Diagnosis and Pathogenesis of Nairobi Sheep Disease Orthonairovirus Infections in Sheep and Cattle.

Nairobi sheep disease orthonairovirus (NSDV) is a zoonotic tick-borne arbovirus, which causes severe gastroenteritis in small ruminants. To date, the virus is prevalent in East Africa and Asia. However, due to climate change, including the spread of transmitting tick vectors and increased animal movements, it is likely that the distribution range of NSDV is enlarging. In this project, sheep and cattle (hitherto classified as resistant to NSDV) were experimentally infected with NSDV for a comparative study of the species-specific pathogenesis. For this purpose, several new diagnostic assays (RT-qPCR, ELISA, iIFA, mVNT, PRNT) were developed, which will also be useful for future epidemiological investigations. All challenged sheep (three different doses groups) developed characteristic clinical signs, transient viremia and virus shedding-almost independent on the applied virus dose. Half of the sheep had to be euthanized due to severe clinical signs, including hemorrhagic diarrhea. In contrast, the course of infection in cattle was only subclinical. However, all ruminants showed seroconversion-implying that, indeed, both species are susceptible for NSDV. Hence, not only sheep but also cattle sera can be included in serological monitoring programs for the surveillance of NSDV occurrence and spread in the future.

Nairobi sheep disease virus/Ganjam virus.

Nairobi sheep disease virus (NSDV) is a tick-borne virus which causes a severe disease in sheep and goats, and has been responsible for several outbreaks of disease in East Africa. The virus is also found in the Indian subcontinent, where it is known as Ganjam virus. The virus only spreads through the feeding of competent infected ticks, and is therefore limited in its geographic distribution by the distribution of those ticks, Rhipicephalus appendiculata in Africa and Haemaphysalis intermedia in India. Animals bred in endemic areas do not normally develop disease, and the impact is therefore primarily on animals being moved for trade or breeding purposes. The disease caused by NSDV has similarities to several other ruminant diseases, and laboratory diagnosis is necessary for confirmation. There are published methods for diagnosis based on polymerase chain reaction, for virus growth in cell culture and for other simple diagnostic tests, though none has been commercialised. There is no established vaccine against NSDV, although cell-culture attenuated strains have been developed which show promise and could be put into field trials if it were deemed necessary. The virus is closely related to Crimean-Congo haemorrhagic fever virus, and studies on NSDV may therefore be useful in understanding this important human pathogen.

Nairobi sheep disease.

Nairobi sheep disease is probably the most pathogenic virus known for sheep and goats. It is transmitted by an Ixodid tick, both trans-stadially and transovarially and causes an acute gastroenteritis. In totally susceptible populations, mortality rates of over 90% regularly occur. The infection also causes abortion. The disease is known to occur in East Africa, Somalia and Rwanda. It may exist in the south east of Ethiopia. No evidence for its existence has been found in those parts of Africa where the principle vector tick, Rhipicephalus appendiculatus has a seasonal breeding cycle. Thus countries like Zambia, Zimbabwe and Botswana appear to be free from the disease.

General review of tick-borne diseases of sheep and goats world-wide.

A general review of the tick-borne diseases of sheep and goats is given, with the emphasis on those thought to be of greatest economic importance. These include babesiosis, theileriosis, cowdriosis, anaplasmosis, ehrlichiosis, Nairobi sheep diseases and tick paralysis. A commented list of tick-borne diseases and their vectors is presented. It is stressed that large gaps remain in our knowledge of the real importance in the field of many of these diseases, especially in local stock.

Investigation of tick-borne viruses as pathogens of humans in South Africa and evidence of Dugbe virus infection in a patient with prolonged thrombocytopenia.

In the course of investigating suspected cases of viral haemorrhagic fever in South Africa patients were encountered who had been bitten by ticks, but who lacked evidence of infection with Crimean-Congo haemorrhagic fever (CCHF) virus or non-viral tick-borne agents. Cattle sera were tested by enzyme-linked immunoassay to determine whether tick-borne viruses other than CCHF occur in the country. The prevalence of antibody in cattle sera was 905/2116 (42.8%) for CCHF virus, 70/1358 (5.2%) for Dugbe, 21/1358 (1.5%) for louping ill, 6/450 (1.3%) for West Nile, 7/1358 (0.5%) for Nairobi sheep disease, 3/625 (0.5%) for Kadam and 2/450 (0.4%) for Chenuda. No reactions were recorded with Hazara, Bahig, Bhanja, Thogoto and Dhori viruses. The CCHF findings confirmed previous observations that the virus is widely prevalent within the distribution range of ticks of the genus Hyalomma, while antibody activity to Dugbe antigen was detected only within the distribution range of the tick Amblyomma hebraeum. Cross-reactivity for the nairoviruses, Hazara, Nairobi sheep disease and Dugbe, was detected in serum samples from 3/72 human patients with confirmed CCHF infection, and serum from 1/162 other patients reacted monospecifically with Dugbe antigen. The latter patient suffered from febrile illness with prolonged thrombocytopenia.

Developmental disorders of the fetus in some arthropod-borne virus infections.

A number of arboviruses have been associated with congenital defects in domestic aminals and man. In this review comparison is made of the temporal association between epidemics of arboviruses affecting man and animals in which there is an obvious relationship between the infection and the fetal defects, and arboviruses which cause no overt clinical symptoms in the vertebrate host but result in deformities of the fetus. The danger to the fetus following the use of live attenuated virus vaccines against several important arbovirus diseases is also examined. It is concluded that arboviruses which are capable of infecting humans or animals without producing overt clinical signs, and attenuated vaccine viruses pose the greatest threat to the fetus.

Nairobi sheep disease in Kenya. The isolation of virus from sheep and goats, ticks and possible maintenance hosts.

Nairobi sheep disease was seen principally upon movement of susceptible animals into the enzootic areas. This occurred most frequently for marketing purposes near the main centres of population. Other outbreaks followed local breakdowns in tick control measures. The disease did not occur in epizootic form during the period under consideration. Nairobi sheep disease was isolated from pools of Rhipicephalus appendiculatus but not from many pools of other tick species. No virus was isolated from the blood or tissues of a range of wild ruminants and rodents.