Influenza Equinos

Influenza Infections

http://www.ivis.org/special_books/Lekeux/daly/chapter_frm.asp?LA=1

In: Equine Respiratory Diseases, Lekeux P. (Ed.) International Veterinary Information Service, Ithaca NY (www.ivis.org), 2001; B0322.1101

Influenza Infections (Last Updated: 13-Nov-2001)
J. M. Daly and J. A. Mumford Animal Health Trust, Lanwades Park, Kentford, Suffolk, Newmarket, UK. Classificationof Equine Influenza Virus Equine influenza viruses are classified as type A influenza viruses, which also include viruses infecting man, birds and swine. Type A influenza viruses are divided into subtypes according to the antigenic characteristics of the surface antigens, haemagglutinin (HA) and neuraminidase (NA). Two subtypes are known to cause disease in equids (H7N7 or equi-1 and H3N8 orequi-2). These differ from the subtypes that cause infection in man (H1N1, H2N2 and H3N2). All the known subtypes (H1 to H15 and N1 to N9) have been isolated from aquatic birds, which are thought to act as a natural reservoir for influenza A viruses. Epidemiology Influenza virus was first isolated from horses in Czechoslovakia in 1956; this was the prototype strain for the H7N7 subtype of equineinfluenza virus A/eq/Prague/56 (H7N7). Virus of the H7N7 subtype was subsequently detected in several European countries and in America. The H7N7 subtype is still represented in vaccines in most countries although the evidence over the last 20 years suggests that H7N7 viruses may only persist at very low level in some parts of the world, or may be extinct. Virus of the H3N8 subtype was first isolated inMiami in 1963, and has continued to cause outbreaks of disease in Europe and North America since. Antigenic variants can give rise to large-scale disease epidemics such as occurred in 1979 to 1981 in Europe and North America. Representative strains from these epidemics were subsequently incorporated into vaccines and annual vaccination for racehorses and competition horses became mandatory inseveral European countries. Following implementation of a mandatory vaccination policy in 1981, equine influenza was not diagnosed in Ireland and the UK for almost a decade. In 1989, an extensive epidemic of equine influenza was experienced in Europe. Antigenic and genetic analysis of a virus isolated in the UK (A/eq/Suffolk/89) demonstrated that it differed significantly from the prototype strain(A/eq/Fontainebleau/79) of the previous European epidemic [1]. A novel strain of equine influenza caused an epidemic in north-eastern China in 1989 with an overall morbidity rate of 80% and a mortality rate of 20%. The prototype virus from this epidemic (A/eq/Jilin/89) was shown to be more closely related to influenza viruses infecting aquatic birds than to contemporary equine influenza viruses,demonstrating that the epidemic was the result of emergence of virus from the avian influenza reservoir [2]. Although a related virus affected a few hundred horses in 1990 and serological evidence suggests that avian-like virus continued to circulate in horses for at least 5 years [3], there have been no reports of deaths due to infection with this virus since the initial outbreak in 1989. Effortsdirected at international surveillance for equine influenza have substantially increased in the past 10 years. The International Collating Centre (ICC) receives reports of episodes of infectious disease from nominated veterinary contacts in participating countries on a quarterly basis and provides regular reports to subscribers around the world. Although limited, the data reported by the ICC are theonly data available for non-notifiable diseases such as equine influenza and provide a global perspective. During the period 1995 to 2000, some European countries reported the laboratory diagnosis of influenza on an annual basis, as did the USA, suggesting that the virus is endemic in these regions. Only Australia, New Zealand and Iceland are known to have remained entirely free from equine...