A Recombinant Vaccine-like Strain of Lumpy Skin Disease Virus Causes Low-Level Infection of Cattle through Virus-Inoculated Feed

Abstract

Since 1989, lumpy skin disease of cattle (LSD) has spread out of Africa via the Middle East northwards and eastwards into Russia, the Far East and South-East Asia. It is now threatening to become a worldwide pandemic, with Australia possibly next in its path. One of the research gaps on the disease concerns its main mode of transmission, most likely via flying insect vectors such as biting flies or mosquitoes. Direct or indirect contact transmission is possible, but appears to be an inefficient route, although there is evidence to support the direct contact route for the newly detected recombinant strains first isolated in Russia. In this study, we used experimental bulls and fed them via virus-inoculated feed to evaluate the indirect contact route. To provide deeper insights, we ran two parallel experiments using the same design to discover differences that involved classical field strain Dagestan/2015 LSDV and recombinant vaccine-like Saratov/2017. Following the attempted indirect contact transmission of the virus from the inoculated feed via the alimentary canal, all bulls in the Dagestan/2015 group remained healthy and did not seroconvert by the end of the experiment, whereas for those in the Saratov/2017 recombinant virus group, of the five bulls fed on virus-inoculated feed, three remained clinically healthy, while two displayed evidence of a mild infection. These results provide support for recombinant virus transmission via the alimentary canal. In addition, of particular note, the negative control in-contact bull in this group exhibited a biphasic fever at days 10 and 20, developed lesions from day 13 onwards, and seroconverted by day 31. Two explanations are feasible here: one is the in-contact animal was somehow able to feed on some of the virus-inoculated bread left over from adjacent animals, but in the case here of the individual troughs being used, that was not likely; the other is the virus was transmitted from the virus-fed animals via an airborne route. Across the infected animals, the virus was detectable in blood from days 18 to 29 and in nasal discharge from days 20 to 42. Post-mortem and histological examinations were also indicative of LSDV infection, supporting further evidence for rapid, in F transmission of this virus. This is the first report of recombinant LSDV strain transmitting via the alimentary mode.

Keywords: lumpy skin disease; recombinant; transmission; virus.

Figure 1

Changes in body temperature of animals exposed to inoculated bread feeding over time for the Saratov/2017 group (the numbers denote the assigned bull numbers).

Figure 2

(A) Necrotised skin lesion (Bull №2) (B) Sequestrated skin lesion (Bull №2).

Figure 3

Prescapular node: edema and hemorrhages (Bull №1).

Figure 4

Pathologies observed in various organs. (A) The hypodermis of the skin (the arrows indicate a lesion) of Bull №2; (B) Prescapular node displaying hemorrhaging, Bull №2; (C) Testes: hyperaemic, Bull №3; (D) Mesenteric lymph node, Bull №3; (E) Mesenteric lymph node, enlarged, Bull №4; (F) Posterior pharyngeal lymph node enlargement of Bull №5.

Scheme 1

Bulls №1 and 3 were fed virus-inoculated bread, whereas Bull №2 was mock-fed.

Figure 5

Histopathologic findings of lumpy skin disease virus in cattle tissue. (A) Subacute liquefactive (pus) necrosis in the derma. H and E staining, magnification X40; (B) Esophageal Necrotic Lesion. H and E staining, magnification X40; (C) Lobular pneumonia with multiple necrotic sites: 1—foci of necrosis; (D) Affected lungs. 1—Atelectases, 2—dislectases, 3—pleurite. H and E staining, magnification X40; (E) Focal purulonecrotic pneumonia; 1—necrosis, 2—thickened interalveolar septa; (F) Muscle dystrophy. 1—edema, 2—muscle fiber dystrophy. H and E staining, magnification X40; (G) The serous membrane. Focal serositis. 1—inflammation, 2—lymphocytic infiltrations. H and E staining, magnification X4.