A080 - RIFT VALLEY FEVER

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A080 - RIFT VALLEY FEVER

Nature of the disease
Rift Valley fever (RVF) is an acute insect-borne virus (arbovirus) zoonotic disease caused by a Bunyaviridae of the gens Phlebovirus. It mainly affects ruminants and humans. 

In ruminants it causes abortion in pregnant animals and high mortality in young animals.
In humans RVF causes a severe influenza-like disease with occasionally more serious complications

Classification
OIE List A disease, high level of biosecurity
Susceptible species
RVF virus is highly pathogenic for sheep and cattle, buffalo camels, mice, rats and humans.

Goats appear to be moderately susceptible and pigs, horses, carnivores and poultry have only transitory viremia.

Distribution
To date RVF is limited to some part of Africa, it is present in Eastern Africa and recently extended to Madagascar and Western African countries (up to Senegal) probably in relation with El Nino climatic phenomena. 
Clinical signs 
Clinical disease occurs in animals of all ages but is most severe in young.

The incubation period is very short (12 hours) and within 36 hours mortality can reach 95% in lambs and 70% in calves.

Affected animals show:

  • Fever (40-42°C) 
  • Anorexia, depression and weakness in adult cattle and lambs,
  • Mucopurulent nasal discharge in sheep and goats,
  • Vomiting, 
  • Diarrhea in cattle,
  • Vomiting and jaundice in sheep,
  • Sometimes haemorrhagic diarrhoea in lambs and sheep,
  • Drop in milk yield in cattle,
  • Abortion may reach 100% in ewes and 85% in cows

In humans there is a severe influenza-like illness that lasts for up to a week. The commonest complication is damage to the retina that can, in some cases, lead to temporary or permanent blindness. Other occasional complications include encephalitis, blood clotting problems and hepatitis. These can be fatal.

Post-mortem findings 
  • Carcases are sometimes jaundiced
  • Small haemorrhages present in the internal organs
  • Liver contains greyish-white necrotic foci, more severe in lambs than adults
  • Variable level of intestinal inflammation
  • Fluid, frequently blood-stained, in body cavities
Differential diagnosis 
  • Wesselbron disease
  • Nairobi sheep disease
  • Bluetongue
  • Hepatotoxins
  • Other viral and bacterial causes of abortion
Specimens required for diagnosis 
RVF can be expected on the basis of clinical history, but laboratory support is required to confirm the diagnosis.
  • Whole blood (in EDTA anticoagulant) collected from febrile animals for virus isolation
  • Tissue specimens — liver and spleen from animals at post-mortem and aborted foetuses — both fresh and preserved (in 10% neutral buffered formalin and in glycerol) samples should be collected
  • Serum can be used for serology based on Virus Neutralisation (3 days after infection), ELISA (1 week after infection)
Transmission   
RVF is transmitted by 8 species of mosquitoes from the Aedes and Culex family.

Little is known about the dynamics of the virus in vectors.

Direct contact with blood and viscera (e.g. during slaughter) and aerosol can cause human infection.

Virus in meat is rapidly inactivated after slaughter due to pH changes. RVF virus is present in milk but is inactivated by pasteurisation.

Virus is present in semen and it is possible that infection could be transmitted this way

Risk of introduction   
RVF could be introduced through the importation of infected vectors or hosts, including humans. Infected vectors could conceivably be introduced by aircraft. Routine disinsection procedures while unlikely to be 100% effective, reduces this risk.
Importation of live animals from endemic countries to the Pacific is unlikely. While a viraemic human could theoretically introduce the disease, for it to establish this person would have to be bitten by a vector and in turn the infection passed on. Little is known about the competency of local mosquito species.
Control / vaccines  
Attenuated and inactivated vaccines are used overseas. Attenuated vaccines produce a high level of immunity, but cause abortions and birth defects. These can be transmitted by insect vectors and there is the potential that they may revert to virulence. Inactivated vaccines give a lower antibody response and regular vaccination is required to maintain immunity. The US Army has produced an inactivated vaccine for human use, although this is only available in limited quantities.
References
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Office International des Epizooties, 2002
  • Rift Valley Fever, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 541-542
  • Rift Valley Fever, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 950-951
  • SWANEPOEL R (1998), Rift Valley Fever In Zoonoses, ed by SR PALMER, Lord SOULSEY and D.I.H. SIMPSON, Oxford University Press, Bath Press, Avon, 1998, p.459-468