Hantavirus Pulmonary Syndrome (HPS) : Diagnostics

by CDC

(Page 11 of 17)

A positive serological test result, evidence of viral antigen in tissue by immunohistochemistry, or the presence of amplifiable viral RNA sequences in blood or tissue, with compatible history of HPS, is considered diagnostic for HPS.

Serologic Assays

At the time of the 1993 outbreak in the Four Corners area, cross-reactive antibodies to the previously known hantaviruses (e.g., Hantaan, Seoul, Puumala, and Prospect Hill viruses) were found in the acute- and convalescent-phase sera of some of the initial HPS patients. Tests based on specific viral antigens from SNV have since been developed and are now widely used for the routine diagnosis of HPS. CDC uses an enzyme-linked immunosorbent assay (ELISA) to detect IgM antibodies to SNV and to diagnose acute infections with other hantaviruses. This assay is also available in some state health laboratories.

An IgG test is used in conjunction with the IgM-capture test. Acute- and convalescent-phase sera should reflect a four-fold rise in IgG antibody titer or the presence of IgM in acute-phase sera to be diagnostic for hantaviral disease. Note that acute-phase serum sent as an initial diagnostic specimen may not yet have IgG. IgG antibody is long lasting, and sera of patients retrospectively identified appear to have retained antibody for many years. The SNV IgG ELISA has therefore been used in serologic investigations of the epidemiology of the disease and appears to be appropriate for this purpose. Investigations of selected populations using this assay have confirmed that infections with the virus are not common and that mild or inapparent infections are rare.

A Western blot assay using recombinant antigens and isotype-specific conjugates for IgM-IgG differentiation has also been developed and its results are generally in agreement with those of the IgM-capture format.

Epitope mapping of SNV antibodies has been used to identify immunodominant epitopes of 43 and 31 amino acids in the nucleocapsid protein and G1 glycoprotein, respectively. The immunodominant epitope of G1 is conserved among SNV strains from a broad geographical area, despite extensive nucleotide sequence heterogeneity, and this feature constitutes the basis of a type-specific assay for SNV-like antibodies. Antibodies from HPS patients separated by more than 3000 km have been shown to react with the dominant G1 epitope.

Also in use is a rapid immunoblot strip assay (RIBA), an investigational prototype assay to identify serum antibody to recombinant proteins and peptides specific for SNV and other hantaviruses.

Serologic confirmation of hantaviral infections has traditionally been done with neutralizing plaque assays, which have been recently described for SNV. However, these specific assays are also not commercially available.

Isolation

Isolation of hantaviruses (see below) from human sources is difficult, and the viruses causing HPS seem to be no exception to this rule. To date, no isolates of SNV-like viruses have been recovered from humans, and therefore virus isolation is not a consideration for diagnostic purposes.

Immunohistochemistry (IHC)

IHC testing of formalin-fixed tissues with specific monoclonal and polyclonal antibodies can be used to detect hantavirus antigens and has proven to be a sensitive method for laboratory confirmation of hantaviral infections. IHC has an important role in the diagnosis of HPS in patients from whom serum samples and frozen tissues are unavailable for diagnostic testing and in the retrospective assessment of disease prevalence in a defined geographic region.

Polymerase Chain Reaction (PCR)

Reverse transcriptase-PCR (RT-PCR) can be used to detect hantaviral RNA in fresh frozen lung tissue, blood clots, or nucleated blood cells. However, RT-PCR is very prone to cross-contamination and should be considered an experimental technique. Differences in viruses in the United States complicate the use and sensitivity of RT-PCR for the routine diagnosis of hantaviral infections.

Epidemiology

HPS an Old Disease, Newly Recognized

Although the high-profile investigation of the HPS syndrome emphasized public health authorities' warnings about new and emerging infectious diseases, HPS has turned out to be a newly identified, but not a "new," disease (see Tracking a Mystery Disease, at http://www.cdc.gov/ncidod/diseases/hanta/hps/noframes/history.htm). In fact, the earliest case of a serologically confirmed SNV infection was in a person who developed an HPS-compatible illness in July 1959 and was found to have IgG antibodies in September 1994. The earliest case of HPS to be confirmed by IHC with direct visualization of hantaviral antigens in postmortem tissue involved a patient who died in 1978.

Risk Factors for Disease

Little is known about activities that lead to a greater risk of infection. However, an early case-control study suggests that increased numbers of rodents in the household is the strongest risk factor for infection. Entering rarely opened or seasonally closed buildings may also contribute to infection. Among the confirmed cases of HPS for which exposure information is available, 70% of the patients in the case control study had exposures closely associated with peridomestic activities, such as cleaning, in homes that showed signs of rodent infestation. Four clusters of HPS cases involving 2-4 persons have been documented; for each cluster, exposure probably occurred within a shared, enclosed structure. Taken together, these observations suggest that disturbing or inhabiting closed, actively rodent-infested structures may constitute an important risk factor for contracting HPS

Potentially occupationally acquired SNV infections have been recognized but are infrequent. Among documented U.S. cases of HPS, patients with potential occupational exposures have included grain farmers, an extension livestock specialist, field biologists, and agricultural, mill, construction, utility and feedlot workers. Many of these individuals had concurrent peridomestic exposures. Among U.S. mammalogists and rodent workers with varying degrees of rodent exposure, the seroprevalence of SNV antibodies was 1.14%. In contrast, a recent HPS seroprevalence study focused on selected occupational groups with frequent contact with rodents and their excreta (e.g., farm workers, laborers, professionals, home repairers, service industry and park service workers, heating and plumbing contractors, utility workers, and technicians) found no evidence of SNV infection.

Travel to and within all areas where hantavirus infection has been reported is not considered a risk factor for infection with HPS. The possibility of exposure to hantavirus for campers, hikers, and tourists is very small and is reduced further if steps are taken to reduce rodent contact.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.