What is Rift Valley Fever?
The Rift Valley fever (Febris Rift-Vallée-lat; Fievre de la valee du Rift-French) is a viral zoonosis that affects mainly animals, but also has the ability to infect humans. Infection can lead to severe illness in both animals and people with high morbidity and mortality rates. The disease also leads to significant economic losses due to death and miscarriages among fever-infected Rift Valley cattle.
Causes of Rift Valley Fever
The Rift Valley fever virus belongs to the genus of phlebovirus, one of the five genera of the Bunyavirus family, the ecological group of arboviruses. The virus was first identified in 1931 during an investigation of an epidemic among sheep on a farm in Rift Valley, Kenya. Since then, epidemics have been recorded in sub-Saharan Africa and North Africa. In 1997–1998, a large outbreak occurred in Kenya, Somalia and Tanzania, and in September 2000, Rift Valley fever was confirmed in Saudi Arabia and Yemen. It was the first recorded occurrence of the disease outside the African continent, which raised concerns about its possible spread to other parts of Asia and Europe.
The virus was found in mosquitoes Culex pipiens, Eretmapodites chrysogaster, Aedes cabbalus, Aedes circurnluteolus, Culex theiler L. It is possible that the disease was brought to Egypt by the mosquitoes Culex pipiens. Although antibodies to the virus are found in wild field rats in Uganda, the reservoir of the disease is unknown. Presumably, the virus may exist due to transovarial transmission among the Aedes keel mosquitoes. Cases of laboratory infection through the respiratory tract are described.
Pathogenesis during the Rift Valley Fever
Pathogenesis is associated with hematogenous dissemination of the pathogen into the central nervous system (brain, organ of vision) and internal organs (liver damage). Characterized by vasculitis and impaired blood coagulation.
Transfer to people
- The vast majority of human infections occur as a result of direct or indirect contact with the blood or organs of infected animals. The virus can be transmitted to humans during manipulations with animal tissues during slaughter or cutting, assisting animals during childbirth, performing veterinary procedures or disposing of corpses and embryos. Therefore, people engaged in certain activities, such as shepherds, farmers, slaughterhouse workers and veterinarians, are at increased risk of infection. The virus infects a person by inserting, for example, when a wound is applied to an infected knife or in contact with damaged skin, or by inhaling aerosols formed during the slaughter of infected animals. Aerosol transmission also leads to infection of laboratory workers.
- There is some evidence of the possibility of human infection with Rift Valley fever when consuming unpasteurized or raw milk of infected animals.
- Infection of people also occurs as a result of bites of infected mosquitoes, most often mosquitoes of the Aedes species.
- It is also possible to transmit the Rift Valley fever virus by hematophagous (blood-fed flies).
- To date, cases of the transfer of Rift Valley fever from person to person are not registered. There are also no reports of the transfer of Rift Valley fever to health care workers while observing standard infection control measures.
- There is no data on outbreaks of Rift Valley fever in urban areas.
Symptoms of Fever Rift Valley
The incubation period is 3-6 days. A sudden start. The patient feels unwell, feeling chilling or a real chill, headache, retroorbital pains, pains in the muscles of the whole body and limbs, pain in the lumbar region. Body temperature quickly rises to 38.3-40 ° C. Later, there is a worsening of appetite, loss of taste, epigastric pain, photophobia. Physical examination noted redness of the face and vascular injection of the conjunctiva. The temperature curve has a biphasic character: the initial rise lasts for 2-3 days, followed by remission and a repeated rise in temperature.
Mild Rift Valley Fever in People
– The incubation period (the time between infection and the onset of symptoms) of Rift Valley fever lasts from two to six days.
– Infected people either do not have any detectable symptoms, or they develop a mild form of the disease, which is characterized by febrile syndrome with a sudden onset of influenza fever, muscle pain, joint pain and headache.
– Some patients develop rigidity (stiffness) of the neck, photosensitivity, loss of appetite and vomiting; in such patients, the disease in its early stages may be mistaken for meningitis.
– Usually the symptoms of Rift Valley fever last from four to seven days, after which you can detect the body’s immune response, manifested in the appearance of antibodies and the gradual disappearance of the virus from the blood.
Severe Rift Valley Fever in Humans
While most human cases are relatively easy, a small proportion of patients develop a much more severe form of the disease. Usually it is accompanied by the appearance of one or more of the three obvious syndromes: eye disease (in 0.5-2% of patients), meningoencephalitis (less than 1%) or hemorrhagic fever (less than 1%).
Eye shape. In this form of the disease, the usual symptoms characteristic of the mild form of the disease are accompanied by damage to the retina. As a rule, eye lesions occur one to three weeks after the first symptoms appear. Patients usually complain of blurred or impaired vision. After 10-12 weeks, the disease can go away on its own without any lasting effects. However, with lesions of the macula in 50% of patients, there is a permanent loss of vision. Death among patients with only one ocular form of the disease is rare.
Meningoencephalitis form. The onset of the meningoencephalitis form of the disease usually occurs one to four weeks after the appearance of the first symptoms of RRF. Clinical symptoms include severe headache, memory loss, hallucinations, confusion, disorientation, dizziness, convulsions, lethargy, and coma. Later (more than 60 days later) neurological complications may occur. The mortality rate among patients with this form of the disease alone is low, but residual neurological deficit, which can be severe, is common.
Hemorrhagic form. Symptoms of this form of the disease appear two to four days after the onset of the disease. First, there are signs of severe liver damage, such as jaundice, then signs of hemorrhage, such as vomiting of blood, blood in feces, red rash or bruising (caused by hemorrhages in the skin), bleeding from the nose and gums, menorrhagia and bleeding from venipuncture sites. Mortality among patients who develop hemorrhagic form of the disease reaches about 50%. Death usually occurs three to six days after the onset of symptoms. Virus in the blood of patients with LRV in the form of hemorrhagic jaundice can be detected within 10 days.
The overall mortality rate for different epidemics varies widely, but, in general, in cases of registered epidemics it does not exceed 1%. Most deaths occur in patients who develop a form of hemorrhagic jaundice.
Complications of severe forms are usually associated with hemorrhagic manifestations – generalized hemorrhages or liver damage (jaundice). With extensive liver necrosis, after 7–10 days after the onset of the disease, death can occur.
In 2-7 days after the onset of fever, loss of vision is possible, including light perception. Edema of the yellow spot, hemorrhage, vasculitis, retinitis and vascular occlusion develop. In 50% of patients, visual acuity is not restored.
In the peripheral blood at the beginning of the disease, the number of leukocytes does not change, but then leukopenia develops with a decrease in the total number of neutrophilic granulocytes and an increase in the stab forms.
Diagnose Rift Valley Fever
The diagnosis of acute fever Rift Valley can be made using various methods. Serological tests, such as standard enzyme-linked immunosorbent assay (ELISA and EIA methods), can confirm the presence of specific IgM antibodies to the virus. The virus itself can be detected in the blood at an early stage of the disease or in tissues taken after death using various techniques, including reproduction of the virus (in cell cultures or inoculated animals), tests for detecting antigens and PCR with reverse transcription.
Differentiate with other phlebovirus fevers (phlebotomy fever, Colorado tick fever, Zika fever).
Rift Valley Fever Treatment
Considering that most cases of people suffering from Rift Valley fever are relatively mild and short-lived, special treatment is not required for these patients. In more severe cases, the predominant treatment is general supportive therapy.
Forecast. Mortality rate is 0.25-0.5%. With severe forms – up to 50%. The survivors have severe residual effects (effects of encephalitis and reduced vision).
Preventing Rift Valley Fever
An inactivated vaccine has been developed for use in humans. However, this vaccine is not licensed and is not available for sale. It is used for experimental purposes to protect veterinarians and laboratory workers who are at high risk of infection with Rift Valley fever. Other candidate vaccines are being tested.
Close contact with animals, especially body fluids, both directly and through aerosols, is the most significant risk factor for Rift Valley fever infection during an outbreak. In the absence of specific treatment and an effective vaccine for people, raising awareness of the risk factors for Rift Valley fever infection, along with taking individual protective measures to prevent mosquito bites, are the only ways to reduce morbidity and mortality in humans.
Health information messages aimed at reducing risk should focus on the following aspects:
- Reducing the risk of transmission from animal to human as a result of unsafe animal husbandry practices or animal slaughter. Gloves and other appropriate protective clothing must be worn and care must be taken when handling sick animals or their tissues, as well as when slaughtering animals.
- Reducing the risk of transmission from animals to humans due to unsafe consumption of fresh blood, raw milk or animal tissue. In areas covered by epizootic diseases, all animal products (blood, meat, and milk) must be thoroughly cooked before eating.
- The importance of protection from mosquito bites at the level of individuals and communities, which should use insecticide-soaked mosquito nets and, if available, individual repellents, wear light-colored clothing (long-sleeved shirts and pants), and avoid outdoor activities in the high season activity of mosquitoes that are carriers of infection.
Infection control in medical institutions
- Although no cases of human-to-human transmission have been identified, there is still theoretically a risk of transmitting the virus from infected patients to healthcare workers as a result of contact with infected blood or tissues. Health care workers caring for patients with suspected or confirmed Rift Valley fever should follow standard precautions when handling specimens taken from these patients.
- Standard precautions determine the working procedures that must be applied to ensure a basic level of infection control. Standard precautions are recommended for the care and treatment of all patients, regardless of whether their infectious status is suspected or confirmed. They concern manipulations with blood (including dried blood) and all other body fluids, secretions and excrement (except sweat), regardless of whether they contain visible blood or not, as well as contact with damaged (intact) skin and mucous membranes.
- As mentioned above, laboratory workers are also at risk. Manipulations with specimens taken for diagnosis in humans and animals with suspected Rift Valley fever should be conducted by trained personnel in properly equipped laboratories.
Vector control
- Other ways to control the spread of Rift Valley fever include vector control and protection against their bites.
- Conducting anti-licking treatment in the breeding sites of mosquitoes is the most effective form of vector control, provided that such places can be clearly defined and their size and length are limited. However, during periods of flooding, when the number and size of breeding sites tend to increase significantly, anti-lichen treatment is impractical.