The role of Rhipicephalus (Boophilus) decoloratus, Rhipicephalus appendiculatus and Amblyomma hebraeum in the transmission of lumpy skin disease virus

Abstract

Lumpy skin disease (LSD) is an economically important and debilitating disease of cattle caused by the lumpy skin disease virus (LSDV), a poxvirus in the genus Capripoxvirus. The disease is of economic importance to farmers in endemic regions and is a major constraint to international trade in livestock and their products. It is characterised by fever, enlargement of superficial lymph nodes, loss of weight, inappetence, salivation, lachrymation and formation of eruptive circumscribed skin lesions. The quality of meat and milk are reduced; there is infertility due to reduced sperm quality, abortions and reduced calving rates. The hides are permanently scarred, thereby reducing their quality and trade may be affected following movement restrictions from affected areas. v Lumpy skin disease has the potential to become an emerging disease because of global climate change and changes in patterns of trade in animals and animal products. The disease has become endemic in Africa except in countries like Libya, Algeria, Tunisia and Morocco, where the disease has never been reported. It has also spread to the Middle East where outbreaks were first reported in Israel (1989), Kuwait (1991), Saudi Arabia (1990) , Lebanon (1993), The United Arab Emirates (2000) and Oman (2010). In endemic areas, LSD outbreaks are common in summer. The persistence of LSDV between inter-epidemic periods has not been determined and there is no carrier state reported in either cattle or wild animals. Transmission of the disease has been associated with a high incidence of biting insects such as in wet conditions. The spread of LSD from Egypt to Israel e.g. was associated with movement of the stable fly, Stomoxys calcitrans. The virus has been recovered from S. calcitrans and Biomya fasciata, caught while feeding on infected animals and transmission by insects is suspected to be mechanical, which has been demonstrated in Aedes aegypti mosquitoes. During the 1957 outbreak of LSD in Kenya, affected animals were observed to have high tick infestations, especially of Amblyomma species. In a pilot trial in 2008 at the University of Pretoria (UP), Department of Veterinary Tropical Diseases (DVTD), Amblyomma hebraeum, Rhipicephalus appendiculatus and R. (B) decoloratus ticks were implicated in the transmission of LSDV. The overall objective of this study was to investigate the vector competence of three common sub-Saharan tick species (R. (B) decoloratus, R. appendiculatus and A. hebraeum) and their potential roles in the epidemiology of LSD. This was achieved by testing for persistence of LSDV in ticks and its subsequent transmission to recipient animals following interrupted feeding, transstadial and transovarial development of the ticks. The over-wintering of LSDV was also investigated during transstadial passage in A. hebraeum and transovarial passage in R. (B) decoloratus. During the study, seven cattle were artificially infected with LSDV to serve as source (donors) of infection to ticks. To test for mechanical / intrastadial transmission and persistence in ticks, adult ticks (A. hebraeum and R. appendiculatus) were partially fed on donor animals and then transferred to recipient animals or collected for testing. To test for transstadial transmission/passage, nymphal stages of A. hebraeum and R. appendiculatus were fed on donor animals until they engorged and dropped. Engorged nymphs were incubated to moult to adults. The emergent adults were placed on recipient animals and also tested for the virus. To test for transovarial transmission and passage R. (B) decoloratus (one- host tick) larvae were fed on donor animals until engorged adults. For R. appendiculatus and A. hebraeum (three-host ticks), adults were fed to repletion on the donor animals. Engorged females were collected and incubated to lay eggs and the eggs were allowed to hatch. The emergent larvae were placed to feed on recipient animals to test for transovarial transmission, while larvae were tested for the presence of the virus. Over-wintering of LSDV in ticks was tested by transstadial passage in A. hebraeum and transovarial passage in R. (B) decoloratus under fluctuating reduced temperatures, simulating wintery climatic conditions. Engorged A. hebraeum nymphs and R. (B) decoloratus females were infected by intracoelomic injection. The presence of the virus in LSDV- infected animals was tested by real-time PCR, virus isolation (VI), and the serum neutralisation test (SNT). Tick saliva was tested by real-time PCR and VI while ticks were tested by immunohistochemistry, transmission electron microscopy, VI and real-time PCR. Mechanical/intrastadial and transstadial transmission is reported in A. hebraeum and R. appendiculatus. Transovarial transmission was reported in A. hebraeum, R. appendiculatus and R. (B) decoloratus. The virus was demonstrated in saliva and tick organs of A. hebraeum and R. appendiculatus adults following both mechanical/intrastadial and transstadial persistence. Transovarial passage of LSDV was demonstrated in R. (B) decoloratus, R. appendiculatus and A. hebraeum larvae. The virus also persisted through cold temperature exposure during transstadial passage in A. hebraeum and transovarial passage in R. appendiculatus. This study confirms the vector competency of A. hebraeum, R. appendiculatus and R. (B) decoloratus ticks for LSDV. It also shows the potential for LSDV to over-winter in ticks and demonstrates that LSDV may persist in ticks during inter-epidemic periods.