Infection, dissemination, and transmission of lumpy skin disease virus in Aedes aegypti (Linnaeus), Culex tritaeniorhynchus (Giles), and Culex quinquefasciatus (Say) mosquitoes

Lumpy skin disease virus (LSDV) is a transboundary viral disease in cattle and water buffaloes. Although this Poxvirus is supposedly transmitted by mechanical vectors, only a few studies have investigated the role of local vectors in the transmission of LSDV. This study examined the infection, dissemination, and transmission rates of LSDV in Aedes aegypti, Culex tritaeniorhynchus, and Culex quinquefasciatus following artificial membrane feeding of 102.7, 103.7, 104.7 TCID50/mL LSDV in sheep blood. The results demonstrated that these mosquito species were susceptible to LSDV, with Cx tritaeniorhynchus exhibiting significantly different characteristics from Ae. aegypti and Cx. quinquefasciatus. These three mosquito species were susceptible to LSDV. Ae. aegypti showed it as early as 2 days post-infection (dpi), indicating swift dissemination in this particular species. The extrinsic incubation period (EIP) of LSDV in Cx. tritaeniorhynchus and Cx. quinquefasciatus was 8 and 14 dpi, respectively. Ingestion of different viral titers in blood did not affect the infection, dissemination, or transmission rates of Cx. tritaeniorhynchus and Cx. quinquefasciatus. All rates remained consistently high at 8-14 dpi for Cx. tritaeniorhynchus. In all three species, LSDV remained detectable until 14 dpi. The present findings indicate that, Ae. aegypti, Cx. tritaeniorhynchus, and Cx. quinquefasciatus may act as vectors during the LSDV outbreak; their involvement may extend beyond being solely mechanical vectors.

Using the basic reproduction number to assess the risk of transmission of lumpy skin disease virus by biting insects

In recent years, lumpy skin disease virus (LSDV) has emerged as a major threat to cattle outside Africa, where it is endemic. Although evidence suggests that LSDV is transmitted by the bites of blood sucking arthropods, few studies have assessed the risk of transmission posed by particular vector species. Here this risk is assessed by calculating the basic reproduction number (R0 ) for transmission of LSDV by five species of biting insect: the stable fly, Stomoxys calcitrans, the biting midge, Culicoides nubeculosus, and three mosquito species, Aedes aegypti, Anopheles stephensi, and Culex quinquefasciatus. Parameters relating to mechanical transmission of LSDV were estimated using new analyses of previously published data from transmission experiments, while vector life history parameters were derived from the published literature. Uncertainty and sensitivity analyses were used to compute R0 for each species and to identify those parameters which influence its magnitude. Results suggest that S. calcitrans is likely to be the most efficient at transmitting LSDV, with Ae. aegypti also an efficient vector. By contrast, C. nubeculosus, An. stephensi, and Cx. quinquefasciatus are likely to be inefficient vectors of LSDV. However, there is considerable uncertainty associated with the estimates of R0 , reflecting uncertainty in most of the constituent parameters. Sensitivity analysis suggests that future experimental work should focus on estimating the probability of transmission from insect to bovine and on the virus inactivation rate in insects.

Risk of introduction of lumpy skin disease in France by the import of vectors in animal trucks

BACKGROUND

The lumpy skin disease virus (LSDV) is a dsDNA virus belonging to the Poxviridae family and the Capripoxvirus genus. Lumpy skin diseases (LSD) is a highly contagious transboundary disease in cattle producing major economic losses. In 2014, the disease was first reported in the European Union (in Cyprus); it was then reported in 2015 (in Greece) and has spread through different Balkan countries in 2016. Indirect vector transmission is predominant at small distances, but transmission between distant herds and between countries usually occurs through movements of infected cattle or through vectors found mainly in animal trucks.

METHODS AND PRINCIPAL FINDINGS

In order to estimate the threat for France due to the introduction of vectors found in animal trucks (cattle or horses) from at-risk countries (Balkans and neighbours), a quantitative import risk analysis (QIRA) model was developed according to the international standard. Using stochastic QIRA modelling and combining experimental/field data and expert opinion, the yearly risk of LSDV being introduced by stable flies (Stomoxys calcitrans), that travel in trucks transporting animals was between 6 x 10-5 and 5.93 x 10-3 with a median value of 89.9 x 10-5; it was mainly due to the risk related to insects entering farms in France from vehicles transporting cattle from the at-risk area. The risk related to the transport of cattle going to slaughterhouses or the transport of horses was much lower (between 2 x 10-7 and 3.73 x 10-5 and between 5 x 10-10 and 3.95 x 10-8 for cattle and horses, respectively). The disinsectisation of trucks transporting live animals was important to reduce this risk.

CONCLUSION AND SIGNIFICANCE

The development of a stochastic QIRA made it possible to quantify the risk of LSD being introduced in France through the import of vectors that travel in trucks transporting animals. This tool is of prime importance because the LSD situation in the Balkans is continuously changing. Indeed, this model can be updated to process new information on vectors and the changing health situation, in addition to new data from the TRAde Control and Expert System (TRACES, EU database). This model is easy to adapt to different countries and to other vectors and diseases.

Transovarial passage and transmission of LSDV by Amblyomma hebraeum, Rhipicephalus appendiculatus and Rhipicephalus decoloratus

Lumpy skin disease (LSD), an acute, sub-acute or inapparent disease of cattle, is caused by lumpy skin disease virus (LSDV), a member of the genus Capripoxvirus in the family Poxviridae. LSD is characterised by high fever, formation of circumscribed skin lesions and ulcerative lesions on the mucous membranes of the mouth, respiratory and digestive tracts. It is an economically important disease due to the permanent damage to hides, the reduction in productivity and trade restrictions imposed on affected areas. Transmission has been associated with blood-feeding insects such as stable flies (Stomoxysis calcitrans) and mosquitoes (Aedes aegypti). Mechanical (intrastadial) and transstadial transmission by Amblyomma hebraeum and Rhipicephalus appendiculatus as well as transovarial transmission by R. decoloratus have been reported. In this study transovarial passage of LSDV to larvae and subsequent transmission to recipient animals were demonstrated. The finding of transovarial passage of LSDV in female ticks shows the potential for A. hebraeum, R. appendiculatus and R. decoloratus to be reservoir hosts for LSDV.

Detection of lumpy skin disease virus in saliva of ticks fed on lumpy skin disease virus-infected cattle

Lumpy skin disease is an economically important disease of cattle that is caused by the lumpy skin disease virus (LSDV), which belongs to the genus Capripoxvirus. It is endemic in Africa and outbreaks have also been reported in the Middle-East. Transmission has mostly been associated with blood-feeding insects but recently, the authors have demonstrated mechanical transmission by Rhipicephalus appendiculatus as well as mechanical/intrastadial and transstadial transmission by Amblyomma hebraeum. Saliva is the medium of transmission of pathogens transmitted by biting arthropods and, simultaneously, it potentiates infection in the vertebrate host. This study aimed to detect LSDV in saliva of A. hebraeum and R. appendiculatus adult ticks fed, as nymphs or as adults, on LSDV-infected animals, thereby also demonstrating transstadial or mechanical/intrastadial passage of the virus in these ticks. Saliva samples were tested for LSDV by real-time PCR and virus isolation. Supernatants obtained from virus isolation were further tested by real-time PCR to confirm that the cytopathic effects observed were due to LSDV. Lumpy skin disease virus was detected, for the first time, in saliva samples of both A. hebraeum and R. appendiculatus ticks. At the same time, mechanical/intrastadial and transstadial passage of the virus was demonstrated and confirmed in R. appendiculatus and A. hebraeum.

Attempted mechanical transmission of lumpy skin disease virus by biting insects

The mosquitoes Anopheles stephensi Liston and Culex quinquefasciatus Say (Diptera: Culicidae), the stable fly Stomoxys calcitrans Linnaeus (Diptera: Muscidae) and the biting midge Culicoides nubeculosus Meigen (Diptera: Ceratopogonidae) were allowed to feed on either lumpy skin disease (LSD) infected animals or through a membrane on a bloodmeal containing lumpy skin disease virus (LSDV). These arthropods were then allowed to refeed on susceptible cattle at various intervals after the infective feed. Virus was detected in the insects by polymerase chain reaction immediately after feeding and at sufficiently high titre to enable transmission to occur. However, no transmission of virus from infected to susceptible animals by An. stephensi, S. calcitrans, C. nubeculosus and Cx. quinquefasciatus was observed.

Mechanical transmission of lumpy skin disease virus by Aedes aegypti (Diptera: Culicidae)

Aedes aegypti female mosquitoes are capable of the mechanical transmission of lumpy skin disease virus (LSDV) from infected to susceptible cattle. Mosquitoes that had fed upon lesions of LSDV-infected cattle were able to transmit virus to susceptible cattle over a period of 2-6 days post-infective feeding. Virus was isolated from the recipient animals in 5 out of 7 cases. The clinical disease recorded in the animals exposed to infected mosquitoes was generally of a mild nature, with only one case being moderate. LSDV has long been suspected to be insect transmitted, but these findings are the first to demonstrate this unequivocally, and they suggest that mosquito species are competent vectors.

Spread of lumpy skin disease in Israeli dairy herds

Fourteen of the 17 dairy herds in Peduyim, an Israeli village, became infected with lumpy skin disease during a period of 37 days in August and September 1989. One cow in one neighbouring village and four cows in another neighbouring village also became infected, probably through being treated by a veterinarian who treated cows in Peduyim. Circumstantial evidence suggests that the original infection was brought to Peduyim and spread by stable flies (Stomoxys calcitrans) carried by the wind from foci of the disease at El Arish in northern Sinai, or at Ismailiya and the Nile delta in Egypt. All the cattle and the small flocks of sheep and goats in the village were slaughtered.

An investigation of possible routes of transmission of lumpy skin disease virus (Neethling)

British cattle were infected with the South African (Neethling) strain of lumpy skin disease virus (LSDV) and their clinical signs monitored over a 3-week period. Different routes of infection were assessed for effect on the clinical characteristics of the disease by using a clinical scoring system. Neither of 2 animals inoculated onto the conjunctival sac showed clinical signs of seroconverted. The intradermal route produced local lesions in 21 of 25 animals, and generalized infection in 4. In contrast the intravenous route produced generalized lesions in 8 of 11 animals. Seven uninfected animals were housed in contact with infected animals for 1 month. None developed clinical signs or produced detectable serum neutralizing antibodies. Six of seven of these animals were then challenged and were fully susceptible to infection. The results suggest that the transmission of LSDV between animals by contagion is extremely inefficient, and that parenteral inoculation of virus is required to establish infection. The high proportion of animals with generalized disease following intravenous inoculation implies that naturally occurring cases of generalized LSD may follow spread by intravenously feeding arthropods.

Observations on the epidemiology of lumpy skin disease in Kenya

Lumpy skin disease virus strains isolated in Kenya over a period of some 20 years have proved to be serologically identical. They were indistinguishable by indirect fluorescent antibody and serum neutralization test from the South African Neethling and West African serotypes. These two serological methods proved of value in studying the antibody responses to infection. While epizootic spread of LSD has occurred in Kenya, most cases are of a sporadic nature and are thought to be the result of accidental contacts with a maintenance cycle. There is evidence of antibody to LSD in the African buffalo (Syncerus caffer) in those areas where LSD is considered to be enzootic in Kenya, and also in small numbers of domestic cattle. No buffalo or bovine sera contained antibody to cowpox virus. An area enzootic for LSD is proposed and it is suggested that the maintenance cycle involves the buffalo. No antibody was found in the other wild ruminant species examined.