Hantavirus Essay, Research Paper

Hantavirus: A Four Corners Study

When a new virus appeared in the Four Corners region, American scientists were stumped. What was causing such a quick death to such healthy people? Was there a potential epidemic on their hands? No one knew, and when they finally determined that a strain hantavirus was involved, many were shocked. This had to be something completely new to the hantavirus family and that was somewhat overwhelming. Normally, the hantavirus only affects the kidneys, but this new virus dealt with the upper respiratory area. This paper takes an in depth look at the history of the hantavirus as well as its infectious nature and replication process.

Hantaviruses first came into existence through human eyes during the Korean War. Over 2000 U.S. soldiers were affected with this unknown virus that was quickly found to be carried through field mice. The natural territory of this virus included parts of Japan, Korea, northeastern China, and southeastern and central Russia. Between 1955 and 1977, this virus caused many more infections along with fatalities. Throughout the 1970s, eleven other strains of hantaviruses were found in Korea and Eurasia. In 1976, Hantaan virus was isolated from the Apodemus agreavius coreae mouse. Using the microscope, one could see the round microbes that were stacked in rows along the epithelial lining of the lungs (CDC website 6). Each infection still involved mild kidney infections. The same type of rodent always carried the virus, and people came in contact with the microbes through skin exposure or inhalation of infected animal feces or urine. Research continued on this virus and in 1981, it was first cultured in human cells.

Many people were worried that since the virus was so prevalent in Korea that it possibly could be spread easily from boat to new land through trade. When rats were tested in various harbors throughout the United States, the Seoul virus, a form of hantavirus, was found. In 1982, a study in Baltimore tested local rats and found that every rat over the age of two had the virus in their system (Garrett 539). Then individuals from two STD clinical-hospitals were tested and four cases of the Korean Hantavirus were found. The researchers took it one step further by testing the blood of humans that were undergoing proteinuria blood chemistry analysis as well as kidney dialysis patients. Amazingly they discovered that 6.5% of the patients on dialysis had serum that reacted with Seoul hantavirus antibodies indicating that there had been an infection at some point.

Cases of hantavirus continued to pop up throughout the United States. A Mexican immigrant died of internal hemorrhaging and kidney failure, and the disease was tied to a new strain of hanta: the Leaky virus. Not only was the disease infecting southwestern U.S., but also deep in America’s inner cities. With the increase of rats in the cities, that increased the likelihood of the hantavirus infecting people.

But on May 14, 1993 there was something new that was infecting Navajo Indians in the Four Corners Area. This disease struck people quickly and it left them behind with lungs so severely fluid-filled that they weighed twice as much as normal (Garrett 529). Symptoms generally started similar to the flu: fever, muscle aches, and headaches. After a few hours or up to two days, the symptoms escalated to coughing and irritation in the lungs. Then within a few hours, the patients would become so hypoxic that they would be unable to absorb oxygen. Slowly, the heart would stop and death would follow either caused by cardiac failure or pulmonary edema. By May 14th, there was a list of five healthy people who had died from this disease. When the lab work returned it showed no signs of plague bacteria in the victim’s blood. Worried, the head scientist pushed people to look through the medical records of all the recent unexplained respiratory deaths. The scientists determined that a disease known as acute respiratory distress syndrome (ARDS) was infecting these people. Normally, this infected the elderly and burn victims, but for the five individuals in this area there was no connection.

When searching for a possible chemical that might cause these symptoms, Cheek came across something known as phosgene. Initially the Germans used it during World War I and it was known that this chemical could cause symptoms of ARDS. This chemical had been banned from the United States. However, a sister compound known as phosphene had been used legally to kill prairie dogs. With the increase in the number of prairie dogs that winter and spring, Cheek thought they had found their solution. But when he searched for this chemical in trailers used to store equipment for extermination throughout the area he found nothing.

After the local doctors and hospitals were notified of these strange occurrences, more cases surfaced. Of the 19 people with similar symptoms, 12 had already died. Working with the CDC, a group of scientists came up with a list of possible explanations: an unknown chemical toxin, a new virulent flu strain, a new coxiella bacterium, anthrax, Crimean-Congo hemorrhagic fever virus, Hantaan virus, or something completely new. When the lab obtained samples from the people from Four Corners, it was suggested that the samples be tested against antibodies for every virus that was available. Finally, on June 3, antibodies against a family of viruses called hantaviruses, cross-reacted in test tubes with blood from the patients. And then patient blood injected into mice from the compound showed an even stronger reaction to the hantavirus reagents. That helped to prove that the virus could reproduce and multiply in mice.

However, there was some doubt that hantavirus was the culprit. Never had a strain of hantavirus ever caused respiratory infections. Usually it was only involved with kidney disease. There was one undeniable factor that was intriguing the scientists: it was noted that during this research and the year previous there had been a 10-fold increase in the number of deer mice in the area. Because people were unconvinced that hantavirus was the culprit, traps were set up that could handle larger animals such as raccoons as well as smaller animals like mice and prairie dogs. All the traps contained a variety of animals, but by far one particular mouse, Peromyscus maniculatus, was found the most. It is a brown, big-eared mouse with a white belly and tail and huge, black eyes deep in the skull. When these mice were tested, it was determined that they carried a strain of hantavirus. And by the middle of June it was evident that the Four Corners strain of hantavirus was one previously undiscovered.

Soon fears of an epidemic came into the spotlight. Other cases of ARDS caused by hantavirus were found in East Texas, Nevada, northern California, Oregon, and then Louisiana and Mississippi. While the initial infections did match the Four Corners strain, the case in Louisiana was unique. The particular deer mouse was not a native inhabitant in Louisiana, and the strain in the patient did not match the Four Corners strain.

Surges of Hantavirus infections were not only occurring in the United States. Outbreaks were occurring in Germany as well as France, Belgium, and the Netherlands. Some believed that the decreased efforts to keep rat and mouse populations low may have been the reason behind the increased infections.

Early in 1994, the CDC reported that there had been a total of fifty-five ARDS cases in sixteen different states. Thirty-two of the infected people died. And also in 1994, the new microbe was officially named Muerto Canyon, after a valley located inside the Navajo nation where the Four Corners virus fist appeared.

Hantavirus is able to induce two different infections in a person’s body. Hemorrhagic fever with renal syndrome (HFRS) is responsible for the infections caused by the Hantaan, Seoul, Puumala, and Prospect Hill strains. HFRS is a generalized infection with fever, hemorrhages, and acute renal insufficiency. Approximately 150,000 to 200,000 cases of HFRS involving hospitalization are reported each year throughout the world, with more than half in China. Death rates vary from 0.1% for HFRS caused by Puumala to 5-10% from HFRS involved with HTN viruses (CDC 2). The clinical course of HFRS follows five overlapping stages: febrile, hypotensive, oliguric, diuretic, and convalescent. The course of action usually starts with a severe headache, backache, fever, and chills. When the febrile stage ends, hypotension can develop and last for hours up to days. This is where nausea and vomiting are most common. One-third of deaths occurs during this phase caused by vascular leakage and acute shock. About one-half of the deaths occur in the oliguric stage because of hypervolemia. Those that survive those stages may progress to the diuretic phase and show improved renal function, but may still die of shock or pulmonary complications. The last stage can last for weeks to months before recovery is complete.

Luckily, there is a somewhat effective treatment for HFRS. Unfortunately, it is only effective if used early in the course of the disease. The treatment known as Ribavirin is a broad-spectrum antiviral drug. It is effective against RNA viruses in vitro and in vivo. This treatment has shown a reduced mortality when administered early enough (Care 1). However, the treatment is not effective when the disease is advanced. There is one adverse side effect of the treatment; a person can possibly develop of anemia. This side effect is reversible once a person stops taking the drugs. Scientists do not believe that this treatment is successful for the other type of infection caused by hantavirus: hantavirus pulmonary syndrome (HPS).

Hantavirus pulmonary syndrome (HPS) was first detected in the United States in 1993. Infection through HPS occurs at a much faster rate. Following a three-day prodromal period, patients quickly developed interstitial pulmonary edema and respiratory failure. The main histopathological features were seen primarily in the lung and spleen. The noncardiogenic pulmonary edema, which is associated with high mortality rate, has been attributed to an increased permeability of the pulmonary capillaries (Bhide 1).

The single most common feature of this illness was a fever that occurred in 98% of the people infected in the Four Corners area. In addition, myalgia, vomiting, weakness/fatigue, coughing, diarrhea, nausea, shortness of breath, and headaches also occurred in patients. The mortality rate of HPS is much higher than HFRS at 50% of people effected dying. Since the symptoms were so general, it is often easy to initially think that something else is the cause of the problems.

In HPS, capillary leakage was largely localized in the lungs, rather than in the retroperitoneal space, and the kidneys are largely unaffected. The Sin Nombre (SN) virus has caused most of the cases in the United States. In HPS, death occurs from shock and cardiac complications, even with adequate tissue oxygenation.

Although there appears to be significant difference between the two types of infection, there are points when the two intermingle. For example, the Bayov virus and the Black Creek Canal virus, both cases of HPS can exhibit renal involvement similar to HFRS. And there have been several cases of HFRS that had pulmonary manifestations. Although there are many differences between the two, they do have something in common: pathogenetic features. Both have capillary injury that can lead to hemorrhages and shock in HFRS or pulmonary edema and suffocation in HPS (Plyusnin 2677).

Hantaviruses belong to the Bunyavirus family. In this family there are five genera: Bunyavirus, Phlebovirus, Nairovirus, Tospovirus, and Hantavirus. Unlike other members of Bunyavirus, which are arthropod-borne, the hantavirus is rodent-borne. Hantaviruses are negative-stranded RNA viruses with a tripartite genome that consists of a large (L), medium (M), and small (S) segment (Rodriguez 99). They are spherical and enveloped. The L protein acts as a replicase, transcriptase, and endonuclease. The M and S code for two surface glycoproteins G1 and G2 and a nucleocapsid protein (N) respectively. The envelope glycoproteins are thought to be the major elements involved in induction of immunity to hantaviruses. Genomic RNA of hantavirus shows relatively few differences in length between the L and M segments. However, the S segment varies in length mainly in its 3′ noncoding region. The noncoding region specific to the S segment is made up of numerous imprecise repeats.

Both the 3′ and 5′ ends of the hantavirus RNA are highly conserved and complementary and therefore are capable of forming panhandle structures. The panhandles are about 17 bp long. In most types of hantaviruses 14 out of the 17 bases are identical in all three genome segments and the complementarity of the termini is incomplete, with a mismatch at position 9 and a pairing of U-G at position 10 (Kukkonen 2615). It is thought that the panhandle-like structures serve a role in the regulation of viral transcription and replication. The ends of the genome contain three nucleotide repeats which are suggested to be involved in the proposed prime-and-realign mechanism of replication resulting in 5′ ends containing only monophosphate.

The S segment 3′ NCR represents the most puzzling aspect of the hantavirus genome. Within a certain hantavirus type the length and sequence of this region does not change thereby leading to the conclusion that the S segment has a functional role. But between different hantavirus types, the 3′ NCR varies widely in length and in sequence. Some believe that the 3′ NCR participates in something like packaging, then there are two possible explanations for the differences. First, there may be a molecular mechanism operating that differ from host to host or the secondary structure of the 3′ NCR is crucial for its proper activity.

The entire genomic sequence of SNV (the Four Corners disease) has been determined by using RNA extracted from autopsy material as well as RNA extracted from cell culture-adapted virus. The L RNA is 6562 nucleotides in length; the M RNA is 3696 nucleotides, and the S RNA is 2059 nucleotides in length.

Replication of the hantavirus occurs in the host cell cytoplasm. Attachment is mediated by an interaction of either G1 or G2 or both integral viral envelopes. In the entry and coating phase, there is endocytosis of virions and a pH-dependent (acidic) fusion of viral membranes, release of three (S, M, and L) RNA segments and proteins in the cell cytoplasm (Bhide 4). Primary transcription occurs with the negative sense vRNA to virus-complement mRNA from the genome templates using host cell derived capped primers and viron-associated polymerase. The association of the L protein with the three nucleocapsid species initiates transcription of viral genes. A Negative-sense genome serves as the template for the L protein to produce positive-strand genome and mRNA. The viral L protein is also thought to have an endonuclease activity that cleaves cellular messenger RNAs for the production of capped primers used to initiate transcription of viral mRNAs. As a result of this “cap snatching,” the mRNAs of hantaviruses are believed to be capped and contain non-templated 5″ terminal extensions.

Translation follows where it is thought that the viral L and S segments are translated at free ribosomes while the M mRNA is translated in the endoplasmic reticulum. Synthesis of cDNA must take place in order to serve as the template for genomic RNA. Switching between transcription and replication may depend on the presence of newly synthesized nucleocapsid protein. The secondary transcription amplifies the synthesis of mRNA species. There is continual translation and replication taking place. In the appendix is a diagram of the coding strategies of the RNA segments. At the morphogenesis stage, an accumulation of G1 and G2 form heterodimers and are transmitted from the endoplasmic reticulum to the golgi complex, where glycosylation is completed. Hantavirions are believed to form by association of nucleocapsids embedded in the membranes of the Golgi,and generally budding goes into the golgi cisternae. Lastly, nascent virions are then transported in secretory vesicles to the plasma membrane and released by exocytosis.

There has been much new research involving hantaviruses. Although there are approximately 30 different sero/genotypes known, only four have been completely sequenced: Hantaan, Puulama, Pirparinen, and Sin Nombre. One article dealt with the mechanisms of completing a gene sequence and how closely related the strains are to each other. The L protein is the longest and is therefore the most difficult to sequence. However, the sequences of the L and termini are essential for establishing a reverse-genetic system. Previously the S and M were sequenced apart from the 5′ and 3′ termini.

The L protein of the strain TUL is the most closely related with the L proteins of two strains of Puulama virus, followed by the L proteins of the Sin Nombre, Hantaan, and Seoul viruses. Close to the carboxyl terminus of the L protein, there is an acidic region of 38 amino acids which have been also described for the L proteins of Puulama virus and Sin Nombre virus.

Completion of the TUL L sequence allowed comparison of the genome segments and proteins of 6 completely sequenced hantaviruses. All the sizes were similar with the exception of the S segment in which the size of the 3′ noncoding varies. This conforms with the established view that hantaviruses have been co-evolving with their natural hosts. The Tula hantavirus was the first hantavirus to be sequenced that is not linked to any human disease.

Hantaviruses are viruses that are able to diverge from their normal sequence. When the outbreak of 1993 occurred in Southwestern United States, no one had a clue about what was causing the death of these people. When hantavirus was found to be the cause of the illnesses, it was hard to believe. But it was true. A mutation had occurred that had caused the hantavirus to change its entire mechanism of infection. From the research of the article above, it becomes apparent that hantaviruses are constantly evolving from each other.

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