African horse sickness in naturally infected,immunised horses

To determine whether subclinical cases, together with clinical cases, of African horse sickness (AHS) occur in immunised horses in field conditions, whole blood samples were collected and rectal temperatures recorded weekly from 50 Nooitgedacht ponies resident in open camps at the Faculty of Veterinary Science, University of Pretoria, Onderstepoort, during 2008–2010. The samples were tested for the presence of African horse sickness virus (AHSV) RNA by a recently developed real‐time RT‐PCR. It was shown that 16% of immunised horses in an AHS endemic area were infected with AHSV over a 2 year period, with half of these (8%) being subclinically infected. The potential impact of such cases on the epidemiology of AHS warrants further investigation.     

非洲马瘟病毒分子生物学研究进展

非洲马瘟病毒属呼肠病毒科环状病毒属,有9个血清型,是一种有10个节段(L1~L3,M4~M6,S7~S10)的双链RNA病毒,编码10个蛋白(VP1~VP7和NS1,NS2,NS3/NS3A)。VP2蛋白在病毒的各蛋白中变异率最大,有15个抗原位点,是病毒的血清型特异性抗原,能与病毒的中和抗体发生反应;VP7蛋白在病毒的各蛋白中最保守,是病毒的血清群特异性抗原。NS1蛋白在感染细胞中形成病毒特异性微管结构;NS3蛋白在病毒各蛋白中变异率位居第二,系统进化分析将NS3分成3个进化群(α,β和γ)。该病毒的分子生物学诊断技术主要有RT-PCR和核酸探针技术。     

Epizootiology and vectors of African horse sickness virus

African horse sickness (AHS) virus causes a non-contagious, infectious, arthropod-borne disease of equines and is enzootic in sub-Saharan Africa. The major vectors are species of Culicoides but mosquitoes and ticks may be involved. Periodically the virus makes excurisons beyond its enzootic zones but until recently has not been able to maintain itself outside these areas for more than 2–3 consecutive years. This is probably due to a number of factors including the absence of a long term vertebrate reservoir, the prevalence and seasonal incidence of the vectors and the efficiency of control measures. The recent AHS epizootics in Iberia and North Africa seem to have established a new pattern in AHS virus persistence. This is probably linked to the continuous presence of adult C. imicola in the area. Culicoides imicola is basically an Afro-Asiatic insect and prefers warm climates. Therefore its continuous adult presence in parts of Iberia may be due to some recent moderation of the climate in these areas.     

African horse sickness virus structure

African horse sickness virus (AHSV), of which there are nine serotypes (AHSV-1, -2, etc.), is a member of Orbivirus genus within the Reoviridae family. Both in morphology and molecular constituents AHSV particles are comparable to those of bluetongue virus (BTV), the prototype virus of the genus. The two viruses have seven structural proteins (VP1–7) organized in two layered capsid. The outer capsid is composed of VP2 and VP5. The inner capsid, or core, is composed of two major proteins, VP3 and VP7, and three minor proteins, VP1, VP4 and VP6. Within the core is the virus genome. This genome consists of 10 double-stranded (ds)RNA segments of different sizes, three large, designated L1–L3, three medium, M4–M6, and four small, S7–S10. In addition to the seven stuctural proteins that are coded by seven of the RNA species, four non-structural proteins, NS1, NS2, NS3 and NS3A, are coded by three RNA segments, M5, S8 and S10. The two smallest proteins (NS3 and NS3A) are synthesized by the S10 RNA segment, probably from different in-frame translation initiation codons. Nucleotide sequences of eight RNA segments (L2, L3, M4, M5, M6, S7, S8 and S10) and the predicted amino acid sequences of the encoded gene products are also available, mainly representing one serotype, AHSV-4. In this review the properties of the AHSV genes and gene products are discussed. The sequence and hybridization analyses of the different AHSV dsRNA segments indicate that the segments that code for the core proteins, as well as those that code for NS1 and NS2 proteins, are highly conserved between the different virus serotypes. However, the RNA encoding NS3 and NS3A, and the two segments encoding the outer capsid proteins, are more variable between the AHSV serotypes. A close phylogenetic relationship between AHSV, BTV and epizootic haemorrhagic disease virus (EHDV), three Culicoides-transmitted orbiviruses, has been revealed when the equivalent sequences of genes and gene products are compared. Recently, the four major AHSV capsid proteins have been expressed using recombinant baculoviruses. Biochemically and antigenically these proteins are similar to the authentic proteins. Since the AHSV VP7 protein is highly conserved among the different serotypes, it has been utilized as a diagnostic reagent. The expressed VP7 protein has also been purified to homogeneity and crystallized for three-dimensional X-ray analysis. The expressed outer capsid proteins, VP2 and VP5, have been purified and used to raise antisera in rabbits. The VP2 antisera neutralize virus infections in vitro indicating the importance of this protein for vaccine development.     

Rapid and sensitive detection of African horse sickness virus by real-time PCR

A highly sensitive and specific TaqMan-MGB real-time RT-PCR assay has been developed and standardised for the detection of African horse sickness virus (AHSV). Primers and MGB probe specific for AHSV were selected within a highly conserved region of genome segment 7. The robustness and general application of the diagnostic method were verified by the detection of 12 AHSV isolates from all of the nine serotypes. The analytical sensitivity ranged from 0.001 to 0.15 TCID₅₀ per reaction, depending on the viral serotype. Real-time PCR performance was preliminarily assessed by analysing a panel of field equine samples. The same primer pair was used to standardise a conventional RT-PCR as an affordable, useful and simple alternative method in laboratories without access to real-time PCR instruments. The two techniques present novel tools to improve the molecular diagnosis of African horse sickness (AHS).     

Adaptive strategies of African horse sickness virus to facilitate vector transmission

African horse sickness virus (AHSV) is an orbivirus that is usually transmitted between its equid hosts by adult Culicoides midges. In this article, we review the ways in which AHSV may have adapted to this mode of transmission. The AHSV particle can be modified by the pH or proteolytic enzymes of its immediate environment, altering its ability to infect different cell types. The degree of pathogenesis in the host and vector may also represent adaptations maximising the likelihood of successful vectorial transmission. However, speculation upon several adaptations for vectorial transmission is based upon research on related viruses such as bluetongue virus (BTV), and further direct studies of AHSV are required in order to improve our understanding of this important virus.     

Re-emergence of bluetongue, African horse sickness, and other Orbivirus diseases

Arthropod-transmitted viruses (Arboviruses) are important causes of disease in humans and animals, and it is proposed that climate change will increase the distribution and severity of arboviral diseases. Orbiviruses are the cause of important and apparently emerging arboviral diseases of livestock, including bluetongue virus (BTV), African horse sickness virus (AHSV), equine encephalosis virus (EEV), and epizootic hemorrhagic disease virus (EHDV) that are all transmitted by haematophagous Culicoides insects. Recent changes in the global distribution and nature of BTV infection have been especially dramatic, with spread of multiple serotypes of the virus throughout extensive portions of Europe and invasion of the south-eastern USA with previously exotic virus serotypes. Although climate change has been incriminated in the emergence of BTV infection of ungulates, the precise role of anthropogenic factors and the like is less certain. Similarly, although there have been somewhat less dramatic recent alterations in the distribution of EHDV, AHSV, and EEV, it is not yet clear what the future holds in terms of these diseases, nor of other potentially important but poorly characterized Orbiviruses such as Peruvian horse sickness virus.     

A review of African horse sickness and its implications for Ireland

ABSTRACT: African horse sickness is an economically highly important non-contagious but infectious Orbivirus disease that is transmitted by various species of Culicoides midges. The equids most severely affected by the virus are horses, ponies, and European donkeys; mules are somewhat less susceptible, and African donkeys and zebra are refractory to the devastating consequences of infection. In recent years, Bluetongue virus, an Orbivirus similar to African horse sickness, which also utilises Culicoides spp. as its vector, has drastically increased its range into previously unaffected regions in northern Europe, utilising indigenous vector species, and causing widespread economic damage to the agricultural sector. Considering these events, the current review outlines the history of African horse sickness, including information concerning virus structure, transmission, viraemia, overwintering ability, and the potential implications that an outbreak would have for Ireland. While the current risk for the introduction of African horse sickness to Ireland is considered at worst 'very low', it is important to note that prior to the 2006 outbreak of Bluetongue in northern Europe, both diseases were considered to be of equal risk to the United Kingdom ('medium-risk'). It is therefore likely that any outbreak of this disease would have serious socio-economic consequences for Ireland due to the high density of vulnerable equids and the prevalence of Culicoides species, potentially capable of vectoring the virus.     

A serological survey of African horse sickness in Botswana

A retrospective serological survey of African horse sickness (AHS) in Botswana covering a 10-year period (1995-2004) is reported. The survey involved horses showing clinical symptoms of the disease; the horses had not been vaccinated against AHS. Over the period surveyed, serological evidence suggestive of infection with AHS virus (AHSV) was found in 99 clinical cases out of which 41.4% (41/99) cases were found during the 1st half (1995-1999) and 58.6 % (58/99) cases were found in the 2nd half of the survey period (2000-2004). These serological findings are discussed in relation to AHSV serotypes isolated from diseased horses in Botswana before and during the period of this serological survey.     

A Retrospective Economic Analysis of the Ontario Red Fox Oral Rabies Vaccination Programme

Ontario initiated a red fox (Vulpes vulpes ) oral rabies vaccination (ORV) programme in 1989. This study utilized a benefit‐cost analysis to determine if this ORV programme was economically worthwhile. Between 1979 and 1989, prior to ORV baiting, the average annual human post‐exposure treatments, positive red fox rabies diagnostic tests and indemnity payments for livestock lost to rabies were 2248, 1861 and $246 809, respectively. After baiting, from 1990 to 2000, a 35%, 66% and 41% decrease in post‐exposure treatments, animal rabies tests and indemnity payments was observed, respectively. These reductions were viewed as benefits of the ORV programme, whereas total costs were those associated with ORV baiting. Multiple techniques were used to estimate four different benefit streams and the total estimated benefits ranged from $35 486 316 to $98 413 217. The annual mean ORV programme cost was $6 447 720, with total programme costs of $77 372 637. The average benefit‐cost ratios over the analysis period were .49, 1.06, 1.27 and 1.36, indicating overall programme efficiency in three of the four conservative scenarios.     

An African horse sickness virus serotype 4 recombinant canarypox virus vaccine elicits specific cell-mediated immune responses in horses

A recombinant canarypox virus vectored vaccine co-expressing synthetic genes encoding outer capsid proteins, VP2 and VP5, of African horse sickness virus (AHSV) serotype 4 (ALVAC(?)-AHSV4) has been demonstrated to fully protect horses against homologous challenge with virulent field virus. Guthrie et al. (2009) detected weak and variable titres of neutralizing antibody (ranging from <10 to 40) 8 weeks after vaccination leading us to hypothesize that there could be a participation of cell mediated immunity (CMI) in protection against AHSV4. The present study aimed at characterizing the CMI induced by the experimental ALVAC(?)-AHSV4 vaccine. Six horses received two vaccinations twenty-eight days apart and three horses remained unvaccinated. The detection of VP2/VP5 specific IFN-γ responses was assessed by enzyme linked immune spot (ELISpot) assay and clearly demonstrated that all ALVAC(?)-AHSV4 vaccinated horses developed significant IFN-γ production compared to unvaccinated horses. More detailed immune responses obtained by flow cytometry demonstrated that ALVAC(?)-AHSV4 vaccinations induced immune cells, mainly CD8(+) T cells, able to recognize multiple T-epitopes through all VP2 and only the N-terminus sequence of VP5. Neither VP2 nor VP5 specific IFN-γ responses were detected in unvaccinated horses. Overall, our data demonstrated that an experimental recombinant canarypox based vaccine induced significant CMI specific for both VP2 and VP5 proteins of AHSV4.     

African horse sickness: The potential for an outbreak in disease‐free regions and current disease control and elimination techniques

African horse sickness (AHS) is an arboviral disease of equids transmitted by Culicoides biting midges. The virus is endemic in parts of sub‐Saharan Africa and official AHS disease‐free status can be obtained from the World Organization for Animal Health on fulfilment of a number of criteria. AHS is associated with case fatality rates of up to 95%, making an outbreak among naïve horses both a welfare and economic disaster. The worldwide distributions of similar vector‐borne diseases (particularly bluetongue disease of ruminants) are changing rapidly, probably due to a combination of globalisation and climate change. There is extensive evidence that the requisite conditions for an AHS epizootic currently exist in disease‐free countries. In particular, although the stringent regulations enforced upon competition horses make them extremely unlikely to redistribute the virus, there are great concerns over the effects of illegal equid movement. An outbreak of AHS in a disease free region would have catastrophic effects on equine welfare and industry, particularly for international events such as the Olympic Games. While many regions have contingency plans in place to manage an outbreak of AHS, further research is urgently required if the equine industry is to avoid or effectively contain an AHS epizootic in disease‐free regions. This review describes the key aspects of AHS as a global issue and discusses the evidence supporting concerns that an epizootic may occur in AHS free countries, the planned government responses, and the roles and responsibilities of equine veterinarians.