GENERAL INTRODUCTION: ORTHOMYXOVIRIDAE

ORTHOMYXOVIRIDAE

A General Introduction has been added to each disease chapter in an attempt to give a brief updated overview of the taxonomic, biological and other characteristics of the virus family or group of bacteria /protozoa that cause disease in livestock and, where relevant, involve wildlife. As the text of the three-volume book Infectious Diseases of Livestock is currently under revision the Editors are aware that there are inconsistencies between the updated introductions to chapters and the content of the chapters themselves. Once the chapters have been updated – a process that is currently underway – these inconsistencies will be removed.

The family Orthomyxoviridae contains viruses that cause important diseases of people and domestic livestock, by far the most infamous being human influenza. Avian influenza, partly because of its zoonotic potential, is the most serious livestock disease caused by these viruses; among non-avian livestock species pigs and horses are worst affected; so too but less regularly marine mammals, mink, dogs, other carnivores and bats.

Seven genera currently comprise the family (Influenzaviruses A-D, Isavirus, Quaranjavirus and Thogotovirus); however, in contrast to their common physico-chemical and genomic properties there are major biological differences between these genera. For example, Influenzaviruses A-D infect mammals and birds almost exclusively, while the only known species of Isavirus causes fatal anaemia in farmed Atlantic salmon. Quaranjaviruses and Thogotoviruses are capable of infecting not only a wide variety of birds and mammals but also arthropods, including ticks and mosquitoes. There is evidence that at least some of these viruses behave as true arboviruses in contrast to Influenza- and Isaviruses that are directly transmitted between infected and susceptible hosts. Serious disease of livestock caused by Quaranja- and Thogotoviruses have not so far been recognised, i.e. they appear to cause mostly asymptomatic infections although pyrexia associated with abortion in sheep has been reported for a Thogotovirus in Kenya.

In contrast to Influenza B, C and D viruses which essentially cause relatively mild human diseases (although an Influenza D virus has recently been shown to infect cattle and cause mild disease), Influenza A viruses infect a wide range of avian and mammalian hosts where birds, especially those associated with aquatic environments (e.g. ducks & gulls), clearly play a pivotal epidemiological role.

Furthermore, cross-species infection with influenza A viruses, although infrequent, occurs periodically. That combined with recombination – enabled by the segmented genomes of influenza viruses – give rise to novel immunological types (so-called antigenic shift that is determined by the haemagglutinin/neurominidase combination of the virus – see below). Thus antigenic shifts give rise to immunologically novel viruses for a particular species and thereby may result in an epidemic or pandemic because the host population is immunologically naive. Even more rarely, such pandemics involve highly pathogenic viruses for one or more species and result in large-scale mortality. The so-called ‘Spanish flu’ that arose at the end of the Second World War (1918) is thought to have killed about 40 million people, the most devastating pandemic in recent history. Similarly, highly pathogenic avian influenza viruses periodically decimate poultry populations around the world.     

Influenza virions are medium-sized (80 to 120 nm in diameter), pleomorphic – usually roughly spherical – although filamentous forms may be many nm in length. A host cell-derived envelope surrounds the virions through which ‘spikes’ formed by haemagglutinin (HA) and neuraminidase (N - not present in all genera) glycoproteins project. The genome consists of six to eight single-stranded RNA segments of negative-sense (total length about 10-14,6 kb) surrounded by a helically-arranged nucleoprotein.  

Both the nucleus and cytoplasm of infected cells are involved in the replication cycle, although viral protein synthesis occurs only in the cytoplasm of host cells. Newly formed virions bud from the plasmalemma.

Reference

  1. International Committee for Taxonomy of Viruses https://talk.ictvonline.org/taxonomy/ (accession date: 26/05/2017)
  2. MacLachlan N.J. & Dubovi, E.J. (eds.), 2016. Veterinary Virology, 5th edition, Academic Press.