|Case Study #15: Kevin Chau, Jere Wilson, Yessenia Valezco
A 2-year-old child with a fever for 2 days was not eating and was crying often. On examination, the physician noted that the mucous membranes of the mouth were covered with numerous shallow, pale ulcerations. A few red papules and blisters were also observed around the border of the lips. The symptoms worsened over the next five days and then slowly resolved, with complete healing after two weeks
The physician suspects that this is a HSV infection. How would the diagnosis be confirmed?
The first step in determining a primary HSV infection is a visual examination of the infection. HSV infection is characterized by watery papules that burst, becoming lesions. These lesions contain a yellow-pus core surrounded by an erythematous border around the region of cell lysis. HSV infections occur commonly in the anterior-oral cavity or genital region. Both viral subtypes can cause infections in either area. Infections can also spread to other epidermal areas via autoinoculation. The doctor may evaluate the patient’s medical and sexual history to develop his initial diagnosis.
Diagnosis of primary HSV infection will need other laboratory tests in order to confirm the initial diagnosis. Viral isolation is a technique that involves the collection of fluid from an intact vesicle from a patient with active lesions. This fluid is transferred to a media and inoculated onto cell cultures. The detection of degenerative changes in the cells classifies the virus in the Herpesviridae family.
Tzanck smear is a very useful technique in the detection of herpes simplex virus and herpes zoster virus. However, it cannot differentiate between the two viruses. This technique includes the collection and staining of the specimen from an opened lesion. The presence of giant multinucleated keratinocytes and ballooning degeneration of the cells confirms the diagnosis of an infection by a Herpesviridae.
Direct fluorescent antibody (DFA) is used to detect viral antigens. This test involves labeling HSV antigens with monoclonal antibody and viral typing.
Polymerase chain reaction (PCR) allows the detection of HSV genome and can be used in determining HSV subtype from the lesion. This technique is very sensitive but expensive.
Enzyme-linked immunosorbent assay (ELISA) is a fast and inexpensive assay, which recognizes the antigenic subtype differences in the viral glycoprotein G1 and G2. However, false-positive results may occur because of the potential for cross-reactivity between the two high homologous glycoproteins.
A Western blot assay is necessary for confirming positive ELISA results as it detects multiple viral proteins using labeled antibodies. Each HSV subtype has a unique banding pattern for its viral proteins.
How would you determine whether this infection was caused by HSV-1 or HSV-2?
Differentiation of HSV-1 and HSV-2 begins with an examination of the patient’s medical and sexual history. A history of unprotected sex, primarily in adults and adolescents, is a strong cue for of HSV-2 infection. On the other hand, HSV-1 infection is most common to the human lifespan, and easily transmitted from immediate contact with close relatives and peers. HSV infection of newborns can be either HSV-1 or HSV-2. Neonatal HSV-1 infection is post-partum and acquired through family contact. Neonatal HSV-2 infection typically occurs during vaginal delivery in which the newborn contacts open sores affecting the birth canal.
There are several diagnostic tests that can be used to confirm initial medical hypothesis. These tests include viral inoculation on tissue culture, histological staining (Tzanck Smear), direct fluorescent assay, viral genomic amplification via PCR, ELISA, and Western blots. Cell culture and histological staining are only effective in determining general herpes infection. More specificity for the HSV subtypes can be obtaining using direct detection of a genomic region unique to only one viral subtype. Likewise, each subtype may have unique antigens that can be detected using ELISA, Western blots, and direct fluorescent assay.
What immune responses were most helpful in resolving this infection and when were they activated?
Primary infections are defined as infections which occur in the absence of antibody developed against Herpes Simplex virus (HSV). The initial immune response is complex and not completely understood, but antibodies to HSV antigens can be detected within 4 - 8 days of the initial infection. The first antibodies detected are IgM followed by IgG, which persists for longer period of time. At the site of of epidermal infection viral antigens are presented on dendritic cells and macrophages (antigen-presenting cells) to CD4+ T helper cells. These CD4+ T helper cells initiate viral clearance by secreting cytokines such as IFN-gamma, stimulating activation of macrophages and natural killer (NK) cells. The immune response effectively induces lysis of the infected cells expressing viral antigen via cytotoxic T cells. Apoptosis occurs through the facilitation of MHC I complex and CD8 + T-cells.
Secondary infections initiate MHC II complex, stimulating specific memory in CD4+ T helper cells and B cell activation. More IFN-gamma is release, and primary defense is cytotoxic apoptosis and detection via MHC II. HSV is thought to compromise MHC I detection of the infection by modifying its viral glycoprotein ICP47 to bind to TAP, the transporter protein associated with antigen processing with MHC I. Binding to tap prevents the antigen processing by MHC I, resulting in no cytotoxic T directed apoptosis of the infected region.
HSV escapes complete immune resolution by causing latent and recurrent infections. What was the site of latency in this child and what might promote future recurrences.
HSV can develop latent infections characterized by dormant viral activity. No new virions are synthesized, and the patient is asymptomatic. HSV latency occurs when the virus retreats into neighboring sensory neurons from the initial site of the primary infection. Oral infections develop latency in the trigeminal ganglia, while genital infections tend to develop latency in the sacral ganglia. The child in the case study had an oral HSV infection, and would exhibit viral latency in the trigeminal ganglia.
Once in the neuronal cells, HSV activates its viral LAT gene in response to viral metabolic stress. This halts viral replication and prevents host cell damage that may results in host apoptosis. Stopping host apoptosis creates a permanent viral reservoir in the host, allowing the virus to persist and spread without destroying its host.
LAT expression during latency can stop during times of physical and emotional stress. Stress can include fever, common cold, severe sunburn, fatigue, trauma, menstruation, pregnancy, or allergies. The stress triggers the down regulation of LAT resulting in the reactivation of the virus, producing secondary infections. Secondary infections show the same clinical symptoms without having a new HSV viral infection.
What were the most probable means by which the baby was infected by HSV?
The 2-year-old child was most likely infected by family contact (HSV-1 infection). Transmission can occur through kisses or sharing oral-contaminated material when open sores are present. The virus could have also been transmitted through genital contact (HSV-2 infection). Though this mode of transmission is more common in adults and sexually active adolescents, a mother can pass the virus to her baby during birth. If a woman has a primary herpes infection (having HSV-2 for the first time) at any point in the pregnancy, there is a 5% possibility of the virus crossing the placenta and infecting the uterus. Likewise, if the mother is unaware of her HSV illness, there is a 90% chance for congenital neonatal herpes. Neonatal herpes is associated with premature births and miscarriages. If left untreated, 60% of infect newborns suffer mortality or survive with severe developmental abnormalities.
Which antiviral drugs are available for treatment of HSV infections? What are their targets? Were they indicated for this child? If not, why not?
There is no cure for HSV infection, but there are medications that can modify the course of the disease. Medication can prevent viral outbreaks, viral transmission, viral shedding, and other complications.
Docosanol is an over-the-counter product approved for topical treatment. This antiviral agent prevents the fusion of the viral enveloped with the cell membrane preventing the entry of the virus into the host cell.
Acyclovir, valacyclovir, and penciclovir are viral agents that target the DNA polymerase and viral DNA replication. Acyclovir and valacyclovir perform selective inhibition of DNA polymerase resulting in termination of further elongation of viral DNA. Penciclovir, on the other hand, halts viral DNA replication by competitive inhibition of DNA polymerase rather than chain termination.
Foscarnet is a pyrophosphate analog that competitively blocks pyrophosphate binding site on DNA polymerase resulting in direct inhibition.
It is very probable that acyclovir was indicated for this child. It is the most commonly used antiviral agent for the treatment of herpes simplex virus in children by the oral and intravenous routes. Administration of oral acyclovir prevents cutaneous recurrence of HSV after primary infection.
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Antiviral Drugs For Herpes Treatment