Rift Valley Fever Dr.Tariq Mustafa Mohamed Ali, Department of Municipalities and Agriculture, Agriculture Sector, Veterinary Laboratory , Al Ain
Definition Rift Valley fever (RVF) is an arthropod-borne viral disease affecting wide variety of mammals . characterized by abortions among pregnant animals, high mortality in neonates, and hepatic necrosis. The Human beings are highly susceptible to the disease . The Disease Classified as an OIE List A disease (A080)
Geographic Distribution RVF has been recognised as an exclusive disease in African countries, with an underlying association with high rainfall and dense populations of vector mosquitoes.
History of the disease First described by Daubny in 1931 in Rift valley area In kenya . Antibodies recorded in human cases from Central africa and the virus isolated from animals raised in Uganda ,Mali, Congo and Gabon by 1936
History of the disease (Cont.) In 1951 reported in South Africa In 1954 reported in French Equatorial Africa In 1956 reported in Cattle in Kenya In 1957 reported in Cattle in Togo In 1958 reported in Rhodesia (Nambia) In 1973 in sudan in Kosti District
History of the disease (Cont.) The only epizootic outbreaks of RVF outside sub-Saharan Africa were recorded in animals and humans in Egypt in 1977-78, Mauritania in 1987 and again in Egypt in 1993.
History of the disease (Cont.) The first confirmed Rift Valley fever outbreak outside Africa was reported in September 2000, in the Arabian Peninsula
History of the disease (Cont.) Laboratory infections have been recorded in other parts of the world
Current situation of the disease( Cont.) Endemic Countries Gambia, Senegal, Mauritania, Namibia, South Africa, Mozambique, Zimbabwe, Zambia, Kenya, Sudan, Egypt, Madagascar, Saudi Arabia, Yemen
Epidemiology ( Cont.)Countries known to have some cases with periodic virus isolation Botswana, Angola , Democratic Republic of the Congo, Congo, Gabon, Cameroon, Nigeria, Central African Republic, Chad, Niger, Burkina Faso, Mali, Guinea, Tanzania, Malawi, Uganda, Ethiopia, Somalia
AETIOLOGY RFV virus belongs to family Bunyaviridae , genus Phlebovirus.
Family Bunyaviridae The largest family of viruses. It includes the most arthropod-born viruses It include more than 350 membres with large diversity Genetic reassortment in infected mosquitoes migt be the cause of this diversity.
Bunyaviruses Group V: (-)sense RNA Viruses Genus Type Species HostsFamily Orthobunyavirus Bunyamwera virus Vertebrates Hantavirus Hantaan virus Vertebrates Nairobi sheep diseaseBunyaviridae Nairovirus Vertebrates virus Sandfly fever Sicilian Phlebovirus Vertebrates virus , RFV Tomato spotted wilt Tospovirus Plants virus
Cryptogram of RVF virus R/1: Σ 3 6 / L- : Se/ E : I,V/C,I,Ve(C)/Ac,Di
CryptogramR/1: Σ 3 6 / L- :Se/ E :I,V/C,I,Ve(C)/Ac,Di RNAV , SS, 3 molecules/ - ve strand Spherical,elongated NC / Enveloped 90 - 100 um , heat labile , ether sensitive . All isolates are serologically similar Host range /mode of transmission / kind of vector if present
Micrograph of RVFV The virion is budding into membrane vesicles of Golgi vesicles in the cytoplasm of a liver cell of an infected rat.The virion is about100 nm (nanometer)
NA of RVF virus Single-stranded RNA Negative sense / ambi-sense Each virion contains, 3 linear segments : L 2.7 X 10 6 M 1.6 X 10 6 S 0.6 X 10 6 Not present in equimolar amounts 5 ends not capped; 3 ends not polyadenylated; Genomic RNA not infectious.
Protein structure of RFV L ~8.5kb / RNA dep.RNA polymerase ( Transcriptase) M ~5.7kb / G1, G2, NSM S ~0.9kb / N, NSS
Effect of Temperature The virus can remain viable for up to 4 months at 4o C. Specimens stored below 0o C will retain infectivity for 8 years . The virus in serum, inactivated by 56°C for 120 minutes Rift Valley fever virus in aerosols has a half- life in excess of 77 minutes at 25o C and 30 percent relative humidity .
Effect of Chemical factors on RFVvirus Rift Valley fever virus is inactivated by lipid solvents (ether and chloroform ), detergents, and low pH..
Effect of Chemical factors on RFVvirus ( Cont.) At neutral or alkaline pH in the presence of protein such as serum, the virus can remain viable for up to 4 months at 4o C. Solutions having a pH of 6.2 (acetic acid) or lower are also effective.
Effect of Disinfectants Inactivated by strong solutions of sodium or calcium hypochlorite (residual chlorine should exceed 5000 ppm)
SurvivalThe RVFV survives in dried discharges and multiplies in some arthropod vectors.
Host range Rift Valley fever virus infects many species of animals and humans . Sheep and cattle are the primary species affected and the primary amplifiers of the virus.
Host Range Neonatal lambs, kids, calves, and puppies are highly susceptible and have a high mortality.
Human being Humans are highly susceptible to RVF virus infection and are readily infected by mosquitoes and aerosols. Humans develop a sufficient viremia to be a source of infection for mosquitoes and thus could introduce the disease into uninfected areas.
Transmission Haematophagous mosquitoes of many genera (Aedes, Anopheles, Culex, Eretmapodites, Mansonia, etc.) can transmit fever as biological, competent vectors. Can replicate extensively in insects - transovarian passage allows overwintering. Mosquitoes (Aedes) migt be the reservoir host Direct contamination: occurs in humans when handling infected animals and meat
Transmission (Cont.)Historically, explosive outbreaks of the disease have occurred simultaneously over a wide area of Africa at 5 to 15 year intervals
Infection of human being Direct and indirect contact with infected animals through nasal discharge, blood, vaginal secretions after abortion in animals, mosquitoes, and infected meat. Aerosol and consumption of raw milk is also possible
Incubation Period Experimentally, the incubation period in newborn lambs, kids, calves, and puppies, is about 12 hours. In adult sheep, cattle, goats, and dogs the incubation period may be as long as 3 days. In humans, the incubation period is 4 to 6 days.
Morbidity and Mortality in an Outbreak Susceptibl Morbdity Mortality CaseSpecies Cases Deaths e cases rate rate fatality rateSheep 9000 1500 105 16.7% 1.2% 7%Goats 10000 1500 95 15% 0.95% 6.3%Cattle 4000 500 30 13% 0.75% 6%Camelidae 4000 500 5 13% 0.13% 1%Animals belong to different herds.70 people have died
Clinical Signs in cattle Adults: fever (40-41°C), excessive salivation, anorexia, weakness, fetid diarrhoea, fall in milk yield. Calves: fever (40-41°C), depression. Mortality rate: 10-70% Abortion may reach 85% in the herd. Mortality rate is usually less than 10%
Fetuses can be aborted at any stage ofgestation.
Clinical Signs in adult sheep & goats Fever (40-41°C), mucopurulent nasal discharge, vomiting . Abortion may reach 100% in pregnant ewes Mortality rate may reach 20-30%
Clinical Signs in lambs and Kids fever (40-42°C), anorexia, weakness, death within 36 hours after inoculation. Mortality rate: for animals under 1 week of age - up to 90%; for animals over 1 week of age - up to 20%
Clinical Signs in other animalsInapparent infections are quite frequent in other species than sheep
PM lesions. Focal or generalised hepatic necrosis (white necrotic foci of about 1 mm in diameter)
PM Lesions ( cont.)Congestion, enlargement, and discoloration of liver with subcapsular haemorrhages
PM Lesions ( cont.) Brown-yellowish colour of liver in aborted fetuses
PM Lesions ( cont.) Widespread cutaneous haemorrhages, petechial to ecchymotic haemorrhages on parietal and visceral serosal membranes
PM Lesions ( cont.) Enlargement, oedema, haemorrhages and necrosis of lymph nodes
PM Lesions ( cont.) Congestion and cortical haemorrhages of kidneys and gallbladder
Severe hemorrhagic and edematous biliary cystitis.
Specimens for Laboratory Heparinised or clotted blood Plasma or serum Tissue samples of liver, spleen, kidney, lymph node, heart blood and brain from aborted fetus. Specimens should be submitted preserved in 10% buffered formalin and in glycerol/saline and transported at 4°C
Differential Diagnosis1. Bluetongue2. Wesselsbron disease3. Enterotoxemia of sheep4. Ephemeral fever5. Brucellosis6. Vibriosis7. Trichomonosis8. Nairobi sheep disease9. Heartwater10. Ovine enzootic abortion11. Towic plants12. Bacterial septicaemias13. Rinderpest and Peste des petits ruminants
Laboratory diagnosis Virus isolation: Inoculation of mice or hamsters - preferred method Inoculation of 1-2-day-old lambs Inoculation of embryonated chicken eggs Tissue culture inoculation (Vero, CER, BHK- 21, mosquito line cells or primary calf, lamb and goat kidney and testis cells)
Laboratory diagnosis (cont.)Viral antigen detection by :1. Immunofluorescence in cryostat sections or in impression smears of liver, spleen and brain.2. Complement fixation and immunodiffusion on tissue suspensions
Laboratory diagnosis (cont.)Antigen detection in blood by :1. Immunodiffusion2. ELISA
Laboratory diagnosis by Serological tests Enyzme-linked immunosorbent assay - IgG and IgM and for Ag.detection Virus neutralisation Fluorescent antibody test Haemagglutination inhibition Plaque reduction neutralisation Complement fixation Immunodiffusion
Control and Eradication Vaccination is the only practical method of preventing low-level losses. Animal movement of from endemic areas to RVF-free areas should be discouraged. Mosquito control during an epizootic is logical but not practical for large areas; Slaughter of sick animals is not recommended
Vaccination Vaccination of all susceptible animals to prevent infection of amplifying hosts and thus infection of vectors is the only way to prevent infection of animals and man Inactivated vaccine containing saponin or peanut oil