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Hemorrhagic Fevers and Diseases

Viral Types And Characteristics

Four main groups of viruses exist that cause hemorrhagic disease or fever: arenaviruses, filoviruses, bunyaviruses, and flaviviruses. Arenaviruses cause Argentine hemorrhagic fever, Bolivian hemorrhagic fever, Sabia-associated hemorrhagic fever, Lymphocytic choriomeningitis, Venezuelan hemorrhagic fever, and Lassa fever. Members of the filovirus group cause Ebola hemorrhagic fever and Marburg hemorrhagic fever. Bunyaviruses cause Crimean-Congo hemorrhagic fever, Rift Valley fever, and Hantavirus pulmonary syndrome. Lastly, Flaviviruses cause tick-borne encephalitis, yellow fever, Dengue hemorrhagic fever, Kyasanur Forest disease, and Omsk hemorrhagic fever.

These viruses differ in structure and in the severity of the symptoms they can cause. They all, however, share common features. All hemorrhagic viruses contain ribonucleic acid (RNA) as their genetic material. The RNA is protected and confined in a membrane called the viral envelope. The envelope is typically made of lipid. Another feature of hemorrhagic viruses, and indeed of all viruses, is the requirement for a host in which to live and produce new viral particles. Hemorrhagic viruses can live in some non-human mammals, such as primates, and in insects. The primates and insects are described as being natural reservoirs of the particular virus. Humans are not a natural reservoir. Epidemiologists (disease trackers) suspect that initial infections of humans occurs only accidentally when humans and the primate or insect come into close contact.

In contrast to the reservoir host, the presence of the hemorrhagic virus in humans typically produces a devastating illness. The symptoms can progress from mild to catastrophic very rapidly (i.e., in only hours). While catastrophic for the victims and difficult to treat, the rapid nature of the outbreaks has an advantage. Because victims succumb quickly, the transmission of the virus from human to human is limited. An outbreak can appear, ravage a local population, and fade away within days or a few weeks.

The viruses that cause the various hemorrhagic fevers and diseases do not survive in the host following the disease (the Human Immunodeficiency Virus, in contrast, is able be latent in the host, and survive for prolonged periods of time before symptoms of infection appear). However, people who are recovering from infections caused by Hantavirus and Argentine hemorrhagic fever can excrete infectious viruses in their urine.

The sporadic appearance of hemorrhagic outbreaks and the fact that they often occur in geographically isolated regions (e.g., interior of Africa) has made the study of the diseases difficult. It is known that there is not any timetable to the appearance of a hemorrhagic fever, such as in one season of the year relative to another season. The only factor that is known clearly is that the viruses are passed from the natural host to humans. How this transfer occurs and why it occurs sporadically are not known.

The viruses do not damage their primate or insect hosts as much as they do a human who acquires the microorganisms. The reasons for this difference are unknown. Researchers are attempting to discover the basis of the natural resistance, as this would help in finding an effective treatment for human hemorrhagic diseases.

The speed at which hemorrhagic fevers appear and end in human populations, combined with their frequent occurrence in relatively isolated areas of the globe has made detailed study difficult. Even though some of the diseases, such as Argentine hemorrhagic fever, have been known for almost 50 years, knowledge of the molecular basis of the disease is lacking. For example, while it is apparent that some hemorrhagic viruses can be transmitted through the air as aerosols, the pathway of infection once the microorganism has been inhaled is still largely unknown.

Hemorrhagic diseases are zoonotic diseases; ones that occur by the transfer of the disease causing agent from a non-human to a human. For some of the hemorrhagic viruses, the reservoir host is known. They include the cotton rat, deer mouse, house mouse, arthropod ticks, and mosquitoes. However, for viruses such as the Ebola and Marburg viruses, the natural host still is not known. Outbreaks with these two viruses have involved transfer of the virus to human via primates. Whether the primate is the natural reservoir host, or whether primates acquire the virus as the result of contact with the true natural reservoir host, is yet another aspect of hemorrhagic diseases that is not clear.

As mentioned, hemorrhagic fevers can rapidly spread through a human population. This is due to human-to-human transmission. This transmission occurs easily, often via body fluids that accidentally contact a person who is caring for the afflicted person. Funeral practices of handling and washing the bodies of the deceased have contributed to human-to-human transmission of Ebola during outbreaks in Sub-Saharan Africa.

Hemorrhagic diseases typically begin with a fever, a feeling of tiredness, and a generalized aching of muscles. In rare instances, symptoms may not progress any further, in which case recovery is rapid. For unknown reasons, however, more serious damage often occurs. Here, symptoms include copious bleeding from the mouth, eyes, and ears. Internal bleeding also occurs, as organs are attacked and destroyed by the infection. Death is typically the result of the overwhelming damage to the organs, and from the failure of the nervous system. Often, victims have seizures and lapse into a coma prior to death.

Hemorrhagic diseases are difficult to treat. One reason is because of the rapid progression of the disease. Another reason is because vaccines exist for only a few of the diseases (i.e., yellow fever and Argentine hemorrhagic fever). For the remaining diseases, supportive care such as keeping the infected person hydrated is often the only course of action.

To prevent outbreaks, the most effective policy is to curb human interaction with the natural reservoir of the microbe. For example, in the case of hantavirus pulmonary syndrome, scientists discovered in the 1990s that the responsible virus was resident in rodent populations, and that these populations exploded in numbers after rainy periods. Thus, limiting contact with places where the rodents live (i.e., barns), particularly after a rainy period, is a wise practice. Insect vectors are controlled by a spraying and common sense steps, such as use of insect repellent, proper clothing, insect netting over sleeping areas.

Hemorrhagic fevers are significant, not only because of the human suffering they cause, but because the viral agents could be exploited as bioweapons. For these reasons, a great deal of research effort is devoted towards understanding the origins and behaviors of the viruses.

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Science EncyclopediaScience & Philosophy: Habit memory: to HeterodontHemorrhagic Fevers and Diseases - Viral Types And Characteristics, Ebola And Other Hemorrhagic Diseases, Vaccine And Treatment