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Silent circulation of coronaviruses in pigs
  1. Stefania Leopardi,
  2. Calogero Terregino and
  3. De Benedictis Paola
  1. OIE Collaborating Centre for Diseases at the Animal/Human Interface, Istituto Zooprofilattico Sperimentale delle Venezie, viale dell’Universitá 10, Padova, Italy
  1. email: pdebenedictis{at}izsvenezie.it

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In the summer of 2018, four intensive pig farms in north-eastern Italy were tested for coronaviruses.1 At the time of sampling, none of the pigs showed clinical signs and no outbreaks of gastroenteritis were reported in the previous or following winter seasons. Of the 65 faecal pools collected from the farms, eight pools tested positive for coronaviruses, namely porcine haemagglutinating encephalomyelitis virus (PHEV) in three of the farms; one of these farms also had coinfection with porcine epidemic diarrhoea virus (PEDV). The fourth farm was negative for coronaviruses. On the three farms with infection, the percentage of positivity ranged from 9.7 per cent to 12.5 per cent of pigs infected with PHEV, while 10 per cent of pigs were infected with PEDV on the coinfected farm.

In Italy, the majority of pig production is located in the north, where PED cases are reported almost every winter with variable morbidity and mortality in piglets.2 Unexpectedly, PED was last reported in the affected farm in February 2017. This finding suggests a potentially long-lasting maintenance, possibly leading to a recrudescence within the farm in the absence of reintroduction through infected animals or fomites.2

On the contrary, the asymptomatic circulation of PHEV is well known,3 likely due to intentional maintenance in sows to avoid lethal encephalitis in naive newborn piglets.4

Subclinical circulation of corona-viruses in pigs might present a risk to animals and, potentially, to human health

Subclinical circulation of coronaviruses in pigs might present a risk to animals and, potentially, to human health. Different coronaviruses might interact during coinfection, influencing both viral replication and pathogenic outcome.5 The cocirculation of coronaviruses is worrisome as it might favour the emergence of potential epidemic strains through recombination events.6 Indeed, a recombinant transmissible gastroenteritis virus-PEDV strain (swine enteric coronavirus) has already been reported in Italy, fortunately without any apparent increase in pathogenicity.7

Following the first imported cases in early 2020, northern Italy is now facing a local circulation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). While there is a need to understand the role of animals in the emergence of SARS-CoV-2 in China, surveillance in domestic animals in the light of human outbreaks should not be overlooked; we need to understand if they play a role in the virus’s epidemiology, including transmission, amplification, evolution or maintenance. Indeed, SARS-CoV-2 has been shown to be able to interact with the angiotensin-converting enzyme II receptor from non-human primates, pigs, cats and ferrets.8,9 Critically, these data suggest that circulation of coronaviruses in pigs might be missed unless they cause severe disease. Most symptomatic coronavirus infections are clinically indistinguishable in pigs, and this might delay early detection of new pathogens, as shown by the swine acute diarrhoea syndrome coronavirus epidemic.10 To aid early detection of unexpected viruses, broad-spectrum molecular techniques followed by genetic characterisation urgently need to be used.

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