Chapter Capsule

A. Viruses are minute parasitic particles consisting of DNA or RNA genomes packaged within a protein capsid; they invade host cells and appropriate the cell’s machinery for mass production of new virions, both in cytoplasm and nucleus; viruses exit the host cell by lysis or budding; budded viruses leave with an envelope; actively infected host cells are usually destroyed.

B. Viruses attack a variety of host and cell types; individual viruses are relatively host/cell specific, due to the need for receptor recognition; target cells include nervous system, blood, liver, skin, and intestine.

C. Diseases range from mild and self-limited to fatal in effects; symptoms are local, systemic, and depend on the exact tissue target; identification and diagnosis are by culture, microscopy, genetic probes, and serology; immunity is both humoral and cell-mediated; limited number of drugs available for treatment.

D. DNA and some RNA viruses persist in inactive state (latent) in host cells; some latent viruses cause recurrent infections, and others integrate into host genome and cause cancerous transformation; some viruses are teratogenic and cause congenital defects; zoonotic viruses transmitted by vectors cause severe human disease.

A. Enveloped Viruses

1. Poxviruses: Large, complex viruses; produce skin lesions called pox (pocks).

a. Variola is the agent of smallpox, the first disease to be eliminated from the world through vaccination; last smallpox case reported in 1977; infection leads to large deep pustules that may scar; vaccination with vaccinia virus is no longer done routinely, except to protect certain populations against animal poxviruses such as monkeypox.

b. Molluscum contagiosum, an unclassified poxvirus, causes waxy nodules on skin; transmitted by direct contact; may be an STD in adults. Humans can also be infected by monkeypox virus, which causes a disease similar to smallpox.

2. Herpesviruses: Persistent latent viruses that cause recurrent infections and may be involved in neoplastic transformations; attack skin, mucous membranes, and glands.

a. Herpes simplex virus (HSV): Types 1 and 2 both create lesions of skin and mucous membranes; migrate into nerve ganglia; are reactivated; cause disseminated disease in newborn infants and immunodeficient patients; cause whitlows on fingers of medical and dental personnel. HSV-1 is transmitted by close contact and droplets and mainly infects lips (cold sores, fever blisters), eyes, and oropharynx. HSV-2 is sexually transmitted and usually affects the genitalia. Complication in newborns infected at birth with either virus is encephalitis. Infections are treated with some form of cyclovir; controlled by barriers and care in handling secretions.

b. Varicella-Zoster Virus (VZV): This herpesvirus causes both chickenpox (varicella) and shingles (zoster); chickenpox is the primary infection, zoster is later recurrence of infection by latent virus. Chickenpox is transmitted through droplets; symptoms are fever, poxlike papular rash. In zoster, the virus has migrated into spinal nerves and is reactivated by surgery, cancer, or other stimuli; causes painful lesions on skin of trunk or head; severe disease treated with acyclovir or famciclovir; immune globulin used to relieve symptoms; vaccine now available.

c. Cytomegalovirus (CMV): One of the most common herpesvirus; spread through close contact with body fluids; congenital infection affects the liver, spleen, brain, and eyes; also causes mononucleosis-like syndrome with fever and leukocytosis; disseminated disease is common in AIDS and transplant patients; antiviral drugs may be of some benefit.

d. Epstein-Barr Virus (EBV): A herpesvirus of the lymphoid and glandular tissue; transmitted by saliva; 95% of all persons develop some form of infection; cause of mononucleosis, a “kissing disease’’ marked by sore throat, fever, swollen lymphoid tissue, leukocytosis; virus is oncogenic; also causes Burkitt lymphoma, a malignancy of the B lymphocytes that swells cheek or abdomen and is prevalent in African children; and causes nasopharyngeal carcinoma, cancer in older men in China, Africa. Disseminated EBV infection is a complication of AIDS and transplant patients.

e. Herpesvirus-6 is the cause of roseola, an acute febrile disease in children; virus may be involved in multiple sclerosis and lymphoma. Herpesvirus-8 is the oncogenic viral agent of Kaposi sarcoma, a tumor of AIDS patients.

3. Hepadnaviruses: One cause of viral hepatitis, an inflammatory disease of liver cells that may result from several different viruses.

a. Hepatitis B virus (HBV) causes serum hepatitis; virus has strong affinity for liver cells, is carried and shed into blood; only a tiny infectious dose is required.

b. The virus is resistant to heat and disinfectants; risks include contact with blood, drug addiction, and homosexuality.

c. Infection is marked by fever, intestinal disturbance, jaundice; chronic carriage may lead to liver disease and liver cancer.

d. Managed by serum globulin, interferon, and vaccines. Delta agent is an associated virus that accompanies HBV and causes a more severe disease.

A. Adenoviruses: Common infectious agents of lymphoid organs, respiratory tract, eyes; spread through close contact with secretions; diseases include common cold with fever and rash; keratoconjunctivitis, a severe eye infection, cystitis, an acute urinary infection.

B. Papovaviruses: Papilloma- and polyomaviruses.

1. Human papillomaviruses (HPVs) cause skin tumors called papillomas, verrucas, or warts; spread by close contact with infected skin or fomites. Common warts are rough, painless lesions on hands; plantar warts are painful, flat, benign tumors on feet, trunk. Genital warts are verrucas that start as tiny bumps on membranes or skin of genitals; a very common STD; may progress to large masses called condylomata acuminata; chronic infection associated with cervical and penile cancer; treatment by surgery and interferon.

2. Polyomaviruses include animal tumor viruses and human viruses. JC virus causes progressive multifocal leukoencephalopathy, a slow destruction of the brain in cancer and immunodeficient patients; BK virus causes complications in renal transplants.

C. Single-Stranded DNA Viruses: Parvoviruses are the tiniest viruses; cause severe disease in several mammals; human parvovirus (B19) causes erythema infectiosum, a mild respiratory infection of children that can be dangerous to fetus.