It is spread mainly by the fecal–oral route due to contamination of water supplies or food; direct person-to-person transmission is uncommon. In contrast to genotypes 1 and 2, genotypes 3 and 4 cause sporadic cases thought to be contracted zoonotically, from direct contact with animals or indirectly from contaminated water or undercooked meat. Outbreaks of epidemic hepatitis E most commonly occur after heavy rainfalls, especially monsoons because of their disruption of water supplies; heavy flooding can causes sewage to contaminate water supplies. The World Health Organization recommendation for chlorine on HEV inactivation, a free chlorine residual of for 30 min (pH, <8.0) Major outbreaks have occurred in New Delhi, India (30,000 cases in 1955–1956), Burma (20,000 cases in 1976–1977), Kashmir, India (52,000 cases in 1978), Kanpur, India (79,000 cases in 1991), and China (100,000 cases between 1986 and 1988). According to Rein et al., HEV genotypes 1 and 2 caused some 20.1 million hepatitis E infections, along with 3.4 million cases of symptomatic disease, and 70,000 deaths in 2005; however the aforementioned paper did not estimate the burden of genotypes 3 and 4. According to the Department for Environment, Food and Rural Affairs, evidence indicated the increase in hepatitis E in the U.K. was due to food-borne zoonoses, citing a study that found in the U.K. that 10% of pork sausages contained the hepatitis E virus. Some research suggests that food must reach a temperature of for 20 minutes to eliminate the risk of infection. The Animal Health and Veterinary Laboratories Agency discovered hepatitis E in almost half of all pigs in Scotland. Hepatitis E infection appeared to be more common in people on hemodialysis, although the specific risk factors for transmission are not clear. Hepatitis E due to genotypes other than 1 and 2 is thought to be a zoonosis, in that animals are thought to be the primary reservoir; deer and swine have frequently been implicated. Domestic animals have been reported as a reservoir for the hepatitis E virus, with some surveys showing infection rates exceeding 95% among domestic pigs. Replicative virus has been found in the small intestine, lymph nodes, colon, and liver of experimentally infected pigs. Transmission after consumption of wild boar meat and uncooked deer meat has been reported as well. The rate of transmission to humans by this route and the public health importance of this are, however, still unclear.