GENERAL INTRODUCTION: BUNYAVIRIDAE

Introduction and history

Rift Valley fever (RVF) is a peracute or acute disease that mainly affects domestic ruminants and humans caused by a mosquito-borne RNA phlebovirus.³⁴⁶ In domestic animals, the disease is most severe in sheep and goats, resulting in high mortality in new-born lambs and kids and high percentage of abortion in pregnant sheep, goats and sometimes cattle. The infection in humans is usually associated with mild to moderate febrile illness (i.e. fever, myalgia, arthralgia, lethargy), but can progress to severe sequelae including ocular lesions and loss of vision, encephalitis, a haemorrhagic disease with necrotic hepatitis and a high case fatality rate in a minority of patients. Humans usually become infected from contact with virus-contaminated tissues and body fluids from infected animals, but mosquito bites can also transmit the virus. Outbreaks of the disease tend to occur when particularly heavy rains favour the breeding of competent mosquito vectors.

Since original isolation in Kenya,¹³⁵ RVF virus (RVFV) has been contained for decades within the African continent, but in the last 30 years emerged outside its historic boundaries⁵⁹³ including crossing significant geographic barriers: the Sahara desert into Egypt in 1977,³¹⁹ the Indian Ocean to Madagascar in 1991,^{439, 440} and the Comoros Islands in 2007,³⁴⁹ and  the Red Sea to the Arabian Peninsula in 2000/01.⁵⁵⁷ The unpredictable and sudden emergence of RVFV outside traditional endemic areas, unavailability of safe and efficacious antiviral treatment and prophylactic immunization, led the World Health Organization (WHO) to identify RVF as a priority disease for development of effective therapeutics and vaccines.⁴¹⁴

There have been multiple reviews including Henning,²⁵⁸ Weiss,⁶⁴⁹ Easterday,¹⁶⁷ Peters and Meegan,⁴⁹⁹ Shimshony and Barzilai,⁵⁵⁶ and Meegan and Bailey.⁴¹⁰ The renewed interest in RVF as a significant zoonotic threat has been a subject of numerous reviews covering various aspects of the disease, including epidemiology, pathogenesis, control/prevention, development of new diagnostic assays, therapeutics and vaccines,^{7, 35, 63, 66, 74, 165, 191, 271, 274, 287, 320, 331, 337, 361, 368, 383, 405, 416, 477, 485, 486, 490, 494, 503, 522, 593} molecular biology and genomics, ^{76, 209, 231, 270} viral and host determinants of virulence,²⁷⁴ surveillance, predictive models and control strategies emphasizing the One Health concept^{67, 203, 246, 331, 429} and ecological, climatic and anthropogenic factors that play a role in RVF emergence, re-emergence and spread.^{107, 384}

An acute and highly fatal disease of lambs associated with heavy rains and accompanied by reports of illness in humans was first recognized in the Rift Valley in Kenya at the turn of the century, but the causative agent was not isolated until 1930.^{134, 431, 579} Major outbreaks were subsequently recorded in Kenya in 1930/31, 1968, 1978/79, 1997/98, 2006/07, 2014/15 and 2019 and lesser outbreaks at irregular intervals during the intervening years.^{145, 146, 410, 666}

The disease was first recorded in southern Africa in the early 1950s when a large epidemic occurred in the western Free State, southern Gauteng and adjacent North West and Limpopo provinces of South Africa, although it was only recognized as RVF early in 1951 when humans became ill after assisting at a necropsy of a bull near Johannesburg.^{15, 453} Sheep farming dominates in some of the affected areas and it was estimated that 100 000 sheep died and 500 000 aborted in the epidemic, with smaller losses occurring in cattle.⁵⁴² Lesser outbreaks of the disease or sporadic isolations of virus were recorded in South Africa in 1952/53, 1955/59, 1969/71, 1981, 1996, and 2018.^{25, 45, 283, 392, 395, 402, 629, 649} After more than a decade with minor reported RVF activity, large outbreaks affecting most of South Africa occurred in 2009/10.^{30, 82, 418, 657}

The diversity of clinical disease presentation in humans was recognized first in Kenya, and during the 1950/51 epidemic in South Africa, when it was recognized that RVF could be accompanied by transient loss of visual acuity and serious ocular sequelae.^{217, 234, 539, 540} Human deaths following natural infection were first recorded in South Africa during the epidemic in 1975 when...