Nairobi Sheep Disease/Ganjam Virus

Nairobi sheep disease (NSD) was first reported from Kenya in the early 1900’s as a cause of severe disease with high mortality in sheep and goats. Subsequently, the virus has been identified more broadly in East and Central Africa. A similar disease called Ganjam was described in India and later in Sri Lanka and parts of China. Genomic sequencing revealed that the two viruses are one and the same.

NSD/Ganjam virus is a member of the Nairovirus family within the Bunyavirales order of viruses. It is closely related to other tick-borne viruses including Crimean-Congo hemorrhagic fever virus, a serious human pathogen, and a number of viruses isolated from diverse mammals and birds. The virus replicates well in a variety of established cell lines.

Transmission and Host Range

These viruses are transmitted by Ixodid (hard) ticks and animal-to-animal contact does not seem to occur. Sheep and goats are the only known susceptible hosts and infection in naïve animals is highly virulent, with mortality rates approaching 90%. Cattle, buffalo, horses and humans do not appear susceptible to infection or disease, although rare cases of mild human disease following laboratory accidents have been reported.

The Disease in Sheep and Goats

The typical incubation period for NSD is 2-6 days. Prominent clinical signs include very high fever, profound depression, conjunctivitis, nasal discharge, profuse and foul-smelling diarrhea that is often hemorrhagic. Affected animals usually die several days after onset of clinicals signs. In some case, peracute death with the absence of clinical signs has been observed. As expected, pregnant animals frequently abort. In the face of an outbreak, the prognosis for animals with clinical disease is extremely poor. The disease is somewhat less severe in goats compared to sheep.

In both Africa and India, distinct differences in the pattern of mortality among indigenous versus exotic breeds of sheep have been observed. For NSD, indigenous breeds seems to suffer higher rates of mortality compared to exotic or crossbred animals, whereas the reverse has been reported for Gangam in India.

Pathogenesis and Pathology

Experimental infection of sheep and cattle with NSD virus inoculated subcutaneously revealed a number of interesting points:

• Viremia was first detected 1-4 days after virus inoculation and was maximal at 3-6 days
• Infected animals shed considerable amounts of virus in oral and rectal swabs, as detected by PCR assay of viral RNA; interestingly, infectious virus was not detected.
• Clinical signs were very similar to those described from field and included profuse, often bloody diarrhea, conjunctivitis and nasal discharge, and inappetence.

Major findings at necropsy of animals that have died of NSD included enlarged and edematous lymph nodes and hemorrhages in the gastrointestinal tract, particularly the glandular stomach (abomasum). Hemorrhages in the colon and cecum often appear as streaks.

Disease Control

The foundation for control of NSD/Ganjam is tick control, implemented through dips or sprays. Experimental vaccines have been developed but are not yet commercially available.

References and Reviews

• Bin Tarif A, Lasecka L, Holzer B, Baron MD. Ganjam virus/Nairobi sheep disease virus induces a pro-inflammatory response in infected sheep. Vet Res. 2012; 43:71.
• Hartlaub J, Gutjahr B, Fast C, Mirazimi A, Keller M, Groschup MH. Diagnosis and pathogenesis of Nairobi sheep disease Orthonairovirus infections in sheep and cattle. Viruses 2021; 13: 1250
• Walker, PJ, Widen SG, Wood TG, Guzman H, Tesh RB, Vasilakis N. A Global genomic characterization of Nairoviruses identifies nine discrete genogroups with distinctive structural characteristics and host-vector associations. Am J Trop Med Hyg. 2016; 94, 1107–1122.

Updated May 2022. Send comments to Richard.Bowen@colostate.edu