Figure 2: Hantavirus pulmonary syndrome cases by state of exposure, United States, 1993-2013. A total of 624 cases occurred in 34 states. The state of exposure was unknown for another 28 cases. Source: Knust B., Rollin PE. Twenty-year surveillance for human hantavirus infections, United States. Emerg Infect Dis 2013; 19: 1934-1937.
tors concluded the dramatic increase in high-desert rainfall associated with the 1992-1993 El Niño weather pattern contributed to the risks of Sin nombre virus exposure in the region and that annual precipitation predictions would be of value in designing disease prevention campaigns. 16 DISCUSSION Like arthropods, rodents serve as animal reservoir hosts and vectors for several pathogens - including bacte- ria, viruses, and protozoa - and are usually immune to any pathogenic effects. The rodent-borne infectious diseases of greatest medical importance include leptospirosis, hanta- virus, monkeypox, lymphocytic choriomeningitis virus,
and the rat-bite fever bacterium, Streptobacillus moniliformis . Although plague is often considered a rodent-borne dis- ease, rodents are only the animal reservoirs of the plague bacterium and not the vectors of Yersinia pestis , which is transmitted by the bites of plague-infected rat fleas. While most other rodent-borne infectious diseases are sporadic and rare in occurrence, regional cluster outbreaks of lepto- spirosis and hantavirus pulmonary syndrome (HPS) have been associated with periods of heavy rainfall and flooding throughout the US. Leptospirosis (LS) is now the most commonly reported zoonotic disease worldwide. 17 Unanticipated regional outbreaks of leptospirosis (LS) following heavy rainfall and flooding events have occurred in the US, such as after extensiveMississippi River flooding in Iowa in 1993. 18 Between 100-200 cases of LS are reported every year in the US, withmost cases reported fromHawaii, where the incidence is increasing during the rainy seasons and on Kauai, the wettest island. 17 Leptospirosis is endemic in Louisiana with reservoirs in rodents and insectivores. 18 Ichinose and colleagues reported one of the earliest clusters of six fatal cases of LS-caused Weil’s disease with hepatorenal failure in Louisiana in 1963. 18
Leptospires are motile spirochetes of the family Leptospiraceae, with nearly 300 serotypes divided into human pathogenic strains and saprophytic strains. 2 The larger group of pathogenic leptospires comprises the Leptospira interrogans sensu lato complex with more than 200 serotypes. 2 Although many wild and domestic mammals serve as reservoir hosts for leptospires in their kidneys; rodents, primarily rats and mice, are the most common reservoirs world- wide. 2 Asymptomatically infected rodents excrete leptospires in their urine that proliferate in freshwa- ter, mud, moist soil, and wet vegetation and remain viable and infectious for months. 7 As heavy rains and
Figure 3: Range of the marsh rice rat ( Oryzomys palustris ), reservoir of Bayou virus, United States. Source: US Centers for Disease Control and Prevention. Hantavirus pulmonary syndrome - United States: Updated recommendations for risk reduction. Morb Mort Week Rep 2002; 51: 1-12.
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