Intraspecific transmission of Hantaan virus, etiologic agent of Korean hemorrhagic fever, in the rodent Apodemus agrarius

Lack of vertical transmission of Hantaan virus from persistently infected dam to progeny in laboratory mice

It is unclear how the hantaviruses are transferred from infected to uninfected rodents. We studied the status of persistently infected laboratory mice and examined the frequency of viral transmission to their offspring. Expression of Hantaan virus nucleocapsid protein was detected in the lungs of persistently infected dams. None of the progeny displayed viral antigen, although they were strongly positive for IgG antibodies against hantavirus. There was neither hantavirus RNA nor virus-specific IgM antibodies or virus-specific CD8(+) T cells in the progeny. These results did not show any indication for a vertical transmission of hantaviruses, at least in the laboratory mouse model studied.

Synthesis of 1-beta-D-ribofuranosyl-3-ethynyl-[1,2,4]triazole and its in vitro and in vivo efficacy against Hantavirus

There are no FDA approved drugs for the treatment of hemorrhagic fever with renal syndrome (HFRS), a serious human illnesses caused by hantaviruses. Clinical studies using ribavirin (RBV) to treat HFRS patients suggest that it provides an improved prognosis when given early in the course of disease. Given the unique antiviral activity of RBV and the lack of other lead scaffolds, we prepared a diverse series of 3-substituted 1,2,4-triazole-beta-ribosides and identified one with antiviral activity, 1-beta-d-ribofuranosyl-3-ethynyl-[1,2,4]triazole (ETAR). ETAR showed an EC(50) value of 10 and 4.4 microM for Hantaan virus (HTNV) and Andes virus, respectively. ETAR had weak activity against Crimean Congo hemorrhagic fever virus, but had no activity against Rift Valley fever virus. Intraperitoneally delivered ETAR offered protection to suckling mice challenged with HTNV with a approximately 25% survival at 12.5 and 25mg/kg ETAR, and a MTD of 17.1+/-0.7 days. ETAR was phosphorylated in Vero E6 cells to its 5'-triphosphate and reduced cellular GTP levels. In contrast to RBV, ETAR did not increase mutation frequency of the HTNV genome, which suggests it has a different mechanism of action than RBV. ETAR is an exciting and promising lead compound that will be elaborated in further synthetic investigations as a framework for the rational design of new antivirals for treatment of HFRS.

Thottapalayam virus, a prototype shrewborne hantavirus

This virus is antigenically and phylogenetically distinct from rodent-borne hantaviruses.

Thottapalayam virus (TPMV) has been placed in the genus Hantavirus of the family Bunyaviridae by virtue of its morphologic features and overall genetic similarities to well-characterized rodentborne hantaviruses. This virus has been isolated from the Asian house shrew (Suncus murinus); however, whether TPMV is naturally harbored by an insectivore host or represents spillover from a rodent reservoir host is unknown. Our analysis of published and unpublished data on the experimental host range, genetics, and molecular phylogeny of TPMV supports coevolution of TPMV with its nonrodent reservoir host. Future studies on the epizootiology of TPMV and investigations of new shrewborne hantaviruses will provide additional insights into the evolutionary origin of hantaviruses in their rodent and insectivore reservoir hosts. Such investigations may also provide clues about determinants of hantavirus pathogenicity and virulence.

Regulatory T cells enhance persistence of the zoonotic pathogen Seoul virus in its reservoir host

Hantaviruses are zoonotic pathogens that maintain a persistent infection in their reservoir hosts, yet the mechanisms mediating persistence remain unknown. Regulatory T cell responses cause persistent infection by suppressing proinflammatory and effector T cell activity; hantaviruses may exploit these responses to cause persistence. To test this hypothesis, male Norway rats were inoculated with Seoul virus and regulatory T cells were monitored during infection. Increased numbers of CD4(+)CD25(+)Forkhead box P3(+) T cells and expression of Forkhead box P3 and TGF-beta were observed in the lungs of male rats during persistent Seoul virus infection. To determine whether regulatory T cells modulate Seoul virus persistence, regulatory T cells were inactivated in male rats by using an anti-rat CD25 monoclonal antibody (NDS-63). Inactivation of regulatory T cells reduced the amount of Seoul virus RNA present in the lungs and the proportion of animals shedding viral RNA in saliva. Because regulatory T cells suppress proinflammatory-induced pathogenesis, pathologic observations in the lungs were evaluated during infection. Subclinical acute multifocal areas of hemorrhage and edema were noted in the lungs during infection; inactivation of regulatory T cells reduced the amount of pathologic foci. Expression of TNF was suppressed during the persistent phase of infection; inactivation of regulatory T cells eliminated the suppression of TNF. Taken together, these data suggest that regulatory T cells mediate Seoul virus persistence, possibly through elevated transcription and synthesis of TGF-beta and suppression of TNF. These data provide evidence of regulatory T cell involvement in the persistence of a zoonotic pathogen in its natural reservoir host.

Ex vivo stability of the rodent-borne Hantaan virus in comparison to that of arthropod-borne members of the Bunyaviridae family

The possible effect of virus adaptation to different transmission routes on virus stability in the environment is not well known. In this study we have compared the stabilities of three viruses within the Bunyaviridae family: the rodent-borne Hantavirus Hantaan virus (HTNV), the sand fly-borne Phlebovirus sandfly fever Sicilian virus (SFSV), and the tick-borne Nairovirus Crimean-Congo hemorrhagic fever virus (CCHFV). These viruses differ in their transmission routes: SFSV and CCHFV are vector borne, whereas HTNV is spread directly between its hosts, and to humans, via the environment. We studied whether these viruses differed regarding stability when kept outside of the host. Viral survival was analyzed at different time points upon exposure to different temperatures (4 degrees C, 20 degrees C, and 37 degrees C) and drying at 20 degrees C. We observed clearly different stabilities under wet conditions, particularly at 4 degrees C, where infectious SFSV, HTNV, and CCHFV were detectable after 528, 96, and 15 days, respectively. All three viruses were equally sensitive to drying, as shown by drying on aluminum discs. Furthermore, HTNV and SFSV partially survived for 2 min in 30% ethanol, whereas CCHFV did not. Electron microscopy images of HTNV, SSFSV, and CCHFV stored at 37 degrees C until infectivity was lost still showed the occurrence of virions, but with abnormal shapes and densities compared to those of the nonincubated samples. In conclusion, our study points out important differences in ex vivo stability among viruses within the Bunyaviridae family.

Emergence and persistence of hantaviruses

Hantaviral diseases have been recognized for hundreds of years but, until 1976, they had not been associated with an infectious agent. When Lee and colleagues isolated what is now known as Hantaan virus, the techniques they introduced allowed further investigations into the etiology of the classical hantavirus disease, hemorrhagic fever with renal syndrome (HFRS), now known to be caused by any of multiple hantaviruses. The discovery of hantavirus pulmonary syndrome (HPS) in the New World, and that it also can be caused by any of multiple hantaviruses (family Bunyaviridae, genus Hantavirus), has opened an entire field of epidemiologic, virologic, molecular, behavioral, and ecologic studies of these viruses. There appears to be a single hantavirus-single rodent host association, such that understanding the idiosyncrasies of each rodent host species and the ecologic variables that affect them are recognized as critical if we are to reduce human risk for infection. This chapter summarizes what is known about hantaviruses with regard to history of these viruses, their taxonomy, recognized geographical distribution, ecologic factors impacting their maintenance and spread of hantaviruses, effect of rodent behavior on hantavirus transmission, influence of host factors on susceptibility to and transmission of hantaviruses, and transmission of hantaviruses from rodents to humans. In addition, we summarize all these complexities and provide suggestions for future research directions.

Prolonged survival of Puumala hantavirus outside the host: evidence for indirect transmission via the environment

The capability of rodent-borne viruses to survive outside the host is critical for the transmission dynamics within rodent populations and to humans. The transmission of Puumala virus (PUUV) in colonized bank voles (Clethrionomys glareolus) was investigated and additional longevity studies in cell culture with PUUV and Tula (TULV) hantaviruses were performed. Wild-type PUUV excreted by experimentally infected donor bank voles was shown to be transmitted indirectly between rodents through contaminated beddings, and maintained its infectivity to recipient voles at room temperature for 12-15 days. In cell culture supernatants, PUUV and TULV remained infectious for 5-11 days at room temperature and up to 18 days at 4 degrees C, but were inactivated after 24 h at 37 degrees C. Interestingly, a fraction of dried virus was still infectious after 1 h at 56 degrees C. These results demonstrated that hantavirus transmission does not require direct contact between rodents, or between rodents and humans, and that the indirect transmission of PUUV through contaminated environment takes place among the rodents for a prolonged period of time. The results also have implications for safety recommendations for work with hantaviruses and for preventive measures.

Social status does not predict responses to Seoul virus infection or reproductive success among male Norway rats

Trade-offs exist among life history strategies that are used to increase survival and reproduction; such that, males that engage in more competitive behaviors may be more susceptible to infection. Hantaviruses are transmitted horizontally between rodents through the passage of virus in saliva during wounding and male rodents are more likely to be infected with hantaviruses than females. To determine whether a trade-off exists between dominance and susceptibility to Seoul virus infection, male Long Evans rats were group housed (3/cage) with a female rat and aggressive and subordinate behaviors were monitored during a 10 day group housing condition. After behavioral testing, males were individually housed, inoculated with Seoul virus, and blood, saliva, and fecal samples were collected. Dominant males initiated more aggressive encounters than subordinate males. Dominant and subordinate males, however, had similar steroid hormone concentrations, anti-Seoul virus IgG responses, and weight gain over the course of infection. A similar proportion of dominant and subordinate males shed virus in saliva and feces during infection. Using microsatellite DNA markers paternity was assigned to pups derived during the group housing period. In contrast to our initial hypothesis, dominant and subordinate males sired a similar percentage of pups. Taken together, host social status may not predict reproductive success or susceptibility to hantaviruses in rodent reservoir populations.

HFRS causing hantaviruses do not induce apoptosis in confluent Vero E6 and A-549 cells

Hantaviruses are known to cause little or no cytopathic effect in vitro, but have been suggested to cause apoptosis. To determine whether different hantaviruses would induce apoptosis to varying degrees, confluent Vero E6 cells were infected with the hemorrhagic fever with renal syndrome (HFRS) causing viruses Hantaan, Dobrava, Saaremaa, and Puumala. However, no difference was found in the percentage of adherent cells, or of cells with condensed nuclei, between non-infected and virus-infected cells at 3, 6, 9, or 12 days after infection. Furthermore, no differences in the percentage of cells with inter-nucleosomal cleavage of DNA between uninfected and Hantaan infected cells could be detected using the TUNEL assay. Possibly, slightly more apoptotic cells, but never more than 5%, were detected after Hantaan infection of non-confluent cells as compared to the negative control. Earlier reported results that Tula hantavirus induces significant apoptosis on Vero E6 cells were also verified, suggesting that non-pathogenic hantaviruses might differ from HFRS-causing strains regarding induction of apoptosis. In conclusion, the results indicated that the HFRS-causing hantaviruses might induce a very low level of apoptosis in dividing cells, but not at all in confluent cells.

Human and rodent humoral immune responses to Andes virus structural proteins

In the present work we identified B-cell epitopes recognized by sera of humans and rodents naturally infected with Andes virus, a hantavirus present in Chile and Argentina. Analysis of patient and rodent sera with overlapping peptides revealed 21 human and rodent epitopes on the three structural proteins. Whereas in the nucleoprotein the region comprising aa 248-260 was shown to be the key determinant of human sera, the major antigenic site of rodent antibody reactivity is located at aa 326-338. In G1, the main epitope recognized by human sera was mapped to aa 14-26, while rodent antibodies bound predominantly to aa 599-611. In contrast, humans and mice had strong responses to three regions in G2 (aa 691-703, aa 918-930, aa 955-967), of which the last two are associated with neutralization of Hantaan virus. This insight affords important information for the development of immunotherapies for the acute phase of hantavirus cardiopulmonary syndrome.

Human recombinant neutralizing antibodies against hantaan virus G2 protein

Old world hantaviruses, causing hemorrhagic fever with renal syndrome (HFRS), still present a public health problem in Asia and Eastern Europe. The majority of cases has been recorded in China. The aim of our study was to generate human recombinant neutralizing antibodies to a hantavirus by phage display technology. To preserve the structural identity of viral protein, the panning procedure was performed on native Hantaan (HTN) (76-118) virus propagated in Vero-E6 cells. In total, five complete human recombinant IgG antibodies were produced in a baculovirus expression system. All of them were able to completely neutralize HTN, and Seoul (SEO) virus in a plaque reduction neutralization test (PRNT). Three of these antibodies could also completely neutralize Dobrava (DOB) virus but not Puumala (PUU) virus. All antibodies bind to Hantaan virus G2 protein localized in the virus envelope. The sequence areas within the HTN (76-118)-G2 protein detected by five selected antibodies were mapped using peptide scans. Two partial epitopes, 916-KVMATIDSF-924 and 954-LVTKDIDFD-963, were recognized, which presumably are of paramount importance for docking of the virus to host cell receptors. A consensus motif 916-KVXATIXSF-924 could be identified by mutational analysis. The neutralizing antibodies to the most widely distributed hantaviruses causing HFRS might be promising candidates for the development of an agent for prevention and treatment of HFRS in patients.

Persistent Sin Nombre virus infection in the deer mouse (Peromyscus maniculatus) model: sites of replication and strand-specific expression

To address Sin Nombre (SN) virus persistence in deer mice, we sacrificed experimentally infected deer mice at eight time points from day 21 to day 217 postinoculation (p.i.) and examined their tissues for viral nucleocapsid (N) antigen expression and both negative-strand (genomic) and positive-strand (replicative/mRNA) viral S segment RNA titers. All the animals that we inoculated developed persistent infections, and SN virus could be isolated from tissues throughout the course of infection. The transition from an acute to a persistent pattern of infection appeared to occur between days 60 and 90 p.i. Beginning on day 60 p.i., the heart, brown adipose tissue (BAT), and lung retained antigen expression and genomic viral RNA the most frequently. We found a statistically significant association among the presence of replicative RNA in the heart, lung, and BAT, widespread antigen expression (in > or =5 tissues), and RNA viremia. Of these three tissues, the heart retained negative-strand RNA and viral N antigen the most consistently (in 25 of 26 animals). During persistence, there were two distinct patterns of infection: restricted versus disseminated tissue involvement. Mice with the restricted pattern exhibited N antigen expression in < or =3 tissues, an absence of viral RNA in the blood, neutralizing antibody titers of < or =1:1,280 (P = 0.01), and no replicative RNA in the heart, lung, or BAT. Those with the "disseminated" pattern showed N antigen expression in > or =5 tissues, neutralizing antibody titers of 1:160 to 1:20,480, replicative RNA in the heart, lung, and BAT at a high frequency, and RNA viremia. Virus could be isolated consistently only from mice that demonstrated the disseminated pattern. The heart, lung, and BAT are important sites for the replication and maintenance of SN virus during persistent infection.

Antigenic drift, antigenic shift and interferon antagonists: how bunyaviruses counteract the immune system

Members of the Bunyaviridae family are amongst the most widespread viruses in the world. They can be found on every inhabited continent at virtually every latitude, and are able to infect a wide range of arthropods, plants and mammals including humans. More than 300 named viruses are contained within the family Bunyaviridae (Virus Taxonomy: Seventh Report of the International Committee on Taxonomy of Viruses (2000) 599), and several members cause significant disease in humans or domestic animals. Despite being recognised as an emerging threat, relatively little is known about their virulence mechanisms. Here, we try to summarise the current state of knowledge about how the viruses of the Bunyaviridae succeed in establishing infection in the face of a powerful immune system.

Viral hemorrhagic fevers including hantavirus pulmonary syndrome in the Americas

The term viral hemorrhagic fever refers to an acute systemic illness with a propensity for bleeding and shock. The viral hemorrhagic fevers endemic in the Americas include yellow fever, dengue hemorrhagic fever, the South American hemorrhagic fevers, hantavirus pulmonary syndrome, and hemorrhagic fever with renal syndrome. Because these diseases are primarily zoonotic, the distribution of any given virus is generally restricted by the distribution of its natural reservoir or arthropod vector. A high index of suspicion, detailed investigation of the travel and exposure history of the patient, and a basic understanding of the incubation periods and distributions of the various reservoirs of hemorrhagic fever viruses are imperative, as are prompt notification and laboratory confirmation. Clinical management is largely supportive, with a special emphasis on safe nursing practices to prevent nosocomial transmission.

Zoonotic infections in travelers to the tropics

Infections that normally occur in animal hosts, zoonoses are transmitted occasionally between animals and humans. Zoonoses occur worldwide, and the traveler may engage in activities increasing the risk of acquiring these otherwise rare infections. This article reviews selected zoonoses in the context of travel to the tropics.

Behavioral, physiologic, and habitat influences on the dynamics of Puumala virus infection in bank voles (Clethrionomys glareolus)

Populations of bank voles (Clethrionomys glareolus) were monitored during a 4-year study in southern Belgium to assess the influence of agonistic behavior, reproductive status, mobility, and distribution of the rodents on the dynamics of Puumala virus (abbreviation: PUUV; genus: Hantavirus) infection. Concordance was high between data from serologic testing and results of viral RNA detection. Wounds resulting from biting or scratching were observed mainly in adult rodents. Hantavirus infection in adults was associated with wounds in the fall, i.e., at the end of the breeding season, but not in spring. In addition, sexually active animals were significantly more often wounded and positive for infection. Hantavirus infection was associated with higher mobility in juvenile and subadult males. Seroconversions observed 6 months apart also occurred more frequently in animals that had moved longer distances from their original capture point. During nonepidemic years, the distribution of infection was patchy, and positive foci were mainly located in dense ground vegetation.

Shedding and intracage transmission of Sin Nombre hantavirus in the deer mouse (Peromyscus maniculatus) model

The mechanism(s) by which Sin Nombre (SN) hantavirus is maintained in deer mouse populations is unclear. Field studies indicate that transmission occurs primarily if not exclusively via a horizontal mechanism. Using an experimental deer mouse infection model in an outdoor laboratory, we tested whether infected rodents shed SN virus in urine, feces, and saliva, whether infected mice transmit infection to naïve cage mates, and whether infected dams are able to vertically transmit virus or antibody to offspring. Using pooled samples of urine, feces, and saliva collected from mice infected 8 to 120 days postinoculation (p.i.), we found that a subset of saliva samples, collected between 15 and 90 days p.i., contained viral RNA. Parallel studies conducted on wild-caught, naturally infected deer mice showed a similar pattern of intermittent positivity, also only in saliva samples. Attempts to isolate virus through inoculation of cells or naïve deer mice with the secreta or excreta of infected mice were uniformly negative. Of 54 attempts to transmit infection by cohousing infected deer mice with seronegative cage mates, we observed only a single case of transmission, which occurred between 29 and 42 days p.i. Dams passively transferred antibodies to neonatal pups via milk, and those antibodies persisted for at least 2 months after weaning, but none transmitted infection to their pups. Compared to other hantavirus models, SN virus is shed less efficiently and transmits inefficiently among cage mates. Transmission of SN virus among reservoir rodents may require factors that are not required for other hantaviruses.

Experimental infection model for Sin Nombre hantavirus in the deer mouse (Peromyscus maniculatus)

The relationship between hantaviruses and their reservoir hosts is not well understood. We successfully passaged a mouse-adapted strain of Sin Nombre virus from deer mice (Peromyscus maniculatus) by i.m. inoculation of 4- to 6-wk-old deer mouse pups. After inoculation with 5 ID(50), antibodies to the nucleocapsid (N) antigen first became detectable at 14 d whereas neutralizing antibodies were detectable by 7 d. Viral N antigen first began to appear in heart, lung, liver, spleen, and/or kidney by 7 d, whereas viral RNA was present in those tissues as well as in thymus, salivary gland, intestine, white fat, and brown fat. By 14 d nearly all tissues examined displayed both viral RNA and N antigen. We noted no consistent histopathologic changes associated with infection, even when RNA load was high. Viral RNA titers peaked on 21 d in most tissues, then began to decline by 28 d. Infection persisted for at least 90 d. The RNA titers were highest in heart, lung, and brown fat. Deer mice can be experimentally infected with Sin Nombre virus, which now allows provocative examination of the virus-host relationship. The prominent involvement of heart, lung, and brown fat suggests that these sites may be important tissues for early virus replication or for maintenance of the virus in nature.

Persistent hantavirus infections: characteristics and mechanisms

Hantaviruses include serious human pathogens that are maintained in nature in persistently infected rodents and that can also persistently infect cultured mammalian cells, causing little or no cytopathology. The mechanisms of hantavirus persistence are only beginning to be explored. Recent data point to subtle changes in the viral genome that might result in the differential regulation of replication and lead to persistence.

Accumulation of terminally deleted RNAs may play a role in Seoul virus persistence

Two independent, long-term infections were analyzed to determine whether changes in viral replication could contribute to the establishment and/or maintenance of persistent Seoul virus infections. Infected cell cultures initially contained high levels of infectious virus and intracellular viral RNA that peaked between approximately 7 to 16 days postinfection and then gradually declined until day 26. After day 26, the viral titers and the levels of the small (S), medium (M), and large (L) viral RNAs varied cyclically until the end of the studies. The changes in the concentrations of the RNAs and titer were similar in pattern and appeared to result from changes in the regulation of replication. Neither internal deletions nor an accumulation of nucleotide changes were found in the RNAs. However, fine mapping and sequence analysis revealed short deletions in some of the RNAs in the conserved complementary terminal sequences believed to contain the signals for initiation of replication and transcription. Deletions at the 3' termini of S, M, and L virus-sense RNAs (vRNAs) accumulated during the acute phase of infection just before the time that the viral titer and the concentration of vRNAs and virus complementary-sense RNAs (cRNAs) began to decline. The absence of deletions at the 5' termini of the S, M, and L cRNAs suggests that the 3'-deleted vRNAs may not be replication competent. Thus, as the percentage of 3'-deleted vRNAs increase in the population, they could potentially compete with standard virus and downregulate viral replication. Deletions at the 3' L cRNA and 5' L vRNA termini were also observed, and the proportion of these deleted RNAs varied cyclically during the infections. We propose a model in which terminal nucleotide deletions arise by nuclease activity of the viral polymerase. In addition, we speculate that cleaved terminal fragments might be used as primers during replication, resulting in the repair of some of the deleted RNAs.

Transmission of Black Creek Canal virus between cotton rats

Black Creek Canal (BCC) virus is a hantavirus associated with hantavirus pulmonary syndrome in southeastern North America. The virus was isolated from the spleen of a cotton rat (Sigmodon hispidus) trapped in southern Florida. Our previous studies have shown that we could consistently infect male cotton rats with BCC virus in the laboratory. These animals became persistently infected and virus could be detected in salivary glands, urine, and feces. In this report we show: (1) female and male cotton rats are equally susceptible to BCC virus infection, (2) susceptibility to infection was not influenced by age, (3) all inoculated rats transmitted the infection to uninoculated cage mates, and (4) offspring of infected rats became infected despite the presence of high maternal antibodies. The course of BCC virus infection, as determined by antibody response and the ability to isolate or detect virus, appeared to be similar regardless of whether the rats obtained their infection by inoculation or contact with inoculated rats. J. Med. Virol. 60:70-76, 2000. Published 2000 Wiley-Liss, Inc.

Apoptosis is induced by hantaviruses in cultured cells

Hantaviruses replicate in primary and cultured animal cells with little or no cytopathic effect. We report here that the cultured Vero E6 cells infected by the Hantaan or by the Prospect Hill viruses exhibited characteristic features of apoptosis, including condensation and segmentation of nuclei and internucleosomal cleavage of nuclear DNA. Apoptosis was not seen in the cells adsorbed by UV-inactivated virus, indicating that the viral replication is required for the induction of apoptosis. Furthermore, level of the proto-oncogenic Bcl-2 protein was significantly reduced, whereas its mRNA level remained unchanged in Hantaan virus-infected cells, suggesting possible involvement and posttranscriptional regulation of this antiapoptotic protein in the process.

Long-term hantavirus persistence in rodent populations in central Arizona

For 35 months, we monitored hantavirus activity in rodent populations in central Arizona. The most frequently captured hantavirus antibody-positive rodents were Peromyscus boylii and P. truei. Antibody-positive P. boylii were more frequently male (84%), older, and heavier, and they survived longer on trapping web sites than antibody-negative mice. The number of antibody-positive P. boylii was greater during high population densities than during low densities, while antibody prevalence was greater during low population densities. Virus transmission and incidence rates, also related to population densities, varied by trapping site. The spatial distribution of antibody-positive P. boylii varied by population density and reflected the species preference for dense chaparral habitats. The focal ranges of antibody-positive P. boylii also demonstrated a patchy distribution of hantavirus.

Sin Nombre virus pathogenesis in Peromyscus maniculatus

Sin Nombre virus (SNV), a member of the Hantavirus genus, causes acute viral pneumonia in humans and is thought to persistently infect mice. The deer mouse, Peromyscus maniculatus, has been identified as the primary reservoir host for SNV. To understand SNV infection of P. maniculatus, we examined wild deer mice for localization of viral antigens and nucleic acid. Morphologic examination consistently revealed septal edema within lung tissue and mononuclear cell infiltrates in portal areas of the liver. Immunohistochemical analysis of SNV-infected deer mice identified viral antigens within lung, liver, kidney, and spleen. The lungs consistently presented with the highest levels of viral antigen by immunohistochemistry and with the highest levels of nucleic acid by reverse transcriptase (RT) PCR. The mononuclear cell infiltrates surrounding liver portal triads were positive for SNV antigens in addition to resident macrophages in liver sinuses. Spleen tissue contained antigens in both the red pulp and the periartereolar region of the white pulp. The kidney presented with no gross pathology, although antigens could be localized to glomeruli. Virus antigen levels within the kidney were highest in deer mice that did not have antibodies to SNV but contained viral nucleic acid detectable by RT PCR. Since transmission is thought to occur via urine, our results suggest that virus transmission may be highest in the early stages of infection. In addition, these results indicate that SNV does cause some pathology within its reservoir host.

Natural history of Sin Nombre virus in western Colorado

A mark-recapture longitudinal study of immunoglobulin G (IgG) antibody to Sin Nombre virus (SNV) in rodent populations in western Colorado (1994-results summarized to October 1997) indicates the presence of SNV or a closely related hantavirus at two sites. Most rodents (principally deer mice, Peromyscus maniculatus, and pinyon mice, P. truei) did not persist on the trapping webs much beyond 1 month after first capture. Some persisted more than 1 year, which suggests that even a few infected deer mice could serve as transseasonal reservoirs and mechanisms for over-winter virus maintenance. A positive association between wounds and SNV antibody in adult animals at both sites suggests that when infected rodents in certain populations fight with uninfected rodents, virus amplification occurs. At both sites, male rodents comprised a larger percentage of seropositive mice than recaptured mice, which suggests that male mice contribute more to the SNV epizootic cycle than female mice. In deer mice, IgG antibody prevalence fluctuations were positively associated with population fluctuations. The rates of seroconversion, which in deer mice at both sites occurred mostly during late summer and midwinter, were higher than the seroprevalence, which suggests that the longer deer mice live, the greater the probability they will become infected with SNV.

New York 1 and Sin Nombre viruses are serotypically distinct viruses associated with hantavirus pulmonary syndrome

New York 1 virus (NY-1) and Sin Nombre virus (SN) are associated with hantavirus pulmonary syndrome (HPS). NY-1 and SN are derived from unique mammalian hosts and geographic locations but have similar G1 and G2 surface proteins (93 and 97% identical, respectively). Focus reduction neutralization assays were used to define the serotypic relationship between NY-1 and SN. Sera from NY-1-positive Peromyscus leucopus neutralized NY-1 and SN at titers of >/=1/3,200 and 16-fold-lower neutralizing titers to NY-1 than to SN. Reference sera to Hantaan, Seoul, and Prospect Hill viruses also failed to neutralize NY-1. These results indicate that SN and NY-1 define unique hantavirus serotypes and implicate the presence of additional HPS-associated hantavirus serotypes in the Americas.

Long-term studies of hantavirus reservoir populations in the southwestern United States: a synthesis

A series of intensive, longitudinal, mark-recapture studies of hantavirus infection dynamics in reservoir populations in the southwestern United States indicates consistent patterns as well as important differences among sites and host-virus associations. All studies found a higher prevalence of infection in older (particularly male) mice; one study associated wounds with seropositivity. These findings are consistent with horizontal transmission and transmission through fighting between adult male rodents. Despite very low rodent densities at some sites, low-level hantavirus infection continued, perhaps because of persistent infection in a few long-lived rodents or periodic reintroduction of virus from neighboring populations. Prevalence of hantavirus antibody showed seasonal and multiyear patterns that suggested a delayed density-dependent relationship between prevalence and population density. Clear differences in population dynamics and patterns of infection among sites, sampling periods, and host species underscore the importance of replication and continuity of long-term reservoir studies. Nevertheless, the measurable associations between environmental variables, reservoir population density, rates of virus transmission, and prevalence of infection in host populations may improve our capacity to model processes influencing infection and predict increased risk for hantavirus transmission to humans.

Evaluation of Puumala virus IgG and IgM enzyme immunoassays based on recombinant baculovirus-expressed nucleocapsid protein for early nephropathia epidemica diagnosis

BACKGROUND

Puumala virus (PUU), a member of Hantavirus genus, is the causative agent of nephropathia epidemica (NE), a milder form of hemorrhagic fever with renal syndrome (HFRS). Rapid diagnosis is essential for clinical management of NE.

OBJECTIVES

To evaluate the usefulness of recombinant protein-based IgM (direct- and mu-capture) and IgG (direct-and antigen (Ag)-capture) enzyme immunoassays (EIA) in early diagnosis of NE in comparison to IgG immunofluorescence assay (IF), and to find out the time limit for PUU-specific antibody seroconversion.

STUDY DESIGN

The specific IgM and IgG antibody responses in serum were analyzed in 109 patients (235 serial sera) and 114 patients (233 serial sera), respectively. The serum panel used was selected from a larger material according to the availability of information concerning the date after onset of symptoms, the panel also containing NE patients who had been IgG-IF negative in their first (early) samples to find out the possible differences between sensitivities of the EIAs and IF.

RESULTS

All NE patients tested became IgM-positive at the latest on the 6th (mu-capture EIA) or 7th (direct-IgM EIA) day after onset of symptoms. Out of a panel of very early NE-patient sera (n = 38) that could not be detected by IgG-IF, 66% were already positive with both direct-IgM EIA and mu-capture EIA. When comparing IgG EIAs and IgG-IF, 98% of IF-positive sera from NE patients were also positive with direct-IgG EIA, and 99% with Ag-capture IgG EIA. Out of a panel of very early NE-patient sera (n = 37) that could not be detected by IgG-IF, 57% were positive with direct-IgG EIA, and 27% with Ag-capture IgG EIA.

CONCLUSIONS

The baculovirus-expressed PUU-N-based IgG and IgM EIAs were found most suitable for NE diagnosis, giving the opportunity in some cases for earlier diagnosis as compared with PUU-IgG IF.

Modes of Seoul virus infections: persistency in newborn rats and transiency in adult rats

To understand the mode of persistent infection of Seoul virus in rodents, we examined the distribution of the virus genome and antibody production in infected rats. When 1-day-old rats were inoculated with the KI-83-262 strain, the S segment of viral genome was detected in sera, clots, lungs and kidneys from 3 to 184 days post inoculation (d.p.i.) by nested reverse transcriptase PCR. On the other hand, when 7-week-old rats were infected with this virus, viral genome was detected only in the lungs from 3 to 50 d.p.i. The neutralizing antibody titers of rats inoculated at 1-day of age were higher than those of rats inoculated at 7 weeks of age. In both age groups, however, the IgG avidity of antibody increased along with the course of infection. We found that urban rats (Rattus norvegicus) infected early in life harbored the virus for more than 6 months.

Pathogenesis of puumala and other hantavirus infections

Hantaviruses are rodent/insectivore-borne negative-stranded RNA viruses which belong to the Bunyaviridae family. They do not cause any symptomatic disease in their adult carrier rodents, but in humans they are aetiologic agents of haemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS), both associated with a significant mortality. In cell culture hantaviruses do not cause cytopathic effects and the mechanisms of disease in man are not well understood. Increased capillary permeability is a central phenomenon in the pathogenesis of hantavirus infections. Although the viruses have in vivo a predilection for endothelial cells, it is presumed that inflammatory mediators of the host immune response play a significant role in the capillary leak that may produce abrupt hypotension and shock in severely ill patients. Mediators released by activated macrophages including NO and TNF-alpha are considered important. The pathogenesis of renal failure in HFRS also awaits to be resolved. This review summarises what is known about these phenomena and discusses also the molecular basis of the putative virulence factors of hantaviruses. Finally, the genetic predisposition and HLA association with severe Puumala virus infection will be discussed. Copyright 1998 John Wiley & Sons, Ltd.

Ecologic studies of rodent reservoirs: their relevance for human health

Within the past few years, the number of "new" human diseases associated with small-mammal reservoirs has increased dramatically, stimulating renewed interest in reservoir ecology research. A consistent, integrative approach to such research allows direct comparisons between studies, contributes to the efficient use of resources and data, and increases investigator safety. We outline steps directed toward understanding vertebrate host ecology as it relates to human disease and illustrate the relevance of each step by using examples from studies of hosts associated with rodent-borne hemorrhagic fever viruses.

A colorimetric PCR-enzyme immunoassay to identify hantaviruses

BACKGROUND

Hantaviruses cause two serious human diseases: hantavirus pulmonary syndrome and hemorrhagic fever with renal syndrome. At least nine hantaviruses are known to be pathogenic for humans and numerous others, with unknown disease potential, have been detected in rodents. Assays to quickly identify specific hantaviruses would be useful both for clinical diagnosis and in risk assessment studies.

OBJECTIVES

The goal of our study was to develop and test a specific and sensitive PCR-based assay for identification and differentiation of hantaviruses.

STUDY DESIGN

We developed an assay that combined RNA-PCR amplification and colorimetric enzymatic detection to identify representative European, Asian, and north American hantaviruses. RNAs from 18 hantavirus strains of nine species were amplified in the presence of digoxigenin-dUTP by using a single pair of oligonucleotide primers and polymerase chain reaction (PCR) performed by using rTth DNA polymerase. Digoxigenin-labeled PCR products were hybridized in solution to virus type-specific biotinilated probes, captured onto streptavidin-coated microtiter plates and detected by horseradish peroxidase-labeled anti-digoxigenin antibodies and a chromogenic substrate.

RESULTS AND CONCLUSIONS

The assay correctly identified each homologous virus type tested. The detection limit of the assay was approximately 15 PFU or at least 50 copies of the viral genome. The assay is simple and strain-specific and is adaptable for automation, making it more practical than other available techniques for accurate and reliable diagnosis and typing of hantaviruses.

Spontaneous gastric carcinoid tumors in the striped field mouse (Apodemus agrarius)

Gastric carcinoid tumors were found in seven of 135 striped field mice (Apodemus agrarius) by routine histopathologic examination. All these carcinoids occurred in mature striped field mice aged 72-100 weeks. Six animals were females and only one was male. Only two of seven tumors were detectable by gross examination. Grossly, tumors were located in the fundus of the glandular stomach. All seven tumors were microscopically single in the stomach and two mice exhibited extragastric metastasis. Tumors from all the mice were characterized by densely packed sheets of round to polygonal cells, subdivided into packets by a fine fibrovascular stroma. The cytoplasm of all tumor cells from all the mice contained argyrophil granules when stained by Grimelius and Sevier-Munger silver procedures. All seven mice with gastric carcinoids exhibited positive immunoreactivity to neuron specific enolase. Psammoma bodies, concentrically laminated microcalcification, were characteristic findings in gastric carcinoids from five mice. There were also a concomitant and independent hepatocellular adenoma in one case and hepatocellular carcinoma in two cases. The present cases provide the first description of spontaneous gastric carcinoid tumors in the striped field mice.

Hantaviruses: a global disease problem

Hantaviruses are carried by numerous rodent species throughout the world. In 1993, a previously unknown group of hantaviruses emerged in the United States as the cause of an acute respiratory disease now termed hantavirus pulmonary syndrome (HPS). Before than, hantaviruses were known as the etiologic agents of hemorrhagic fever with renal syndrome, a disease that occurs almost entirely in the Eastern Hemisphere. Since the discovery of the HPS-causing hantaviruses, intense investigation of the ecology and epidemiology of hantaviruses has led to the discovery of many other novel hantaviruses. Their ubiquity and potential for causing severe human illness make these viruses an important public health concern; we reviewed the distribution, ecology, disease potential, and genetic spectrum.

Genetic variation in Tula hantaviruses: sequence analysis of the S and M segments of strains from Central Europe

Hantavirus carried by the European common vole Microtus arvalis from Moravia (Czech Republic) was analyzed by RT-PCR-sequencing and by reactivity with a panel of monoclonal antibodies (MAbs). Sequencing of the full-length S segment and the proximal part of the M segment showed that the virus belonged to genotype Tula (TUL) we discovered earlier in Microtus arvalis from Central Russia. This finding supported the concept of host dependence of hantaviruses. Phylogenetic analyses suggested a similar evolutionary history for S and M genes of TUL strains; thus far there is no evidence for reassortment in TUL. Geographic clustering of TUL genetic variants was observed and different levels of the genetic variability were revealed resembling those estimated for another hantavirus, Puumala (PUU). Comparison of the deduced N protein sequence from Russia and from Moravia showed that genetic drift in TUL occurred not only by accumulation of point mutations but also by the deletion of a nucleotide triplet. It encoded Ser252 which was located within a highly variable hydrophilic part of the N protein carrying B-cell epitopes and presumably forming a loop. Analysis of naturally expressed TUL N-antigen derived from lung tissue of infected voles with MAbs indicated antigenic heterogeneity among TUL strains.

A vaccinia virus-vectored Hantaan virus vaccine protects hamsters from challenge with Hantaan and Seoul viruses but not Puumala virus

To investigate the ability of a vaccinia virus-vectored vaccine expressing the M and the S segments of Hantaan (HTN) virus (C. S. Schmaljohn, S. E. Hasty, and J. M. Dalrymple, Vaccine 10:10-13, 1992) to elicit a protective immune response against other hantaviruses, we vaccinated hamsters with the recombinant vaccine and challenged them with HTN, Seoul (SEO), or Puumala (PUU) virus. Neutralizing antibodies to HTN virus were found in all vaccinated hamsters both before and after challenge. Neutralizing antibody titers to SEO virus were present at low levels or were undetectable after two immunizations with the vaccine but were positive in all vaccinated hamsters after challenge with SEO virus and were also positive in control animals that were not challenged. Neutralizing antibodies to PUU virus were observed only in hamsters previously challenged with PUU virus. To assay for virus in the blood and tissues of the hamsters, we developed a nested reverse transcriptase (RT)-PCR with cross-reactive outer primers and serotype-specific inner primers. The RT-PCR specifically detected as little as 1 PFU of virus in serum containing high-titer neutralizing antibodies and was more sensitive than immunofluorescent antibody staining for detecting virus in lung and kidney specimens of infected hamsters. By using the RT-PCR, we found that vaccinated hamsters, challenged with HTN or SEO virus, neither were viremic nor had evidence of virus in their lungs or kidneys. In contrast, vaccinated hamsters challenged with PUU virus were viremic and had PUU virus-specific nucleic acid in their organs.

Hantavirus pulmonary syndrome: Report of four Alberta cases

Four Alberta cases of hantavirus pulmonary syndrome are reported. Three cases required intensive care, with one experiencing a fulminant course resulting in death. A fourth case with milder illness was identified after epidemiological investigations. Ribavirin was used in one patient who experienced a successful outcome. A recent open label trial has not supported the efficacy of this drug. The epidemiology of Peromyscus maniculatus, the primary rodent host, and the clinical features of this syndrome are summarized.

Sensitive detection of hantaviruses by biotin-streptavidin enhanced immunoassays based on bank vole monoclonal antibodies

Antigen capture enzyme-linked immunosorbent assays (ELISAs) based on detection of the viral nucleocapsid protein (N) were designed for rapid diagnosis of hantavirus infections. Several combinations of bank vole (Clethrionomys glareolus) monoclonal antibodies with various N-epitope specificities were used for the development of two double-antibody sandwich forms of ELISA; PUU virus AG-ELISA for an exclusive detection of Puumala-related viruses, and Hantavirus AG-ELISA for a more extensive detection of all serotypes of hemorrhagic fever with renal syndrome (HFRS) viruses. The biotin-streptavidin system, in combination with horseradish peroxidase, rendered the assays' sensitivities similar to or even higher than immunoblotting. Calculated detection limits for the PUU virus and the Hantavirus AG-ELISAs were 405 and 50 focus forming units or 80 and 10 infected Vero E6 cells, respectively. The assays were evaluated and found to be suitable for convenient and routine detection of hantaviruses in cell cultures and in infected animal tissue.

Hantaviruses: clinical, microbiologic, and epidemiologic aspects

Hantaviruses comprise a genus of the family Bunyaviridae. Bunyaviruses are enveloped viruses with a negative-sense, tripartite RNA genome. Hantaviruses are etiologic agents for two acute and severe illnesses of man, hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Each hantavirus is primarily associated with a single rodent host species or genus, and is transmitted to man through accidental inhalation or ingestion of virus-contaminated rodent excreta. The distribution of hantaviruses is worldwide. HFRS is caused by infection with Hantaan, Seoul, Dobrava/Belgrade, and Puumala hantaviruses, all of which are enzootic in murid rodents of Old World origin. HPS is caused by any of several hantavirus species associated with indigenous New World rodents of the subfamily Sigmodontinae, family Muridae. HFRS and HPS have numerous common epidemiologic, clinical, and laboratory characteristics. Common features include fever, myalgia, thrombocytopenia, neutrophilia, and a profound capillary leak syndrome associated with hypotension, decreased cardiac output, and shock. Worldwide, HPS is much less common than HFRS but is associated with a higher mortality rate. Recovery from hantavirus disease is generally complete, although chronic renal insufficiency may be a rare sequel of HFRS.

Hantavirus pulmonary syndrome: the Four Corners disease

OBJECTIVE

To review the epidemiology, pathogenesis, clinical features, diagnosis, and treatment of hantavirus infections, focusing on the recent outbreak of hantavirus pulmonary syndrome in the US.

DATA SOURCES

A MEDLINE search (1966 to present) of English language literature pertaining to hantaviruses was performed. Additional literature was obtained from reference lists of pertinent articles identified through the search.

STUDY SELECTION AND DATA EXTRACTION

All articles were considered for possible inclusion in the review. Pertinent information, as judged by the authors, was selected for discussion.

DATA SYNTHESIS

Hemorrhagic fever with renal syndrome (HFRS) has long been recognized in Eurasia and is the predominant disease manifestation of hantavirus infection worldwide. Hantavirus pulmonary syndrome (HPS) recently has been described in the US and exhibits greater pulmonary involvement and mortality than HFRS. Historically, 4 hantavirus serotypes (Hantaan, Seoul, Puumala, Prospect Hill) are recognized; however, additional serotypes have been proposed as distinct serogroups, including the serotype responsible for HPS in the Four Corners area: the Four Corners virus (FCV). Phylogenetic analysis shows that FCV is most closely related to Prospect Hill virus, another hantavirus previously isolated in the US that has not yet been identified with human disease. Additional hantavirus serotypes isolated in the US may provide insight into the prevalence of hantavirus infection and disease in this country. Inhalation of aerosolized virus is the predominant mechanism of hantavirus infection. Diagnosis is based primarily on clinical findings and serologic evidence of hantavirus antibody or direct evidence in clinical tissue specimens. Limited clinical studies evaluating ribavirin as a therapeutic modality demonstrated that the agent improves clinical outcome in HFRS. However, the role of ribavirin in the treatment of HPS remains to be determined.

CONCLUSIONS

Hantavirus infections are becoming increasingly recognized as a cause of disease worldwide. Recognition of hantavirus disease in the US suggests enzoonosis of pathogenic hantaviruses. In the absence of a well-established cure, early diagnosis is imperative so that aggressive supportive care can be initiated.

Protection against hantavirus infection by dam's immunity transferred vertically to neonates

Antibodies to hantavirus, Seoul type B-1 strain, vertically transferred to rat neonates prevented lethal as well as persistent infection. When relatively high titer viruses were inoculated into neonates, the mother's antibodies protected all the neonates from lethal virus infection. However, the antibodies could not protect all of the neonates from persistent infection but only half of them underwent persistent infection. The other half was completely cured but also became persistently infected when rechallenged with the active viruses after reaching maturity.