The herpesvirus family includes DNA viruses that infect humans and various animal species. These viruses are characterized by their relatively large size and complex structure, containing a double-stranded DNA genome encased within a protein shell and an outer lipid envelope. Many individuals carry one or more types of herpesviruses, often without experiencing noticeable symptoms.
What Makes Herpesviruses Unique?
Herpesviruses exhibit specific biological features. They possess a relatively large, double-stranded DNA genome housed within an icosahedral capsid, surrounded by a protein layer called the tegument, and enclosed by a lipid envelope derived from the host cell membrane. This complex structure allows them to establish lifelong infections.
A defining characteristic of herpesviruses is their ability to enter a state of dormancy known as latency following initial infection. During latency, the viral genome persists within specific host cells, often neurons, without actively producing new virus particles. This allows the virus to evade the host’s immune system for extended periods.
Latency is not a permanent state, as these viruses can periodically reactivate, leading to the production of new viruses and, in some cases, recurrent symptoms. Reactivation can be triggered by various factors, including stress, illness, hormonal changes, or exposure to ultraviolet light. This cycle of latency and reactivation is a hallmark of herpesvirus infections, influencing their persistence and clinical manifestations.
Common Herpesviruses Affecting Humans
There are nine distinct human herpesviruses (HHVs) that commonly infect people, each associated with specific conditions. Herpes Simplex Virus (HSV) types 1 and 2 are well-known, with HSV-1 typically causing oral herpes, often presenting as cold sores around the mouth, though it can also cause genital herpes. HSV-2 is primarily linked to genital herpes, a sexually transmitted infection, but it can also lead to oral infections.
Varicella-Zoster Virus (VZV), also known as HHV-3, is responsible for two distinct diseases. Primary infection with VZV results in chickenpox, a highly contagious illness characterized by an itchy, vesicular rash that commonly affects children. After the initial infection resolves, VZV can reactivate later in life, often decades later, to cause shingles (herpes zoster), which presents as a painful rash typically affecting a single nerve pathway.
Epstein-Barr Virus (EBV), or HHV-4, is widely prevalent, with most individuals exposed by adulthood. It is the primary cause of infectious mononucleosis, commonly referred to as “mono” or “glandular fever,” particularly in adolescents and young adults. Symptoms of mononucleosis can include extreme fatigue, fever, sore throat, and swollen lymph nodes. EBV has also been associated with several cancers, including certain lymphomas and nasopharyngeal carcinoma.
Cytomegalovirus (CMV), or HHV-5, is another common herpesvirus, with a significant portion of the population infected at some point. In healthy individuals, CMV infection often causes mild or no symptoms, but it can manifest as an illness resembling mononucleosis. CMV poses serious risks for newborns if acquired congenitally, potentially leading to hearing loss, developmental delays, or other significant health problems. It can also cause severe disease in individuals with weakened immune systems, such as transplant recipients or those with HIV.
Other human herpesviruses include HHV-6 and HHV-7, which are closely related and are the main causes of roseola infantum, also known as exanthem subitum or sixth disease. This common childhood illness typically presents with a high fever followed by a characteristic rash. HHV-8, or Kaposi’s sarcoma-associated herpesvirus (KSHV), is linked to Kaposi’s sarcoma, a type of cancer that affects the skin, lymph nodes, and internal organs, particularly in individuals with compromised immune systems, such as those with AIDS.
How Herpesviruses Spread and Persist
Herpesviruses spread primarily via direct contact with infected bodily fluids or lesions. Herpes Simplex Viruses (HSV-1 and HSV-2) are transmitted through intimate person-to-person contact, including kissing for oral herpes and sexual contact for genital herpes. Transmission can occur even when visible sores are not present, due to asymptomatic viral shedding from skin or mucosal surfaces. Varicella-Zoster Virus (VZV) spreads through direct contact with blisters or via airborne respiratory droplets.
Epstein-Barr Virus (EBV) is transmitted through saliva, earning mononucleosis the nickname “kissing disease,” though sharing utensils or drinks can also spread it. Cytomegalovirus (CMV) is shed in various bodily fluids, including urine, saliva, semen, and breast milk, and can be transmitted through close contact, sexual activity, blood transfusions, organ transplants, or from mother to child during pregnancy or birth. HHV-6 and HHV-7 are also spread through saliva.
Once a herpesvirus infects a host, it establishes a lifelong presence by entering a latent phase. For HSV and VZV, this typically involves the virus migrating along nerve pathways to sensory nerve ganglia, where it remains dormant within nerve cells. EBV maintains latency in B lymphocytes and salivary glands, while CMV can persist in white blood cells, kidneys, and secretory glands. During reactivation, the virus begins to replicate, leading to recurrent symptoms or asymptomatic shedding, allowing for further transmission.
Managing Herpesvirus Infections
Diagnosing herpesvirus infections involves clinical assessment and laboratory tests. Viral culture identifies the presence of the virus from a lesion sample, although its sensitivity can vary depending on the stage of the lesion. Polymerase Chain Reaction (PCR) tests are sensitive and specific, detecting viral DNA in samples from sores or bodily fluids, even in the absence of visible lesions. Blood tests that detect antibodies specific to different herpesviruses, such as HSV-1, HSV-2, or VZV, can indicate a past or current infection, helping to differentiate between virus types.
While there is no cure for most herpesvirus infections, antiviral medications can effectively manage symptoms and reduce the frequency and severity of outbreaks. Antivirals include acyclovir, valacyclovir, and famciclovir, which work by inhibiting viral replication. These medications are used for initial severe infections, for episodic treatment to shorten outbreaks, or as suppressive therapy to prevent recurrent episodes. Topical creams or ointments containing antivirals like acyclovir or penciclovir are also used for localized symptoms such as cold sores.
Prevention strategies for herpesvirus infections involve avoiding direct contact with lesions during outbreaks, such as refraining from kissing or sexual activity. Consistent and correct use of condoms can reduce the risk of sexually transmitted herpesviruses like HSV-2, though they do not offer complete protection. Vaccines are available for certain herpesviruses; for example, vaccines protect against VZV, preventing chickenpox in children and shingles in adults. No approved vaccines currently prevent HSV-1 or HSV-2 infection, but research into both prophylactic (preventative) and therapeutic (symptom-reducing) vaccines is ongoing.