Meet the most relevant arboviruses in Animal Health

In this step, we will learn the main characteristics of the viruses that are transmitted by arthropods, and some of the most important diseases that they cause in animals. Arboviruses are transmitted, exclusively or predominantly, by arthropods, that act as vectors. These arthropods can be different types of mosquitoes, culicoides insects (midges), and ticks or fleas, which feed on blood. Currently we know of more than 400 different arboviruses, of which only 50 are capable of producing disease, both in domestic animals and in wild animals. In addition, some can also affect people, which means they are zoonotic.

Each arbovirus is capable of infecting and replicating in a particular arthropod. The arthropod ingests the virus when it is feeding on the blood of a vertebrate animal. The virus replicates in tissues of the vector, mostly in its salivary glands. In this way, when it bites a new host, it transmits the virus with the saliva. Despite the virus replicating in the vector, it does not cause disease.

There are three types of susceptible vertebrate hosts: amplifying, reservoir and incidental hosts. Amplifying hosts have very large amounts of virus in their blood (in other words, viremia) and can infect an arthropod that bites them. In addition, they may develop disease. Reservoir hosts do not develop illness despite having a high viremia. This is the case of many types of wild birds. The virus passes continuously from reservoir hosts to vectors, staying undetected in nature. “Incidental or dead-end” hosts do not have a large enough amount of virus in their blood as to infect a new vector that bites them. These animals cannot transmit the infection but do develop the disease. Animal arboviruses belong to seven different virus families.

In three of them, the transmission by vectors is not the main form of contagion, so we will not talk about them. The most interesting arboviruses belong to the families Togaviridae, Flaviviridae, Bunyaviridae and Reoviridae. All are RNA viruses, which implies a high mutability and variability. Let’s learn about some of these viruses and the diseases they cause. In the family Togaviridae, within the genus Alphavirus, the most important viruses are those that produce equine encephalitis (Eastern, Western and Venezuelan), that also affect many other species. They can cause encephalitis in horses and in people. In the family Flaviviridae, the genus Flavivirus includes a large number of important viruses in human and animal health.

Among them, we highlight two zoonotic viruses that cause encephalitis: Japanese encephalitis virus (that affects pigs, horses and birds) and West Nile fever virus (that affects horses and birds, and is an emerging disease worldwide). Other emerging viruses are the Usutu virus (that affects birds) and louping ill virus, which affects sheep and is transmitted by ticks. The family Bunyaviridae includes many arboviruses of importance in Animal Health, associated with abortions and congenital malformations.

Some of these are: Rift Valley fever virus, which affects domestic and wild ruminants and people. It was first described in this African region, but currently it has extended considerably. And Schmallenberg virus, a new virus described in Germany in 2011, which affects ruminants and is considered an emerging disease in Europe. Lastly, in the family Reoviridae, the genus Orbivirus includes some of the most important arbovirus affecting Animal Health. The African horse sickness virus is endemic in large areas of central Africa and South Africa, although it has spread to other geographical regions. It affects horses and zebras, which are its natural reservoirs. And Bluetongue virus that is emerging around the world. It mainly affects sheep, causing significant economic losses.

In this step, we have learned that animal arboviruses are important, both in animal medicine and in human health, since many are zoonotic. Of all of them, we are going to choose as examples Bluetongue virus and West Nile Fever virus, which are considered emerging due to the big dissemination around the world. In the following steps, we will learn more about these viruses, the diseases they cause, and how they can be prevented and controlled.