70       Picornaviridae

 

Picornaviruses (Spanish pico, very small), which are icosahedral and non-enveloped, contain a molecule of single-stranded RNA. Virions are 30 nm in diameter. The capsid is composed of 60 identical subunits, each containing four major proteins, VP1, VP2, VP3 and VP4. The VP4 protein is located on the inner surface of the capsid. Viral replication occurs in the cytoplasm in membrane-associated complexes and infection is usually cytolytic. The family has expanded greatly in recent years and now comprises 29 genera. Viruses of veterinary importance are contained in the genera Enterovirus, Cardiovirus, Aphthovirus, Avihepatovirus, Erbovirus, Sapelovirus, Tremovirus and Teschovirus. Several enteroviruses of pigs and poultry have been reassigned: porcine enteroviruses 1 to 7 and 11 to 13, which are associated with nervous disease and reproductive problems in pigs, have been reassigned to the genus Teschovirus while avian encephalomyelitis virus has been placed in the newly created genus Tremovirus. The genus Rhinovirus has been removed and human rhinoviruses have been placed in the genus Enterovirus.

Viruses of veterinary importance in the family Picornaviridae are presented in Table 70.1. Important human pathogens in the family include hepatitis A virus (genus Hepatovirus) and enterovirus C (genus Enterovirus) the cause of poliomyelitis, a serious neurological disease in humans. Picornaviruses are resistant to ether, chloroform and non-ionic detergents. Individual genera differ in their thermal lability and pH stability. Aphthoviruses are unstable at pH values below 6.5. Viruses in the other genera are stable at acid pH values.

Table 70.1 Picornaviruses of veterinary importance.

Genus	Virus	Comments
Enterovirus	Swine vesicular disease virus (porcine variant of enterovirus B)	Produces mild vesicular disease in pigs, clinically indistinguishable from foot-and-mouth disease
	Enterovirus E, F (bovine enteroviruses group A, group B)	Isolated from both normal cattle and animals with enteric, respiratory and reproductive disease
Teschovirus	Porcine teschovirus	Thirteen serotypes. Virulent strains of PTV-1 which occur in eastern Europe and Madagascar cause severe encephalomyelitis (Teschen disease); mild strains of PTV-1 are more widely distributed and cause endemic posterior paresis (Talfan disease). Infections with other seroptypes of PTV are often asymptomatic but may be associated with encephalomyelitis, SMEDI, pneumonia and diarrhoea
Sapelovirus	Porcine sapelovirus (porcine enterovirus A, porcine enterovirus 8)	Usually an asymptomatic infection but may be associated with SMEDI
Tremovirus	Avian encephalomyelitis virus	Avian encephalomyelitis is of considerable economic importance in chickens. Horizontal and vertical transmission occurs. Nervous signs seen in birds at 1 to 2 weeks of age. Control is achieved by vaccination of breeding flocks
Aphthovirus	Foot-and-mouth-disease virus	Seven serotypes are recognized: A, O, C, Asia 1, SAT 1, SAT 2, SAT 3. Economically important, highly contagious vesicular disease of even-toed ungulates
	Equine rhinitis A virus	Systemic infection, often subclinical but may be associated with respiratory signs
	Bovine rhinitis B virus	Widely distributed. Capable of causing mild respiratory disease
Cardiovirus	Encephalomyocarditis virus	Wide host range; rodents are considered to be the natural hosts. Infection in pigs is often subclinical but sporadic deaths and minor outbreaks may occur
Erbovirus	Equine rhinitis B virus	Considered a minor respiratory pathogen of horses
Avihepatovirus	Duck hepatitis A virus	Severe, fatal disease of young ducklings

Clinical infections

With the exception of foot-and-mouth disease virus and encephalomyocarditis virus, picornaviruses typically infect a single, or a limited number of, host species. Transmission usually occurs by the faecal–oral route but may also occur by fomites or by aerosols. Some picornaviruses, notably foot-and-mouth disease virus and swine vesicular disease virus, can produce persistent infections. Antigenic variation, which may contribute to the development of persistent infection, has been attributed to a number of molecular mechanisms including genetic recombination. Mixed infections with different serotypes of foot-and-mouth disease virus are known to occur in individual animals, particularly in African Cape buffaloes. Porcine teschovirus infections are widespread in pig populations and frequently subclinical in nature but may result in encephalomyelitis, diarrhoea, pneumonia, pericarditis/myocarditis and reproductive disorders.

Foot-and-mouth disease

This highly contagious disease of even-toed ungulates is characterized by fever and the formation of vesicles on epithelial surfaces. Foot-and-mouth disease (FMD) is a listed disease of the OIE. It is of major importance internationally on account of its rapid spread and the dramatic economic losses which it causes in susceptible animals. Isolates of foot-and-mouth disease virus (FMDV) are grouped in seven serotypes with differing geographical distributions. Infection with one serotype does not confer immunity against the other serotypes. A large number of subtypes is recognized within each serotype.

Cattle, sheep, goats, pigs and domesticated buffaloes are susceptible to FMD. Several wildlife species including African buffaloes, elephants, hedgehogs, deer and antelopes are also susceptible. Large numbers of virus particles are shed in the secretions and excretions of infected animals. Transmission can occur by direct contact, by aerosols, by mechanical carriage by humans or vehicles, on fomites and through animal products. Infected groups of animals, particularly pigs, shed large quantities of virus in aerosols. Under favourable conditions of low temperature, high humidity and moderate winds, virus in aerosols may spread up to 10 km over land. Turbulence is generally less marked over water than over land. In 1981, virus was carried a distance of more than 200 km from France to the south coast of England. Foot-and-mouth virus can persist in the pharyngeal region of carrier animals which have recovered from FMD.

The incubation period ranges from 2 to 14 days, but is generally shorter than a week. Infected cattle develop fever, inappetence and a drop in milk production. Profuse salivation, with characteristic drooling and smacking of lips, accompanies the formation of oral vesicles which rupture, leaving raw, painful ulcers. Ruptured vesicles in the interdigital cleft and on the coronary band lead to lameness. Vesicles may also appear on the skin of the teats and udders of lactating cows. Although the ulcers tend to heal rapidly, there may be secondary bacterial infection which exacerbates and prolongs the inflammatory process. Infected animals lose condition. Mature animals seldom die. Young animals, especially calves and lambs, may die from acute myocarditis. In pigs, foot lesions are severe and the hooves may slough. Marked lameness is the most prominent sign in this species. The disease in sheep, goats and wild ruminants is generally mild, presenting as fever accompanied by lameness which spreads rapidly through groups of animals.

Foot-and-mouth disease clinically resembles other vesicular diseases of domestic animals, including vesicular stomatitis in cattle and pigs, swine vesicular disease and vesicular exanthema in pigs. Consequently, FMD requires laboratory confirmation. Diagnosis is based on the demonstration of FMDV antigen in samples of tissue or in vesicular fluid by ELISA, CFT, RT-PCR or virus isolation. Demonstration of specific antibody by virus neutralization or ELISA can be used to confirm a diagnosis in unvaccinated animals. In endemic areas, interpretation of antibody titres may prove difficult.

In countries which are free from FMD, it is a notifiable disease and affected and in-contact animals are usually slaughtered. Following an outbreak, movement restrictions are applied and infected premises must be thoroughly cleaned and disinfected. Mild acids, such as citric acid and acetic acid, and alkalis such as sodium carbonate are effective disinfectants. Reserves of inactivated virus are maintained in several countries to provide an adequate supply of vaccine at short notice in the event of a major outbreak of the disease. Although ring vaccination around an affected premises may help to limit the spread of the disease, it may also allow the development of the carrier state in animals subsequently exposed to the virus. In countries where FMD is endemic, efforts are generally directed at protecting high-yielding dairy cattle by a combination of vaccination and control of animal movement. Vaccines for FMD, incorporating adjuvant, are derived from tissue culture-propagated virus which has been chemically inactivated. They are usually multivalent, containing three or more virus strains. Protection against antigenically similar strains of virus is satisfactory and lasts for up to six months.