Hemorrhagic Fever Diseases
A number of agents that infect the blood and lymphatics cause extreme fevers, some of which are accompanied by internal hemorrhaging. The diseases are grouped into the category of “hemorrhagic fevers” and are covered in this section. The following section deals with diseases in which the main symptom is fever—without the hemorrhagic part. All hemorrhagic fever diseases described here are caused by viruses in one of three families: Arenaviridae, Filoviridae, and Flaviviridae. Bunyaviridae is a fourth family with members that cause hemorrhagic fevers, but we do not discuss examples of these here. All of these viruses are RNA enveloped viruses, the distribution of which is restricted to their natural host’s distribution.
- Yellow Fever : This disease is caused by an arbovirus, a single-stranded RNA flavivirus that is generally called the yellow fever virus. It currently occurs only in parts of Africa and South America. Two patterns of transmission are seen in nature. One is an urban cycle between humans and the mosquito Aedes aegypti, which reproduces in standing water in cities. The other is a sylvan (forest) cycle, maintained between forest monkeys and mosquitoes. The presence of the virus in the bloodstream causes capillary fragility and disrupts the blood-clotting system, which can lead to localized bleeding and shock. Infection begins acutely with fever, headache, and muscle pain. In some patients, the disease progresses to oral hemorrhage, nosebleed, vomiting, jaundice, and liver and kidney damage with significant mortality rates. Most cases occur during the rainy season.
- Dengue Fever: Dengue fever is caused by a single-stranded RNA flavivirus that is also carried by Aedes mosquitoes. Although mild infection is the usual pattern, a form called dengue hemorrhagic shock syndrome can be lethal. Dengue fever is also called “breakbone fever” because of the severe pain it induces in muscles and joints (it does not actually cause fractures). The illness is endemic to Southeast Asia and India, and several epidemics have occurred in South America and Central America, the Caribbean, and Mexico. The Pan American Health Organization has reported an ongoing epidemic of dengue fever in the Americas that increased from 390,000 cases in 1984 to more than 1 million cases in 2008. In Mexico, cases have increased 600% since 2001. Researchers in Thailand, where dengue fever is one of the leading causes of child mortality, have developed a live attenuated vaccine, which is being tested in clinical trials. A low-tech approach has led to big successes in Vietnam. There, health officials urged local citizens to round up tiny crustaceans that are common in natural water sources and to put them in water tanks and wells. The crustaceans, which are not harmful to humans, eat the mosquitoes that carry dengue. Officials reported a complete elimination of the disease in communities where the strategy was used.
- Chikungunya: The Chikungunya virus was discovered in 1955 and has caused sporadic outbreaks of disease since then. The name comes from an African phrase meaning “that which bends up,” a reference to the arthritic stance people infected with this virus often assume. It is an alphavirus that is transmitted by Aedes mosquitoes, just like dengue fever. Symptoms are similar to dengue fever with the additional complication of severe joint pain, sometimes lasting for years. There is growing concern about this virus, since it has established itself in mosquitoes in Western Europe—the first time one of these hemorrhagic viruses has done so.
- Ebola and Marburg: Unlike the two viruses causing yellow fever and dengue fever, the Ebola and Marburg viruses are filoviruses (Family Filoviridae). The two viruses are related and cause similar symptoms, although Ebola has received the greatest share of media attention. Its gruesome symptoms are extreme manifestations of the same kind of hemorrhagic events described for yellow fever and dengue fever. The virus in the bloodstream leads to extensive capillary fragility and disruption of clotting. Patients bleed from their orifices, even from their mucous membranes, and experience massive internal and external hemorrhage. Very often they manifest a rash on their trunk in early stages of the disease. The mortality rate is between 25% and 100%, and there is no effective treatment. It is not known how humans acquire these viruses. They are both indigenous to Africa. In August of 2007 researchers found the virus in a cave-dwelling fruit bat. It is thought that bats are the natural reservoir of these viruses. Direct contact with an infected person or with their body fluids will transmit the virus. Hospital workers caring for Ebola patients are at high risk of becoming infected. Two major outbreaks of Ebola occurred in Kikwit, Zaire, in 1995 and in Gulu, Uganda, in 2000. Outbreaks with Marburg virus are also rare, but individuals have been infected sporadically since it was first recognized in 1967. In 2005, the largest Marburg outbreak in history occurred in and around a hospital in Angola. Sixty-three people died during the 5-month outbreak. Symptoms are similar to Ebola virus infection. There is no treatment and no vaccine for Ebola or Marburg, though some promising research is being conducted.
- Lassa Fever: The Lassa fever virus is an arenavirus. Several related arenaviruses cause the diseases Argentine hemorrhagic fever, Bolivian hemorrhagic fever, and lymphocytic choriomeningitis (an infection of the brain and meninges). Lassa fever virus is found in West Africa. In most cases infection with this virus is asymptomatic, but in 20% of the cases a severe hemorrhagic syndrome develops. The syndrome includes chest pain, hemorrhaging, sore throat, back pain, vomiting, diarrhea, and sometimes encephalitis. Patients who recover suffer from deafness at a significant rate. The reservoir of the virus is a rodent found in Africa called the multimammate rat. It is spread to humans through aerosolization of rat droppings, urine, hair, and so forth. Eating food contaminated by rat excretions also transmits the virus. Infected persons can spread it to other people through their own secretions. Vertical transmission also occurs, and the disease leads to spontaneous abortions in 95% of infected pregnant women. This hemorrhagic fever has been shown to respond to the antiviral agent ribavirin, especially if administered in the early stages of infection. There is no vaccine.