Outbreaks Of African Horse Sickness Research Articles

Repellent effect of topical deltamethrin on blood feeding by Culicoides on horses

African horse sickness (AHS) is a vectorborne disease spread by Culicoides biting midges. The UK's Department for Environment, Food and Rural Affairs currently suggests using topical deltamethrin for AHS control;...

Protection of horses against Culicoides biting midges in different housing systems in Switzerland

Species belonging to the Culicoides complexes (Diptera, Ceratopogonidae), obsoletus and pulicaris, in Switzerland, are potential vectors of both bluetongue virus (BTV) and African horse sickness virus (AHSV). The epidemic of BTV in 2006 and 2007 in Europe has highlighted the risk of introduction and spread of vector-borne diseases in previously non-endemic areas. As a measure of prevention, as part of an integrated control programme in the event of an outbreak of African horse sickness (AHS), it is of utmost importance to prevent, or substantially reduce, contact between horses and Culicoides. The aim of the present study was to compare the effect of three protection systems, net, fan, repellent, or combinations thereof, with regard to their potential to reduce contact between horses and Culicoides. Three different equine housing systems, including individual boxes (BX), group housing systems (GR), and individual boxes with permanently accessible paddock (BP) were used. The efficacy of the protection systems were evaluated by comparing the total number counts of collected female Culicoides, of non-blood-fed and blood-fed Culicoides, respectively, with UV black light traps. The study was conducted over 3 summer months during 2012 and 2013 each and focused on the efficacy and practicality of the protection systems. The repellent was tested in 2012 only and not further investigated in 2013, as it showed no significant effect in reducing Culicoides collected in the light traps. Net protection system provided the best overall protection for the total number of female Culicoides, non-blood-fed and blood-fed Culicoides in all tested housing systems. The net, with a pore size of 0.1825mm2, reduced the total number of Culicoides collected in the housing systems BP, GR and BX by 98%, 85% and 67%, respectively. However, in the GR housing system, no significant difference between the effectiveness of the fan and the net were determined for any of the three Culicoides categories. The results of the present study demonstrated that horse owners can substantially reduce their horses’ exposure to Culicoides, by using net protection in the housing systems BX, BP and GR. In GR housing systems, protection against Culicoides using a fan is also recommended.

African horse sickness outbreaks in Namibia from 2006 to 2013: clinical, pathological and molecular findings.

African horse sickness (AHS) is a vector‑borne viral disease of equids, endemic in Sub‑Saharan Africa. This article reports the clinic‑pathological and laboratory findings observed in the framework of passive surveillance during the AHS outbreaks which occurred in Namibia between 2006 and 2013. This study was conducted in the framework of the collaboration among the Istituto Zooprofilattico Sperimentale dell'Abruzzo e del Molise (Teramo, Italy), the Namibian Ministry of Agriculture Water and Forestry, and the Namibian National Veterinary Association. A total of 92 horses were investigated, showing different clinical form of AHS: peracute/acute (n = 43), sub‑acute (n = 21) and mild AHS fever (n = 19). Clinical data were not available for 9 horses, because they were found dead. Pathological findings have been recorded for 35 horses. At necropsy, pulmonary and subcutaneous oedema, haemorrhages and enlargement of lymph nodes were mainly observed. Diagnosis was confirmed by laboratory testing, AHS virus (AHSV) was isolated from 50 horses and the identified serotypes were: 1, 2, 4, 6, 7, 8, and 9. The phylogenetic analysis of the S10 genome sequences segregated the Namibian AHSV strains in the same clusters of those circulating in South Africa in recent years. The description of AHS clinical, pathological, and laboratory features of AHS provided in this article is of value for differential diagnosis and control of AHS, especially in areas currently free from this disease.

Orbivirus detection from Culicoides collected on African horse sickness outbreaks in Namibia.

African horse sickness (AHS), a non-contagious infectious disease caused by a RNA virus in the Orbivirus genus within the Reoviridae family affecting all equids, is endemic in sub-Saharan Africa. The virus is transmitted by some species of biting midges in the genus Culicoides (Diptera: Ceratopogonidae). In April 2011, 8 Culicoides collections were performed in 6 districts of 4 regions of the Republic of Namibia (Africa), all within a 400 km radious from the capital Windhoek. Six farms - Khomas (Windhoek and Steinhausen), Erongo (Karibib and Omaruru), Otjozondjupa (Okahandja), and Omaheke (Gobabis) involved in the AHS outbreaks, were sampled. Overall 194,211 Culicoides were collected and identified. Culicoides imicola was largely the most abundant species at all farms (99.4%). A total of 18,687 parous and gravid Culicoides females were assayed for AHS virus (AHSV) by real time RT-PCR. Of the 248 assayed pools, 227 consisted of C. imicola, 13 of Culicoides pycnostictus and 5 of Schultzei complex. Only 1 pool each of Culicoides nivosus, Culicoides leucostictus, and Culicoides tropicalis was assayed. Of the 248 pools examined by real time RT-PCR, 81 tested positive for AHSV, all consisting of C. imicola collected at Omaruru, resulting in a field vector infection rate of 0.91%. No viable AHSV could be isolated from 88 of the tested pools (n = 1,463). However, bluetongue virus (BTV) serotype-1 and 10 were isolated from 3 of these pools, each consisting of 100 C. imicola collected at Windhoek. The present study confirms the relative low infection prevalence in field collected Culicoides and the strict relationship between the high abundance of C. imicola and outbreaks of AHSV.

Culicoides species abundance and potential over-wintering of African horse sickness virus in the Onderstepoort area, Gauteng, South Africa.

In South Africa, outbreaks of African horse sickness (AHS) occur in summer; no cases are reported in winter, from July to September. The AHS virus (AHSV) is transmitted almost exclusively by Culicoides midges (Diptera: Ceratopogonidae), of which Culicoides imicola is considered to be the most important vector. The over-wintering mechanism of AHSV is unknown. In this study, more than 500 000 Culicoides midges belonging to at least 26 species were collected in 88 light traps at weekly intervals between July 2010 and September 2011 near horses in the Onderstepoort area of South Africa. The dominant species was C. imicola. Despite relatively low temperatures and frost, at least 17 species, including C. imicola, were collected throughout winter (June-August). Although the mean number of midges per night fell from > 50 000 (March) to < 100 (July and August), no midge-free periods were found. This study, using virus isolation on cell cultures and a reverse transcriptase polymerase chain reaction (RT-PCR) assay, confirmed low infection prevalence in field midges and that the detection of virus correlated to high numbers. Although no virus was detected during this winter period, continuous adult activity indicated that transmission can potentially occur. The absence of AHSV in the midges during winter can be ascribed to the relatively low numbers collected coupled to low infection prevalence, low virus replication rates and low virus titres in the potentially infected midges. Cases of AHS in susceptible animals are likely to start as soon as Culicoides populations reach a critical level.

The threat of midge-borne equine disease: investigation of Culicoides species on UK equine premises

There are concerns that outbreaks of exotic or novel vector-borne viral diseases will increasingly occur within northern Europe and the UK in the future. African horse sickness (AHS) is a...

The 2011 outbreak of African horse sickness in the African horse sickness controlled area in South Africa

African horse sickness (AHS) is a controlled animal disease in South Africa, and as a result of the high mortality rates experienced, outbreaks in the AHS controlled area in the Western Cape Province have a significant impact on affected properties as well as on the exportation of live horses from the AHS free zone in metropolitan Cape Town. An outbreak of AHS serotype 1 occurred in the surveillance zone of the AHS controlled area of the Western Cape during the summer of 2011. The epicentre of the outbreak was the town of Mamre in the magisterial district of Malmesbury and the outbreak was confined to a defined containment zone within this area by movement control of all equids and a blanket vaccination campaign. A total of 73 cases of AHS were confirmed during this outbreak, which included four confirmed subclinical cases. The morbidity rate for the outbreak was 16%with a mortality rate of 14%and a case fatality rate of 88%. Outbreak disease surveillance relied on agent identification using polymerase chain reaction (PCR)-based assays, which is novel for an AHS outbreak in South Africa. The source of this outbreak was never confirmed although it is believed to be associated with the illegal movement of an infected animal into the Mamre area. This detailed description of the outbreak provides a sound scientific basis to assist decision making in future AHS outbreaks in the AHS controlled area of South Africa and in countries where AHS is an exotic or emerging disease.

Can Europe learn lessons from African horse sickness in Senegal?

AFRICAN horse sickness (AHS) is an insect-transmitted, non-contagious disease of equids caused by African horse sickness virus (AHSV), with a mortality of up to 95 per cent in immunologically naive...

Outbreaks of African horse sickness in Senegal, and methods of control of the 2007 epidemic

Since first being detected in Nigeria in January 2007, African horse sickness virus serotype 2 (AHSV-2) has spread throughout the northern hemisphere, and was first reported in Senegal. A retrospective...

African Horse Sickness Outbreaks Caused by Multiple Virus Types in Ethiopia

African horse sickness (AHS) is associated with high morbidity and mortality in equids, especially horses. A retrospective analysis was carried out concerning 737 AHS outbreaks that occurred during 2007-2010 in Ethiopia. A total of ten outbreaks were investigated in the study period. All four forms of the disease (pulmonary, cardiac, horse sickness fever and the combined form) were observed, with the cardiac form being the most prevalent. Multiple African horse sickness virus serotypes (AHSV-2, AHSV-4, AHSV-6, AHSV-8 and AHSV-9) were detected by molecular methods (type-specific real-time RT-PCR assays), and fourteen isolates were derived from blood and tissue samples collected during 2009-2010. This is the first report of AHSV-4, AHSV-6 or AHSV-8 in Ethiopia.

Predicting the abundance of African horse sickness vectors in South Africa using GIS and artificial neural networks

African horse sickness (AHS) is a disease that is endemic to sub-Saharan Africa and is caused by a virus potentially transmitted by a number of Culicoides species (Diptera: Ceratopogonidae) including Culicoides imicola and Culicoides bolitinos. The strong association between outbreaks of AHS and the occurrence in abundance of these two Culicoides species has enabled researchers to develop models to predict potential outbreaks. A weakness of current models is their inability to determine the relationships that occur amongst the large number of variables potentially influencing the population density of the Culicoides species. It is this limitation that prompted the development of a predictive model with the capacity to make such determinations. The model proposed here combines a geographic information system (GIS) with an artificial neural network (ANN). The overall accuracy of the ANN model is 83 %, which is similar to other stand-alone GIS models. Our predictive model is made accessible to a wide range of practitioners by the accompanying C. imicola and C. bolitinos distribution maps, which facilitate the visualisation of the model’s predictions. The model also demonstrates how ANN can assist GIS in decision-making, especially where the data sets incorporate uncertainty or if the relationships between the variables are not yet known.

Clinicopathological Features Associated with the Outbreak of African Horse Sickness in Lagos, Nigeria

An outbreak of African horse sickness involving 17 horse stables in Lagos, Nigeria, was investigated from December 20 to February 21, 2007. The investigation revealed high and sudden mortality of 40 horses, affecting both the local and Argentine breeds from December 20, 2006 to January 30, 2007. The clinical and postmortem signs were predominantly circulatory and respiratory, as described elsewhere, and seldom digestive. Clinical and pathologic findings coupled with epidemiologic findings tentatively resulted in the diagnosis of African horse sickness, which was confirmed by complement fixation test and reverse transcription polymerase chain reaction.

Investigaciones sobre brotes de peste equina en la zona de vigilancia de Sudáfrica

Confirmed outbreaks of African horse sickness (AHS) occurred in the surveillance zone of the Western Cape in 1999 and 2004, both of which led to a two-year suspension on the export of horses. Light trap surveys in the outbreak areas showed that known vector competent Culicoides species, notably C. imicola, were abundant and present in numbers equal to those in the traditional AHS endemic areas. Isolations of AHS virus serotypes 1 and 7, equine encephalosis virus, and bluetongue virus from field-collected C. imicola in the surveillance zone demonstrated that this species was highly competent and could transmit viruses belonging to different serogroups of the Orbivirus genus. Molecular identification of recovered virus isolates indicated that at least two incursions of AHS into the surveillance zone had taken place in 2004. The designation of an AHS-free zone in the Western Cape remains controversial since it can be easily compromised, as evidenced by the two recent outbreaks. In light of the results reported in the present study, the policy of maintaining a large population of unvaccinated horses in the surveillance zone should be reconsidered, as it leaves them vulnerable to infection with AHS virus, which is the most pathogenic of all equine viruses.

An epidemiological investigation of the African horsesickness outbreak in the Western Cape Province of South Africa in 2004 and its relevance to the current equine export protocol

African Horsesickness (AHS) is a controlled disease in South Africa. The country is divided into an infected area and a control area. An outbreak of AHS in the control area can result in a ban of exports for at least 2 years. A retrospective epidemiological study was carried out on data collected during the 2004 AHS outbreak in the surveillance zone of the AHS control area in the Western Cape Province. The objective of this study was to describe the 2004 outbreak and compare it with the 1999 AHS outbreak in the same area. As part of the investigation, a questionnaire survey was conducted in the 30 km radius surrounding the index case. Spatial, temporal and population patterns for the outbreak are described. The investigation found that the outbreak occurred before any significant rainfall and that the main AHS vector (Culicoides imicola) was present in abundance during the outbreak. Furthermore, 63% of cases occurred at temperatures < or = 15 degrees C, the Eerste River Valley was a high risk area, only 17% of owners used vector protection as a control measure and 70% of horses in the outbreak area were protected by means of vaccination at the start of the outbreak. The study revealed that the current AHS control measures do not function optimally because of the high percentage of vaccinated horses in the surveillance zone, which results in insufficient sentinel animals and the consequent failure of the early warning system. Alternative options for control that allow continued export are discussed in the paper.

Climate change and malaria: the debate is heating up

The logic is straightforward. The distribution of malaria is restricted at altitude because of low temperatures. Climate change makes the world warmer. Therefore, climate change should explain recent upsurges in the incidence of malaria in highland regions. No less a body than the International Panel on Climate Change accepts this notion but a recent paper by S. Hay et al. suggests that the argument is too simplistic [1xClimate change and the resurgence of malaria in the East African highlands. Hay, S.I et al. Nature. 2002; 415: 905–909Crossref | PubMed | Scopus (277)See all References][1]. Hay et al. report a careful statistical analysis which shows that in four highland locations in East Africa, where malaria has been increasing in recent decades, there was no evidence that warming has actually occurred, either during the study period or, indeed, during the whole of the last century. Instead, the authors suggest that the causes of malaria resurgence should be sought elsewhere such as changes in land use or human demography, or increased resistance to antimalarial drugs.This study is the latest of several which have explored the relationship between infectious diseases and climate variation. Baylis et al. found that the outbreaks of African horse sickness in South Africa were closely correlated with El Nino events [2xHorse sickness and ENSO in southern Africa. Baylis, M et al. Nature. 1999; 397: 574Crossref | PubMed | Scopus (53)See all References][2], whereas Linthicum et al. failed to demonstrate that outbreaks of Rift Valley fever in Kenya were related to climate and vegetation data [3xClimate and satellite indicators to forecast Rift Valley fever epidemics in Kenya. Linthicum, K.J et al. Science. 1999; 285: 397–400Crossref | PubMed | Scopus (367)See all References][3].These are challenging analyses to attempt for several reasons: infectious disease surveillance data are often of poor quality; disease outbreaks have multiple causes and it is not always clear which climate variables are the right ones to measure. As a consequence, it is often difficult to detect any signal among the noise. But, as Hay et al. point out, climate change remains both a scientifically and politically controversial subject and it is important that its conjectured implications are tested as rigorously as possible. However, their analysis perhaps does not entirely decide the issue, even for malaria. For all four of the sites studied by Hay et al. there was an upward trend in the Garnham index of the number of ‘suitable’ months for malaria, although the trend was statistically significant only in southwest Uganda. We might wonder what a follow up analysis in a few decades time will reveal.

The 1996 outbreak of African horse sickness in South Africa--the entomological perspective.

During the 1996 summer season (January-April) in South Africa an estimated 500 horses died of African horse sickness (AHS); 80% of deaths were due to AHS virus serotypes 2 and 4. Nearly all cases occurred in the northern, north-eastern and central parts of South Africa. This study reports the first attempt to verify the involvement of the biting midge Culicoides imicola in a field outbreak of AHS in southern Africa. In light-trap collections made at 47 sites over 12 weeks, C. imicola comprised 94.2% of 4.78 million Culicoides. Culicoides imicola was the most prevalent of 34 species captured and was the only species whose distribution matched that of the disease. Record catches of C. imicola were made, and reveal that in years of above average rainfall its numbers can show a 200-fold increase over those in dry years. Soil type appeared to determine strongly the distribution of C. imicola. The largest populations of C. imicola were found in areas with clayey, moisture-retentive soils whereas the lowest numbers, or none, occurred in areas where the soils were sandy and quick-draining. The deaths of two horses (confirmed AHS) in a sandy area were perplexing as they occurred in a region known to be free of C. imicola. The probable origin of these infections was established.

Modelling the distribution and abundance of Culicoides imicola in Morocco and Iberia using climatic data and satellite imagery.

Relative abundances of Culicoides imicola at 22 sites in Morocco were compared with climatic variables, altitude and the NDVI (Normalised Difference Vegetation Index, a satellite derived measure of photosynthetic activity) of the same sites. Abundances were negatively correlated with wind speed and positively correlated with the average and minimum NDVI (NDVImin). There were no significant correlations with air or soil temperatures, relative humidity, saturation deficit, rainfall, altitude or mean annual maximum or range of NDVI. The best 2-variable model, which combined wind speed and NDVImin as predictors, explained over 50% of the variance in abundance. It is suggested that wind speed affects the abundance of C. imicola by causing adult mortality while NDVImin provides a measure of the availability of C. imicola breeding sites. Data from 27 sites in Iberia yielded broadly similar results to those found in Morocco although the great abundance of C. imicola at Milfontes (Portugal) could not be accounted for. Several northern sites where the NDVImin is sufficiently high to suggest the presence of C. imicola but where it appears to be absent indicate that there may be a northern limit to the distribution of C. imicola in Iberia that is independent of NDVImin. The potential use of NDVImin to predict the distribution of outbreaks of African horse sickness was investigated using data from the 1989-1990 epizootic in northern Morocco. Within the cluster of outbreaks in Larache province is a corridor of very low NDVImin in which few or no outbreaks were reported.

The seasonal and geographical distribution of Culicoides imicola, C. pulicaris group and C. obsoletus group biting midges in central and southern Spain.

Pirbright-type light traps were used to collect Culicoides biting midges (Diptera: Ceratopogonidae) at fifteen sites in twelve provinces of central Spain and Andalusia. A total of 293,625 Culicoides were collected in 1,387 samples over a two year period. These comprised approximately 9.2% Culicoides imicola, 11.4% C. pulicaris group, 1.6% C. obsoletus group and 12.2% C. circumscriptus. Culicoides imicola was present at ten of the fifteen sites; the five sites from which it was absent were the most eastern of the fifteen. The greatest abundance of this species was at Navalmoral in Caceres Province. Culicoides pulicaris group were present at all sites; C. obsoletus group were present at twelve sites. The annual peaks in abundance were: C. imicola, August-October; C. pulicaris group, May-June; and C. obsoletus group, March-June. The geographical and seasonal distributions of C. imicola are consistent with those of the outbreaks of African horse sickness (AHS) and bluetongue (BT) during epizootics in Spain, and support the contention that C. imicola was the major vector of AHS and BT viruses.

Simulation studies of vaccination strategies in African horse sickness

A simulation model including two hosts (horses and donkeys) and one vector ( Culicoides imicola) for African horse sickness in Spain is extended to consider vaccination strategies. If hosts were protected prior to virus introduction, elimination of simulated epidemics was related nonlinearly to the fraction protected. Protecting donkeys as well as horses increased the effectiveness of vaccination. Prevention of 50% of epidemics required 75% coverage of horses and donkeys or 90% coverage of horses only. Protection after the introduction of the virus was rarely successful in preventing outbreaks. If horses alone were protected, the number of donkeys was the most significant factor determining the level of protection needed to prevent an epidemic. If both hosts were protected, the abundance of other hosts for vector blood meals was the most significant factor. These results suggest that prophylactic vaccination of both horses and donkeys with high coverage is necessary to prevent outbreaks of African horse sickness in Spain.

The transmission and geographical spread of African horse sickness and bluetongue viruses.

African horse sickness virus (AHSV) and bluetongue virus (BTV) are dsRNA viruses within the genus Orbivirus. Both are able to cause non-contagious, infectious arthropod-borne diseases in their respective vertebrate hosts. AHSV infects equines and occasionally dogs, whereas BTV replicates in ruminants. The disease caused by AHSV is usually at its most severe in horses, whereas certain breeds of sheep are particularly sensitive to BTV infection. AHSV is endemic in sub-Saharan Africa but periodically makes brief excursions beyond this area. BTV occurs much more widely and can be found in a band around the World, stretching from approximately 40 degrees N to 35 degrees S. In the wild, both viruses are transmitted between their vertebrate hosts almost entirely via the bites of arthropod vectors, although dogs can occasionally acquire AHSV by eating virus-contaminated meat and BTV may be infrequently transmitted via infected semen or transplacentally. Because of their reliance upon arthropod vectors, BTV and AHSV have a global distribution which is limited not only by the requirement for susceptible vertebrates but also by the necessity for competent arthropod vectors. The major vectors of AHSV and BTV are certain species of Culicoides biting midge, which are true biological vectors but mosquitoes and/or ticks may also be involved to a greater or lesser extent. Until recently, AHSV has apparently been unable to survive beyond its traditional endemic zones in sub-Saharan Africa for more than 2-3 years at most. This has been interpreted as being due to a number of factors, including the absence of a long-term vertebrate reservoir, a lower prevalence, shorter, seasonal incidence and decreased transmission efficiency of the local vectors and also possibly to the effect of control measures (vector abatement, vaccination). The recent outbreaks of African horse sickness (AHS) in Spain, Portugal and Morocco, which persisted for at least 5 years (1987-1991) therefore seem to have established a new pattern in AHSV survival in an epidemic zone. This extended persistence may be due to the 'all-year-round' presence in the area of adult Culicoides imicola, the major AHSV vector. This is basically an Afro-Asiatic species and its continuous presence in parts of Iberia and may be due to some recent moderation in the climate. Further northerly extensions in the range of Culicoides imicola, in response to 'climatic moderation', cannot be ruled out and could substantially increase the area of Europe 'at risk' to AHS.(ABSTRACT TRUNCATED AT 400 WORDS)