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Diseases caused by herpesviruses
The herpesviruses are a family (herpesviridae) of enveloped DNA viruses, of which 9 members are known to infect humans with 5 species being widely spread. Herpes viruses can cause a large spectrum of diseases severity, from mild to potentially lethal. The viruses are highly infective and remain dormant in our bodies after the primary infection. In some cases, reactivation of the virus can occur, causing a secondary infection, which may also be different in nature than the primary disease.
Herpes simplex viruses (HSV-1, HSV-2) are known to infect oral and genital mucosa, respectively. HSV-1 causes gingival, oral and labial herpes (cold sores). The primary disease is represented by vesicular eruption on the lips, oral cavity and gingiva or nose nostrils. The virus is contracted in many cases as early as childhood by human-to-human contact. HSV-2 causes genital herpes – vesicular eruptions on the genitalia (penis, vagina, anus) and is contracted from human to human by vaginal, anal or oral sex. Use of condom during sex can prevent transmission of the virus. Both viruses remain dormant in our nerves and reactivate to form a secondary similar infection. In some cases, primary or secondary infections can occur in the brain and/or its protective layers (meninges), causing severe neurological-infectious diseases – herpes encephalitis and/or herpes meningitis. A special form or herpes meningitis is Mollaret’s meningitis, caused by HSV-2 primarily, leading to a recurrent meningitis in some patients (3-10 episodes throughout life).
Varicella zoster virus (VZV, HHV-3) is the cause of varicella, known as chickenpox, one of the 6 most common rash-causing childhood diseases. The virus causes vesicular eruption all over the body, with distinctive feature of having vesicles in different stages present at the same time. The virus remains dormant in the nerves and its reactivation in older age, then named herpes zoster – painful vesicular eruption in a skin area innervated by the same nerve in which the virus resided (dermatome). VZV can also cause severe encephalitis and meningitis.
Epstein-Bar virus (EBV, HHV-4) is the main cause of the “kissing disease”, infectious mononucleosis. The virus is being transmitted via saliva droplets (coughing, sneezing, sharing cutlery). The virus was recognized as oncogenic (contributing to development of specific cancers).
Human cytomegalovirus (HCMV, HHV-5) is a minor causative agent related to infectious mononucleosis. Moreover, the virus can infect the liver causing transient hepatitis and jaundice (yellowing of the skin and eyes due to accumulation of bilirubin), infection of the lungs or gut, and is a major concern in the very young population and in the immunocompromised patients, such as HIV-infected or those posttransplant.
Human herpesviruses 6 and 7 (HHV-6, HHV-7) can cause mild erythematous skin diseases in young children (erythema infantum, erythema infectiosum), manifested by red skin eruption and high fever. In some cases, the infected child may develop fever seizures that pass after the disease subsided.
Human herpesvirus 8 (HHV-8, KSHV) – was connected to cancerous tumors of the connective tissue (Kaposi’s sarcoma) in immunocompromised patients, mainly in AIDS patients and in some lymphomas.
Transmission of herpesvirus infections
HSV-1 and HSV-2 are transmitted via contact with mucosal and damaged skin surfaces. HSV-1 is mainly tropic for orofacial areas, such as the lips, mouth and nose. In adults, it can be transmitted through saliva during kissing or by sharing food, eating utensils, razors, etc. with an infected person. In some cases, HSV-1 can also cause genital infection through oral-genital contact during sexual intercourse. Although, the risk of transmission increases when there are active sores, virus transmission can occur by healthy skin prior to appearance of ulcers. On the other hand, HSV-2 is mainly tropic for the genital areas. It can be spread through contact with genital surfaces, lesions, sores or fluids of an infected person during sexual intercourse. However, HSV-2 can also cause cold sores in the facial area. In neonates, there is a risk of transmission of HSV if the mother is infected. Prior to, during and shortly after delivery are periods during which the risk of transmission is greatly increased. In addition, mothers with primary HSV infection are more likely to transmit the virus to the neonate.
Transmission of VZV is achieved by direct contact or through the respiratory system via inhalation of virus particles contained in air droplets. The virus mainly infects the respiratory tract (nasopharynx, oropharynx, upper respiratory tract), then leads to viremia, and generalized vesicular lesions.
EBV, the major cause of infectious mononucleosis, is transmitted primarily via direct contact. Exchange of saliva (kissing), sneezing, coughing and sharing food, eating utensils etc. are some practices that facilitate the transmission of the virus from seropositive people. The virus can also be transmitted by blood transfusion.
HCMV is also a cause of mononucleosis, but mainly causes a diverse set of symptoms in immunocompromised patients. The main means of transmission is through saliva, but it can also occur through transfer of infected cells. The most frequent way to transmit HCMV is through sexual intercourse, daily contact between children in school or day-care centres, blood perfusion, organ transplantation, and breast-feeding. Finally, it is important to mention that HCMV is one of the few viruses that can be transmitted from mother to child during pregnancy.
HHV-6, 7 and 8 are mainly transmitted through saliva, where most quantity of these viruses is found, while other modes of transmission are currently under research.
Want to learn more about the immune system?
The immune system is our fundamental defense mechanism. It has aided the evolution of our species, enabling us protection from pathogens such as bacteria, viruses and fungi. Without this complex system of immune cells and defense proteins, we would quickly succumb to lethal infections.
Challenges and opportunities
Despite available drugs against some herpesviruses there are still insufficient options for treatment and prevention of infections with these viruses. With the recent advance in the understanding of immune response to infections, there are new opportunities to use this knowledge.