Many people around the world rely heavily on livestock farming for a living. In 2022, the worldwide cattle population was estimated to be around one thousand million head, which serves to meet the daily needs for milk, milk products, and meat.1
However, the recent spread of lumpy skin disease (LSD) in cattle across nations has threatened this source of livelihood, affecting the stakeholders in the dairy and meat industries substantially, especially the poor farmers. When there is an outbreak of LSD, international trade restrictions are imposed to safeguard the unexposed farm animals. In addition, the United Nations Food and Agriculture Organization have classified it as a transboundary animal disease (TAD) of livestock.2 Though immunization can be used to limit the spread of LSD, is it readily available? A specific vaccine must be available on the market to completely prevent this outbreak. However, we’ve found three different vaccines that can control or stop this outbreak in animals. But are they made especially for LSD? If not, how effective are they at preventing this disease?
In the following blog, we will examine the findings of three separate vaccine comparison studies to see how truly effective different vaccines are at preventing the occurrence of LSD.
To begin with, lumpy skin disease is a vector-borne pox disease of domestic cattle and Asian water buffalo.2 It is caused by the lumpy skin disease virus (LSDV) which is a member of the family Poxviridae (genus: Capripoxvirus (CaPV)). LSDV has a large and stable double-stranded DNA which shows very little genetic variability. The incubation period in cattle is 4–14 days.
The infected cattle develop nodules on the skin and other parts of the body that may become ulcerated and infected, aggravating the condition of the affected animals. They also exhibit fever, lacrimation (secretion of tears), and hyper salivation. The virus has a drastic effect on reducing the milk yield, abortion in pregnant cows, poor coat condition, and bull sterility, resulting in a significant economic impact on the cattle rearers and devastating them.3
Previously, LSD was prevalent and endemic throughout Africa. However, after 2000, LSD spread throughout the Middle East and, in 2013, into Turkey and many countries in the Balkans.3 In July 2019, LSD first appeared in Bangladesh, followed by reports of cases in China and India. Figure 1 depicts the morbidity and mortality in countries recently affected by LSD in 2020.4
As shown in the graph, the LSD-associated mortality rate is low. Yet the trade restrictions affect the dairy business globally on a large scale. Hence, all affected countries are making desperate efforts to protect their livestock to reduce the impact on their dairy industries. In fact, since the sheep pox virus (SPPV) and goat pox virus (GTPV) are closely related but phylogenetically distinct members of the Poxviridae family, many countries are relying on the sheep pox vaccination to protect their livestock from this disease. Is this vaccination, however, truly effective against LSD?
In the first case study, which took place in Egypt between 2017 and 2018, 1050 sheep pox vaccinated cattle were examined to determine vaccine efficacy. Despite having been vaccinated, these cattle exhibited classic clinical signs of LSD on clinical examination. Skin samples were taken for PCR to determine the pathophysiology of lesions. Multiple clinical complications and deaths were observed. Figure 2 shows the clinical cases, complications, and deaths in vaccinated LSDV-infected animals.5
The above graph shows that despite the prevalence of complications such as pneumonia, enteritis, and bloody urine, the death rate in immunized cattle was only 6.86%. Hence, it is clear that the sheep pox vaccine can help reduce the severity of the disease and prevent LSD-related mortality in animals.
In the second study to assess the safety and efficacy of vaccines in Turkey, animals were divided into four groups to compare the efficiency of 10 doses of SP vaccine, 5 doses of SP vaccine (new SheepPox vaccine produced in MDBK cells), and Penpox-M vaccine (commercially available SP vaccine). Two cattle were left unvaccinated. All animals were infected with the virulent LSDV on day 31 after vaccination. Thereafter, blood and swab samples were collected and analyzed with the Enzyme-Linked Immunosorbent Assay (ELISA). The average ELISA optical density results of experimental animals are shown in figure 3.6
The graph above clearly shows that the antibody response to viral infection was significantly stronger in vaccinated animals than in unvaccinated animals, whose serum antibody concentration only increased by 21 DPC (Day Post-Challenge). Moreover, viremia was absent in Penpox-M-vaccinated cattle but evident in SP-vaccinated animals. In addition, LSDV-DNA was not found in any vaccinated animals’ nasal swabs, but it was found in unvaccinated animals’ ocular or nasal swabs. It shows that, while the SP vaccination strain did not provide complete protection against LSD, it did prevent the disease from spreading. It proves that even though the SP vaccine strain can provide protection against LSD infections, it is less effective than the Penpox-M vaccine.
The efficacy of live attenuated Neethling LSDV vaccine and live attenuated Romanian Sheep pox virus (SSPV) vaccine was compared against a recently circulating LSDV field isolate in another study. In this study, three different batches of Neethling LSDV and Romanian SSPV virus were taken.
In batches 1 and 2, the LSDV vaccination titers are significantly higher than the SPPV vaccine titers for all parameters. In batch 3, the antibody titers for tissue culture and SNT antibodies were similar in both vaccines, while ELISA response and challenge titers were higher in the LSDV vaccine. These findings show that the live attenuated LSDV vaccine provided better immunity in cattle than the live attenuated SPPV vaccine. Detailed representation of the study is shown in Figure 4.7
It is clear from the above discussion that the live attenuated LSDV vaccine provides the best immunity against LSD, whereas each variant of the sheeppox vaccine provides a different level of partial immunity against LSD, which is far less effective than the LSDV vaccine. Of all the sheeppox vaccines, the commercially available Penpox-M vaccine provided the best immunity. So, it can be used for the vaccination of unexposed animals, if the LSDV vaccine is unavailable.
Even if these animals become infected with LSD, they can be treated by the administration of suitable antibiotics to control secondary infections. Moreover, good nursing care may help reduce the severity of complications.3
The above discussion helps us to conclude that immunization can help prevent disease and death in animals due to LSD. However, more research is still needed to make the vaccines more potent and efficacious. Moreover, disease surveillance should be intensified to identify the possible routes of transmission to curb its spread.
In addition, though the LSDV vaccine gives the best immunity against LSD, its widespread availability might be an issue for the affected countries. In that case, the widely available Penpox-M vaccine can be used for immunization as it helps to lower the mortality and severity of LSD in cattle. In fact, both heterogeneous and homologous vaccines against any virus from the genus Capripoxvirus (CaPV) can be used for immunization against LSD. Such interventions would help to reduce the financial losses until the LSDV vaccines become widely available.
Reviewer: priyanka
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