Infection with Rift Valley fever virus

The first warning is usually a wave of abortions in pregnant sheep, goats and cattle together with deaths of newborn animals, often alongside flu-like illness in the people handling them. Newborn lambs show fever, anorexia, weakness, abdominal pain and rapid breathing and may die within 24-36 hours. Older lambs and adult sheep can die suddenly or show fever, vomiting, bloody or fetid diarrhoea, mucopurulent nasal discharge and occasional icterus. Adult cattle are often subclinical apart from abortion, fever and a fall in milk yield. At necropsy the liver is enlarged, discoloured and studded with small necrotic foci, with widespread haemorrhages.

Rift Valley fever is caused by Rift Valley fever virus, a negative-sense, three-segmented RNA virus of the genus Phlebovirus (family Phenuiviridae, order Bunyavirales). A single serotype exists, but strains vary in virulence. After a mosquito delivers the virus, it produces an acute or peracute systemic infection whose most consistent target is the liver, where it causes focal to generalised hepatic necrosis, most severe in the young. Widespread haemorrhages on serosal surfaces, enlarged oedematous lymph nodes and spleen, and haemorrhagic enteritis follow. The virus crosses the placenta, so pregnant animals abort regardless of stage of gestation, and abortion products carry large amounts of virus. The infection does not produce a carrier state.

Sheep, goats and cattle are the main amplifying hosts; wild ruminants also become infected, usually without disease. Aedes and Culex mosquitoes are the key vectors, and Aedes can pass the virus through its eggs, so explosive epizootics follow heavy rain or flooding that hatches large numbers of infected mosquitoes, or new irrigation and dam schemes. The disease is endemic across much of Africa; major outbreaks struck Egypt (1977-78, 1993), and the first outbreaks outside Africa occurred in Saudi Arabia and Yemen in 2000, with sentinel animals in Saudi Arabia still showing sporadic infection. It is zoonotic: people are infected by handling blood, tissues and abortion material, by aerosols, or by mosquito bites. It is a WOAH-listed disease, and suspected cases should be reported under national rules.

WOAH lists bluetongue, Wesselsbron disease, enterotoxaemia of sheep, ephemeral fever, brucellosis, vibriosis, trichomonosis, Nairobi sheep disease, heartwater, ovine enzootic abortion, toxic plants, bacterial septicaemias, rinderpest, peste des petits ruminants and anthrax. Clinical signs are non-specific; the pattern that points to Rift Valley fever is an abortion storm with high neonatal mortality, diffuse hepatic necrosis at necropsy and concurrent human illness after heavy rain. Confirmation needs RT-PCR, antigen ELISA, histopathology with immunostaining, or IgM serology.

Liver, gallbladder, lymph nodes, spleen, kidneys, gastrointestinal tract, serosal membranes, uterus and placenta, fetus

Live attenuated (Smithburn, Clone-13) or inactivated vaccines in endemic areas ahead of rain-driven outbreaks; Smithburn can cause abortion in pregnant stock. Movement control, draining or treating mosquito breeding water, protection at slaughter.

No specific treatment; care is supportive. Newborn lambs and kids: 70-100% mortality. Adult sheep: 20-70% (WOAH card) vs 10-30% (CFSPH). Adult cattle usually under 10%. Camels: resistant per WOAH, but fatal haemorrhagic cases per CFSPH.