Bluetongue

Bluetongue (BT) is an arthropod-borne viral infectious disease affecting both livestock and wild ruminants. The causative agent is the bluetongue virus (BTV), classified within the Reoviridae family under the Orbivirus genus. Globally, BT has raised concerns about livestock health, witnessing multiple outbreaks across diverse regions. Comprising a robust team, BioVenic offers specialized services for diagnostic development, pioneering research and development of vaccines, and comprehensive drug development services to address the challenges posed by BT.

Cause and Effect

The cause of BT is the transmission of the BTV through the bites of infected Culicoides spp. In the United States, mainly C. sonorensis and C. insignis. When these midges feed on viremic animals, they acquire the virus and subsequently transmit it to susceptible animals during subsequent blood meals. This transmission cycle establishes a cause-effect relationship between the presence of infected midges and the occurrence of BT in susceptible hosts.

The effect of BT on animals can be significant. Sheep, in particular, may develop cyanosis of the tongue, giving rise to the characteristic "bluetongue" appearance. Severe cases can lead to high morbidity and mortality rates, impacting animal welfare and productivity. The effect of BT is not limited to individual animals but can also have broader consequences. Outbreaks of BT can result in economic losses for the livestock industry.

Virology

BTV is a non-enveloped virus whose nucleic acid is composed of 10 segmented double-stranded RNAs, encoding 12 proteins. Based on differences in coat proteins, BTV strains are classified into 29 serotypes with varying virulence and host ranges. Within the BTV, VP2 and VP5 are the primary antigens associated with specific serotypes, while VP7 serves as the main group-specific antigen crucial for serological detection. It is noteworthy that viruses of distinct serotypes can exchange gene segments during natural infection, contributing to the dynamic nature of the virus and potential variations in its genetic makeup. Almost all ruminants can be infected with BTV. Although occurrences of BT must be reported to the OIE, it is not a zoonotic disease and poses no threat to public safety.

Fig. 1 Schematic representation of predicted BTV structure.¹

Distribution

Multiple factors influence the occurrence of BT, including climate, vectors, and infected animal activities. BT was first discovered in Africa and appeared in Asia in 1943. In 1952, the BT epidemic occurred in California, USA. During 1956-1957, BT caused mass sheep die-offs in Europe. BT successfully invaded Australia in 1976, and BTV was identified and isolated on every continent except Antarctica. So far, BT has become popular all over the world.

Hosts and Transmission

BTV primarily infects livestock ruminants, with sheep being the most susceptible, followed by cattle and goats. Among wild ruminants, deer exhibit the highest susceptibility, while antelopes and giant horned sheep can also be infected. Although sheep are regarded as the most susceptible host species, cattle are generally considered the primary mammalian host for BTV.

Bluetongue is not transmitted by direct or indirect contact, mainly by Culicoides spp. Second, BT can be transmitted to ewes or cows through semen, and vertically through the placenta.

The occurrence of BT has obvious seasonality, which is mainly related to the activities of various Culicoides.

Fig. 2 Schematic diagram of the transmission modes of BTV.²

Pathogenesis

After the virus infects ruminants, it first replicates in regional lymph nodes, and then enters other lymph nodes, lymphoreticular tissue, and vascular endothelial and perithelial cells, resulting in vascular damage and subsequent clinical symptoms. The immune response triggered by the virus contributes to the development of inflammatory processes and various pathological changes.

Signs and Symptoms

In sheep, symptoms of BT are typically distinctive and severe, encompassing,

Cattle and other ruminants are usually asymptomatically infected. Overt infection is usually mild with long-term persistent viremia. Symptoms include,

• Temporary loss of appetite.
• Nasal, nasal, and oral congestion.
• Slight erosions of lips, mouth lids, gums, etc.

Laboratory Diagnosis

• Isolation and Identified
  
  • Laboratory confirmation of BT can be performed using chicken embryo or cell culture to isolate BTV.

In addition to virus isolation and identification, we provide customers with the following diagnostic kit development services based on immunology and molecular science.

• Immunodiagnostics

We provide customers with development services such as virus neutralization assay, agar gel immunodiffusion (AGID), and competitive ELISA to help detect BTV-specific antibodies and antigens.

• Molecular Diagnostics

PCR is our longest-running diagnostic development service for customers, and it enables early diagnosis of BTV quickly and accurately.

Treatment

There is no effective treatment for BT. For sick ruminants, nutrition should be strengthened, supportive care should be given, and sulfonamides can be used.

Prevention and Control

• Vaccination
  
  • Vaccination plays a vital role in preventing betongue. Vaccination strategies should be tailored based on the local epidemiology and serotype distribution. We can develop custom multivalent vaccines for our clients.
• Vector Control
  
  • Controlling Culicoides spp. is crucial for disease prevention. Measures include reducing breeding sites, using insecticides, and employing physical barriers to minimize Culicoides spp. contact with susceptible animals.
• Movement Restrictions and Biosecurity
  
  • In areas without BT, it is prohibited to introduce susceptible animals from epidemic areas. Strengthen customs quarantine and transportation quarantine, and it is strictly forbidden to import cattle, sheep, or frozen semen from countries or regions with BT.

BT remains a major global livestock health problem. Continuous monitoring of the prevalence, serotype distribution, and vector populations is required. To effectively combat BT, BioVenic is committed to providing customers with vaccines, diagnostic kits, and drug development services. For more information, please feel free to contact us.

References

1. Rojas, José M., et al. "Vaccination as a strategy to prevent bluetongue virus vertical transmission." Pathogens 10.11 (2021): 1528.
2. Saminathan, Mani, et al. "An updated review on bluetongue virus: epidemiology, pathobiology, and advances in diagnosis and control with special reference to India." Veterinary Quarterly 40.1 (2020): 258-321.