disease (no attempt has been made to vaccinate against Por- cine Respiratory Coronavirus). Vaccines against coronaviruses may protect against disease, but killed and subunit/RNA vaccines may not adequately stimulate the local protective responses in the gut or respira- tory tract necessary for full protection, which may only be provided by a suitable attenuated vaccine. Apparent failure of vaccination can occur when mutant strains arise, as in infec- tious bronchitis in chickens, and when maternal antibody is present at vaccination. Few coronavirus vaccines eliminate all virus or protect against subclinical infection.

Pigs are not susceptible to SARS-CoV-2 In the meantime, however, it has become crystal clear that pigs are not susceptible to the latest addition to the group of coronaviruses. At least two different research groups tested the susceptibility of various species of animals to the virus. The German Federal Research Institute for Animal Health, the Friedrich-Loeffler-Institut (FLI), was the first to report that pigs and chickens are not susceptible to SARS-CoV-2 in- fection, whereas fruit bats and ferrets are. The FLI examined whether or not the animals could be in- fected, whether the pathogen replicates and whether the animals showed clinical signs of disease. It also tested if ani- mals excrete the pathogen and thus pose a potential risk to human health. Under experimental conditions, neither pigs nor chickens were found to be susceptible to infection with SARS-CoV-2. In the infection studies, the animals were inocu- lated nasally with SARS-CoV-2 to mimic the natural route of

infection in humans via the nasopharyngeal route. Not much later, trials in China showed a similar pattern. Sci- ence published a report from 21 Chinese authors attached to the Harbin Veterinary Research Institute, part of the Chinese Academy of Agricultural Sciences (CAAS), the National Insti- tute for Viral Disease Control and Prevention, as well as the National High Containment Laboratory for Animal Diseases Control and Prevention in Harbin. The team tested a range of animals for susceptibility to SARS-CoV-2 and concluded that “it replicates poorly in dogs, pigs, chickens and ducks”.

The descriptions given in the boxes are on the basis of the Pig Progress Health Tool, compiled by Dr David Taylor. More infor- mation:

Transmissible Gastroenteritis virus (TGEv)

How does it cause disease? TGE is caused by a coronavirus that produces cell damage. The virus multiplies in the cells lining the small intestine. Mature absorptive cells are lost, leading to shortening of the villi and to reduction in the ability to digest food. Osmotic diarrhoea results; piglets die from de- hydration. Specific IgA antibodies in the colos- trum and milk of immune sows protect piglets against infection.

Mode of transmission Infection is oral, from virus present in the faeces of infected pigs. As the diarrhoea in affected pigs is so profuse and watery, the faeces can be distributed widely within a herd by contact and aerosol, and spread within a building is rapid. The virus can sur- vive for long periods when frozen and

remains stable in neutral conditions. Trans- mission between farms is usually by the movement of infected pigs, but transport, clothing and aerosol spread have all been documented.

Clinical signs TGE begins as an explosive outbreak of diar- rhoea which involves pigs of all ages within a few days in non-immune herds. The diarrhoea is watery and yellowish green. Vomiting may occur in piglets under three weeks of age. Af- fected pigs become dehydrated, resulting in death in piglets within 24–48 hours. The dis- ease occurs within three to four days of birth in susceptible piglets. In a non-immune herd where water is not available to the piglets, mortality between days 0 and 7 is 100% and is rare in piglets aged three weeks or more.

Where the disease is enzootic, pigs aged be- tween ten days and six weeks develop diar- rhoea and growth depression. Finishing pigs may only have growth depression.

Treatment and prevention There is no treatment. Affected piglets can be supported using electrolyte replacer solutions. Early weaning, high temperatures and hygiene may all help reduce mortality, as does cross-suckling of litters onto immune sows. Water should be available to weaned pigs. Prevention of entry to farms relies on isolation, exclusion of visitors or lorries and use of showers or dips. Vaccines based on live attenuated strains, inactivated virus and virus subunits are available in some countries, alone or combined with those for PEDv.

▶PIG PROGRESS | Volume 36, No. 4, 2020 9

Pigs may not be susceptible for SARS-CoV-2, but the pig industry has felt the ef- fects as meat- packers all across the globe had backlogs.

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