Case 1 introduction

To gain entry into the host cell, the viral fiber proteins bind to the coxsackie adenovirus receptor on host cell surfaces and become internalized by receptor-mediated endocytosis. The virus then lyses the endosome, and the viral DNA is delivered to the host nucleus by the capsid. Viral DNA replication occurs in the nucleus, and the viral capsid proteins are made in the cytoplasm and then transported to the nucleus for viral assembly. A single viral replication cycle takes approximately 32-36 hours and produces around 10,000 new virions. Figure 24-1 shows the replication cycle. However, errors in assembly and replication are common, resulting in a much lower number of infectious viral particles.

Adenoviruses primarily infect children under 3 years and appear clinically with a variety of symptoms including fever, cough, nonstreptococcal exudative pharyngitis, cervical adenitis, conjunctivitis, or gastroenteritis. Symptoms can last from 3-5 days. More severe respiratory diseases include laryngitis, bronchiolitis, and pneumonia. Reactivated viral disease occurs primarily in immune compromised individuals. Adenoviral follicular conjunctivitis, or "pink eye," outbreaks in children often involve swimming pools as a common source of infection. Gastroenteritis is also a major clinical manifestation of adenoviral infection. Adenoviral types 40, 41, and 42 have been shown to be associated with gastrointestinal disease in infants and hospitalized patients.

In addition to clinical presentation of infection, laboratory diagnostic tests, including cell culture, ELISA (enzyme-linked immunosorbent assay), PCR, and DNA-probe analysis are available and can be used to detect the viral type in clinical samples and tissue culture. However, their primary use is for epidemiological studies, and they are not used widely in clinical practice for diagnostic purposes. Typically, diagnosis is made by clinical presentation and by history.

Treatment and Prevention

Currently, there is no treatment for adenoviral infection. Live oral vaccines have been developed for adenovirus types 4 and 7, which cause acute respiratory tract infections, and have been used primarily in military settings. However, because some adenoviruses are oncogenic, such vaccines have not been made available to the general population. Thus, prevention is the most important aspect involving careful hygiene, handwashing, and isolation of infected individuals.

[24.1] An 11-year-old boy attending summer Boy Scout camp develops symptoms of sore throat, headache, fatigue, and conjunctivitis. He is seen by the camp medical staff and on examination is found to have a slight fever of 39.8C (103.6F), but no rash. Within the next 1-2 days, several of the other campers develop similar symptoms, which last for 5-7 days. The larger number of campers with similar symptoms indicates that a common source of infection is causing the outbreak. Which of the following activities is the most likely source of the campers' infection?

[24.2] The causative agent in the question above was determined to be an adenoviral infection. Which of the following best describes this viral agent?

[24.3] A 2-year-old child attending daycare develops diarrhea and gastroenteritis as a result of an adenoviral infection. Which of the following adenoviral serotypes would most likely be responsible for this girl's illness?

[24.1] D. The campers symptoms are consistent with adenoviral conjunctivitis, which is commonly spread through contaminated swimming pools or ponds; answers A, B, C, and E are incorrect.

[24.2] A. Adenoviruses are nonenveloped, double-stranded DNA viruses with fiber structures projecting from their vertices or penton bases; answers B, C, D, and E are incorrect: (B) describes polyoma viruses such as human papillomavirus; (C) describes viruses such as rhabdoviruses, orthomyxoviruses, paramyxoviruses, and the like; (D) describes herpes viruses; (E) describes hepatitis B virus.

[24.3] E. Adenoviral types 40, 41, and 42 have been shown to be associated with gastrointestinal disease in infants; answers A, B, C, and D are incorrect: Adenoviral types 4 and 7, commonly cause upper respiratory infections in military recruits; adenoviral types 19 and 37 have been implicated in causing epidemic keratoconjunctivitis.

You are called to examine a 1-day-old male because the nurse is concerned that he is jaundiced. He was born by spontaneous vaginal delivery to a 19-year-old gravida1 para1 after a full-term, uncomplicated pregnancy. The mother had no illnesses during her pregnancy; she did not use tobacco, alcohol, or drugs; and the only medication that she took was prenatal vitamins. She denied any significant medical history, and there is no family history of genetic syndromes or illnesses among children. The infant is mildly jaundiced but has a notable abnormally small head circumference (microcephaly). His cardiovascular examination is normal. His liver and spleen are enlarged on palpation of the abdomen. Neurologic exam is notable for the lack of a startle response to a loud noise. CT scan of his head reveals intracerebral calcifications. The pediatrician explains to the child's mother that the virus involved is the most commonly transmitted transplacental viral infection in the United States.

 What is the most likely cause of this infant's condition?

 How did he likely acquire this?

 What is the test of choice to confirm the diagnosis?

ANSWERS TO CASE 25: Cytomegalovirus

 Most likely cause of this infant's condition: The infant is most likely suffering from a congenital infection with Cytomegalovirus (CMV).

 Likely acquisition of this infection: Transplacental spread of the virus during a primary CMV infection of the pregnant mother.

 Test of choice to confirm the diagnosis: The definitive diagnostic test to confirm CMV infection in this patient is to demonstrate the presence of CMV in the infant's urine.

Human Cytomegalovirus (CMV) is the largest member of the human Herpesviridae family. It is lymphotrophic and commonly produces asymptomatic infections in immune competent hosts. However, it can cause serious primary and recurrent infections in immunosuppressed individuals and neonates. CMV is the most common transplacentally transmitted infection in the United States.

CMV belongs to the Betaherpesvirinae subunit of the Herpesviridae family and is the largest known virus to infect humans. The genome of linear, double-stranded DNA is housed in an icosadeltahedral capsid. Between clinical cases the envelope and the capsid is a layer called the tegument, a phosphoprotein-containing matrix that plays a role in initiating replication. CMV, like other herpesviruses, has a lipid envelope that contains glycoproteins that facilitate attachment and entry into host cells. The virus often establishes latent infection in lymphocytes, leukocytes, and organs like the kidney, heart, and lung. Cell-mediated immunity is required for the control of CMV infections. Suppression of the immune system by medications or infection, such as AIDS, can result in reactivation of the virus and severe, symptomatic disease.

CMV has a ubiquitous distribution and approximately 10-15 percent of children are infected before the age of 5. Most CMV infections in immune competent hosts are asymptomatic, although occasionally a mononucleosis-like syndrome can occur. Yet, even in subclinical infections, CMV can be isolated from saliva, cervical secretions, semen, urine, and white blood cells for months to years following infection. Although CMV is found in many host secretions, its major routes of transmission are via contact with blood, oral secretions, sexual contact, organ transplant, or congenital infection. CMV is the most common viral cause of congenital disease and infection, and its spread is thought to occur via transplacental transfer. The risk to the fetus is particularly high when the mother has a primary infection during her pregnancy.

Diagnosis

Although most CMV infections during childhood and in adults are asymptomatic, infants and immunocompromised patients can develop severe clinical symptoms from either primary infection or reactivation. Of the 1 percent of infants infected in utero or during delivery, 90 percent will develop asymptomatic infections, while the other 10 percent will develop symptomatic infections with congenital defects or disorders. Nearly all of the infants with symptomatic infections are born of mothers with primary infections during their pregnancies. Congenital CMV can cause a devastating syndrome that includes microcephaly, intracerebral calcifications, hepatosplenomegaly, thrombocytopenia, chorioretinitis, deafness, mental retardation, jaundice, and rash. Many of the infants with severe CMV congenital syndrome die within a short time, and those who survive have been shown to have persistent neurologic deficits. Reactivation of a latent infection during pregnancy confers a much lower risk, as the fetus is protected by the maternal immune response.

In addition to assessing the clinical symptoms, more definitive approaches to diagnosing CMV infection include direct detection of CMV antigen or DNA in tissues or fluids via immunoassays or quantitative PCR. Diagnosis of CMV infection can be confirmed by identification of the virus in the infant's urine during the first week of life. Histologically, CMV infection can also be detected by its ability to produce characteristic enlarged cytomegaly of infected cells with pronuclear inclusions, or "owl's eyes."

Treatment and Prevention

CMV infections are primarily treated with ganciclovir, immune globulin plus ganciclovir, or foscarnet. Treatment with ganciclovir has been used to prevent CMV disease in AIDS patients and transplant recipients. Use of this agent also reduces the severity of CMV syndromes such as retinitis and gastrointestinal disease. Treatment with both immune globulin and ganciclovir has been used to reduce the high mortality of CMV pneumonia in bone marrow transplant patients.

Unfortunately, congenital and perinatal transmission of CMV cannot be prevented once acquired by the pregnant woman. Thus, hygiene and handwashing may play a role in prevention. Isolation of infants with CMV infections can prevent spread to other infants. Prevention of transplantation-acquired CMV infection can be obtained by transplanting organs and blood products from seronegative donors into seronegative recipients. In situations where it is not possible to use organ or blood products from seronegative donors, prophylactic treatment of all transplant patients or preemptive therapy of those patients with evidence of active CMV infection should be used. Such therapies include the use of hyperimmune CMV globulin, anti-CMV agents or a combination of both. Additionally, safe sex practices also reduce transmission of new CMV infections. CMV vaccines are currently under development; however, none are currently available.

COMPREHENSION QUESTIONS

[25.1] An 18-year-old female presents to her physician with a 1 week history of fever, sore throat, fatigue, and myalgia. Physical examination reveals enlarged tonsils and exudative pharyngitis. Based on her clinical presentation, her physician diagnoses her with infectious mononucleosis. Because there are multiple causes of infectious mononucleosis-like illnesses, which of the following diagnostic assays would rule out CMV as the causative agent of this patient's infection?

[25.2] A previously healthy 8-year-old boy develops a classic childhood illness as a result of a primary viral infection. Which of the following agents would most likely produce symptomatic disease in a boy of this age?

[25.3] A 32-year old HIV infected male is noted to have acute cytomegalovirus infection causing acute gastrointestinal symptoms. The treating physician has ordered that antiviral therapy be administered. Which of the following is most likely to be targeted by the antiviral agent?

[25.1] D. Both CMV and EBV infections can cause infectious mononucleosis disease; however, only EBV produces heterophile antibodies that would result in a positive Monospot test; answers A, B, and C, incorrect: A negative Gram stain of the patient's throat culture would rule out group A Streptococcus; atypical lymphocytes are commonly present in EBV infection but not CMV-related infections; and cytomegaly is typically present in CMV infections.

[25.2] E. VZV is a classic childhood disease that produces symptomatic primary infections; answers A, B, C, and D are incorrect: Most primary CMV, EBV, and poliovirus infections are asymptomatic, whereas HSV-2 infections would rarely occur in a child of this age.

[25.3] C. Ganciclovir has been used primarily to treat severe CMV infections, and its method of action involves inhibition of DNA synthesis; answers A, B, D, and E are incorrect methods of antiviral therapy for CMV infection.

 CMV is the most common viral cause of congenital infection in the United States with the mechanism being primarily transplacental infection.

CASE 26

A 17-year-old female is brought to the office for evaluation of a sore throat and fever. Her symptoms started about 1 week ago and have been worsening. She has been extremely fatigued and has spent most of the last 3 days in bed. She denies any ill contacts. She has no significant medical history, takes no medications, and has no allergies. On examination, she is tired and ill appearing. Her temperature is 38.5C (101.3F). Examination of her pharynx shows her tonsils to be markedly enlarged, almost touching in the midline. They are erythematous and covered with white exudates. She has prominent cervical adenopathy, which is mildly tender. A cardiovascular examination is normal, and her abdomen is soft, nontender, and without palpable organomegaly. A rapid streptococcal antigen test in the office is negative. You send a throat culture and decide to start amoxicillin for strep pharyngitis, assuming that the office test was a false negative. Two days later, you get a call from her mother stating that she has had an allergic reaction to the amoxicillin, and she now has a red rash from head to toe.

 What is the most likely diagnosis of this patient?

 What is the most likely cause of her infection?

 In what human cells can this virus replicate? In what cells can it cause latent infection?

ANSWERS TO CASE 26: Epstein-Barr virus

 Most likely diagnosis: Infectious mononucleosis.

 Most likely etiology: Epstein-Barr virus (EBV).

 In what human cells can this virus replicate, and in what cells can it cause latent infection? EBV preferentially replicates in epithelial cells and B cells, and is known to cause latent infections in B cells.

Epstein-Barr virus (EBV) is a member of the human herpesvirus family, and more specifically a member of the Gammaherpesvirinae subfamily. Humans are the only known natural host for these viruses. EBV infections are most commonly known for causing infectious mononucleosis in adolescents and young adults, and it is often referred to as the "kissing disease." Viral transmission occurs via repeated close intimate contact or through the sharing of items contaminated with saliva, because virus is intermittently shed in the saliva of most seropositive individuals. Secondary attack rates with family and household contacts tend to be low because 90-95 percent of adults have previously been exposed to EBV. Most primary infections are asymptomatic, whereas symptomatic infections are marked with fever, fatigue, pharyngitis, tender lymphadenitis, and possible hepatosplenopathy. Infections with these symptoms can be mistakenly diagnosed as streptococcal pharyngitis, and the resulting inappropriate treatment with amoxicillin can produce an allergic rash.

Similar to other members of the Herpesviridae (Table 26-1), EBV is an enveloped virus with a double-stranded linear DNA genome that is approximately 172 kb in size and encodes more than 70 viral proteins. The DNA core is surrounded by an icosadeltahedral nucleocapsid, with a protein tegument located between the capsid and viral envelope, containing viral enzymes and proteins necessary for replication. The outer membrane of EBV contains virally encoded glycoprotein spikes, important for host cell attachment to human B cells and epithelial cells of the oro- and nasopharynx via the receptor for the C3d component of the complement system. As an enveloped virus, EBV is easily disrupted by acids, detergents, and desiccation and, thus, is effectively transmitted via intimate contact and saliva.

There are two infectious subtypes of EBV, EBV-1 and EBV-2, which are closely related except for differences in their nuclear antigens. The various EBV antigens are expressed in different phases of productive viral replication or in latent infection and can be used in diagnoses. Early EBV antigens, such as early antigens (EAs) and nuclear antigens (NAs), are nonstructural proteins expressed at the onset of lytic viral infection and are followed by the expression of late viral antigens, including the structural components of the viral capsid (VCA) and membrane (MA). Latent phase antigens are expressed in latently infected B cells and include Epstein-Barr nuclear antigens (EBNAs), latent proteins (LPs), and latent membrane proteins (LMPs).

EBV was first discovered in association with African Burkitt lymphoma, a common malignancy of young children in sub-Saharan Africa. The highest occurrence of Burkitt lymphoma appears to occur in regions with high incidence of malaria, indicating malaria as a possible disease cofactor. Other EBV-related diseases include nasopharyngeal carcinoma and, in immunocompromised patient populations, B-cell lymphomas, interstitial lymphocytic pneumonia, and hairy leukoplakia of the tongue.

EBV can cause lytic infections of epithelial cells and latent infection or immortalization of B cells. The lytic infection of epithelial and B cells promotes virus shedding into the saliva of the host, allowing for viral transmission to other hosts and spread within the host. In B cells, EBV promotes cell growth and prevents apoptosis. The proliferating B cells produce an IgM antibody to the Paul-Bunnell antigen, called a heterophile antibody, which serves as a diagnostic indicator of infectious mononucleosis. In this stage of infection, antibody is produced against several EBV antigens, and a T-cell response is mounted. This response contributes to the symptoms and signs of mononucleosis, such as lymphadenopathy, splenomegaly, and atypical lymphocytosis. Latent infection of B cells may occur after the resolution of the acute infection, with periodic reactivation and shedding of the virus in the saliva for months, years, or even lifetime. Persons with inadequate T cell immunity may not be able to suppress EBV infection and may progress to lymphoproliferative disease, B-cell lymphomas, or Hodgkin disease. Nasopharyngeal carcinoma, seen primarily in Asian and Aleutian populations, is thought to be associated with EBV infection in conjunction with some other genetic or environmental component.