadenopathy is lymph node enlargement (> 1 cm) and is benign and self-limited in most patients. Etiologies include malignancy, infection, and autoimmune disorders, as well as iatrogenic causes such as the use of certain medications. Generalized lymphadenopathy often indicates underlying systemic disease. Lymphadenopathy:
    • Cervical
    • Postauricular
    • Occipital Occipital Part of the back and base of the cranium that encloses the foramen magnum. Skull: Anatomy

Diagnosis and Management

Diagnosis

The diagnosis is clinical, and testing is rarely indicated. An exception is made in the case of severe disease in immunocompromised immunocompromised A human or animal whose immunologic mechanism is deficient because of an immunodeficiency disorder or other disease or as the result of the administration of immunosuppressive drugs or radiation. Gastroenteritis individuals (e.g., encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis, myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis).

• PCR PCR Polymerase chain reaction (PCR) is a technique that amplifies DNA fragments exponentially for analysis. The process is highly specific, allowing for the targeting of specific genomic sequences, even with minuscule sample amounts. The PCR cycles multiple times through 3 phases: denaturation of the template DNA, annealing of a specific primer to the individual DNA strands, and synthesis/elongation of new DNA molecules. Polymerase Chain Reaction (PCR) to detect viral DNA DNA A deoxyribonucleotide polymer that is the primary genetic material of all cells. Eukaryotic and prokaryotic organisms normally contain DNA in a double-stranded state, yet several important biological processes transiently involve single-stranded regions. DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine). DNA Types and Structure
• Serology Serology The study of serum, especially of antigen-antibody reactions in vitro. Yellow Fever Virus
  • Indirect immunofluorescence
  • ELISA ELISA An immunoassay utilizing an antibody labeled with an enzyme marker such as horseradish peroxidase. While either the enzyme or the antibody is bound to an immunosorbent substrate, they both retain their biologic activity; the change in enzyme activity as a result of the enzyme-antibody-antigen reaction is proportional to the concentration of the antigen and can be measured spectrophotometrically or with the naked eye. Many variations of the method have been developed. St. Louis Encephalitis Virus

Management

Treatment is generally not necessary, as symptoms are self-limiting Self-Limiting Meningitis in Children. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship with severe disease may be given:

• Ganciclovir Ganciclovir An acyclovir analog that is a potent inhibitor of the herpesvirus family including cytomegalovirus. Ganciclovir is used to treat complications from aids-associated cytomegalovirus infections. Antivirals for Herpes Virus
• Cidofovir Cidofovir An acyclic nucleoside phosphonate that acts as a competitive inhibitor of viral DNA polymerases. It is used in the treatment of retinitis caused by cytomegalovirus infections and may also be useful for treating herpesvirus infections. Antivirals for Herpes Virus
• Foscarnet Foscarnet An antiviral agent used in the treatment of cytomegalovirus retinitis. Foscarnet also shows activity against human herpesviruses and HIV. Antivirals for Herpes Virus
• Note: Data for these treatments are limited.

Comparison of Herpesviruses

The following table compares the 9 herpesviruses considered endemic in humans; there are 115 different total known species of herpesviruses, grouped into 3 families: 

• Alpha (infect epithelial cells and produce latent infection in post-mitotic neurons Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology)
• Beta (infect and produce latent infection in a variety of cell types)
• Gamma (produce latent infection, mainly in lymphoid cells)

Comparison of Common Childhood Rashes