Prevention and control of tick born disease in sheep and goat

26/05/2023

Tick-borne viruses affecting sheep and goats can be be locally important diseases. Viruses of the tick-borne encephalitis complex occur across Europe to North-East Asia and are the cause of significant losses in small ruminant production. Thogoto virus which can be transmitted by Ixodid ticks has been associated with abortion storms in sheep, it may contribute to the large proportion of abortions which are due to unknown causes. Nairobi sheep disease is the most pathogenic virus disease known for sheep and goats in East Africa. Ganjam virus in India may ultimately be shown to be a significant cause of disease in small ruminants. Some unidentified Orbiviruses have been found both in ticks and sick sheep, and are thought to be a cause of some losses in African sheep and goats. Small ruminant populations in both Africa and Asia are vertebrate host for Congo-Crimean haemorrhagic fever, an important human pathogen, and for many Arboviruses some of which are of zoonotic importance.

08/10/2022

Vaccines
Formalin-inactivated veterinary vaccines against Rift Valley fever and Nairobi sheep disease have been available for a number of years, but vaccines for use in humans are still experimental. Laboratory workers immunized with inactivated Rift Valley fever virus vaccines develop high levels of neutralizing antibodies and remain free from infection. Attenuated strains of Rift Valley fever virus have been developed with a view to their possible use as live viral vaccines. Hantavirus vaccines are being developed: one prepared using brains of newborn mice has been used in South Korea with no significant adverse reactions.

08/10/2022

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.

08/10/2022

Clinical Findings of Nairobi Sheep Disease
The incubation period of Nairobi sheep disease is usually 2–6 days. Clinical signs begin with a steep rise in body temperature (41°–42°C [105.8°–107.6°F]), which persists for 1–7 days. Leukopenia and viremia usually coincide with the febrile phase. Common clinical signs include depression, anorexia, mucopurulent to hemorrhagic nasal discharge, occasional conjunctivitis, and fetid dysentery that causes painful straining. Diarrhea usually occurs 1–3 days after the onset of fever and worsens as the illness progresses. Pregnant animals frequently abort. Death tends to occur 2–7 days after the initial clinical signs, with some animals succumbing before the characteristic clinical signs appear; however, some sick animals survive up to 11 days. Death in the later stages of the illness is usually associated with dehydration and debilitation from diarrhea. The prognosis is poor in animals with severe clinical signs.
Breed-related differences in susceptibility have been reported in the field, although not necessarily substantiated by studies in experimentally infected sheep. In Africa, the mortality rate in the field is as high as 70%–90% for indigenous breeds of sheep; however, it is 30% for exotic and cross-breeds. Conversely, Ganjam virus causes higher mortality in exotic than indigenous breeds in India. The clinical signs in goats are similar to those in sheep but usually less severe, although mortality up to 80% has been reported. The presence of colostral immunity not only protects lambs and kids from exposure early in life but also allows development of active immunity, enabling survival in tick-infested areas.

08/10/2022

Etiology and Transmission of Nairobi Sheep Disease
Nairobi sheep disease virus (NSDV) is classified in the genus Orthonairovirus, family Nairoviridae. It is a member of a group of serologically and genetically related bunyaviruses such as Dugbe virus, Hazara virus, and Crimean-Congo hemorrhagic fever virus.
The principal vector for NSDV in Africa is the brown ear tick (Rhipicephalus appendiculatus) in which the virus can survive up to 800 days. Transovarial and transstadial transmission occurs via this vector. The unfed adult ticks can transmit the virus for >2 years after infection. Haemaphysalis intermedia is the principal tick vector in Asia; however, Ganjam virus also occurs in other members of Haemaphysalis, which could be important in specific geographic areas. Additional ticks, including Rhipicephalus spp and Amblyomma variegatum ticks in Africa, also may be involved in transmission of these viruses. Nairobi sheep disease virus is shed in urine and f***s; however, the disease is not transmitted via contact between animals.
Clinical cases tend to occur when naive animals are moved into endemic areas or when climatic conditions, such as heavy and prolonged rainfall, allow the tick population to increase or expand into nearby areas

08/10/2022

Nairobi sheep disease (NSD) is a tickborne viral disease of sheep and goats characterized by fever, hemorrhagic gastroenteritis, abortion, and high mortality. It is possibly the most pathogenic virus of small ruminants. Most other species do not appear susceptible; however, a few clinical cases have been reported in blue duikers (Cephalophus monticola). The African field rat (Arvicanthus abyssinicus nubilans) is a potential reservoir host. Reports of endemic NSD are mainly clustered in eastern and central Africa from Ethiopia to Democratic Republic of the Congo; however, serologic evidence suggests that this or a similar virus might also occur elsewhere in Africa (eg, Botswana and Mozambique). A variant known as Ganjam virus, once thought to be a separate virus, circulates in parts of Asia including India, Sri Lanka, and China.

08/10/2022

In endemic areas, clinical cases are infrequent unless susceptible animals are introduced. Such animals should be vaccinated, as should those exposed when the range of the tick vector extends. Two types of experimental vaccines have been developed—a modified-live virus vaccine attenuated in mouse brain and an inactivated oil adjuvant vaccine. A single dose of the modified-live virus vaccine produces rapid immunity; however, revaccination is necessary to maintain full protection. Two doses of the inactivated vaccine are required to elicit good protection. Neither of these vaccines is produced commercially.
Acaricides used short-term in areas bordering enzootic areas as a barrier to range expansion of the tick vector. Long-term tick control for prevention of NSD is not feasible

10/06/2022

Nairobi sheep disease virus (NSDV) infects sheep and goats and is transmitted by ticks. The virus belongs to the family Bunyaviridae, in the genus Nairovirus. NSDV has a single-stranded RNA genome, that is surrounded by a capsid and an envelope.
Associated diseases:
NSDV causes Nairobi sheep disease which is one of the most pathogenic diseases of sheep and goats. High morbidity and mortality are seen in both sheep and goats, although goats tend to have less severe clinical signs than sheep.
Clinical Signs:
Fever
Reduction in white blood cells
Rapid respiration
Anorexia
Profound depression
Diarrhoea
Drop in body temperature
Pregnant animals frequently abort
Death - in some cases within 12 hours of the onset of the fever
Disease transmission:
NSDV is transmitted through tick bites. The most important vector is the tick Rhipicephalus appendiculatus, but it can be transmitted through other vectors such as R. pulchellus, R. simus, and Amblyomma variegatum. After infection, adult ticks can transmit this virus for more than two years.
Disease prevalence:
NSD is found in East and Central Africa and the disease may also be present in Botswana and Mozambique. A variant of NSD (Ganjam virus) has been reported from parts of Asia including India and Sri Lanka.
Impact for Society – what are we doing?
Research at the Institute is ongoing to discover how NSDV causes disease in the host and investigating the species specificity of disease by comparing host responses to the virus in sheep and cattle. Effort is also going into determining how NSDV interferes with host defences and using the information to create a weakened virus that may be useful as a vaccine in the field.

12/05/2021

07/05/2021

Evolutionary relationships of nairoviruses
The evolutionary history of the viruses was inferred from the known
full-length N protein sequences of the indicated viruses using the
neighbour-joining method (19). Sequences were aligned using ClustalW
(20). The tree is drawn to scale, with branch lengths in the units of the
number of amino acid substitutions per site, which were computed using
the James–Taylor–Thornton matrix-based method (21). The percentage
of replicate trees in which the associated taxa clustered together in the
bootstrap test (1,000 replicates) are shown next to the branches (22).
Evolutionary analyses were conducted in MEGA6 (23). For each sequence,

07/05/2021

the genus Nairovirus, and is thus an enveloped virus with a
genome consisting of three segments of negative-sense RNA,
S (small), M (medium) and L (large); these segments encode,
respectively, the nucleocapsid (N) protein, the surface
glycoproteins (Gn and Gc) and the viral RNA polymerase
(L). Sequencing showed that the N and L proteins of NSDV
and GANV are almost identical, with only around 10% or
2% (respectively) difference at the nucleotide level, leading
to the conclusion that they represent different isolates of
the same virus (16, 17). Notably, the sequences of the viral
surface glycoproteins differed more between GANV and
NSDV isolates than did the core proteins (16), possibly
reflecting the use of different tick hosts by viruses from
different continents (18). The relationships of NSDV and
GANV to each other and to other viruses of the same genus,
based on the sequence of the viral genomes, are illustrated
in Figure 2. It is notable that the Indian and African isolates
do not appear to be distinct lineages (16), but are located in
the phylogenetic tree as members of a single clade.
NSDV/GANV therefore represents a virus that is widespread
in India and in East Africa, and may also be found in other
parts of the Indian subcontinent if/when it is looked for.
The virus appears to exist without causing notable disease
in native sheep/goats in India, yet causes a severe disease in
imported European animals in India, and in local breeds of
sheep (often derived from Persian fat-tailed sheep) in East
Africa. This suggests that the virus is well adapted to its
mammalian hosts in India, but is not (yet) adapted to other
breeds. Coupled with the fact that East Africa lacks native
wild sheep/goat species which might have been the original
hosts, these observations suggest that it is possible that the
virus originated in India and was brought into Africa more
recently, either via a bird-borne tick or on animals shipped
from India, that being the main livestock trading direction
during the 18th and19th centuries; this was, for example,
the route by which rinderpest virus was originally brought
into Africa.
Diagnosis
In susceptible animals, the virus causes a fever which lasts
for three to seven days. Diarrhoea, frequently containing
blood, is seen one to three days after the fever starts.
Animals rapidly become lethargic and unresponsive and die
about a week after the fever starts. Goats show a similar
pattern to sheep, just with milder signs and much lower
mortality (1, 2). Haemorrhagic petechiae can be seen
in the nasal mucosa and in the coronary band above the
hoofs. Post-mortem examination shows typical signs of
haemorrhage in the longitudinal folds of the lower gastric
tract, most severe in the caecum and colon, and sometimes
in the respiratory mucosa. The mesenteric lymph nodes and
spleen can be swollen. The clinical picture of NSD is usually
distinguishable from PPR by the absence of mouth lesions
in the former, and the later onset of diarrhoea in the latter,
often after the febrile period (authors’ own observations).
Peste des petits ruminants virus will have higher morbidity,
unless the tick infestation is unusually high. Paratyphoid
dysentery can have a similar clinical picture to NSD, as can
Rift Valley fever, if there is gastrointestinal involvement.
The easiest and most sensitive method of laboratory
confirmation of NSDV/GANV infection is reversetranscription polymerase chain reaction (RT-PCR). The
virus can be detected by isolation from blood only during
the febrile period (1), but, using PCR, the authors have
found virus genome in blood even after temperatures have
returned to normal (24). Another advantage of RT-PCR

05/04/2021

Nairoviruses are tick-borne viruses that includes pathogens of man, such as Crimean Congo hemorrhagic fever virus (CCHF) and livestock animals, such as Nairobi sheep disease virus (NSDV). CCHFV is almost completely insensitive to IFN-α (Andersson et al., 2008), but not IFN-β (Andersson et al., 2006; Weber and Mirazimi, 2008). Replication of CCHFV delays the IFN response, and it has been speculated that this may occur through interfering with the activation pathway of IRF3, although the mechanism has not been demonstrated (Andersson et al., 2008; Weber and Mirazimi, 2008).

Ubiquitin (Ub) and interferon-stimulated gene product 15 (ISG15) reversibly conjugate to proteins and mediate important innate antiviral responses. OTU domain-containing proteases from the viral RNA polymerase of CCHFV hydrolyze Ub and ISG15 from cellular target proteins, and antagonize the antiviral effects of ISG15 (Garcia-Sastre et al., 2007). NSDV was also shown to inhibit the induction of both type I and type II IFNs through the ovarian tumor-like protease domain (OTU) found in the NSDV RNA polymerase (Holzer et al., 2011). Among viruses of the genus Bunyavirus are LaCrosse and Akabane viruses, the etiologic agents of important diseases of humans and livestock, respectively, and at least 165 others; these are transmitted by mosquitoes or culicoid flies. The genus Phlebovirus includes Rift Valley fever and the sandfly fever viruses, etiologic agents of human, livestock, and wild animal diseases of considerable importance, as well as 50 others; these are transmitted by biting flies (the phleboviruses) and ticks (the uukuviruses). Thirty-four viruses comprise the genus Nairovirus, to which belong Congo–Crimean hemorrhagic fever and Nairobi sheep disease viruses, the etiologic agents of diseases of livestock and of humans; these are transmitted by ticks. Tomato spotted wilt virus, a plant pathogen, and three other viruses, comprise the genus Tospovirus; these are transmitted by thrips. Another genus of the family is Hantavirus, comprising 28 viruses. Consequent to the great numbers of viruses, their wide geographic distributions, their capacity to replicate in a wide variety of hosts, and their remarkable genomic plasticity, the family Bunyaviridae must be considered one of the most remarkable of all RNA virus taxa.

An example of the extraordinary capacity of these viruses is Congo–Crimean hemorrhagic fever (C–CHF) virus. Isolated in 1956 from a human with fever, headache, nausea, vomiting, backache, joint pains, and photophobia in the Democratic Republic of Congo (the former Zaire), the virus has been recovered from humans, livestock, and many species of ticks. In 1967 it was shown that the etiologic agent of Crimean hemorrhagic fever was a virus named after that disease in Asia. When it was realized that the two viruses (Congo virus and Crimean hemorrhagic fever virus) were the same, the names and data were merged. C–CHF virus is now recognized to occur from Africa to Asia, including portions of southern Europe. C–CHF virus may occur focally, with human and livestock disease unrecognized, or may be recognized as a persistent zoonosis with periodic outbreaks.

The principal natural vertebrate hosts of C–CHF virus are hares and hedgehogs (hosts for the immature stages of the tick vectors) and livestock (hosts for the adult stages of the tick vectors). Birds may serve as hosts for some of these tick species and may provide a means for transportation of C-CHF virus over wide geographic areas.

Human disease caused by C-CHF virus occurs in rural areas, usually related to infections of cattle infested with virus-infected ticks. Human infections usually occur in livestock handlers slaughtering cattle and bitten by infected ticks, but secondary human infections occur when hospital personnel handle virus-contaminated blood from hemorrhaging patients. Thus, the ecology of this virus involves a variety of vertebrate and invertebrate hosts in defined but focal situations. Macroecology and Microecology of Crimean–Congo Hemorrhagic Fever Virus (family Bunyaviridae, genus Nairovirus)
The virus family Bunyaviridae is comprised of more than 300 viruses, making it the largest family of vertebrate viruses. Viruses within each genus have in common the different 3′ and 5′ termini of each of their three RNA species. However, viruses of different genera have different 3′ and different 5′ termini (sequences of 3′ termini of bunyaviruses, hantaviruses, nairoviruses, and phleboviruses are shown in Table III). These characteristics serve as the basis for genus placement within the family Bunyaviridae, but, ultimately, they reflect divergent evolutionary events, a result of chance mutations and adaptation to defined hosts by selection of the winners.

TABLE III. 3' Nucleotide Sequences of L, M, and S RNA Segments of Representative Viruses of Genera within the Family Bunyaviridae

Genus Virus RNA segment 3' terminusa
Bunyavirus LaCrosse S 3' UCAUCACAUGAGGUG
M 3' UCAUCACAUGAUGGU
L 3' UCAUCACAUGAGGAU
Hantavirus Hantaan S 3' AUCAUCAUCUGAGGG
M 3' AUCAUCAUCUGAGGC
L 3' AUCAUCAUCUGAGGG
Nairovirus Qalyub S 3' AGAGAUUCUGCCUGC
M 3' AGAGAUUCUUUAUGA
L 3' AGAGAUUCUUUAAUU
Phlebovirus Rift valley fever S 3' UGUGUUUCGG
M 3' UGUGUUUCUGCCACGU
L 3' UGUGUUUCUG
a Nairoviruses
The Nairovirus genus contains two important human pathogens: the Crimean-Congo hemorrhagic fever group, which includes Crimean-Congo hemorrhagic fever virus (CCHFV) and Hazara virus, and the Nairobi sheep disease group, which includes Nairobi sheep disease virus (NSDV) and Dugbe virus. Both CCHF and NSD group viruses are transmitted primarily by ticks though the virus has been isolated from culicoides flies and mosquitoes. CCHFV is widely distributed throughout Central Asia and Africa and NSDV in parts of Africa. Both are maintained in the environment by domestic animals as sheep, goat, and cattle, with humans becoming infected either through the bite of an infected tick or by inoculation from the slaughtering of infected animals. CCHF is transmitted to humans by the bite of infected Hyalomma species ticks. Domestic animals serve in the transmission cycle as the viral amplifying host or reservoir. Ticks maintain the virus in the environment and can remain infected for long periods and infect their progeny by transovarial transmission. Risk for human infection primarily involves behavior that increases the potential exposure of bites from infected ticks, handling of infected carcasses, and nosocomial transmission. In a study conducted in northern Senegal, antibody prevalence to CCHFV was similar among the s*xes, increased with age, and was related to herding activities, sleeping outdoors, bite by a tick, or contact with sick animals. Abattoir workers are at particular risk. An investigation of an outbreak largely associated with workers in Saudi Arabia and the United Arab Emirates demonstrated both clinical disease and high antibody seroprevalence rates; Pakistan has experienced 13 outbreaks of CCHF since 1976 that were associated with significant nosocomial transmission among health care professionals due to contact with infected body fluids and blood. CCHF in humans is associated with hemorrhagic fever with a 3–7 day incubation period, and the time between the onset of fever to hemorrhagic manifestations is brief. The time from the onset of fever, chills, headache, and muscle pains to the time of severe hemorrhage can be 3 days, with death occurring on the sixth day of illness. Other signs and symptoms associated with CCHF are vomiting, diarrhea, and throat pain. Mortality from CCHF can be as high as 30%, with human to human transmission through contact with infected body fluids responsible for nosocomial outbreaks.

The high mortality rate associated with CCHFV infections and the lack of vaccines and antivirals against this pathogen restrict the research with this virus to laboratories that have high biocontaiment facilities (BSL4). In addition, there is no animal model of the disease caused in humans by CCHFV. Thus, very little is known on the molecular determinants of pathogenesis of CCHFV. Recently, a protease domain in the N-terminal region of the viral-encoded RNA polymerase has been characterized and shown to deconjugate ubiquitin and ubiquitin-like molecules from target proteins. Since ubiquitin and ubiquitin-like molecules are involved in many diverse cellular processes, including host defense, it is likely that this viral protease domain disarms host antiviral responses and contributes to virulence. In contrast to CCHFV, which usually causes inapparent infections in vertebrate hosts different from humans, NSDV causes disease in sheep and goats that is associated with virus replication in endothelial cells and necrosis of capillary walls. NSDV has been isolated from febrile patients in Uganda, but its impact on human health is unclear. Animal disease: an important disease of ruminants, mainly sheep and goats—fatal hemorrhagic gastroenteritis, starting with high fever, depression, respiration problems, myocarditis and tubular nephritis, often pulmonary edema; mortality rate is very high, 30–90%. Abortions in pregnant ewes and goats, as well as developmental defects have been observed (Parsonson, Della-Porta, O'Halloran, et al., 1981a; Parsonson, Della-Porta, & Snowdon, 1981b; Parsonson, Della-Porta, & Snowdon, 1981c). NSD is a notifiable disease (OIE, 2012). The disease may appear as a result of introduction of naive livestock into an endemic area.

Prevention: attenuated vaccine.

Human disease: occasional cases (at least six reported). BSL-2/3.

Geographic distribution: Africa (Kenya, Uganda, Nigeria, Central African Republic, DR Congo, South Africa), India (Ganjam strain).
NAIROBI SHEEP DISEASE VIRUS
Nairobi sheep disease virus, a member of the genus Nairovirus, is highly pathogenic for sheep and goats. The virus is enzootic in eastern Africa, and closely related viruses occur in Nigeria (Dugbe virus in cattle), and in India and Sri Lanka (Ganjam virus in sheep and goats). The virus is not contagious among mammals; rather, it is transmitted by all stages of the brown ear tick, Rhipicephalus appendiculatus, in which there is transovarial and trans-stadial infection and very long-term carriage in adult ticks (up to 2 years). The vertebrate reservoir host of the virus remains unknown; the virus has not been found in wild ruminants or other animals in enzootic areas.

In Kenya, sheep and goats acquire the infection when they are transported from northern districts to the Nairobi area. After a short incubation period, there is high fever, hemorrhagic enteritis, and prostration. Affected animals may die within a few days, and pregnant ewes abort. Mortality in sheep is up to 90%. Subclinical infections also occur, and recovered animals are immune. Diagnosis is made clinically and by gross pathologic examination; it may be confirmed by virus isolation and identification of isolates immunologically, or by simple immunodiffusion tests on tissue extracts utilizing hyperimmune virus-specific antisera. Control depends primarily on acaricidal treatments to control the vector tick, which is also the vector of the economically important protozoan disease, East Coast fever. Both live-attenuated and inactivated vaccines are effective in preventing the disease in sheep.

The virus is not considered to be a significant human pathogen, although it was isolated from a person with a mild febrile disease.

05/04/2021

The genus Nairovirus within the family Bunyaviridae includes 34 predominantly tick-borne viruses. These viruses are enveloped, with a tripartite negative-sense, single-stranded RNA genome. Among the members of this genus, the most important pathogens are the Crimean–Congo hemorrhagic fever virus (CCHFV), which causes severe and often fatal hemorrhagic fever in humans, Crimean–Congo hemorrhagic fever (CCHF), and Nairobi sheep disease virus (NSDV), which circulates in Africa and Asia and causes acute hemorrhagic gastroenteritis in sheep and goats.

CCHF was first recognized during a large outbreak among soldiers and agricultural workers in the mid-1940s in the Crimean peninsula but the etiologic agent was not isolated at that time. In 1956 a virus named Congo virus was isolated from a febrile patient in the Belgian Congo (now the Democratic Republic of the Congo) and it was later recognized that Congo virus was the same as Crimean hemorrhagic fever virus, isolated in 1967 from a patient with this disease in (now) Uzbekistan. The disease now occurs sporadically throughout much of Africa, Asia, and Europe and results in an approximately 30% case–fatality rate. CCHF is characterized by a sudden onset of high fever, chills, severe headache, dizziness, and back and abdominal pains. Additional symptoms can include nausea, vomiting, diarrhea, neuropsychiatric disorders, and cardiovascular changes. In severe cases, hemorrhagic manifestations, ranging from petechiae to large areas of ecchymosis, develop. Ixodid ticks of numerous genera serve both as vector and reservoir for CCHFV. Ticks in the genus Hyalomma are particularly important in the ecology of this virus and exposure to these ticks represents a major risk factor for contracting disease. Other important risk factors include direct contact with blood and/or body fluids from infected patients or animals. The highly pathogenic nature of CCHFV has restricted research on the virus to biosafety level 4 (BSL-4) laboratories and has led to the fear that it might be used as a bioweapon.

NSDV, which is an important cause of veterinary disease, was originally isolated from sheep from Nairobi, Kenya in 1910. Dugbe virus was originally isolated from adult male Amblyomma variegatum ticks collected from cattle in Ibadan, Nigeria in 1964. Both Dugbe and Ganjam viruses (a variant of NSDV) have been isolated repeatedly from ticks removed from domestic animals and have both caused febrile illnesses in humans. Most other nairoviruses have been isolated from ixodid ticks or from argasid ticks, which are ectoparasites of seabirds and other birds, and their medical or veterinary significance is not known.

05/04/2021

Genus Nairovirus
The nairoviruses have a much larger genome than members of the other genera, primarily because the L segment is twice the size of those of the other genera of animal viruses (Table 4.10). They are named for Nairobi sheep disease virus, now considered a strain of Dugbe virus. There are seven species recognized, all of which consist of multiple strains with distinct names. The viruses are tick-borne although a few can also be transmitted by culicoid flies or mosquitoes. Nairobi sheep disease virus causes acute gastroenteritis with hemorrhagic symptoms in sheep and goats, with mortality rates over 90% in some populations. It was first identified as the causative agent of the disease in 1917 and is transmitted by the tick Rhipicephalus appendiculatus. Humans can be infected by the virus but suffer only mild illness. There is a close relative of the virus called Ganjam virus present in India which also causes disease in sheep and goats; it is transmitted by the tick Haemaphysalis intermedia.

Crimean-Congo hemorrhagic fever virus (CCHF) is the most important nairovirus in terms of human disease. It was first identified in the 1940s in the Crimean region of the former USSR and in the Democratic Republic of Congo. The virus is now known from at least 30 countries. It is found from southern Africa through Eastern Europe and the Middle East to western China. The principal vector is Hyalomma ticks, but Dermacentor and Rhipicephalus ticks can also transmit the virus. Sheep, goats, cattle, ostriches, wild herbivores, and hares become infected by CCHF but most infections result in subclinical disease. In contrast, infection of humans results in severe hemorrhagic fever with a 30% mortality rate. Humans are infected by the bite of a tick or by contact with blood or tissues of infected livestock. Transmission to hospital personnel treating infected patients has occurred.

05/04/2021

Nairovirus Genus
Nairoviruses are named after Nairobi sheep disease virus, the type virus. There are more than 30 viruses in the genus classified into seven serogroups. The most distinctive feature of nairoviruses is that they are tick-borne, although a few have been isolated from mosquitoes. Crimean–Congo hemorrhagic fever is the most medically important of these viruses.

Crimean–Congo hemorrhagic fever (CCHF) virus is transmitted by Hyalomma ticks, and is found over the wide geographic range of these ticks, extending from South Africa to the Middle East and into western China. CCHF virus is probably maintained in nature by two cycles, a transovarial and trans-stadial cycle in ticks, and a tick–vertebrate cycle. In South Africa the wildlife hosts include large herbivores (wild and domestic) and hares. Infection of humans is relatively infrequent even in enzootic areas, and occurs either by tick bite or by direct contact with infected animals or their tissues. Agricultural and abattoir workers are particularly at risk. In addition to tick and zoonotic transmission, person to person spread can also occur and has caused many small nosocomial outbreaks. In humans, CCHF is a serious disease with a mortality of 10–50%. After an initial febrile and influenza-like illness, hepatitis and hemorrhagic signs develop, that often progress to a fatal shock syndrome. Although CCHF virus will cause encephalitis in laboratory animals, there is no model for the hemorrhagic syndrome seen in humans.