Update: Hantavirus Pulmonary Syndrome -- United States, 1999

Hantavirus pulmonary syndrome (HPS) is a rodentborne viral disease characterized by severe pulmonary illness and a case-fatality ratio of 43%. Sin Nombre virus is the primary hantavirus that causes HPS in the United States, and the deer mouse (Peromyscus maniculatus) is its predominant carrier. CDC-sponsored studies of rodent populations since 1994 have yielded data that suggest an increased risk for infection for humans in some areas of the southwestern United States during the summer of 1999. This report describes increases in human cases during January-May 1999, current hantavirus prevalence in rodent populations, the need for renewed attention to reduce the risk for hantavirus exposure, and the importance of physician awareness and early detection in the treatment of HPS.

HPS is clinically defined as a febrile illness and the presence on a chest radiograph of bilateral infiltrates resembling acute respiratory distress syndrome (1). As of May 28, 1999, CDC had confirmed 217 cases of HPS in 30 states (Figure 1). From January through May 1999, seven cases of HPS were confirmed in Colorado, New Mexico, New York, and Washington. An additional 11 suspected cases with preliminary clinical and serologic evidence of HPS were reported in Arizona, California, Idaho, Iowa, Montana, New Mexico, and Washington. Eight of the confirmed and suspected cases are from Arizona, Colorado, and New Mexico. In the same 5-month period during each year from 1995 through 1998, this area averaged approximately two cases each year.

Since 1994, CDC has sponsored continuous monitoring studies of rodent populations at nine sites in Arizona, Colorado, and New Mexico (2). Population densities of deer mice at New Mexico monitoring sites during January-May 1999 were lower compared with densities during spring 1998; however, densities at one site in Colorado in May 1999 were greater than 50% higher than 1 year earlier.

Hantavirus antibody prevalences in deer mouse populations surveyed during spring 1999 were 35%-45% in some populations in New Mexico and up to 40% in Colorado. In comparison, prevalences during the population peaks of spring 1998 were less than 10% in New Mexico and approximately 20% in Colorado. These figures were comparable with a prevalence of 10%-15% in deer mouse populations sampled throughout the United States since 1993; during the 1993 outbreak, prevalences of 30% were detected (3).

Reported by: M Leslie, DVM, Arizona Dept of Health Svcs. C Fritz, DVM, California Dept of Health Svcs. C Calisher, PhD, B Beaty, PhD, Arthropod Borne and Infectious Diseases Laboratory, Colorado State Univ, Ft. Collins; J Pape, Colorado Dept of Public Health and Environment. L Tengelsen, DVM, C Hahn, MD, Idaho Dept of Health and Welfare. K Buechler, Iowa Dept of Public Health. J Murphy, DrPH, Montana Dept of Public Health and Human Svcs. T Yates, PhD, Museum of Southwest Biology and Dept of Biology, Univ of New Mexico, Albuquerque; P Ettestad, DVM, New Mexico Dept of Health. D White, PhD, New York State Dept of Health. A Weltman, MD, Pennsylvania Dept of Health. M Goldoft, MD, J Grendon, DVM, Washington Dept of Health. J Cheek, MD, Indian Health Svc. Special Pathogens Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note:

Hantavirus infection can occur after inhaling infectious aerosols from rodent saliva or excreta. HPS typically begins as a prodrome of headache, fever, and myalgia soon followed by pulmonary edema, which often leads to severe respiratory compromise. Thrombocytopenia, presence of immunoblasts, and hemoconcentration are characteristic laboratory findings. Other than supportive care, no treatment exists for hantavirus infection. The probability of surviving HPS increases with early recognition, hospitalization, and aggressive pulmonary and hemodynamic support (4). The highest concentration of HPS cases has occurred in the western United States, and CDC rodent monitoring has focused on this area. However, hantavirus reservoir species occur throughout the United States, and cases of HPS have occurred nationwide. All primary health-care providers are strongly encouraged to become familiar with the signs and symptoms of HPS (5) and to immediately report suspected cases to their state health departments.

Risk for human disease is proportional to the frequency with which persons come into contact with infectious rodents, and rodent population density and the prevalence of infection in rodents may help to quantify risk for communities. Both population densities and prevalences 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. Prevalences of hantavirus infection in deer mouse populations occasionally have been greater than 60% at specific sites in the southwestern United States, California (6), 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 United States, 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 interpretation. Although increased physician awareness of HPS cannot be ruled out, the number of confirmed cases this year exceeds the average number identified during the same periods in 1995 through 1998 and suggests that the increase is real.

The importance of adherence to risk-reduction measures should be emphasized by increased efforts to educate the public, especially among residents of rural areas of the southwestern United States. The most effective way to decrease the risk for HPS is to limit exposure to rodents and their excreta. Most persons with HPS who had high-risk exposures are thought to have been infected in and around their homes; therefore, limiting opportunities for peridomestic exposure is particularly important. Measures to prevent HPS can be divided into four areas: eliminating rodent harborage (7), controlling rodent populations, properly cleaning up rodent infestation, and avoiding rodents in outdoor settings (see box).

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
2. Control rodent populations by maintaining snap traps and/or using rodenticides; in areas where plague occurs, control fleas with insecticides
3. 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
4. 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.

Clinical principles of recognition and support for HPS were reviewed in a video conference in May 1999; a videotape of this conference is available through CDC's Special Pathogens Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, (404) 639-1510. Additional information on HPS is available from local or state health departments; through the hantavirus hotline, telephone (877) 232-3322; on the World-Wide Web at the "All About Hantavirus" web site, http://www.cdc.gov/ncidod/diseases/hanta/hps/index.htm; and by mail to CDC's Special Pathogens Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Mailstop A-26, 1600 Clifton Road, N.E., Atlanta, GA 30333.

References

1. CDC. Case definitions for infectious conditions under public health surveillance. MMWR 1997;46(no. RR-10):16.
2. Mills JN, Yates TL, Ksiazek TG, Peters CJ, Childs JE. Long-term studies of hantavirus reservoir populations in the southwestern United States: rationale, potential, and methods. Emerg Infect Dis 1999;5:95-101.
3. Childs JE, Ksiazek TG, Spiropoulou CF, et al. Serologic and genetic identification of Peromyscus maniculatus as the primary rodent reservoir for a new hantavirus in the southwestern United States. J Infect Dis 1994;169:1271-80.
4. Hallin GW, Simpson SQ, Crowell RE, et al. Cardiopulmonary manifestations of hantavirus pulmonary syndrome. Crit Care Med 1996;24:252-8.
5. Duchin JS, Koster FT, Peters CJ, et al. Hantavirus pulmonary syndrome: a clinical description of 17 patients with a newly recognized disease. N Engl J Med 1994;330:949-55.
6. Graham TB, Chomel BB. Population dynamics of the deer mouse (Peromyscus maniculatus) and Sin Nombre virus, California Channel Islands. Emerg Infect Dis 1997;3:367-70.
7. Hoddenbach G, Johnson J, Disalvo C. Mechanical rodent proofing techniques [a training guide for National Park Service employees]. Washington, DC: US Department of the Interior, National Park Service, 1997.

Figure 1


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