How many herpesviruses have been discovered?

The herpesvirus constitute one of the families with the highest number of pathogenic species, producing a variety of diseases.

Remember that there are three kinds of herpesvirus: alpha, beta and gamma. Their main features were described in the video What is a herpesvirus? The infections that herpesviruses cause are often asymptomatic or benign, especially in cases in which the virus has become highly adapted to its host. Most alphaherpesvirus produce lesions localized in the skin or in mucous surfaces of the respiratory and genital tracts, and neurological syndromes when they establish latency in the sensory nerves, from which they reach the central nervous system inside the neurons. In addition, gammaherpesvirus (mainly) can transform to be cancerous in the cells in which they establish latency, originating lymphoproliferative and neoplastic syndromes.

Herpesvirus have been described in a large variety of hosts, most of them vertebrates but a species of herpesvirus has also been isolated from bivalves. Herpesvirus are known to affect mammals including primates, carnivores, artiodactyls, perissodactyls, rodents and cetaceans. But there are also others have been described that affect numerous birds, both poultry (chickens, turkeys) and wild birds, such as pigeons. Herpesvirus have also been isolated in reptiles and amphibians such as turtles, crocodiles, snakes, frogs, or fish of different families such as carp, salmon, trout, catfish and eel. Most alphaherpevirus that affect farm animals can cause great economic losses.

Some examples are: Aujeszky’s disease is one of the most important viral diseases that affects farm swine because it originates a neurological syndrome with close to 100% mortality in piglets. Due to its importance, we will develop this in another step.

Two diseases have been described in poultry: Marek’s disease and avian infectious laryngotracheitis, both of worldwide distribution. Birds affected by laryngotracheitis show respiratory signs that may lead to localized necrosis, ulcerations and bleeding that may cause death by asphyxia. Mortality rates can reach up to 70%. Marek’s disease is a lymphoproliferative process in chickens and hens. It can have different clinical forms, including the development of lymphoid tumours in a large number of organs. Other herpesviruses cause respiratory diseases in cattle (such as infectious bovine rhinotracheitis) and in horses (equine herpesvirus) that can be subclinical, mild or severe. Both infections are associated with the appearance of abortions and produce large economic losses. Both are notifiable diseases. Wildlife can also be affected by herpesvirus.

Well-known examples are two herpesviruses very well-adapted to their natural hosts. The first example affects apes. To date, more than 100 different herpesviruses in non-human primates have been described. Some of the herpesviruses found in monkeys produce asymptomatic infections or mild mucocutaneous lesions, but can produce encephalomyelitis and death in species of monkeys other than macaques. The second is the endotheliotropic herpesvirus of elephants, described for the first time in 1999 in benign lesions of African elephants. The same virus has been identified as a cause of severe endotheliotropic infections with multi-organ failure and death in Asian elephants. In some cases, herpesvirus infections can pose a threat to specific species.

The infection of curly oysters (Cassostrea gigas) by herpesvirus has spread around the world (USA, Europe, Australia, Japan, China), causing 10-30% mortality in larvae and juveniles. In some outbreaks mortality has reached 60-100%. especially coinciding with slight elevations of the sea temperature. Losses reach billions of dollars in Australia or New Zealand. Another species of herpesvirus is causing high mortality rates in hatcheries of ornamental carp (Koi) and common carp. The infection produces death within the first 24 or 48 hours post-infection. Fish present mottled gills and sunken eyes. Since the first cases in the United Kingdom in 1996, the disease has spread all over the world with the exception of Australia.

In short, new herpesviruses are being exponentially isolated, especially thanks to the use of powerful molecular diagnosis techniques. But we still don’t know many things about the pathogenicity of these viruses, especially in the case of beta- and gamma-herpesvirus that cause mild or subclinical infections.