Infectious diseases - Rift Valley Fever

>Clinical and Epidemiological aspects
>RVF Vírus
>RVF Vectors
>RVF Virus in non-human hosts
>Transmission to humans
>Clinical characteristics
>Clinical features of serious cases
>Diagnosis and treatment
>Prevention and control

>Clinical and Epidemiological aspects

Rift Valley Fever (RVF) is a zoonosis , a disease that mainly affects animals , but occasionally it can infect human beings. It can cause serious illnesses in a range of animals, in human beings and carries a high rate of morbidity and mortality. Death from RVF often leads to substantial economic loss.

Since 1930, when the virus was isolated during an epidemiological investigation in sheep on a farm in the Rift Valley, Kenya, a number of occurrences were noted in North Africa and the Sub-Sahara region. In 1997/98, a series of more substantial outbreaks took place in Kenya and Somalia. In September 2000, RVF was reported for the first time outside the African continent. Cases were confirmed in Saudi Arabia and Yemen. This area in the Arabian Peninsula which had been formerly free from RVF virus Arabic Peninsula greatly increased the risk of the disease spreading to other parts of Asia and into Europe.

A number of different species of mosquitoes can be vectors of the RVF virus. Therefore, a potential epizootiasis (epidemic affecting animals) is associated with human epidemics arising from the introduction of the virus into a new area where these vectors are present. This has been demonstrated in the past and continues to be a serious threat to the present day.

> RVF Vírus
The causative virus of RVF is a member of the Phlebovirus gender, one of the five genders of the Bunyaviridae family.

>RVF Vectors

• The RVF virus is spread first among animals through the bite of infected mosquitoes.
  
• A wide variety of mosquitoes can serve as vectors for transmitting the virus. It has been noted that in different areas a number of species of mosquitoes are predominant vectors of the virus. In addition, various species of vectors have a substantial role in maintaining transmission of the virus.
  
• Aedes, for example, can acquire the virus by feeding on infected animals and are capable of transovarian transmission (transmission of the virus from an infected female directly through her eggs). The next generation of infected mosquitoes will feed off the same eggs.
  

This means that the virus can survive for long periods in the wild since the eggs of the mosquitoes are able to survive for periods of several years in dry conditions. During wet seasons, when the breeding grounds are flooded, the eggs break and the mosquito population tends to increase.

The virus subsequently spreads to the animals they feed on.
Previously, non-infected Aedes and other species of mosquitoes fed on infected animals viraemiae (viruses circulating in the bloodstream) and thus perpetuated the spread of transmission of the virus to animals on which they subsequently fed.

>RVF Virus in non-human hosts

• Several species of animals can be infected by RVF. The disease can cause severe problems among domesticated animals, including sheep, camels, cattle, and goats. However, sheep are generally more susceptible than cattle, and members of the goat family tend to be less susceptible to infection.
  
• Exotic breeds of animals recently introduced into endemic areas tend to be more liable to attack than breeds which have been around much longer and which have as a result adapted better to local conditions.
  
• Animals of different ages also present varying degrees of susceptibility. More than 90% of all young sheep infected with RVF die, while the mortality rate of adult sheep is under 10%.
  
• The abortion rate among infected sheep is 100%. RVF epizootic (animal epidemic) is usually first noticed due to the occurrence of a large number of unexplained abortions. This is normally the first sign of the beginning of an epidemic.

>Transmission to humans

• During the epizootic, people can be infected by RVF as the result of infected mosquito bites or through contact with blood, secretions or the organs of infected animals.
  
• Such contact can occur when animals are being transported or moved, when infected animals are being slaughtered or possibly through drinking milk that has not been properly boiled.
  
• The virus can infect humans through inoculation (for example if the skin has been scratched or through a wound caused by any material that has come into contact with the infection) or through the inhalation of suspended droplets (aerosols). This latter mode of transmission has been noted especially among laboratory workers.

>Clinical characteristics

• The incubation period ( the interval between infection and emergence of the symptoms) of RVF can vary from 2 to 6 days.
  
• The fever begins with symptoms similar to flu, involving a sudden attack of fever, severe headaches, myalgia (muscular pain) and backache. Some patients develop stiffness in the nape, sensitivity to light and vomiting. These early stages of the disease can easily be confused with the symptoms of meningitis.
  
• The symptoms of RVF remain for about 4 to 7 days, followjng which the immunologic response is detectable through antibodies IgG and IgM.

>Clinical features of serious cases

• Whereas most human cases of RVF are relatively moderate, a small proportion of patients develop serious illness from the disease. The main symptoms are generally one of the following: ocular disease, meningo-encephalitis or hemorrhagic fever. Within the group of patients who develop one or more of these three symptoms, the proportions vary from 0.5% to 2% presenting ocular diseases and less than 1% present symptoms of meningo-encephalitis and hemorrhagic fever syndrome.
  
• The fever and other symptoms described in the above paragraph can appear together with ocular disease, which characteristically appears in the form of lesions in the retina. The development of ocular disease normally takes around two to three weeks . When the lesions are in the macula, this will result in some degradation of the patient's sight. Death is unusual in patients presenting only ocular lesions.
  
• Other symptoms can appear spontaneously with acute neurological disease such as the meningo-encephalitis. These symptoms usually take from one to three weeks to appear. Patients who only have meningo-encephalitis are unlikely to die from RVF.
  
• RVF can manifest itself as hemorrhagic fever. Two or four days after the appearance of the disease, the patient starts to present serious liver disease, accompanied by jaundice and hemorrhage. The patient passes blood, blood also appears in the feces and the patient develops purple- coloured lumps (caused by skin hemorrhaging) and bleeding from the gums. Patients with the RVF syndrome remain viremic for over ten days. The mortality rate for patients who develop hemorrhagic diseases is high, often as high as 50%.
  
• Most deaths occur in patients that have developed hemorrhagic fever. The number of fatal cases recorded varies widely in the epidemiological literature , but it is almost always less than 1%.

>Diagnosis and treatment

• Several methods can be undertaken to diagnose acute RVF. Serologic tests involving immunologic analysis (" ELISA" or "EIA" method) can show up the specific presence of IgM antibodies for the virus. The same virus can be detected in the blood during the viremic phase of the disease or in the tissues of corpses by employing a variety of techniques, including viral propagation in cell cultures, tests for detecting antigens and by molecular methods to detect the viral genome.
  
• The antiviral drug ribavirin has been shown to be a viral growth inhibitor in experiments, but this has not yet been clinically evaluated. Most cases of human RVF are relatively mild and of short duration, and do not require any specific treatment. In the more serious cases however support therapy is strongly recommended.

>Prevention and control

• RVF can be prevented through a prolonged programme of animal vaccination. Live vaccines require only one dose and produce a long lasting effect, but the vaccines at present available on the market can cause abortion if applied to pregnant animals. So-called dead vaccines do not cause abortion but a number of doses are required to obtain immunity from the disease. This can be problematic in endemic areas.
  
• An inactive vaccine has been developed for use in humans. This however has not been officially authorized and is not therefore commercially available. However it has been used experimentally to protect veterinarians and laboratory staff at high risk of exposure to RVF. Other forms of vaccines are still at the experimental phase.
  
• The risk of transmission by blood or contaminated tissues exists for people working with infected animals or for those in contact with individuals who have the disease. Gloves and other appropriate protective clothing must be worn and precautions taken when handling infected animals and/or their tissues. Health workers looking after patients suspected of having the disease, or actual carriers, must adopt strict precautions when dealing with the blood or excrement of such patients. Hospitalized patients must be fed using hospital barriers. All workers in the health area dealing with such patients must be correctly trained and equipped to do so.
  
• The principal method of controlling the disease is through elimination of the vector - the mosquito transmitting the disease. Individual protection is important and effective. Wherever possible, people must use protective clothing, with long-sleeved shirts and long trousers, mosquito nets to protect while sleeping, and insect repellents on the skin. Avoid staying out of doors during the periods when the mosquitoes are most active.

Indispensable ways of bringing the disease under control include the elimination of the mosquito breeding grounds, the use of insecticides and information made available for the population at risk .
New systems to monitor climatic changes are being used to give appropriate warning before events occur that can lead to an increase in the mosquito population. These warnings will enable the appropriate authorities to implement measures that could avoid an epidemic taking hold.