Rift Valley Fever

Also Known As: RVF

Caused By: Rift Valley Fever Virus — RVFV

Introduction

Rift Valley Fever (RVF) is a viral zoonotic disease belonging to the family Bunyaviridae in the Phlebovirus genus, possessing a segmented negative sense RNA genome. The disease has an episodic occurrence remerging ever 5-25 years and is seasonal in its occurrence. The occurrence of non immune animal populations every 5-25years combined with the introduction of RVF (due to rainfall) accounts for the explosive cyclical nature of the disease. RVF primarily affects animals but can infect humans and has the capacity to cause severe disease in both.

RVF has a wide economic impact due to livestock loss and trade restrictions as well as public health implications. It is a notifiable disease.

Distribution

RVF virus was first identified in 1831 in the Rift Valley in Kenya during an investigation on a sheep farm and has since spread throughout Sub Saharan Africa emerging into North Africa in the 1970’s. the outbreak in Egypt in 1977-78 is considered to be the largest outbreak with 200,000 human cases reported. In September 2000 it was reported for the first time outside of Africa, in Saudi Arabia and Yemen, probably introduced through infected livestock or mosquitoes. The increase in cases in South Africa may be due to the end of an inter epizootic period. Outbreaks are frequently reported though there is no evidence that it has spread to previously uninfected countries in the last 10 years, though it is hard to monitor changes in disease occurrence due to the cyclical occurrence of epidemics. Most recently RVF was reported in Mauritania in November 2012. A map detailing current outbreaks can be found here

A number of mosquito species (Aedes, Culex, Mansonia, Anopheles) are implicated as vectors of RFV, the most important being Aedes and Culex spp. They are responsible for both maintenance and amplification of RVF.

Mosquitoes can be infected via feeding on infected animals. Vertical transmission can also occur (particularly in Aedes spp); female infected mosquitoes lay virus infected eggs leading to a new generation of infected mosquitoes. Vertical transmission is important in the survival of the virus as the eggs laid by the female can survive for many months in dry conditions, hatching after a period of rain and so increasing spread post rainfall leading to epizootics. Once animal infection has occurred mosquitoes are then responsible for amplifying infection. Aedes mosquito numbers decrease following rain but Culex tend to breed in more permanent water sites, hence the continuation of virus spread.

RVF affects 4 areas:
Dambos (e.g. East Africa) – Shallow depressions, often near rivers, that fill with water during the rainy season. Vertical transmission in mosquitoes occurs here.

Semi-arid (e.g. Senegal, Mauritania) – At temporary water points. It is unknown how the virus persists here, presumably either via vertical transmission or reintroduction of virus through visiting herds.

Irrigated areas (e.g. Nile Delta and Senegal River Valley). Yearlong viral transmission occurs here as the permanent water favours Culex breeding.

Temperate and Mountainous (e.g. Madagascar) – Virus is transmitted by vectors through cattle movement.

No outbreaks have been reported in urban areas.

Zoonotic transmission occurs through direct or indirect contact with infective blood or organs through slaughter, assisting with births and carcass disposal amongst other means. Faecal shedding of virus also occurs, as does spread through nasal and ocular secretions. Aerosol infection has also occurred within laboratory workers. Consuming unpasteurised or uncooked milk has also been associated with infection and seropositivity. Mosquito bites have resulted in infection as well, and blood feeding flies also have the potential to transmit infection. There is no evidence of human to human transmission.

Virus particles are shed in milk but animals have not been infected via suckling or ingestion of milk.

It is currently unknown if there are animal reservoirs of RVF between outbreaks. The namaqua rock rat and bats have been implicated and have been shown to be capable of infection but the potential impact of this is unknown. Low levels of circulation between livestock or wild ruminants and mosquitoes (sylvatic cycle) is also likely to occur.

Outbreaks occur after heavy rain and flooding due to favourable breeding conditions for mosquitoes.

RVF can also be spread by the introduction of infected livestock into previously unaffected areas where mosquitoes are present.

Signalment

A large number of animal hosts are susceptible, many producing high enough levels of viraemia to then infect mosquitoes.

RVF causes severe disease in animals, mainly cattle, sheep, goats and camels, with sheep being more susceptible. Bos Taurus cattle and other European breed imported into Africa appear highly susceptible to RVF.
<br Age is an important factor in determining the severity of the disease, young stock are more susceptible – 90% of infected lambs die whereas in adult sheep mortality can be as low as <10%. Small ruminants are also more susceptible. Pigs are resistant to low doses of RVF but high doses can cause viraemia. During an outbreak in Egypt RVF virus was also isolated from horses as well as camels.

Other species (e.g. dogs and cats) have been infected experimentally and have become viraemic. The only species that are resistant are reptiles, birds and amphibians.

Clinical Signs

RVF has an incubation period of 1-6 days (12-36 hrs in lambs). Once in the lymph nodes viral replication occurs which leads to viraemia and systemic infection. Spontaneous abortions are seen as the hallmark of RVF outbreaks (ref 2). Pregnant animals can abort at any stage often with 100% of stock aborting.

Newborn lambs and kids are highly susceptible to RVF, presenting with pyrexia and anorexia shortly followed by death 24-36hrs after infection. In newborn lambs hepatocytes of the liver are the predominant target cell with hepatic necrosis being a significant post mortem finding. Other organs affected include the gall bladder (haemorrhage and oedema), gastrointestinal tract haemorrhage, lymph node haemorrhage, cutaneous haemorrhage and haemothorax.

Signs in older lambs, kids, calves and adults vary from acute to subclinical (20-70% mortality), Signs can include fever (lasts 24-96hrs), weakness, bloody diarrhoea, abdominal pain, photosensitivity, anorexia, excessive salivation and decreased milk production. Signs in adult cattle are most often subclinical with less than 10% mortality.

Camels display signs similar to those seem with Pasteurellosis infection, though infection can also be subclinical or asymptomatic. Abortions can also occur. During the 2010 outbreak in Mauritania 2 forms of disease were observed in camels; a hyperacute form causing sudden death in <24hrs and an acute form causing fever, ataxia, respiratory signs, icterus, oedema, foot lesions and neurological signs. If haemorrhagic signs were observed death occurred in a few days.

Differential diagnosis should include: bluetongue, Wesselbron disease, Enterotoxaemia of sheep, Ephemeral fever, Brucellosis, Vibrosis, Trichomonosis, Nairobi sheep disease, Heartwater, Ovine enzootic abortion, plant toxicity, bacterial septicaemias, Rinderpest, Anthrax.

Humans develop malarial-like disease. High risk individuals include farmers, veterinarians and abattoir staff. Mild disease is most common but severe hepatitis, encephalitis and ocular damage can develop. The usual presentation is of sudden onset fever, myalgia, biphasic behaviour and gastrointestinal disease.

Diagnosis

Following infection viraemia is often high (though short lived) so the virus can be easily detected in the blood shortly after. Tissue samples can also be used in dead animals or aborted foetuses.

The nucleocapsid protein is used as the antigen of choice in serological assays. Blood samples can be used to detect the virus during the early phase using virus propagation, antigen detection and RT-PCR.

During the acute stages ELISA or EIA can be used to confirm the presence of IgM antibody to the virus, which allows recent infections to be diagnosed. ELISA’s based on recombinant RVF virus proteins have been developed which negates the need for biosecure facilities and are used in a number of species. Cross reactions may occur with other phleboviruses.

RT-PCR is the standard method used in most laboratories as it has a high sensitivity. This is useful for rapid diagnosis and can also be used to detect RVF virus in mosquito pools.

Virus neutralisation tests (VNT) are very specific and sensitive and can be performed in a biosecure laboratory. They are also the prescribed test for international trade, though it cannot differentiate between vaccinated and infected animals. It is the only method to detect functional antibodies though a low level of cross reaction to some other phleboviruses has been observed. Plaque reduction neutralisation assays are the most commonly used VNTs and involve incubating the virus and heat inactivated serum allowing the virus to infect. 4-6 days later the presence of cytopathic plaques is observed.

Haemagglutination inhibition (HI) and complement fixation assays are available but show extensive cross reactivity with other phlebovirus species. HI assays are used in non endemic areas but animals previously infected with other phleboviruses may show a positive result. Immunofluorescence can also be used.

Definitive confirmation can be carried out by virus isolation, however due to the zoonotic risk this can only be carried out in biosecure facilities.

Histopathology on tissue samples will show cytopathology and immunostaining can be used to identify RVF antigen in cells. On post mortem during the viraemic stage, widespread petechiae and ecchymoses on serous surfaces and organs will be seen and present in the body cavities. In older animals, the liver is enlarged and inflamed, with many foci of necrosis which are bronzed and jaundiced. The gall bladder may also be distended and haemorrhagic. Lymph nodes are enlarged and their germinal centres may be necrotic on closer examination. Extensive subcapsular haemorrhage in the spleen is usual. Renal changes include oedema and congestion. Epicardial and endocardial haemorrhages are often present on the heart.

Treatment

No treatment is available.

Control

Modified live and inactivated vaccines are available. Live vaccination is only recommended in non-pregnant animals due to its ability to cause abortion and neurological deficits in lambs. In epizootic situations though, this risk may well be worth taking.

Inactivated vaccines are ineffective during epizootics and therefore less widely used than modified live strains.

Mosquito and larval control is extremely valuable. Slow release larvicides such as methoprene can be applied to well-defined mosquito breeding areas.

Sentinel cattle are used for epidemiological surveillance, and are tested 2-3months after the seasonal rains.

Rift Valley Fever Learning Resources
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References

This article was originally sourced from The Animal Health & Production Compendium (AHPC) published online by CABI during the OVAL Project. The datasheet was accessed on 8 June 2011.

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