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Causes
By Mayo Clinic staffMost of the viruses associated with VHFs are zoonotic, which means they reside in an animal or insect host and are dependent on that host for their survival. In general, humans acquire the virus when they come into contact with an infected host, but in some cases, person-to-person transmission can occur after the initial infection. Human epidemics occur sporadically, usually because of a combination of climatic, ecological and social factors.
Arenaviruses
Lassa fever, a common arenavirus, is endemic in West Africa, where as many as 500,000 cases occur each year. It may spread in the following ways:
- Wild rodents. The multimammate rat is the main source of Lassa virus infection. You can acquire the virus from direct contact with infected rat droppings or urine, from touching surfaces or objects or eating food contaminated with these excretions, or possibly by inhaling airborne particles that contain the virus (airborne or aerosol transmission).
- Person-to-person contact. You can contract the virus if you come in contact with the blood, throat secretions or urine of an infected person, especially during the acute fever stage of the illness.
- Sexual contact. Because the virus can be transmitted in semen long after infection, experts recommend that men who have recovered from Lassa fever refrain from sexual activity for at least three months.
- Contaminated needles and syringes. This form of transmission is most likely to occur through an accidental needle stick or in a hospital in a developing nation where equipment may be used on more than one person.
Argentine, Bolivian, Brazilian and Venezuelan hemorrhagic fevers are less pervasive arenavirus illnesses, occurring only in South America. These particular arenavirus illnesses are carried by rodents.
Bunyaviruses
The diseases in this group spread by contact with an infected insect or animal:
- Mosquitoes. Rift Valley fever primarily affects animals, especially sheep, cattle, camels and goats, but it can also cause severe disease in humans. It's spread by a number of mosquito species, some of which transmit the virus to their offspring. Noninfected mosquitoes acquire the virus when they feed on sick animals and then pass it on to their next victim, perpetuating the cycle of infection. Humans contract the virus through a mosquito bite or through contact with the blood, organs, body fluids and possibly the raw milk of infected animals.
- Ticks. Crimean-Congo hemorrhagic fever, a severe VHF that's often fatal, is endemic in many countries in Africa, Europe and Asia. It's spread by ticks, some of which transmit the virus to their offspring via eggs. More often, ticks acquire the virus when they feed on infected animals, especially cattle, sheep and goats. Humans may become infected from a tick bite or from direct contact with the blood or tissues of sick livestock, especially during the first week after the animal has been exposed to the virus.
- Rodents. The hantaviruses that are responsible for hantavirus pulmonary syndrome are typically spread by rodents and occur in the United States, Canada and South America. In most cases, you contract the virus when you breathe the aerosolized droppings of infected rodents. The risk is especially high for farm workers, campers and other people who spend time outdoors in endemic areas, but you can also be exposed to the virus when rodents invade your home during cold weather or nest near your house.
Filoviruses
The natural reservoir of these diseases isn't known, but once a person is infected, the virus can be transmitted in several ways:
- Person-to-person contact. You can become infected if you're exposed to the blood, body fluids or organs of someone who's severely ill with Ebola or Marburg virus. Most at risk are health care workers and family members caring for a sick relative.
- Contaminated needles and syringes. This form of transmission is most likely to occur in a hospital when equipment is reused or through an accidental needle stick.
- Sexual contact. The Ebola virus and Marburg virus can be transmitted through semen, even when the affected person is no longer sick. For that reason, experts recommend refraining from sexual activity for at least three months after recovery.
- Contact with nonhuman primates. In a few instances, researchers have contracted Marburg virus after handling monkeys infected with the disease.
Flaviviruses
In most cases, insects are responsible for the spread of this family of viruses:
- Mosquitoes. Yellow fever and dengue hemorrhagic fever are virulent forms of flaviviruses, and both are spread by a particularly efficient species of mosquito: Aedes aegypti. The mosquitoes thrive in and near human habitations where they breed in even the cleanest water. Although most cases of yellow fever and dengue fever occur in South America, Asia and parts of Africa, isolated cases have been reported in the United States. The Aedes aegypti mosquito is now found in a large portion of the United States, and in some Caribbean and European countries, increasing the possibility of epidemics in these areas.
- Ticks. Kyasanur Forest disease and Omsk hemorrhagic fever are far more obscure and isolated illnesses — Omsk hemorrhagic fever, for example, occurs only in parts of western Siberia where it mainly affects muskrat trappers. Although ticks spread both diseases, you can also contract Omsk hemorrhagic fever by handling infected muskrats or by drinking water that contains the virus.
