Nairobi Sheep Disease
Nairobi Sheep Disease is a tick-borne viral disease, causing a haemorrhagic gastro-enteritis (bloody diarrhea) with high fever, pulmonary oedema (lung damage), abortion and high mortality - in susceptible sheep up to 90%.
Nairobi Sheep Disease occurs in Eastern Africa from Ethiopia , Somalia , through Kenya , Uganda and Tanzania , to Mozambique , Botswana and the Congo .
The causal nairovirus, of the family Bunyaviridae, is probably the most deadly virus known for sheep and goats. It is identical to, or closely related to, Ganjam virus, a tick-borne infection of sheep, goats and people in India; and is serologically related to Dugbe virus, another tick-borne infection occurring in cattle in West Africa, and to Crimean-Congo Haemorrhagic Fever virus of man. Man may be infected by all of these viruses, and with the exception of Crimean-Congo Haemorrhagic Fever, which causes a severe haemorrhagic disease in man, the symptoms are those of a mild febrile illness, characterized by generalized abdominal pain, headache and backache.
Mode of Spread
The principal tick vector is the Brown Ear Tick, Rhipicephalus apendiculatus, but other tick species, such as Amblyomma may also be responsible for transmitting the disease. The virus can survive for at least 18 months in unfed ticks. This means that eradication is very difficult.
The virus cycles silently between indigenous sheep and goats and brown ear ticks. Clinical disease occurs when sheep and goats reared in tick-free areas are moved into tick-infested areas, or when sheep and goats from tick-infested areas enter tick-free areas while still harbouring infected ticks, which in turn feed on and infect susceptible sheep and goats.
- Inapparent natural infections also occur in antelopes, and there is some evidence that the peri-domestic rat of East Africa is also naturally infected.
- Cattle appear to be resistant.
- Although the antibody prevalence in man is high in Eastern and Southern Africa , clinical symptoms rarely occur.
- Although the virus is shed in urine and faeces, the disease is NOT spread by contact.
Signs of Nairobi Sheep Disease
The incubation period is about 4-5 days, followed by a fever lasting for 1-3 days.
Illness is manifested by
- lack of appetite,
- pus/blood-stained nasal mucus discharge and
- asmelly diarrhea causing painful straining.
- Pregnant animals frequently abort.
- Some animals may die within 3 days. In others the disease is prolonged for 5-9 days.
Animals reared in areas infested with infected ticks are usually resistant. In contrast symptoms in susceptible sheep from previously non infected areas is serious, and mortality may reach 90%. The reactions in young lambs and kids are not as severe as in adults.
A natural attack gives survivors with a life-long immunity with the production of antibodies.
Post-mortem reveals a severe haemorrhagic gastro-enteritis and enlargement of all lymphoid tissue. In addition small haemorrhages often stud the outside and inside of the heart. Skin haemorrhages have been seen in the fetuses of pregnant ewes which die. Haemorrhages have also been seen in several other organs, such as the caecum, colon, gallbladder and kidney. Conjunctivitis and dried crusts around the nostrils are also often seen.
The post-mortem signs, together with a history of movement of susceptible animals into a tick-infested area, should give rise to a presumptive diagnosis of Nairobi Sheep Disease.
Confirmation requires the detection of virus or viral antigen or antibodies. The preferred specimens are blood in anti-coagulant or serum from febrile animals, mesenteric lymph nodes and spleen, forwarded to the laboratory on wet ice. Various laboratory tests are available to confirm the diagnosis.
Diseases with similar symptoms
This includes Peste des Petit Ruminants, Rift Valley Fever, Heartwater, Salmonellosis, Parasitic Gastro-enteritis and Coccidiosis.
In endemic areas infections with Nairobi Sheep Disease virus are silent and here infection does not warrant the application of control measures. However, sheep and goats imported into tick-infested areas should be vaccinated prophylactically before movement, or as soon as possible after they arrive.
Attenuated and inactivated vaccines are available, the latter being the more effective.
Treatment and Control
There is no specific treatment and any treatment is purely symptomatic, and supportive.
Unaffected animals in a flock may be treated with acaricides to try to lessen infection by infected ticks.