HEV Epidemiology
The 2 common patterns of clinical illness from HEV infection depend on the genotype and how it is transmitted in humans.
Genotypes 1 and 2
Genotypes 1 and 2 account for cases of epidemic HEV in developing countries and are principally transmitted by the fecal-oral route after flooding or a severe drought that results in increased fecal concentration in rivers and streams. The reservoirs of genotype 1 and 2 virus that sustain the disease in highly endemic regions appear to be infection in humans and continuous viral contamination of sewage coupled with impaired sanitation and contaminated drinking water.
The attack rate for genotype 1 clinical disease appears to be 1%-15% and affects young adults more than children. If children are infected, they may be anicteric. Men are also more likely to develop clinical disease than women, although attack rates are higher in pregnant women. Genotype 1 infection in pregnant women during the second or third trimester carries the greatest risk for fulminant hepatitis, with a mortality rate of up to 20%. In addition to epidemic forms of genotype 1, HEV infection accounts for up to 70% of acute sporadic hepatitis in such countries as India.
Genotypes 3 and 4
Genotypes 3 and 4 are transmitted chiefly by zoonotic routes. Genotype 3 HEV is the common form in the United States, and 21%-30% of adults are positive for IgG anti-HEV antibody, indicating previous infection with HEV. In addition to humans, genotype 3 HEV is found in domestic animals, such as pigs, cows, goats, and horses, and in wild animals, such as deer, rabbits, mongoose, and wild boar. Ingestion of uncooked or inadequately cooked meat from these animals results in human infection. Sporadic HEV genotypes 3 and 4 have reservoirs of virus in both domestic and wild animals that sustain episodic infection of humans.
Infection has also been caused by transfusion (genotypes 1 and 3), transplanted organs, and vertical transmission from mother to child. Although uncommon, person-to-person spread may occur in highly endemic areas. Chronic HEV disease caused by genotype 3 is seen in immunosuppressed patients, such as transplant patients, patients receiving chemotherapy, or those with HIV disease.