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HEAL ▶▶▶TH


at a new site; a few may be able to cause disease, and fewer still may be able to survive and infect other animals. The coro- naviruses have been isolated from a wide range of animals and birds in which they may cause recognisable disease or have no effect. Bats carry a wide range of coronaviruses with- out obvious effect.


Four distinct groups of coronaviruses Analysis of the RNA sequences and proteins of the viruses re- sult in their classification into four distinct groups: Alpha- coronaviruses, Betacoronaviruses, Gammacoronavi- ruses and Deltacoronaviruses. When two individual coronaviruses infect the same host cell, it is possible for their RNA to combine in the particles released and to produce new viruses. TGEv and PEDv have been shown to recombine in pigs, and these recombinants have been isolated from diar- rhoeic animals. All coronaviruses can infect cells, but, where those cells are


not damaged by the infection or too few are damaged, no signs of disease can be detected and infection is inapparent. Recent studies in Italy have shown that HEv and PEDv are cir- culating in Italian pig herds without causing clinical signs.


Vaccination against coronaviruses If coronaviruses have been around for a long time, what has been the previous experience with vaccines? They certainly were made for coronaviruses, and have been used for many years to control infectious bronchitis in chickens, in bovine coronavirus (a Betacoronavirus like SARS-CoV-2 and HEv of pigs) and TGE and PED in pigs. Successful vaccines have in- cluded live, attenuated viruses which multiply in the host without causing disease while producing immunity; killed whole virus vaccines; and vaccines made from viral fragments or individual proteins or the genes coding for them. The actu- al vaccines used against any one virus will depend on effica- cy, convenience of use, cost and the importance of the


Porcine Epidemic Diarrhoea virus (PEDv)


How does it cause disease? A classical serotype of this type of coronavi- rus has been present for many years in Eu- rope and Asia, but in the last ten years, vari- ant genotypes have developed in China and have spread to the USA and Central America. In suckling piglets, the virus causes reduced levels of digestive enzymes. The villi con- tract, and absorption of nutrients decreases, allowing undigested food and fluid to pass to the large intestine, giving rise to diarrhoea.


Mode of transmission Infection is oral, from virus present in the fae- ces of infected pigs. As the diarrhoea in affect- ed pigs is watery, the faeces can be distribut- ed widely within a herd by contact and aerosol, and spread within a building is rapid. The virus can survive for long periods when frozen and remains stable in neutral condi- tions. Transmission between farms is usually by the movement of infected pigs. The virus has been identified in feed.


Clinical signs PED occurs as an explosive outbreak of diar- rhoea in non-immune weaned pigs or in pigs of all ages. After an incubation period of one to three days, piglets develop watery


8


Typical picture of PED – piglets huddling together and choosing to lie on their recumbent mother.


diarrhoea and may vomit. Illness occurs in al- most all non-immune piglets on the farm and mortality rates among the youngest pig- lets can approach 100%. About 20–30% of the older pigs may be affected with vomiting and diarrhoea. Inappetence may occur in breeding stock. In herds with partial immunity, suckling and young weaned pigs under 32kg live weight are rarely affected. Affected weaned pigs are dull and unwilling to rise. Fever is rare. The acute stage of vomiting and diarrhoea lasts about three days.


▶PIG PROGRESS | Volume 36, No. 4, 2020


Treatment and prevention There is no specific cure, just supportive thera- py for affected pigs. Suckling pigs and recently weaned pigs should be given glucose:glycine electrolyte solutions. Adequate drinkers should be provided. The effects of an outbreak of the disease may be reduced by isolating all sows within 14 days of farrowing and by infecting all those due to farrow in more than 14 days. Vac- cines based on live attenuated strains, inacti- vated virus and virus subunits are available in some countries, but those for classic PEDv do not protect against more recent strains.


PHOTO: JOHN CARR & MARK HOWELLS


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