Hemorrhagic Fever and Arboviruses

History and Epidemiology

In 1934, the prototypic arenavirus, LCM virus, was first isolated during serial monkey passage of human material that was obtained from a fatal infection in the first documented epidemic of St Louis encephalitis, a totally unrelated virus. LCM virus was the first recognized cause of aseptic meningitis in humans. Arenaviruses have been divided into two groups based on whether the virus is found in the Old World (eastern hemisphere) or the New World (western hemisphere). Of the 15 arenaviruses known to infect animals, all living in tropical regions, two are related to human infection in the Old World and only Lassa virus is related to dermatologic findings. LCM virus is the only arenavirus to exist in both areas, but it is classified as an Old World virus ( Table 12-1 ).

LCM and Lassa viruses are associated with Old-World rats and mice (family Muridae, subfamily Murinae). The restricted areas affected by Lassa virus in West Africa may reflect the geographic distribution of their natural host. The endemic regions are near forests and still have a low population density. However, progressive occupation of previously wild areas will expose humans to new cases of Lassa fever. In recent years, significant numbers of LCM infections have been attributed to silently infected pet hamsters and field mice ( Mus musculus ) in biomedical laboratory colonies, which explains the different distribution of LCM virus.

Pathogenesis and Etiology

Knowledge of the multiplication of arenaviruses is fragmentary. Most of what is known comes from studies with LCM virus. This virus replicates in a wide variety of cell types. Although the virus receptor has not been identified, it must be highly conserved and widely distributed. Transcription of the genome and replication are confined to the cytoplasm. The small RNA in the virion encodes in the negative sense a nucleoprotein, and in the positive or message sense a precursor GP, which is cleaved into two virion GPs (GP1 and GP2). The large RNA in the virion encodes in the negative sense an RNA-dependent RNA polymerase, and in the positive sense a zinc-binding protein that binds to the ribonucleoprotein complex. The virus buds from the plasma membrane, incorporating host lipids into the virus membrane.

The onset of the hemorrhagic fevers caused by Lassa virus may be insidious: the disease may present 7–14 days after infection simply as pyrexia, headache, sore throat, and myalgia. Lassa virus can be recovered from the blood and serum for up to 3 weeks after the onset of the infection, and from the urine for up to 5 weeks. Hemorrhagic phenomena, heralded by unremitting high fever, can begin after day 5 of illness and are followed by dehydration and hemoconcentration, shock, hemorrhagic manifestations, and cardiovascular collapse.

Compared with the dramatic clinical course and mortality, the gross pathology is unimpressive and of little help in constructing a pathogenetic scheme. Complete autopsies have not been performed on patients with Lassa fever ; however, autopsies performed on patients with Argentine hemorrhagic fever show a lack of deposited immunoglobulin and complement component C3 in the kidneys and small blood vessels. Mediators released from infected cells have a potential role in the pathogenesis of dysfunction of some target organs. Although LCM virus can produce severe human disease, characterized by prominent neurologic manifestations, pathologic lesions have not been studied extensively. However, in the mouse model the immune response against LCM virus (specifically in the T-cell compartment) is central to the development of fatal neurologic disease. Furthermore, mice infected with a lethal dose of this virus can invariably be saved by treatment with antibody to interferon-α / β (IFN-α / β), raising the possibility that endogenous interferon-α / β enhances the immunopathology.

Antibodies develop following overt human infection with arenaviruses and are detectable by enzyme-linked immunosorbent assay (ELISA), complement fixation, neutralization, and fluorescent antibody techniques. The humoral response is exceptionally slow, but ultimately a long-lasting and vigorous production of antibodies occurs. Usually, antibodies demonstrable by immunofluorescence are the first to appear, followed by complement-fixing antibodies. The complement-fixing antibodies are short lived, with titers diminishing rapidly 5–12 months after onset. In contrast, neutralizing antibodies remain detectable for many years. Cell-mediated immunity is important in arenavirus infections of experimental animals; it is sometimes harmful, but is probably beneficial in human infections, at least for Lassa fever. In Lassa fever passive transfer of early-convalescent-phase human antibodies does not protect monkeys or guinea pigs, whereas late antibodies neutralize virus and are protective. All evidence suggests that viral clearance in humans is complete and that chronic infection is not established. Reinfection with Lassa virus is possible, but appears to be uncommon.

Clinical Features

The incubation period is around 2 weeks. Disease onset usually begins with insidious progression of general malaise and fever over a 5-day period. In clinical illness the onset is gradual, with fever, malaise, headache, sore throat, cough, nausea, vomiting, diarrhea, myalgia, and chest and abdominal pain. The fever may be either constant or intermittent with spikes. Inflammation of the throat and eyes is commonly observed.

Progression beyond this stage is not common for Lassa fever. In severe cases, hypotension or shock, pleural effusion (fluid in the lung cavity), hemorrhage, seizures, encephalopathy, and swelling of the face and neck are frequent. Approximately 15% of hospitalized patients die. The disease is more severe in pregnancy, and fetal loss occurs in more than 80% of cases.

Hemorrhages, neurologic signs and symptoms, leukopenia and thrombocytopenia are commonly present. Hair loss and loss of coordination may occur in convalescence. In addition, deafness occurs in 25% of patients, with only half recovering some hearing function after 1–3 months. Immunity to reinfection occurs following infection, but the length of this period of protection is unknown.

Other hemorrhagic fevers of viral origin, such as Ebola infection and hemorrhagic dengue, should be considered in the differential diagnosis. Meningococcemia and other diseases leading to sepsis, with disseminated intravascular coagulation and shock, can be confused with the Latin American hemorrhagic fever complex.

Diagnosis and Differential Diagnosis

Lassa virus can be isolated by inoculation of Vero cells. All arenaviruses appear to share antigenic determinants in the ribonucleoproteins, as well as antigenically distinct determinants in their outer GPs. Positive immunofluorescent staining of acetone-fixed infected cells is definitive for more than just family identification, since Old-World arenaviruses can be readily distinguished from New-World viruses with limiting dilutions of antibody. Arenavirus species may be identified by their unique surface GPs and infectivity neutralization. Most cases, however, are diagnosed by the epidemiologic and clinical data. Lassa fever must be suspected if arenaviruses are prevalent in geographic areas where infections have occurred and in regions known to harbor reservoir rodent species.

Treatment and Prophylaxis

Although several classes of antiviral compounds have been found with specific in vitro activity against arenaviruses, only ribavirin has been proven to be effective against Lassa fever. It may be used at any point in the illness, as well as for postexposure prophylaxis. The adult dose for Lassa fever (with hepatitis and / or hemorrhagic manifestations) is 2 g (30 mg / kg) IV initially; 1 g (15 mg / kg) IV q 6h for 4 days, and then 500 mg (7.5 mg / kg) IV q 8 h for 6 days. The suggested prophylactic dose is 600 mg PO qid for 10 days. Ribavirin is contraindicated in pregnancy. Systemic ribavirin use causes dose-related anemia and hyperbilirubinemia related to extravascular hemolysis and, at higher doses, bone marrow suppression of the erythroid elements may occur.

Vaccines are under development. A Lassa virus GP gene has been cloned and expressed in vaccinia virus. This vaccine has offered a high degree of protection against disease and death in monkeys challenged with the intact Lassa virus. Use of plasma is not yet indicated for patients with Lassa fever. At least seven serologically distinct strains of Lassa virus have been isolated; animal studies suggest that effective therapy should involve geographic matching of immune plasma and virus strain. In view of the frequency of Lassa virus transmission from person to person in a hospital setting, strict measures must be taken to isolate patients who have or are suspected of having the disease. Isolation of patients with the other pathogenic arenaviruses is desirable.

History and Epidemiology

This family is indigenous to Africa. Marburg and Ebola both cause severe hemorrhagic fevers. Marburg virus was first recognized in laboratory workers exposed to tissues and blood from African green monkeys ( Cercopithecus aethiops ) in Marburg, Germany, in 1967. Since then, sporadic, virologically confirmed Marburg disease cases have occurred in Zimbabwe, South Africa, and Kenya. Ebola virus first emerged in two major disease outbreaks, with mortality rates of 88%, in Zaire and Sudan in 1976. Sporadic cases and minor outbreaks occurred again in the same locations previously affected and also in Gabon, the Ivory Coast, and Uganda, after 1977.

The Ebola Reston virus was first discovered in 1989 in monkeys imported from the Philippines, which had died in a holding facility in Reston, Virginia, just outside Washington, DC. Whereas monkeys suffer a severe disease, often leading to death, the limited information available indicates that humans may not become clinically ill. However, this is based only on the isolation of Reston virus from one asymptomatically infected animal handler identified during the original outbreak and a few seroconversions that were not associated with clinical disease. A formal quarantine procedure for imported monkeys was developed following the original Reston episode and it is this system that apparently identified the current cases. The Ebola Reston virus has also been isolated from two non-human primates ( Cynomolgus ) held at a quarantine facility in Texas, USA. The monkeys had also been imported from the Philippines.

Ebola infection has emerged as a potential global public health threat in the last few years. The most recent outbreak erupted in Guinea in December 2013, but soon spread into neighboring Liberia and Sierra Leone with a reported 70% case fatality rate. It is unprecedented in terms of number of cases, mortality rate, and deleterious socioeconomic impact. Thereupon the World Health Organization (WHO) declared Ebola to be a “public health emergency of international concern” on 8th August 2014. Although there had been only two cases of Ebola transmission inside the United States and both patients had survived, a November 2014 opinion poll revealed that the US public ranked Ebola as the third-most-urgent health problem facing the country – just below cost and access and higher than any other disease, including cancer or heart disease.

Serologic studies suggest filoviruses are endemic in many countries of the central African region. Although serologic data based on ELISA are of only limited reliability, they at least suggest the possible occurrence of subclinical infections caused by known or unknown filoviruses. The mode of primary infection in any natural setting is unknown for Marburg and Ebola viruses. EBOV is an often-fatal RNA virus and highly virulent: as few as 10 viral particles are needed for infection. Fruit bats (family Pteropodidae) are the suspected natural reservoir. EBOV is introduced into the human through close contact with body fluids and other secretions of infected wild animals, such as chimpanzees, gorillas, fruit bats, and monkeys. Once infection is established, it can be transmitted between humans via direct contact with the blood, secretions, organs, or other bodily fluids (including but not limited to urine, saliva, sweat, feces, vomit, breast milk, and semen) of infected humans and animals (fruit bats or primates [apes and monkeys]). Human handling and consumption of contaminated bushmeat and / or Ebola-infected animals has been cited as a major source of transmission to humans.

All secondary cases, however, have been due to intimate contact with infected patients. After hospitalization of an infected person, the disease spreads rapidly via contaminated needles and contact with blood, which seems to be the most important route of contamination. In each Ebola outbreak in Africa, the initial patient spread the disease to close family members through intimate contact with them.

Pathogenesis and Etiology

Filoviridae viruses are usually recovered from acute-phase sera and have also been found in throat washes, urine, soft-tissue effusions, semen, and anterior eye fluid. They have also been regularly isolated from autopsy material, such as spleen, lymph nodes, liver, and kidney. Clinical and biochemical findings support anatomic observations of extensive liver involvement, renal damage, changes in vascular permeability, and activation of the clotting cascade. Visceral organ necrosis is the consequence of virus replication in parenchymal cells. However, no organ is sufficiently damaged to cause death. Fluid distribution problems and platelet abnormalities indicate dysfunction of endothelial cells and platelets. The shock syndrome in severe and fatal cases seems to be mediated by virus-induced release of humoral factors such as cytokines. Filovirus GPs carry a presumably immunosuppressive domain, and immunosuppression has been observed in infected monkeys.

Clinical Features

Filoviruses cause a severe hemorrhagic fever in both human and non-human primates. Following an incubation period of 4–15 days, onset is sudden, marked by high fever, fatigue, headache, erythematous transient rashes, and myalgia. Abdominal pain, sore throat, nausea, vomiting, cough, arthralgia, diarrhea, and pharyngeal and conjunctival vasodilatation may follow these symptoms. Patients are dehydrated, apathetic, and disoriented. They may develop a characteristic, non-pruritic, maculopapular centripetal rash ( Fig. 12-1 ) associated with varying degrees of erythema, which desquamates by days 5–7 of the illness.

Hemorrhagic manifestations develop at the peak of the illness, and are of prognostic value. Bleeding into the gastrointestinal tract is the most prominent, apart from petechiae and hemorrhages from puncture wounds and mucous membranes. Most patients develop severe hemorrhagic manifestations in the following few days, with bleeding from multiple sites such as the gastrointestinal tract, oropharynx, and lungs. The skin and mucous membranes are also affected with echymoses, disseminated non-palpable petechiae, and massive gingival bleeding that usually herald a fatal outcome.

Diagnosis and Differential Diagnosis

Laboratory parameters are less characteristic, but the following are associated with the disease: leukopenia (as low as 1000 / ml, left shift with atypical lymphocytes), thrombocytopenia (50 000–100 000 / ml), markedly elevated serum transaminase levels (typically aspartate aminotransaminase exceeding alanine aminotransferase), hyperproteinaemia, and proteinuria. Prothrombin and partial thromboplastin times are prolonged, and fibrin split products are detectable. At a later stage, secondary bacterial infection may lead to elevated white blood counts. Some patients suffer fever, but eventually recover in about 5–9 days. In cases ending in death, clinical signs occur at an early stage and the patient dies between days 6 and 16, from hemorrhage and hypovolemic shock. The mortality rate is between 30% and 90%, depending on the virus, and the highest rate has been reported for Ebola Zaire. Ebola Reston seems to possess a very low pathogenicity for humans, or may even be apathogenic. Convalescence is prolonged and sometimes associated with myelitis, recurrent hepatitis, psychosis, or uveitis. An increased risk of abortion exists for pregnant women, and clinical observations indicate a high death rate for children of infected mothers.

In tropical settings, filoviral hemorrhagic fever may be difficult to identify, since the most common causes of severe, acute, febrile disease are malaria and typhoid fever. The differential diagnosis should also include other viral hemorrhagic fevers, such as yellow fever, dengue infection, or arenavirus hemorrhagic fevers, as well as meningococcemia, leptospirosis, and idiopathic thrombocytopenic purpura. Travel, treatment in local hospitals, and contact with sick persons or wild and domestic monkeys are useful historical features in returning travelers, especially in those from Africa. Diagnosis of single cases is extremely difficult, but occurrence of clusters of cases with prodromal fever followed by cases of hemorrhagic diatheses and person-to-person transmission are suggestive of viral hemorrhagic fever, and containment procedures must be initiated. In filoviral hemorrhagic fever, prostration, lethargy, wasting, and diarrhea are usually more severe than is observed with other viral hemorrhagic fever patients. The rash is characteristic and extremely useful in the differential diagnosis.

Laboratory diagnosis can be achieved in two different ways: by measuring the host-specific immunologic response to the infection and by detecting viral antigen and genomic RNA in the infected host. The most commonly used assays to detect antibodies to filoviruses are the indirect immunofluorescence assay (IFA), immunoblot, and ELISA (direct immunoglobulin G [IgG] and IgM ELISA, and IgM capture assay). Direct detection of viral particles, viral antigen, and genomic RNA can be achieved by electron microscopy (negative contrast, thin-section), immunohistochemistry, immunofluorescence on impression smears of tissues, antigen detection ELISA, and reverse transcriptase polymerase chain reaction PCR (RT-PCR).

Attempts to isolate the virus from serum and / or other clinical material should be performed using Vero or MA-104 cells (monkey kidney cells). However, most filoviruses do not cause extensive cytopathogenic effects on primary isolation. The most useful animal system, besides non-human primates, is guinea pigs, which develop fever within 10 days upon primary infection.

Treatment and Prophylaxis

There is no standard treatment for Ebola and Marburg infection and virus-specific treatment does not exist. Supportive therapy should be directed towards maintaining effective blood volume and electrolyte balance. Shock, cerebral edema, renal failure, coagulation disorders, and secondary bacterial infection must be managed. Heparin treatment should be considered only when there is clear evidence of disseminated intravascular coagulopathy. Filoviruses are resistant to the antiviral effects of interferon, and interferon administration to monkeys has failed either to increase survival rate or to reduce virus titer. Ribavirin does not affect filoviruses in vitro and thus is probably not of clinical value, in contrast to its efficacy against other viral hemorrhagic fevers. Isolation of patients is recommended, and protection of medical and nursing staff is required. Monkeys caught in the wild are an important source of the introduction of filoviruses. Quarantine of imported non-human primates and professional handling of animals will help to prevent introduction into humans.

Even though filoviral hemorrhagic fever outbreaks have been rare and were mainly restricted to a small number of cases, vaccines would be of value both for medical personnel in Africa and for laboratory personnel. Reports of cross-protection among different Ebola subtypes in experimental animal systems suggest a general value of vaccines. Inactivated vaccines have been developed by treatment with formalin or heat of cell culture-propagated Marburg ( Fig. 12-2 ) and Ebola ( Fig. 12-3 ) subtypes Sudan and Zaire. Protection, however, has only been achieved by carefully balancing the challenge dose and virulence. Because of the biohazardous nature of the agents, recombinant vaccines would be an attractive approach in the future.

Experimental drugs and vaccines are undergoing testing, but no definitive treatment for EBOV exists. The unprecedented 2014 Ebola epidemic has prompted an international response to accelerate the availability of a preventive vaccine. Two vaccine candidates, viz., chimpanzee adenovirus serotype 3 (ChAD3-ZEBOV) and recombinant vesicular stomatitis virus (rVSV-ZEBOV), are currently being tested in humans. Both vaccines have been shown to be safe and efficacious in animals.

There is no safe and effective vaccine widely available for human use against both viruses. The experimental Ebola drug called ‘ZMapp’ has been administered to a limited number of Ebola victims and the results are quite promising. Notably the drug ZMapp has proved to be a potent antiviral agent in monkeys. However, further clinical trials are yet to be conducted to demonstrate its safety and efficacy in humans.

History and Epidemiology

Although primarily a zoonosis, sporadic cases and outbreaks of CCHF affecting humans do occur. The disease is endemic in many countries in Africa, Europe, and Asia, and, during 2001, cases or outbreaks were recorded in Albania, Iran, Kosovo, Pakistan, and South Africa. CCHF was first described in the Crimea (Central Asia) in 1944 and given the name Crimean hemorrhagic fever. In 1969 it was recognized that the pathogen causing Crimean hemorrhagic fever was the same as that responsible for an illness identified in 1956 in the Congo, and linkage of the two place-names resulted in the current name for the disease and the virus. CCHF is a severe disease in humans, with a high mortality rate. Fortunately, human illness occurs infrequently, although animal infection may be more common. The CCHF virus may infect a wide range of domestic and wild animals. Many birds are resistant to infection, but ostriches are susceptible and may show a high prevalence of infection in endemic areas. Animals become infected with CCHF from the bite of infected ticks. The most efficient and common vectors for CCHF appear to be members of the Hyalomma genus. Humans who become infected with CCHF acquire the virus from direct contact with blood or other infected tissues from livestock, or they may become infected from a tick bite. The majority of cases have occurred in those involved with the livestock industry, such as agricultural workers, slaughterhouse workers, and veterinarians.

RVF virus was first isolated near Lake Naivasha in Kenya in 1931. Since then, the virus has been shown to be widespread in sub-Saharan Africa and Egypt. Major epidemics / epizootics occurred in Egypt in 1977 (200 000 human infections with 600 deaths) and 1993, in Mauritania in 1987 (200 human deaths), Madagascar in 1991, and in eastern Africa (89 000 infections and more than 500 deaths reported so far), with outbreaks in 1997–1998 in Kenya, Tanzania, and Somalia. In 2000, the Ministries of Health of Yemen and Saudi Arabia received reports of unexplained hemorrhagic fever in humans and associated animal deaths on the first confirmed occurrence of RVF outside Africa. More than 315 persons with suspected severe RVF have been reported from primary health-care centers and hospitals, and at least 66 (21%) patients have died with hemorrhagic manifestations. The epidemiology of RVF consists of both epizootic and interepizootic cycles. Epizootics of RVF in Africa have often occurred when unusually heavy rainfall was observed. During an epizootic, virus circulates among infected arthropod vectors and mammalian hosts, particularly cattle and sheep, which represent the most significant livestock amplifiers of RVF virus. The interepizootic survival of RVF virus is believed to depend on transovarial transmission of the virus in flood-water Aedes mosquitoes. Virus can persist in mosquito eggs until the next period of heavy rainfall, when they hatch and yield RVF virus-infected mosquitoes. Although RVF has a more circumscribed distribution than CCHF, the disease presents a greater potential of dissemination because of its more widespread vector. In two studies, field populations of Aedes canadensis , A. cantator , A. excrucians , A. sollicitans , A. taeniorhynchus , A. triseria tus, Anopheles bradleyi-crucians , Culex salinarius , C. tarsalis , and C. territans perorally exposed to 10 (6.2)–10 (7.2) plaque-forming units (PFU) of RVF virus readily became infected. Infection rates ranged from 51% (65 / 127) for C. salinarius to 96% (64 / 67) for Aedes canadensis . Disseminated infection rates were generally greater at 14 days than at 7 days after the infectious blood meal and, with the exception of Anopholes bradleyi-crucians , were not significantly different from the pooled rate of 59% for each species tested. For most species, about half of the mosquitoes with a disseminated infection transmitted an infectious dose of virus to hamsters. Although all species, with the exception of A. bradleyi-crucians , transmitted virus, Aedes canadensis , A. taeniorhynchus , and Culex tarsalis had the highest vector potential of the species tested. Following inoculation of approximately 10 (1.6) PFU of virus, 100% of the mosquitoes of each species became infected. For most species, transmission rates were similar for inoculated individuals and those that developed a disseminated infection following peroral infection. Viral titers of transmitting and non-transmitting disseminated individuals were similar for all species tested.

Clinical Features

The clinical presentation of both diseases is quite similar. The incubation period is about 6 days, with a documented maximum of 13 days. Onset of symptoms is sudden, with fever, myalgia, dizziness, neck pain and stiffness, backache, headache, sore eyes, and photophobia. There may be nausea, vomiting, and sore throat early on, which may be accompanied by diarrhea and generalized abdominal pain. Over the next few days, the patient may experience sharp mood swings, and may become confused and aggressive. After 2–4 days, the agitation may be replaced by sleepiness, depression, and lassitude, and the abdominal pain may localize to the right upper quadrant, with detectable hepatomegaly. Other clinical signs that emerge include tachycardia, lymphadenopathy, and a petechial rash both on internal mucosal surfaces, such as in the mouth and throat, and on the skin. The petechiae may give way to ecchymoses and other hemorrhagic phenomena such as melena, hematuria, epistaxis, and bleeding from the gums. There is usually evidence of hepatitis. The severely ill may develop hepatorenal and pulmonary failure after the fifth day of illness.

The mortality rate from CCHF is approximately 30%, with death occurring in the second week of illness. In those patients who recover, improvement generally begins on the 9th or 10th day after the onset of illness. RVF presents a more benign course, with severe eye disease (2%), meningoencephalitis, and hemorrhagic fever syndrome in 1% of patients.

Diagnosis and Treatment

Diagnosis of suspected CCHF and RVF is performed in especially equipped, high-biosafety-level laboratories. IgG and IgM antibodies may be detected in serum by ELISA from about day 6 of illness. IgM remains detectable for up to 4 months, and IgG levels decline but remain detectable for up to 5 years. General supportive therapy is the mainstay of patient management in both diseases. Intensive monitoring to guide volume and blood component replacement is required. Ribavirin has been used in the treatment of established CCHF infection, with apparent benefit. Both oral and intravenous formulations seem to be effective. The value of immune plasma from recovered patients for therapeutic purposes has not been demonstrated, although it has been employed on several occasions.

Persons living in endemic areas should use personal protective measures that include avoidance of areas where tick vectors and mosquitoes are located. When patients with CCHF are admitted to the hospital, there is a risk of nosocomial spread of infection. In the past, serious outbreaks have occurred in this way and it is imperative that adequate infection control measures be observed to prevent this outcome. Patients with suspected or confirmed CCHF should be isolated and cared for using a barrier nursing technique. Specimens of blood or tissues taken for diagnostic purposes should be collected and handled using universal precautions. Sharps (needles and other penetrating surgical instruments) and body wastes should be safely disposed of using appropriate decontamination procedures.

Epidemiology

Hantaviruses are endemic in most parts of the Old World, affecting some regions of Southeast Asia and southeast Europe that have a mild tropical climate ( Table 12-2 ). However, the distribution of the natural reservoir has a great impact on the geographic distribution of a disease that can reach remote areas of Scandinavia, western Russia, and even far-east Russia (see Table 12-2 ).

Hantaviruses are transmitted by aerosols of rodent excreta, saliva, and urine. The most common mode of transmission is inhalation of dust or dried particles that carry dried saliva or waste products of an infected rodent.

Pathogenesis and Etiology

Once infected, a rodent experiences a brief viremia that lasts 5–10 days. Following this stage, the viral antigens remain present in many major organs for weeks to months. In spite of the antibody presence in the rodent's serum, infectious virus is shed in the rodent's saliva, urine, and feces, possibly for the rest of its life. Mice appear to be most infectious 40 days after their infection with the virus. No arthropod vector has been established for hantaviruses.

In general, there are two seasonal peaks for almost all outbreaks of hantavirus diseases: a small one in spring, and a large one in fall. It is suspected that this corresponds to seasonal increases in the infection rate of the rodents, and with farming cycles, during which farmers are exposed to rodents in the fields during planting and harvest periods. Unusually high rainfall in dry parts of the country results in increased food sources for rodents, and subsequently increased rodent populations. Fall / winter outbreaks, such as those in Greece, correspond to the movement of rodents from the fields into artificial structures.