Microbiology for Medical Students

Herpesviruses

1. Characteristics of Herpesviruses

a. Structure:

i. all herpesviruses have an icosahedral core surrounded by a lipoprotein envelope.

ii. the genome is linear double-stranded DNA.

iii. the virion does not contain a polymerase.

iv. they are large (120-200nm).

b. They replicate in the nucleus, form intranuclear inclusions, and are the only viruses that obtain their envelopes by budding from the nuclear membrane.

c. Herpesviruses cause latent infections:

i. in these infections, the acute disease is followed by an asymptomatic period during which the virus remains in a quiescent state.

ii. when the patient is exposed to an inciting agent or immunosuppression occurs, reactivation of virus replication and disease can occur.

iii. with herpes simplex virus, the symptoms of the subsequent episodes are similar to those of the initial one while with varicella-zoster virus, they are different.

d. Herpes simplex viruses types 1 and 2, and varicella-zoster virus, cause a vesicular rash, both in primary infections and in reactivations while cytomegalovirus and Epstein-Barr virus do not cause a vesicular rash.

e. Primary infections are usually more severe than reactivation.

f. Association with Cancer:

i. Epstein-Barr virus: Burkitt’s lymphoma, B cell lymphoma, nasopharyngeal carcinoma.

ii. Human herpesvirus 8: Kaposi’s sarcoma.

iii. HSV-2: cervical carcinoma.

Herpes type	Virus	Primary infection	Site of latency	Recurrent infection	Route of transmission
1	HSV-1	Gingivostomatitis	Cranial sensory ganglia	Herpes labialis Encephalitis Keratitis	Respiratory secretions Saliva
2	HSV-2	Herpes genitalis Perinatal disseminated disease	Lumbar or sacral sensory ganglia	Herpes genitalis	Sexual contact Perinatal infection
3	VZV	Varicella	Cranial or thoracic sensory ganglia	Zoster	Respiratory secretions
4	EBV	Infectious mononucleosis	B lymphocytes	None	Respiratory secretions Saliva
5	CMV	Congenital infection Mononucleosis	Lymphoid cells Epithelial cells	Asymptomatic shedding	Intrauterine infection Transfusions Sexual contact Saliva, urine
6	HHV-6	Exanthem subitum: mild facial rash in small babies. Mononucleosis with cervical lymphadenopathy Hepatitis in transplant patients Leukopenia with lymphocytosis	Unknown Incubation period: 1-2 weeks	Unknown	HHV-6 in serum and throat washings. Oral secretions
7	HHV-7	Without disease association	Unknown	Unknown	Persists in saliva
8	HHV-8	Found in cells of Kaposi’s sarcoma in AIDS patients	Unknown	Unknown	Unknown