Hantavirus A Threat ... Rodent Disease on the Rise

Last Editorial Review: 8/7/1999

August 1999 -- The CDC (Centers for Disease Control) warns that it has confirmed seven new cases and suspects at least eleven more cases of the hantavirus lung (pulmonary) syndrome, a dread disease contracted from rodents.

Eight of the 18 confirmed or suspected cases of hantavirus pulmonary syndrome (HPS) came from the Southwest -- Arizona, Colorado, and New Mexico. The remaining ten cases were in California, Idaho, Iowa, Montana, New York State and Washington State.

HPS is a viral disease characterized by fever, a severe pulmonary (lung) illness and often a fatal outcome. The case-fatality ratio with HPS is 43% (meaning 43% of people with HPS die).

There are several related viruses that can cause HPS. The main one in the United States is called the Sin Nombre virus. In the United States the deer mouse (Peromyscus maniculatus) is the predominant carrier of the Sin Nombre virus and therefore the usual animal responsible for carrying the disease to people in the U.S.

The current outbreak of HPS is not a surprise. Research on rodent populations had, in fact, predicted an increased risk for infection for humans in some areas of the southwestern U.S. during the summer of 1999.

Hantavirus Infection

Hantavirus infection can occur after inhaling infectious aerosols from rodent saliva or excreta. HPS typically begins with headache, fever, and myalgia (muscle pain) soon followed by pulmonary edema (fluid in the lungs), which often leads to severe respiratory compromise. Laboratory findings include low platelets (thrombocytopenia), presence of immature immunologically active cells (immunoblasts), and high levels of red blood cells (hemoconcentration). Other than supportive care, no treatment exists for hantavirus infection. The probability of surviving an attack of HPS increases with early recognition, hospitalization, and aggressive pulmonary and hemodynamic (blood) support.

The highest concentration of HPS cases has occurred in the western United States, and rodent monitoring has focused on this area. However, hantavirus carrying rodents occur throughout the United States, and cases of HPS have occurred nationwide.

The risk for human disease is proportional to the frequency with which persons come into contact with infectious rodents. Both population densities and prevalence vary from site to site and can change markedly from season to season and from year to year. Population densities may vary 10-fold within 2 or 3 months. Prevalence of hantavirus infection in deer mouse populations occasionally have been greater than 60% at specific sites in the southwestern United States, California, and Montana. Infrequently, environmental conditions result in the simultaneous occurrence of high rodent population densities and a high prevalence of hantavirus infection among rodents. This combination, which appears to be occurring this year in some rodent populations in the southwestern U.S., results in a greater number of infected mice and leads to a higher risk for transmission to humans. The increased number of HPS cases reported in the Southwest this year supports this concept.

Recommendations for Preventing Hantavirus Pulmonary Syndrome

  1. Eliminate rodent harborage

    • Keep cooking, eating, and food storage areas clean

    • Cover human food and animal feed

    • Contain and elevate garbage

    • Seal holes and cracks in dwellings to prevent entrance by rodents

    • Clear brush and trash from around homes and outbuildings

    • Control rodent populations by maintaining snap traps and/or using rodenticides; in areas where the virus occurs, control fleas with insecticides

    • Safely clean up rodent-infested areas

      • Air out infested spaces before cleanup

      • Spray areas of infestation and all excreta, nesting, and other materials with household disinfectant or 10% bleach solution, then clean up, seal in bags, and dispose

      • Avoid sweeping, vacuuming, or stirring dust until the area is thoroughly wet with disinfectant

      • Wear rubber gloves; disinfect gloves before removal, and wash hands afterwards

      • In areas where plague occurs, spray insecticide on trapped rodents and nesting materials to prevent fleas from abandoning rodents to find new hosts

      • Avoid rodents when outdoors

        • Do not disturb rodent droppings or camp or sleep near burrows or areas where trash is present

        • Avoid feeding or handling rodents, even if they appear friendly

No restriction of travel to areas where HPS has been reported is necessary. However, activities that may disrupt rodent burrows or result in contact with rodents or aerosolization of rodent excreta should be avoided.

Source: Morbidity and Mortality Weekly Report 48 (24): 521-5, 1999 (The Morbidity and Mortality Weekly Report, better known as MMWR, is published by the CDC and is essential reading for all state, county and city health departments in the U.S.).


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