The Herpesviridae family contains over a 100 species of enveloped DNA viruses that affects humans and animals. They are characterised by the ability to establish latent infections, enabling the virus to persist indefinitely within infected hosts and to undergo periodic activation.
The herpesvirus capsid is icosahedral, composed of 162 capsomeres, and enclosing the core containing the linear dsDNA genome. The nucleocapsid is surrounded by the lipid envelope derived from the host cell nuclear membrane. The envelope carries surface spikes about 8nm long. Between the envelope and capsid is an amorphous structure called the tegument, containing several proteins and enzymes which aids in replication. The enveloped virion measures about 200nm and the naked virion about 100nm in diameter. Herpesviruses replicate in the host cell nucleus. Like other enveloped viruses, herpesviruses are susceptible to fat solvents like alcohol, ether, chloroform and bile salts. They are heat labile and have to be stored at -700C.
1. Herpes simplex- The Herpes simplex virus (HSV) occurs naturally only in humans, but produce experimental infection in many laboratory animals. There are two types of HSV- HSV type 1, is usually isolated from lesions in and around the mouth and is transmitted by direct contact or droplet spread. HSV type 2 is responsible for the majority of genital tract infections and is commonly transmitted venerally. The 2 types cross react serologically. They can be differentiated by:
Antigenic differences can be made out using type specific monoclonal antibodies.
· Type 2 strains form larger pocks on chick embryo.
· Type 2 strains replicate well in chick embryo fibroblast cells, while type 1 poorly.
· Type 2 strains are more neurovirulent in laboratory animals than type 1.
· Type 2 strains are more resistant to antiviral agents.
Both HSV-1 and HSV-2 have an incubation period of 4-10 days, cause the same kind of lesions, and have been isolated from skin and mucus membranes of oral and genital lesions. Genital herpes is the most common and most severe of the herpes simplex virus infections.
In both HSV-1 and HSV-2 infections, vesicles form under keratinized cells and fill with fluid from virus- damaged cells, particles of cell- debris and inflammatory cells. Vesicles are painful but they heal completely in 2-3 weeks without scars if there is no secondary bacterial infections. Adjacent lymph nodes enlarge.
Latency is a hallmark of herpes infections. Within 2 weeks of an active infection, the viruses travel via sensory neurons to ganglia, where they replicates slowly or not at all. They can be reactivated spontaneously or by fever, UV radiation, stress, hormone imbalance, menstrual bleeding, a change in the immune system etc., The infection can spread to and kill cells in the adrenal glands, liver, spleen and lungs. In fatal herpes encephalitis, soft, discolored lesions appear in both grey and white matter of the brain.
After reactivation, the virus moves along the nerve axon to the epithelial cells, where it replicates causing recurrent lesions. These lesions always recur in the same place as original infection occurred, but smaller, shed fewer viruses, contain more inflammatory cells, also heals rapidly than primary lesions. While the virus is in a neuron , neither humoral or cellular immunity can combat it, but once it reaches target epithelial cells and starts to replicate, antibodies can neutralize the viruses, T cells can eliminate virus-infected cells. HSV shedders, people who shed viruses while remaining asymptomatic, pose a significant problem, especially pregnant women pose a serious threat to the infant they bear.
In cases of genital herpes, in females the vesicles appear on the mucus membranes of the labia, vagina and cervix. In males tiny vesicles appear on the penis and are accompanied by a watery discharge. Women infected with genital herpes may be be subject to :
· The incidence of miscarriages.
· If mother infected, the infant must be delivered by caesarean section.
· Infected women have an increased risk of becoming infected with the AIDS virus.
The virus also causes lesions on the cornea, fingers, lips, skin, eyelids etc.,
Laboratory Diagnosis : By microscopy, antigen/DNA detection, virus isolation.
Microscopy: Smears are prepared from lesions and stained with 1% aqueous solution of toluidine blue for 15 secs. Multinucleated giant cells constitute a positive smear. The antigens may be demonstrated by fluorescent antibody technique.
Virus isolation: Inoculation in mice, on chick embryo, tissue culture. Typical cytopathic effects may appear as early as in 24-48 hours.
Serology: Demonstrated by ELISA, neutralisation tests etc.,
Chemotherapy: Idoxuridine used topically ineye and skin infections. Acyclovir and Vidarabine used in deep and systemic infections. Valaciclovir and Famciclovir are effective oral agents.
2. Varicella – Zoster- One Virus- Two Diseases. The Varicella-Zoster virus (VZV), a herpes virus causes both chicken pox(varicella) and shingles(zoster). Chicken pox is a highly contagious disease that causes skin lesions and usually occur in children, however occur at any age. Adult chicken pox is more serious. The source of infection is a chicken pox or herpes zoster parient. Infevtivity is maximum during the initial stages of the disease when the virus is present abundantly in the upper respiratory tract. The buccal lesions which appear in the early stage of the disease and the vesicular fluid are rich in virus content. The virus enters the upper respiratory tract and conjunctiva and replicates at the site of entry. New viruses are carried in blood to various tissues, where they replicate several more times. Release of these viruses cause fever and malaise. In 14-16 days after exposure, small, irregular, rose coloured skin lesions appear. The fluid in them becomes cloudy,dry and crust over in a few days. They start on the scalp and trunk and spread to the face and limbs, mouth, throat, vagina, occasionally to the respiratory and gastrointestinal tracts. The lesions are portals of entry for secondary infections, especially with Staphylococcus aureus . In some cases chicken pox can be fatal, as the viruses invade and damage cells that line small blood vessels and lymphatics. Death is due to extensive blood vessel damage in the lungs and the accumulation of RBC and WBC in alveoli. Cells in the liver, spleen and other organs die because of damage to blood vessels with in them. Chicken pox in pregnancy can be dangerous for both mother and baby.
Multinucleated giant cells and intranuclear inclusion bodies can be seen in smears prepared by scraping the base of the early vesicles, stained with toluidine blue, Giemsa stain. The virus antigen can be detected in scrapings from skin lesions by immunofluorescence, ELISA, PCR also are in use. A live varicella vaccine, developed in Japan, induces good antibody response, but has to be stored between 20C and 80C, recommended for 1-12 year children as a single subcutaneous disease, for older 2 doses. It is safe and effective, but not safe in pregnancy. Antiviral agents are being tested on infections in immunosuppressed patients and those with disseminated disease.
In shingles(herpes zoster), painful lesions like those of chicken pox, usually confined to a single region supplied by a particular nerve. Such eruptions arise from latent viruses, acquired during chicken pox. During the latent period, these viruses reside in ganglia in the cranium and near the spine. When reactivated, the viruses spread from a ganglion along the pathway of associated nerves. Pain and burning of the skin occur before lesions appear. The viruses damage nerve endings, cause intense inflammation, and produce clusters of skin lesions. Also symptoms include mild itching to continuous, severe pain, headache, fever and malaise. Lesions usually appear on trunk but can also infect face and eyes. Shingles is most severe in individuals with immune disorders, where lesions spread to internal organs and can be fatal. Diagnosis and treatment are as for chicken pox.
3. Cytomegaloviruses: CMV are the largest viruses in the herpesviridae family, 150-200nm in size. CMV have been identified in human beings, monkeys, guinea pigs. Human CMV can be grown in human fibroblast cultures. Cultures have to be incubated for prolonged periods, upto 50 days, as the cytopathic effects are slow in appearance.
Initially the virus can be recovered from the oropharynx, virus infected neutrophils last for months. The viruses replicate and have low pathogenicity but are excreted intermittently, during which they infect others. Viruses are shed in all body fluids such as saliva, semen, breast milk, urine. The symptoms include malaise, myalgia, protracted fever, abnormal liver function, lymph node inflammation without swelling. In a synptomatic primary attack of CMV, cell mediated immunity is depressed. Later, during convalescence, the immune system returns to normal. Large quantities of long lasting antibodies are produced in response to CMV infection, but does not prevent viral shedding. It can also be spread by blood transfusions, organ transplants, sexual intercourse.
In fetuses and infants, CMV infections can be life threatening because the virus disseminates widely to variuos organs. Fetuses infected by viruses that cross the placenta. Maternal antibodies also cross the placenta and inactivate small quantities of viruses. In severe CMV fetal infections, retardation of growth within the uterus, severe brain damages happens. Also many babies have hearing loss, jaundice with liver damage, impaired vision.
Laboratory Diagnosis : Diagnosis by the identification and recovery of virus from the clinical specimens by inoculating human fibroblast cultures. Monoclonal antibodies used to detect viral antigens. Nucleic acid probes in which the base sequence unique to CMV nucleic acids is used for identification.
Prophylaxis & Treatment : Screening of blood and organ donors and administration of CMV immunoglobulins have been employed in prevention. Acyclovir is useful in prophylaxis, Ganciclovir, Foscarnet effective and used for the treatment of CMV disease in AIDS patients. No effective vaccine exists.
4. Epstein- Barr Virus – In 1962 Dennis Burkitt suggested that a virus caused the lymphoid malignancy, Burkitt's lymphoma, found in the children in East Africa. This herpes virus is known as Epstein- Barr Virus(EBV). Infection with EB virus leads to latency, periodic reactivation. EBV causes Burkitt's lymphoma, infectious mononucleosis oral hairy leukoplakia(a disease seen in AIDS patients). This virus primarily infects human B lymphocytes. It replicates and derives its envelope from the inner nuclear membrane of the host cell. The virus has large number of genes, ie., more than 50 different proteins are produced by complete expression of the DNA.
The source of infection is usually the saliva of infected persons who shed the viruses in oropharyngeal secretions for months. The EB virus is not highly contagious and droplets and aerosols are not efficient in transmitting infection. Intimate oral contact, as in kissing appears to be the predominant mode of transmission. The virus enters the pharyngeal epithelial cells. It multiplies locally, invades the blood stream and infects B lymphocytes, where the virus becomes latent and the lymphocytes undergoes indefinite growth or releses mature virions by lysing the infected B cells. The viruses invade lungs, bone marrow, lymphoid organs, where they infects mature B lymphocytes. The proliferation of EBV infected lymphocytes is limited by cytotoxic T cells and cells that make humoral antibodies. If these defenses fail to limit lymphocyte proliferation, uncontrolled B cell proliferation leads to B cell cancer or Burkitt’s lymphoma.
Infectious mononucleosis is an adult disease seen in non immune young adults following primary infection with the EB virus. The incubation period is 4-8 weeks. The disease is characterized by headache, fever, malaise, sore throat. Lymphatic tissues become inflammed, some liver cells become necrotic and monocytes accumulate in liver. The spleen is enlarged and cells in lymphoid tissues in the oropharynx multiply. The tonsils are coated with a grey exudate, and the soft palate may be covered with petechia (pin point size hemorrhages, common in skin folds). Secondary infection with ß-hemolytic streptococci frequently occurs. In most cases spontaneous resolution of the disease occurs in 2-4 weeks, while in some it may be prolonged, and lead to a mental & physical fatigue.
Laboratory Diagnosis- Blood examination for the abnormal mononuclear cells characterized by deeply basophilic vacuolated cytoplasm and kidney shaped nuclei. Tests are also available for the demonstration of specific EB virus antibodies. Immunofluorescence and ELISA are commonly employed.
Treatment- Infectious mononucleosis is treated with bed rest and antibiotics for secondary infections. Ampicillin in not used because it causes a rash in infectious mononucleosis patients. No vaccine is available.