Nairobi Sheep Disease Virus

Nairobi Sheep Disease Virus Background

Nairobi sheep disease (NSD) virus, the prototype tick-borne virus of the genus Nairovirus, family Bunyaviridae is associated with acute hemorrhagic gastroenteritis in sheep and goats in East and Central Africa and the disease may be present in Botswana and Mozambique. It causes 70-90 per cent mortality among a susceptible population (Sudeep et al. 2009). Members of the family Nairoviridae produce enveloped virions with three single-stranded RNA segments comprising 17.1 to 22.8 kb in total. The negative-sense RNA molecules are termed S (small), M (medium; if present), and L (large). These RNAs encode respectively, in the virus-complementary sense, N, the GP precursor (GPC), and L (containing RdRP, helicase, and endonuclease domains) (Garrison et al., 2020).

NSDV is transmitted through tick bites. The most important vector is the tick Rhipicephalus appendiculatus, but it can be transmitted through other vectors such as R. pulchellus, R. simus, and Amblyomma variegatum. After infection, adult ticks can transmit this virus for more than two years (Baron & Holzer, 2015). Crimean-Congo hemorrhagic fever virus is also tick-borne and is endemic in most of Asia, Africa, Southern and Eastern Europe.

NSDV causes Nairobi sheep disease which is one of the most pathogenic diseases of sheep and goats. High morbidity and mortality are seen in both sheep and goats, although goats tend to have less severe clinical signs than sheep (Baron & Holzer, 2015). Symptoms of an animal infected with NSDV may include fever, reduction in white blood cells, rapid respiration, anorexia, profound depression, diarrhea, drop in body temperature, premature abortion, and eventually death (Baron & Holzer, 2015).

Ganjam virus (GANV) is antigenically related to African NSDV, and recent molecular studies have demonstrated that GANV is an Asian variant of NSDV and both these viruses are related to Crimean Congo haemorrhagic fever (CCHF) group viruses. GANV has been associated with both febrile illness in humans and disease in livestock (Yadav et al., 2011). The versatility of the virus to replicate in different arthropod species, its ability to infect sheep, goat and man makes it an important zoonotic agent (Sudeep et al. 2009).

References

• Baron M D, Holzer B. Nairobi sheep disease virus/Ganjam virus. Rev Sci Tech. 2015 Aug;34(2):411-7.
• Garrison et al., Consortium IR. ICTV Virus Taxonomy Profile: Nairoviridae. J Gen Virol. 2020 Aug;101(8):798-799.
• Sudeep et al. Ganjam virus. Indian J Med Res. 2009 Nov;130(5):514-9.
• Yadav et al. Genomic analysis reveals Nairobi sheep disease virus to be highly diverse and present in both Africa, and in India in the form of the Ganjam virus variant. Infect Genet Evol. 2011 Jul;11(5):1111-20.