Tick Borne Viruses: Nairobi Sheep Disease/Ganjam Disease

Introduction

Nairobi sheep disease virus (NSDV) is a one of the viruses transmitted by tick vectors and causes Nairobi sheep disease (NSD) – a disease of small ruminants. The disease was first identified as a problem in sheep and goats in Kenya in 1910. The virus belongs to the genus Nairovirus and family Bunyaviridae (Fenner, 2017). An equally pathogenic variant of NSDV is Ganjam virus (GANV), which is found in India and Sri Lanka (bin Tarif et al ., 2012). Both viruses are closely related to the human pathogen Crimean-Congo haemorrhagic fever virus (CCHFV) (Burt et al.,1996) a fever transmitted through infected tick bites or through contact with infected animal blood or tissues during and immediately after slaughter. Other members of the genus include Dugbe virus, Hazara virus and Kupe virus, both isolated from cattle ticks in East Africa. The virus is spread by hard (Ixodid) ticks, and appears to be dependent on the tick vector for dissemination, with no direct transmission between animals (bin Tarif et al., 2012).

The transmission of Tick Borne Viruses TBV occurs primarily during tick feeding and is a complex process, known to be promoted by tick saliva constituents. However, the underlying molecular mechanisms of TBV transmission are poorly understood. Immuno-modulatory properties of tick saliva helping overcome the first line of defense to injury and early interactions at the tick-host skin interface appear to be essential in successful TBV transmission and infection of susceptible vertebrate hosts. The local host skin site of tick attachment, modulated by tick saliva, is an important focus of virus replication (Kazimírová et al., 2017).

Tick borne viruses affect animals, humans and other vertebrate hosts causing various diseases in different parts of the world. The viruses are found in six different virus families (Asfarviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, Bunyaviridae, and Flaviviridae) Bunyavirales (International Committee on Taxonomy of Viruses ICTV, 2018). and in at least 9 genera (Fenner F, Peter A. Bachmann, 1987) . From recent studies, some yet to be assigned tick-borne viruses are thought to a seventh family, the Arenaviridae. With only one exception (African swine fever virus, family Asfarviridae) all tick-borne viruses (as well as all other arboviruses) are RNA viruses (Nuttall & Labuda, 2004). Until recently, the NSDV virus was only known to exist in parts of Africa and on the Indian subcontinent. However, viral RNA was recently identified in ticks in China identified by viral metagenomic analysis of ticks collected from the northeast region of the People’s Republic of China (Gong et al., 2015).

NSDV is the most pathogenic tick borne viral diseases of small ruminants. In susceptible animals, this tick-borne viral infection results in a hemorrhagic gastroenteritis with very high morbidity and mortality rates of up to 90%. Other accompanying clinical manifestations include Fever, reduction in white blood cells, rapid respiration, anorexia, profound depression, diarrhea, drop in body temperature, pregnant animals frequently abort, death - in some cases within 12 hours of the onset of the fever (Montgomery, 1917).

In humans, NSDV infection can cause febrile illness, headache, nausea, and vomiting (Yadav et al., 2011). Vaccines have been developed which showed considerable promise when tested in the laboratory, but there is no demand for these in the field. The obligate vector nature of ticks causing NSD may explain why the virus is not seen as a major economic threat, since young animals in endemic areas tend to be protected by maternal antibodies through the period where they are first exposed to the virus via a bite from an infected tick, after which they have their own immune protection. The disease tends to be only noticed on introduction of naive livestock into an endemic area, e.g. for the purposes of improving local breeds by crossing (bin Tarif et al., 2012). This is due to the presence of a steady enzootic state which ensures that high levels of immunity are present in the small-ruminant populations at risk. No interventions such as vaccination, or tick control are indicated except during outbreaks when tick control is advised. There is no effective treatment, and eradication is generally not feasible once the virus has become established in ticks (Calisher & Fenner, 2000; Cobo, 2014; Craighead, 2000; Payne, 2012; Strauss & Strauss, 2008)