Rift Valley fever: a mosquito-borne and livestock-linked virus

Outbreaks have been reported across sub-Saharan Africa since the virus was identified. WHO describes a 1977 outbreak in Egypt linked to livestock trade along the Nile irrigation system, and a major 1997 to 1998 epidemic in Kenya, Somalia and Tanzania after flooding that followed El Nino. In September 2000 the disease reached Saudi Arabia and Yemen, the first reported occurrence outside Africa.

Epidemics track unusually heavy rainfall, because floodwater lets mosquito populations explode. The first sign in an outbreak is often a wave of unexplained abortions in sheep, goats and cattle, which can warn veterinary and public health teams before human cases appear.

Direct or indirect contact with infected animal blood or organs is the main route. Slaughtering, butchering, helping animals give birth, veterinary procedures and disposing of carcasses or fetuses are all risk activities, so herders, farmers, slaughterhouse workers and vets are most exposed. WHO notes some evidence of infection from unpasteurised or uncooked milk.

Mosquito bites, most commonly from Aedes and Culex species, are the other route, and blood-feeding flies may also transmit it. WHO reports no documented human-to-human transmission and no spread to healthcare workers where standard infection control was applied.

Most infected people have no symptoms or a sudden flu-like illness with fever, muscle and joint pain and headache. Some also develop neck stiffness, light sensitivity, loss of appetite and vomiting, which early on can resemble meningitis.

WHO describes three severe patterns that affect only a minority: an eye form with retinal lesions (0.5 to 2 per cent of patients), a meningoencephalitis form with severe headache, confusion, convulsions and coma (under 1 per cent), and a haemorrhagic form (under 1 per cent) that begins with liver impairment and progresses to bleeding. When eye lesions involve the macula, WHO says half of those patients have permanent loss of vision. Neurological effects of the brain form can appear weeks or months later and can be long lasting.

Early illness is hard to tell from malaria, typhoid, shigellosis, yellow fever and other viral haemorrhagic fevers. Confirmation relies on RT-PCR, antibody ELISA tests or virus culture, and because samples are a serious biohazard, laboratory work on unprocessed specimens needs maximum containment. In the UK, a traveller who becomes unwell after visiting an affected region should tell clinicians exactly where they have been.

Mild cases need no specific treatment. Severe cases receive early intensive supportive care, including fluid management and treatment of individual symptoms. WHO notes there is no licensed human vaccine.

WHO emphasises avoiding bites and avoiding animal contact. Practical measures include long-sleeved, light-coloured clothing, repellent, impregnated nets, and avoiding outdoor activity when the local mosquitoes bite. Anyone handling sick animals or tissues should use gloves and protective equipment, wash hands, and cook all animal products, including blood, meat and milk, thoroughly in affected regions.

Animal vaccination, given before an outbreak and not during one, is the main control measure for livestock, and limiting livestock movement can slow spread.

Government guidance states there has never been a known case of Rift Valley fever in Great Britain. It is a notifiable animal disease, so suspected illness in sheep, goats or cattle must be reported to the authorities. Guidance describes the risk to humans as moderate to severe flu-like illness, in an infection usually found in Africa.