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Determination of the role of hard (Ixodid) ticks in the transmission of lumpy skin disease virus in cattle

Reference	BB/H009361/1
Principal Investigator / Supervisor	Ms Eeva Tuppurainen
Co-Investigators / Co-Supervisors	Professor Jacobus Coetzer, Professor Philip Mellor, Dr Chris Oura, Dr Wilhelm Stoltsz, Mrs Milana Troskie, Professor Estelle Venter
Institution	The Pirbright Institute
Department	Vector Borne Diseases
Funding type	Research
Value (£)	332,236
Status	Completed
Type	Research Grant
Start date	01/04/2010
End date	31/05/2013
Duration	38 months

Abstract

Lumpy skin disease (LSD) is an economically devastating poxviral infection of cattle in Africa and the Middle East. The virus is secreted in saliva, ocular and nasal discharge and semen of infected animals but direct contact is considered to be a relatively ineffective route of transmission. The occurrence of disease outbreaks has been connected to wet and warm weather conditions, and to the abundance of insect populations. However, little is known of the importance of insects as field vectors of the disease and no studies have been undertaken on the role of hard ticks in the transmission of the disease. The aim of the proposed research is to investigate the transmission of lumpy skin disease virus (LSDV) between cattle by different life stages of A. hebraeum, R. appendiculatus and B. decoloratus ticks. All of these ticks are common in Africa. Laboratory-bred larvae, nymphs and adults will be placed to feed on the skin of the cattle experimentally infected with LSDV. Post-feeding the partially engorged adult R. appendiculatus and A. hebraeum males, and larvae hatched from eggs laid by females, previously fed on the skin lesions of cattle with LSD, will be transferred to feed on the skin of non-infected recipient animals. Infected A. hebraeum and R. appendiculatus nymphs will be allowed to moult to adults and then transferred to feed on the skin of non-infected cattle. The recipient animals will be monitored for clinical signs of LSD. Samples will be collected at regular intervals to detect viraemia and seroconversion, and live virus in the skin of the previously non-infected animals. Also, the survival mechanisms of the virus during the moulting of the ticks and the influence of ambient temperature on virogenesis in nymphal stages of ticks and the possible presence of live virus or viral antigen in tick salivary glands, the cells of the midgut and eggs will be investigated.

Summary

Lumpy skin disease (LSD) is an economically important viral infection of cattle occurring in most African countries and in the Middle East. It may cause significant economical losses to the farmers due to decrease in milk yield, abortions, infertility problems in males and females, and a loss in body weight and the value of hide. In endemic countries the loss of production, the costs of control and eradication programmes and, restriction or ban on the international trade of live animals or animal products following an outbreak are substantial. In general, poxviruses have been shown to enter the host through the skin or respiratory tract. Direct contact between infected and susceptible animals is considered to be a relatively inefficient route of the transmission of lumpy skin disease virus (LSDV). The virus is secreted in saliva, ocular and nasal discharges, and semen of the infected animals and therefore contaminated food or water and artificial insemination may serve as a source of infection. The occurrence of the disease is closely connected to warm and wet weather conditions and the abundance of insects. However, little is known about the importance of the different insect vectors in the transmission of LSDV during natural outbreaks. So far, no studies on the potential role of hard ticks in the transmission of LSDV have been carried out. In late 2008 a pilot study on the potential role of hard ticks in the transmission of LSDV was undertaken by the Capripoxvirus Reference Laboratory, IAH, UK and the Department of Veterinary Tropical Diseases, University of Pretoria, South Africa. LSD viral antigen was demonstrated in mouthparts, salivary glands and gut of three tick species, commonly found in Africa, post-feeding on the skin lesions of experimentally infected cattle. The viral antigen was also demonstrated in skin biopsies collected from the feeding sites of the infected ticks on non-infected recipient animals. In addition, viral DNA was demonstrated in eggs laid by female ticks previously fed on the skin lesions of infected cattle. Although these findings do not prove that the virus found in the ticks was still infective, it suggests that the mechanical, transstadial and transovarial transmission of LSDV may occur and further investigations are required. The aim of this study is to investigate the transmission of LSDV by three African ticks species. Laboratory-bred larvae, nymphs and adult ticks will be placed to feed on the skin of cattle experimentally infected with LSDV. Post-feeding partially engorged adult males and females or larvae hatched from eggs laid by females previously fed on LSD skin lesions will be placed to feed on the skin of non-infected animals. Nymphs will be allowed to develop to adults and then the unengorged adults will be transferred to feed on the skin of noninfected animals. The recipient animals will be closely monitored for clinical signs of LSD, samples will be collected at different intervals to detect viraemia, seroconversion and the presence of live virus in the skin of the non-infected animals. Detailed studies will be carried out on the persistence of the virus in different cell types of larvae, nymphs and adult ticks. Also, the effect of the severity of the infection in the host and of the environmental variables (temperature and humidity) on the transmission of LSDV by tick vectors and on viral multiplication rates and dissemination in ticks will be studied. The impact of the study will include the improved control of the LSD and therefore the improved welfare of the farming communities, families, nutritional and education status of the children in endemic countries. In addition, it will affect governmental policies on the international trade of live ruminants and on the potential spread of the disease caused by climate-change and an increase in abundance of tick populations.

Impact Summary

No previous studies on the transmission of LSDV by hard ticks have been undertaken. This study will provide fundamental data on the transmission of LSDV and it will significantly improve the understanding of the epidemiology of LSD. The insect vector has been demonstrated to be able to transmit the virus but the importance of this phenomenon in field conditions is not fully understood. In case LSDV is transmitted by hard tick vector, further investigation on the potential reservoir for the virus between outbreaks such as wild ruminants infested by the same tick species as cattle will be undertaken. Due to the close relatedness of the members of the Capripoxvirus genus hard ticks may transmitt sheep pox and goat pox viruses as well. Therefore further studies are required in order to investigate the transmission of sheep pox and goat pox virus by hard tick vectors. In addition to northern Africa sheep and goat pox is endemic also in the Middle East, Indian subcontinent and in most Asian countries which make the potential impact of this study substantial. The other poxviruses of veterinary importance, such as camel pox and ORF will benefit on the data obtained from this study. Camel pox virus belongs to the genus Orthopoxvirus within the Poxviridae family. The clinical signs and the epidemiology of camel pox are very similar to Capripox virus infections. Wernery et al. (1997) were able to isolate camel pox virus on cell cultures and the virus was also detected by electron microscopy in samples obtained from camel ticks (Hyalomma dromedarii) collected from infected camels. Previously several poxviruses have been showed to be transmitted by insect vectors but this study on hard tick vector is novel. The impact of this study for the community can be divided into three components, namely impact to the farming community, to the work of field veterinarians and the preventative health care of the cattle and to the work of governmental veterinary authorities. The impactof this study for the livelihood of poor farmers, small holders and the cattle farming industry as a whole is significant. If the ticks will be shown to be able to transmit the disease the eradication and control measures of the disease can be targeted to improve tick treatment of cattle in addition to other control measures. Fields veterinarians in Africa and in the Middle East should be aware of the potential role of ticks in the transmission of LSDV between cattle. This study will have direct effect on tick treatment policy recommended by the veterinary authorities. Governmental veterinary authorities responsible for the export and import policy for live domestic and wild ruminants should be informed on the potential role of hard tick vector in the transmission of LSDV. Scientists working with the tick research and vector-borne viruses will obtain fundamental data from this study. African swine fever virus (ASFV) is the other large, enveloped virus, in which the genome consists of a single molecule of linear double-stranded DNA. Both LSDV and ASFV replicate in the cytoplasm of the cells. In addition to the oronasal route of transmission, ASFV is transmitted by soft ticks of the Ornithodoros genus. The virus replicates in ticks and transstadial, transovarial and sexual transmission have been demonstrated in these ticks. The transmission of ASFV by soft tick vector has previously been studied in detail but since LSDV and ASFV share common features this study may provide new data for ASF as well. Female students will be encouraged to participate in the project. In addition to the tick expertise, the PhD project will provide the student with skills to perform the diagnostic tests for the detection of LSD antigen and antibodies. In countries where LSD is endemic, a rapid laboratory confirmation or tentative diagnosis of the disease is crucial for the swift implementation of the effective control measures which in turn may prevent the further spread of the diseas

Committee	Research Committee A (Animal disease, health and welfare)
Research Topics	Animal Health, Microbiology
Research Priority	X – Research Priority information not available
Research Initiative	Combating Infectious Diseases of Livestock for International Development (CIDLID) [2009]
Funding Scheme	X – not Funded via a specific Funding Scheme

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