An update on porcine coronavirus

Introduction

Viruses in the coronavirus family are RNA viruses that belong to the genus Nidovirales, which has two subfamilies:

• Coronaviridae, which is comprised of the genera Alphacoronavirus, Betacoronavirus and Gammacoronavirus.
• Toroviridae, which includes the genera Torovirus and Bafinivirus.
• These viruses cause five diseases in pigs that, in chronological order, are

1. Transmissible gastroenteritis (TGE - 1946)
2. Haemagglutinating encephalomyelitis (HEV - 1962)
3. Porcine epidemic diarrhea (PED - 1977)
4. Porcine respiratory coronavirus (PRCV - 1984)
5. Deltacoronavirus (PDCoV - 2009)

Three porcine coronaviruses are associated with digestive disorders (TGE, PED and PDCoV). Porcine respiratory coronavirus (PRCV) is associated with respiratory problems, and the HEV virus gives rise to two different syndroms, vomiting and wasting disease and encephalomyelitis.

There have been no reported cases of human infection with porcine coronavirus.

Transmissible gastroenteritis virus

The TGE virus is a coronavirus that has rarely been found since the late 20th century. First reported in 1946, it reached its highest prevalence in the 1970, 80s, and 90s in both Europe and the USA, with over 95% of European farms seropositive in the 80s. It is a highly infectious virus that causes diarrhea, dehydration, and occasionally vomiting as a distinctive symptom, and causes high mortality in young pigs. The TGE virus is completely sequenced and only one serotype is known. Homologies exist between TGE virus and bovine and human coronavirus, as well as with porcine respiratory coronavirus. From an epidemiological point of view, there are both epidemic and endemic cases. It is a relative thermostable virus, and is resistant to low pH and to many disinfectants. At 37ºC it is inactivated in less than two hours. It is very sensitive to light and highly resistant to freezing. The virus survives for long periods of time in frozen carcasses. This means that most outbreaks of the disease occur during cold months (the winter season).

Pathogenesis

The route of transmission is fecal-oral. The virus multiplies mainly in the cells of the gastrointestinal tract, and is found present in the cytoplasm of the infected cells after 4-5 hours, mainly at the base of the villi of the small intestine. It causes atrophy of the villi which significantly reduces the absorption of nutrients and causes osmotic diarrhea which is worsened by reduced sodium and glucose transport, leading to hypoglycemia in affected animals. At the same time it multiplies in different extra-intestinal organs such as the lungs (alveolar macrophages), mammary tissue, and lymph nodes. IgA in colostrum protects piglets for up to 6-12 weeks of age. Active antibodies appear one week post-infection, which in fattening pigs can last for 6 months and in sows for up to two years. The virus can persist in pigs after clinical signs have disappeared and they can excrete the virus via their feces for 10 weeks, therefore remaining asymptomatic carriers with the risk of spreading infection. Breeding sows can transmit the virus through their milk.

Other routes of transmission are trucks, boots, slurry, and even dogs and cats since the virus stays in their digestive tract for two weeks and can be excreted. The incubation period is only 1-2 days, and the duration of the clinical symptoms is between 7-10 days.

Clinical signs

The most characteristic presentation in TGE-free farms becoming infected is the onset of peracute and acute cases, showing a sudden and dramatic diarrhea affecting all ages in a matter of days. The diarrhea is severe, watery, yellowish green and sometimes foul-smelling. In nursing piglets, vomiting is common accompanied by a lack of appetite, without fever or nervous signs. Dehydration with high mortality occurs in 24-48 hours. Mortality can reach 100 % in piglets less than one week old, 50 % in the second week of lactation, and up to 25 % in the third week. The piglets die dehydrated, with a distended stomach, milk and hemorrhagic petechiae in the small intestine, hypertrophied mesenteric nodes (characteristic lesion), atrophy of intestinal microvilli (duodenum, jejunum and ileum) with thinner walls, and necrosis of enterocytes in the jejunum with reduced enzymatic activity and yellowish content. In piglets, co-infections with gastrointestinal bacteria such as E. coli and Clostridium spp are frequent, worsening and prolonging the clinical picture.

In pigs over one month old, symptoms are similar but much less severe, with high morbidity and very low mortality. On farms where the virus is enzootic, pigs are affected at a later age, uo to the time they were moved to the fattening stage. The main effects on fattening pigs are slow growth and poor conversion rates. In breeding sows, apart from rapidly spreading diarrhea, there can only be noted a slight increase in abortions and sow mortality restricted to a period of 1-3 weeks. Anorexia and occasional agalaxia can be seen in lactating sows.

Diagnosis

The clinical signs and lesions are quite evident, however, we must make the differential diagnosis with cases caused by strains of enterotoxigenic Escherichia coli, Clostridium spp, Rotavirus, Coccidia, and Cryptosporidium. The ages of the infected pigs and the development of the clinical picture will also help us to narrow the diagnosis down to coronavirus. Confirmation by laboratory analysis is determined by:

• ELISA with specific monoclonal antibodies in pig serum to detect the antigen in faeces and intestinal contents.
• Electron microscopy to detect the virus in intestinal contents.
• Seroneutralization to detect antibodies 7-8 days post infection and for 18 months thereafter.
• Immunoperoxidase in fixed intestinal tissues to detect the virus in piglets at very young ages.
• PCR isolation of the virus, from the intestine, tonsils, lymph nodes, as well as in the feces.

Treatment

Treatment can only be symptomatic. No commercial vaccines are available. There used to be one in the US, but it is no longer commercially available today. Hydration is the main treatment measure and controlling concomitant infections helps us to reduce the impact, also reinforcing biosecurity measures, down time of the facilities and strict control of the environmental conditions, which are so critical for young piglets. The primary objective is to infect all the animals on the farm as quickly as possible to acquire active immunity. To control the process, the feedback of feces is common practice.

The main control measures are based on prevention:

1. Biosecurity: location, clothing only for farm use, control of visitors, feed transport, and animals.
2. Strict AI-AO in the different production areas. Cleaning, washing with hot water at 65ºC and disinfection.
3. Preventing the entry of dogs and cats onto farms.
4. Effective disinfection, rodent and pest control programs.
5. Entry of negative replacement gilts, as well as boars in the breeding facilities.
6. Isolation, aclimation during 8-9 week for the future breeding sows with strict observation of any gastrointestinal symptoms.
7. Protocol for the sanitation of the drinking water.
8. Allowing the piglet to consume colostrum from its own dam during the first 24 hours before fostering.

Hemagglutinating encephalomyelitis virus

It is widespread in North American and European pig populations, but clinical disease is rare. This is because most sows that have been infected, so they are immune, and pass antibodies to their piglets through colostrum, which protects them during the period when they are vulnerable. Although the virus can infect susceptible pigs of any age, it only causes clinical disease in piglets less than 4 weeks old. The strains vary in virulence, leading to two different diseases, vomiting and wasting disease or encephalomyelitis. Both have a sudden onset, at around 4 days of age, and affect entire litters. This is why it is always important to differentiate between infection and disease.

Porcine epidemic diarrhea

It is a highly contagious disease. The virus has a genomic structure and replication very similar to those affecting other animals and humans. The virulence of the viruses isolated in different countries during clinical cases was similar, with minimum genetic dispersion. Only one serotype is known. The epidemic spread throughout Europe in the 1970s, causing severe diarrhea in nursing piglets and recurrent diarrhea in weaned piglets and fattening pigs. New epidemic outbreaks in both Europe and Asia occurred in the 1980s, 1990s, and 2000s until 2009 (Italy, Korea, Thailand), with intermittent occurrences in most pig producing countries worldwide.

Pathogenesis

The main route of transmission is the direct or indirect fecal-oral route, through the entry of infected positive animals, as well as by contaminated vehicles and visitors. The duration of virus excretion in infected pigs is 7-9 days in the feces, and the length of time the virus remains in carrier pigs is not well known. Thus, after an acute outbreak of the disease, the persistence or disappearance of the virus on a farm is highly variable. The virus is destroyed at temperatures above 60°C and is stable at 37°C. The incubation period is only a day or a day and a half. Virus replication takes place in the cytoplasm of the cells of the epithelium of the small intestine villi. Here it produces a degeneration of enterocytes with a distinct atrophy of intestinal villi and a high reduction in the capability to absorb nutrients.

Clinical signs

Morbidity at any stage of production can reach 100%, and is more variable in breeding sows. Mortality in nursing piglets can be as high as 100%, with rates normally between 30 and 50%. The main clinical sign is watery diarrhea leading to death from dehydration within 2-4 days in nursing piglets, in addition to vomiting and poor appetite. Weaned piglets and fattening pigs can recover from diarrhea after one week, showing anorexia, lethargy, and considerable growth retardation. After a clinical infection in breeding sows, they acquire a strong immunity, which they pass on to their piglets via colostrum. The course of the disease on a breeding farm usually lasts a maximum of one month, varying according to farm size and production system. This period can be prolonged if secondary complications arise from gastrointestinal infectious agents such as Escherichia coli, Clostridium perfringens, Rotavirus, Isospora suis, Salmonella spp, Lawsonia intracellularis or Brachispira spp. It should be noted that the lesions are found exclusively in the small intestine, which appears distended with yellowish watery contents, and the stomach is found empty. There is up to 70% atrophy of intestinal villi and vacuolization of enterocytes with reduced enzymatic activity.

Diagnosis

The clinical signs, although not pathognomonic, are quite evident. The diagnosis must be confirmed in the laboratory by sending appropriate samples for antigenic isolation. The most appropriate samples are knotted sections of small intestine refrigerated on the first day of the diarrhea, and from a significant number of animals. Samples must be taken from several ages of animals showing clinical signs. The techniques of direct immunofluorescence and immunohistochemical tests are very specific. ELISA techniques detect the antigen in the feces and the PCR also allows to differenciate PED from TGE viruses. Antibodies appear one week after infection, peaking on infected farms 4 months after infection, and decline from there.

Treatment

Treatment is only through supportive care, with the priority being hydrating nursing and weaned piglets using saline, electrolyte agents, and milk replacers. To establish strong immunity, the transmission of specific IgA via colostrum from the sow to the piglets is critical. Therefore, the rapid exposure of the virus to gestating sows, infecting them with piglet feces to stimulate rapid lactogenic immunity (depending on the production phase where the infection started), allows us to shorten the duration of the clinical picture and lessen the spread to the different production areas as much as possible. We can also stop the spread between production phases by taking the piglets at weaning to a site 2 off the farm, or by taking the piglets at the end of the nursery to a site 3. No vaccine is available in Spain, although there are vaccines in Asia and the United States.

Porcine respiratory coronavirus

Its incidence and prevalence internationally is very low, and it currently has a very low economic impact on pig production. This virus is a variant of the TGE virus family that infects the respiratory tract, and is not excreted via feces. Porcine respiratory coronavirus produces antibodies that neutralize the TGE virus. The virus infects animals of all ages, either by direct contact or by aerosol transmission, being more prevalent in areas with a high pig density. The presentation is subclinical, so we can find many seropositive animals without any clinical signs in most countries of the world. Its genomic structure and replication are very similar to those of other animal species and humans.

Pathogenesis

The route of transmission is direct by oral contact or indirect due to airborne transmission. The virus replicates in the respiratory tract (nasal mucosa and lungs) and infects cells in the nasal epithelium, trachea, bronchi, bronchioles and alveoli. The virus does not cross the placental barrier, although it can be found in the semen of infected boars at 6 days post-infection. The passive immunity from colostrum usually lasts 10-15 weeks, coinciding with the time of entry to the fattening stage and the mixing of animals, when there is the highest risk of infection among them. After infection, the time of nasal excretion of the virus is about 7-15 days. Its prevalence is seasonal, increasing in the cold months and decreasing in the warm ones.

Clinical signs

Clinical signs include coughing, dyspnea, abdominal breathing, lethargy, anorexia and slight growth retardation; symptoms similar to most problems within the porcine respiratory disease complex (PRDC). Worsening of the symptoms occurs in cases combined with PRRS virus or bacterial infectious agents, which in these cases cause pneumonia that can be severe. The respiratory coronavirus can be located in both the upper and lower respiratory tract. The most characteristic lesions, which are not pathognomonic, are:

• Lung consolidation
• Broncho-interstitial and broncho-catarrhal pneumonia
• Bronchiolar epithelial hyperplasia with loss of epithelial cells
• Infiltration of leukocytes, lymphocytes and macrophages into the alveolar septum

Diagnosis

The clinical signs are not pathognomonic. The lesions help, but it is difficult to make a definitive diagnosis based on the symptoms, and this must be confirmed by sending appropriate samples to the laboratory for virus isolation, focusing on lung tissue, nasal mucosal epithelium, and nasal fluids. PCR allows us to differentiate between gastrointestinal and respiratory coronaviruses. ELISA and virus neutralization detect neutralizing antibodies that, neither quantitatively nor qualitatively, can be discerned if they come from one virus or another. Serology serves to check replacement animals in order to confirm their negative status. In the case of VN, antibodies are detected a week after infection, and persist 18 months after infection in the case of TGE, although it is not well known how long they last against respiratory coronavirus.

Prevention and control

Early infection in nursing and weaned piglets by respiratory coronavirus results in immunity to PRCV, but also creates partial immunity against gastrointestinal problems from the TGE coronavirus. There are no antibiotic or antiviral treatments, only treatments against respiratory symptoms and secondary aggravating agents. The first preventative measure is to prevent the entry of the virus via the replacement animals, through discussion with the genetic supplier and testing during quarantine. Internal and external biosecurity measures are one of the greatest safeguards to keep free of the disease. Standards for downtime and all-in-all-out systems are equally recommended. There are currently no commercial vaccines available for porcine respiratory coronavirus.