Bluetongue Virus: Transmission, Diagnosis, Symptoms, And Treatment

Introduction

Bluetongue virus is from the genus Orbivirus in the family Reoviridae. Bluetongue virus is a non-zoonotic disease of ruminants like sheep, cattle, and deer that is believed to have originated in South Africa as early as the 18th century. This virus was first known as the ‘epizootic catarrh’ and was later named ‘malarial catarrhal fever of sheep. ’ However, in 1905 it was renamed as ‘bluetongue’ due to its swollen, cyanotic tongue characteristic in infected animals. Bluetongue is a vector borne disease that can infect most ruminants and camelids but it is most common in white tailed deer and sheep breeds found in the United States. Bluetongue virus is closely related to hemorrhagic diseases but has different characteristics than epizootic hemorrhagic disease virus (EHD). Hemorrhagic disease is caused by two orbiviruses known as epizootic hemorrhagic disease virus and bluetongue virus.

Prevalence of the Disease in USA and Worldwide

The spread of Bluetongue Virus throughout the world corresponds with the geographical and physical distribution of vector species of Culicoides biting midges, which are the only significant natural transmitters of the virus. In the United States, prevalence is high in southern and western regions due to frequent or diverse serotype exposure. However, most infections are reported with mild or inapparent disease. The disease is most often found in sheep, sometimes in goats, and occasionally in cattle. However, the bluetongue virus is commonly seen in white-tailed deer in the Kentucky and Tennessee areas, with a few cases reported in cattle in Texas and Ohio. In places like Africa, bluetongue viruses are broadly divided into western and eastern lineages based on phylogenetic analysis of nucleotide sequences. The prevalence of blue tongue is most closely related to tropical and subtropical regions where certain climate conditions can support large amounts of the midges species that depend largely on the distribution of vector competent species and climate conditions.

Mechanisms of Transmission

Bluetongue virus is a non-contagious disease that is transmitted by insects called biting midges. Midges become infected with Bluetongue virus by ingesting blood from infected ruminants. Once midges ingest a blood meal, they transmit the infection to their next host which is how bluetongue spreads. In the United States, midges are found in the late summer and early fall months and temperatures. There are roughly 1, 400 species of these biting midges of the Culicoides genus identified, however only 30 have been proven to transmit bluetongue virus. Biting midges are commonly found in the mud around or near ponds, lakes, and other water sources for animals. When there are heavy droughts, animals do not get all the hydration they need from the forage they graze so they search for water sources more often which makes them at higher risk of infection by the biting midges. Until recently, it was thought that the disease was not spread by direct contact between animals. However, there is a possibility this might no longer be accurate. Although it is known that bluetongue virus is exclusively transmitted by the female hematophagous midges, there has been a newly identified strain that may very well be transmitted via direct contact.

Transplacental infection can occur and depending on time of gestation to when the fetus is infected it can cause stillbirths and abortions. Bluetongue virus infects endothelial cells of blood vessel walls and mononuclear phagocytic and dendritic cells. This results in vascular thrombosis, tissue infection, necrosis, and hemorrhage. Rates of infection are higher than ever before due to the recent increase in drought conditions have created the perfect environment for these insects to spread the disease.

Diagnosis

There are many different ways to diagnose bluetongue virus. Diagnostics relies heavily on laboratory techniques including isolation of BTV and demonstrations of antigens, viral nucleic acids, and antibodies. The use of blood samples, or samples taken from the spleen, lung, or lymph node tissue are a small selection. However, bluetongue is more often diagnosed based on the signs and symptoms of each species it infects. Specimens such as semen from male animals may also be used for diagnostic purposes if they were collected at the peak of viremia. Bluetongue may also be diagnosed by petechiae, ecchymoses, and hemorrhages in the wall of the base of the pulmonary artery as well necrosis of the papillary muscle of the left ventricle of the heart. Pathogenesis Bluetongue virus infects monocytes both in vivo and in vitro and can interact with target cell surface by VP2 trimers that bind to the cell surface glycoproteins. Once BTV replicates in the lymph nodes where it drains sites of inoculation, it runs to secondary sites such as the lungs and spleen. Here, it replicates into endothelium and mononuclear phagocytes. From then, Bluetongue virus can disperse into lymph and the blood where it can be found in the intracellular vesicles of erythrocytes. At this point, it does not replicate but persists in the folds of cell membranes. In sheep and cattle BTV can be detected in the blood for 35 to 60 days and viral structures for up to 160 days. It can be said that erythrocytes could be the critical mechanism of cattle to serve as a natural reservoir host of bluetongue virus because BTV RNA is remotely similar to the erythrocyte lifespan in cattle.

Signs and Symptoms

Deer infected with bluetongue virus have been known to be depressed, feverish, emaciated, and remain near water in late summer or early fall. In many instances, the hooves of the infected animals become painful, making it very difficult for the animals to move. In some cases the hooves actually slough off. Blue tongue virus may also cause swelling of the head, neck, tongue, and eyelids, respiratory distress, and internal hemorrhaging. The swelling of the tongue can cause it to turn blue due to the lack of blood supply. In sheep, vascular endothelial damage is often seen which results in capillary permeability changes and coagulation. Typical clinical signs of sheep can also include edema, congestion, inflammation, necrosis, and hemorrhage. Young lambs have a higher mortality rate and can show signs of lameness and depression. Infected sheep have been known to eat less and even hold their food in their mouth to soften it before chewing because of oral soreness. In sheep, mortality ranges from 2-30% and can occasionally reach 70%.

Clinical signs of cattle can include ocular discharge, oral mucosal congestions, conjunctivitis, necrotic lesions on in the mouth, and edema. Goats are seen with an acute drop in milk production, edema, nasal discharge, and erythema of the skin and udder. Clinical signs can vary from subclinical infections to severe death of infect animals. In general, clinical signs include pyrexia, tachypnoea, muscle necrosis, and lethargy. Animals that survive may develop chronic dermatitis as well as vesicular and erosive lesions at interdigital and mucosal surfaces. Clinical signs usually can be seen 5-20 days and typically subside within a month. Animals infected during pregnancy have been known to abort or deliver malformed offspring. These malformations can lead to ataxia and blindness at birth.

Treatment, Control, and Prevention

There are a select few protective vaccines for ruminants available that are attenuated or inactivated and their protective activities are serotype specific. However, vaccination strategies depend on serotypes that are causing the infection so using certain vaccinations may not provide any protection at all. In South Africa, there are now three pentavalent attenuated vaccines have been developed for use of protecting sheep herds. However, farmers have been advised to use good husbandry by enclosing infected sheep in smaller areas with soft food, water, and shade and let them recover from the disease on their own, from there antimicrobial and anti-inflammatory drugs can be administered if needed. Due to the fact the disease is spread by midges, implementing insect control in moist areas and removing animals from infested areas helps in controlling the spread of the disease. Unfortunately, insecticide control alone will not completely eradicate the disease and should be used alongside a vaccination program for domestic animals. Another natural occurring control is the oncoming of cold weather; frost will kill the insects, which in turn results in less deaths in the animal population.

Summary and Conclusion

Bluetongue virus, although not spoken of often, has been known to eradicate large populations of deer species. It had not been a large issue in North America until in recent years. This virus originated in Africa and has spread to different regions of the world like North America and Europe. Bluetongue virus is transmitted by biting midges that are commonly found in the mud near water sources especially during droughts. This virus most commonly infects ruminants like sheep, deer, and cattle. Bluetongue virus is known for tissue necrosis, cyanotic mucous membranes and tongue, as well as hemorrhage. This disease is commonly diagnosed by isolation of the antigen through blood samples or tissue samples. There are a few vaccines available to use in domestic ruminants, however researchers have said to just isolate the infected and let them rest to get over the disease. Although there are different vaccinations for prevention, you have to have a serotype specific vaccine in order to protect animals from getting the disease. I chose to write my paper on Bluetongue virus because honestly, I had never heard of it until I told a friend I had to pick a virus to write a report on. My friend began to explain small details of this particular virus and it intrigued me to learn more. I’ve learn through my research that this virus is prominent in my home state of Kentucky. Since I enjoy participating in deer hunting, I will be looking more closely this year at the white tailed deer in our area. Hopefully, I won’t see any of them sticking their tongue out at me. And although research documents that it’s okay to eat a deer infected with the disease “as long as it smells okay, ” I personally will not partake in a meal from that animal.

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