Isolation and purification of Nairobi sheep disease virus for development of a thermostable vaccine
Nairobi sheep disease virus (NSDV) causes severe illness in sheep and goats, with fatalities reaching up to 90% in affected animals. This disease is widespread in Eastern Africa and in Indian subcontinent where it is known as Ganjam virus. It causes considerable losses, especially to small scale farmers. Sheep and goats are also an important source of proteins and provide income through sales. The control of NSDV will thus make a positive contribution to the rural poor. This project aims to contribute towards control and eradication of NSDV, by developing a vaccine that is effective against NSDV. Field studies will be done to obtain tick samples, NSDV isolated from the ticks, and grow in the laboratory. The purified NSDV isolates will be inactivated using formalin to generate an inactivated vaccine candidate and this vaccine will then be tested in sheep to confirm that the vaccine is indeed protective against NSDV.
Nairobi sheep disease virus (NSDV), the prototype tick-borne virus of the genus Nairovirus, family Bunyaviridae is associated with fever, acute haemorrhagic gastroenteritis, abortion, high morbidity in naïve animals with a mortality up to 90% and is endemic in East and Central Africa. In human, NSDV causes mild, influenza-like disease. The virus has no vaccine or specific antiviral treatment.
This study aimed to genetically characterize NSDV and other tick-borne arboviruses from the ticks collected from selected parts of Kenya, purify one of the NSDV isolates, and use it as a vaccine candidate in sheep. The ticks that were collected from selected sites in Kenya were identified morphologically to species using available taxonomic keys and then pooled up to eight ticks per pool, by collection site, species and sex. The ticks collected per site were: 36.5% in Narok, 46.2% in Laikipia and 17.3% in Kiambu. The most abundant tick species in all the three sites were Rhipicephalus evertsi evertsi and Rhipicephalus appendiculatus. The cultured homogenized tick pools, after identifying the infecting viruses using RT-PCR and sequencing, will provide useful genetic information that will determine the evolutionary and genetic diversity of these circulating pathogens.
The vaccine candidate was inactivated using beta-propiolactone and animals were vaccinated and boosted with similar formulation 14 days post immunization for one group. Antibody titres assayed with highest titre at 2048 most averaging 2 logs less for the animals that received single shots. For Animals that received a single shot, titre averaged between 64 and 128 with highest at 512. Our vaccination experiment in a sheep model tested the protective effect of NSDV Vaccine candidate and it has shown that the double shot of the vaccine has a potential to protect the animals from NSDV attack.