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Ebola and Other Viral Hemorrhagic Fevers
Viral hemorrhagic fevers are a loosely defined group of clinical syndromes in which hemorrhagic manifestations are either common or especially notable in severe illness. Both the etiologic agents and clinical features of the syndromes differ, but coagulopathy may be a common pathogenetic feature.
Etiology
Six of the viral hemorrhagic fevers are caused by arthropod-borne viruses (arboviruses) ( Table 297.1 ). Four are caused by togaviruses of the family Flaviviridae: Kyasanur Forest disease, Omsk hemorrhagic fever, dengue (see Chapter 295 ), and yellow fever (see Chapter 296 ) viruses. Three are caused by viruses of the family Bunyaviridae: Congo fever, Hantaan fever, and Rift Valley fever (RVF) viruses. Four are caused by viruses of the family Arenaviridae: Junin fever, Machupo fever, Guanarito fever, and Lassa fever. Two are caused by viruses in the family Filoviridae : Ebola virus and Marburg virus, enveloped, filamentous RNA viruses that are sometimes branched, unlike any other known virus.
Table 297.1
Viral Hemorrhagic Fevers
| MODE OF TRANSMISSION | DISEASE | VIRUS |
|---|---|---|
| Tick-borne | Crimean-Congo hemorrhagic fever (HF) * | Congo |
| Kyasanur Forest disease | Kyasanur Forest disease | |
| Omsk HF | Omsk | |
| Mosquito-borne † | Dengue HF | Dengue (4 types) |
| Rift Valley fever | Rift Valley fever | |
| Yellow fever | Yellow fever | |
| Infected animals or materials to humans | Argentine HF | Junin |
| Bolivian HF | Machupo | |
| Lassa fever * | Lassa | |
| Marburg disease * | Marburg | |
| Ebola HF * | Ebola | |
| HF with renal syndrome | Hantaan |
* Patients may be contagious; nosocomial infections are common.
† Chikungunya virus is associated infrequently with petechiae and epistaxis. Severe hemorrhagic manifestations have been reported in some cases.
Epidemiology
With some exceptions, the viruses causing viral hemorrhagic fevers are transmitted to humans via a nonhuman entity. The specific ecosystem required for viral survival determines the geographic distribution of disease. Although it is commonly thought that all viral hemorrhagic fevers are arthropod borne, seven may be contracted from environmental contamination caused by animals or animal cells or from infected humans (see Table 297.1 ). Laboratory and hospital infections have occurred with many of these agents. Lassa fever and Argentine and Bolivian hemorrhagic fevers are reportedly milder in children than in adults.
Crimean-Congo Hemorrhagic Fever
Sporadic human infection with Crimean-Congo hemorrhagic fever in Africa provided the original virus isolation. Natural foci are recognized in Bulgaria, western Crimea, and the Rostov-on-Don and Astrakhan regions; disease occurs in Central Asia from Kazakhstan to Pakistan. Index cases were followed by nosocomial transmission in Pakistan and Afghanistan in 1976, in the Arabian Peninsula in 1983, and in South Africa in 1984. In the Russian Federation, the vectors are ticks of the species Hyalomma marginatum and Hyalomma anatolicum, which, along with hares and birds, may serve as viral reservoirs. Disease occurs from June to September, largely among farmers and dairy workers.
Kyasanur Forest Disease
Human cases of Kyasanur Forest disease occur chiefly in adults in an area of Mysore State, India. The main vectors are two Ixodidae ticks, Haemaphysalis turturis and Haemaphysalis spinigera. Monkeys and forest rodents may be amplifying hosts. Laboratory infections are common.
Omsk Hemorrhagic Fever
Omsk hemorrhagic fever occurs throughout south-central Russia and northern Romania. Vectors may include Dermacentor pictus and Dermacentor marginatus, but direct transmission from moles and muskrats to humans seems well established. Human disease occurs in a spring–summer–autumn pattern, paralleling the activity of vectors. This infection occurs most frequently in persons with outdoor occupational exposure. Laboratory infections are common.
Rift Valley Fever
The virus causing RVF is responsible for epizootics involving sheep, cattle, buffalo, certain antelopes, and rodents in North, Central, East, and South Africa. The virus is transmitted to domestic animals by Culex theileri and several Aedes species. Mosquitoes may serve as reservoirs by transovarial transmission. An epizootic in Egypt in 1977-1978 was accompanied by thousands of human infections, principally among veterinarians, farmers, and farm laborers. Smaller outbreaks occurred in Senegal in 1987, Madagascar in 1990, and Saudi Arabia and Yemen in 2000-2001. Humans are most often infected during the slaughter or skinning of sick or dead animals. Laboratory infection is common.
Argentine Hemorrhagic Fever
Before the introduction of vaccine, hundreds to thousands of cases of Argentine hemorrhagic fever occurred annually from April through July in the maize-producing area northwest of Buenos Aires that reaches to the eastern margin of the Province of Cordoba. Junin virus has been isolated from the rodents Mus musculus, Akodon arenicola, and Calomys laucha. It infects migrant laborers who harvest the maize and who inhabit rodent-contaminated shelters.
Bolivian Hemorrhagic Fever
The recognized endemic area of Bolivian hemorrhagic fever consists of the sparsely populated province of Beni in Amazonian Bolivia. Sporadic cases occur in farm families who raise maize, rice, yucca, and beans. In the town of San Joaquin, a disturbance in the domestic rodent ecosystem may have led to an outbreak of household infection caused by Machupo virus transmitted by chronically infected Calomys callosus, ordinarily a field rodent. Mortality rates are high in young children.
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