Viral Infections

Arthropod-borne (arbo) viruses causing encephalitis are members of the toga-, flavi-, and bunyavirus families. The medically important togaviruses include rubella virus, which is discussed in site, and the equine encephalitis viruses. The flavivirus family, which includes St.

Parvoviruses are widespread bird and mammalian viruses. More than 50 types have been identified, but the most common identified human pathogen is parvovirus B19. It is the cause of erythema infectiosum (Fifth disease) and is responsible for episodes of aplastic crises in patients with chronic anemia, especially those with hemolytic anemia and AIDS (Box 1). B19 is also associated with arthritis and intrauterine infection.

Poxviruses are a large, complex family of viruses that cause disease in humans and other animals (Table 1). Of the many genera in this family, only species of Orthopoxvirus and Molluscipoxvirus are associated specifically with humans.

Two major groups of retroviruses are considered in this chapter: the oncoviruses (“onco-,” related to a tumor) and the lentiviruses (“lenti-,” slow). Oncoviruses have long been associated with a variety of cancers in animals, including leukemia, lymphoma, and sarcoma; however, until recent years, oncoviruses had not been found to infect humans. The first human retrovirus, human T-lymphotropic virus type 1 (HTLV-1), was discovered in the late 1970s. It was shown to cause adult T-cell leukemia, a rare malignancy found only in Japan, Africa, and the Caribbean (although serologic evidence shows that the virus also occurs in the United States and may be associated with some chronic neurologic conditions).

The causes of hepatitis are varied and include viruses, bacteria, and protozoa, as well as drugs and toxins (eg, isoniazid, carbon tetrachloride, and ethanol). The clinical symptoms and course of acute viral hepatitis can be similar, regardless of etiology, and determination of a specific cause depends primarily on the use of laboratory tests (Box 1). Hepatitis may be caused by at least six different viruses whose major characteristics are summarized in Table 1. Non-A-non-B (NANB) hepatitis is a term previously used to identify cases of hepatitis not caused by hepatitis A or B.

Delta hepatitis is spread just as hepatitis B and is most prevalent in groups at high risk of hepatitis B infection. Injection drug users are those at greatest risk in the western world, and ~ 50% of such individuals may have IgG antibody to the delta virus antigen. Delta virus infection is rare in the U. S., northern Europe, and Japan but largely prevalent in southern Europe, Africa, and South America.

Hepatitis B infection is found worldwide, with prevalence rates varying markedly among countries. Chronic carriers constitute the main reservoir of infection: in some countries, particularly in the Far East, ~ 5-15% of all persons carry the virus, though most are asymptomatic. Of patients with HIV infection, 10% are chronic carriers of hepatitis B.

Hepatitis A virus is the cause of what was formerly termed infectious hepatitis or short-incubation hepatitis. It was first detected in the early 1970s in stools of patients incubating the disease. Humans appear to be the major natural hosts of hepatitis A virus.

Even in developed countries, antibodies to caliciviruses are nearly universal by age 5. Infections tend to occur in family or community outbreaks. In underdeveloped countries, infection by these viruses occurs presumably as a result of poor sanitation. In developed countries, outbreaks that occur year-round have been described in schools, resorts, hospitals, nursing homes, restaurants, and cruise ships.