The current pandemic due to a new coronavirus (COVID-19) has revealed great global interest in the study of coronaviruses. Within them, there are some species of crucial importance because they affect animal production. In this article, we will show you the most important ones and their main characteristics.

What are coronaviruses?

Coronaviruses are a family of infectious agents of viral origin so-called because under the microscope their proteins give them the appearance of a crown (in latin corona) in the outermost part. This family of viruses is divided into two main genera: Coronavirus and Torovirus. In turn, the Coronavirus genus is classified into three types. The following table summarizes the most important ones that affect animal production.

Table 1. Important coronaviruses that cause disease in domestic and production animals.

Its basic structure is composed of: genetic material which is made of a positive-sense single-stranded RNA; also, the virus has a lipid envelope (which facilitates its removal with soapy agents). There are four basic proteins present in all coronaviruses; in fact, vaccines sometimes target them:

• Protein S (spike): allows the virus to bind the cells of the animal for getting into them and gives the virus the characteristic crown shape.
• M protein (membrane protein): it is important in the virus assembly process and generates an immune response from the animal through the production of neutralizing antibodies and interferon, in other words, it generates both humoral and cellular immunity.
• Protein N (nucleocapsid): serves as the support for the nucleocapsid which has a helical shape. Also, it is sometimes the target of the PCR diagnostic test.
• Protein E (envelope): it is a protein that participates in the formation of the virus envelope, it is present in low amounts.

Despite their high mutation capacity, the coronaviruses of domestic animals reviewed here are not zoonotic, that is, they do not cause infection in humans. However, it is important to know and monitor them because the interaction of coronaviruses, in general between wild and domestic animals, can favor the appearance of new pathogens. Taking into account that in many farms the contact between production and wild animals are inevitable, the monitoring and its knowledge are even more important.

Porcine coronavirus (TGEV y PEDV)*

*Since they are closely related, they will be described in the same part.

History and impact

Porcine transmissible gastroenteritis (TGEV) was first described in the United States in 1946 by the scientists Doyle and Hutchings, who  isolated it. However, porcine epidemic diarrhea (PEDV) was classified as such in the year 1971 in England. Such is the impact of swine coronaviruses in the world that the OIE has classified them as a notifiable animal disease.

Transmission

Transmission is mainly through the oral route. The virus can enter into the farm through new animals and the entry of fomites (inanimate objects) such as vehicles, personnel from outside the farm, and external elements. Another relevant transmission route is through the birds that enter in the farm and  contact with feces of infected pigs. Lactating sows and fattening pigs are considered the main carriers of the virus. During lactation, the virus is transmitted to the piglets who are the most sensitive. Intrauterine transmission has not been reported. Infections by this coronavirus occur more frequently in winter.

Symptomatology

Piglets under three weeks have the most severe signs of the entire herd. After being infected, the virus replicates in their small intestine (jejunum and ileum), where it severely damages the cells of these intestinal portions. This damage manifests itself in the form of severe diarrhea (due to poor absorption), vomiting and dehydration; after 3 to 5 days of presenting these signs, the animal dies. Piglet mortality can reach 100% and cause serious economic impact on the production system.

In adult animals such as lactating sows, the virus replicates in their mammary tissue, which explains the transmission through milk to piglets. Also, signs are milder in adults, which suffer from diarrhea for 2 to 4 days and overcome the disease after that. Therefore, mortality in adults is below 5%.

There are two main forms of presentation: the epidemic form causes high mortality in a short time; the endemic form has milder signs and lower mortality in a similar period.

Diagnosis and control

To diagnose porcine transmissible gastroenteritis and swine epidemic diarrhea, samples of intestinal tissue are sent to the laboratory, where immunofluorescence diagnostic test is done to detect antigens (virus particles). Hemagglutination and ELISA tests are also used to detect antibodies (defense molecules synthesized by animals to defend themselves against pathogens). The confirmatory test is viral isolation in cell culture. Also, the RT-PCR test (reverse transcription polymerase chain reaction, where viral RNA is transformed into DNA to be detected in the laboratory) has great sensitivity.

Regarding its control and prevention, a vaccine with an attenuated or inactivated virus can be used to immunize pregnant sows, and thus generate antibodies (defenses) that can be transmitted in colostrum to piglets, thus providing them with passive immunity against the virus. Also, it is recognized that after an eventual outbreak, in which almost all young susceptible animals die and adults survive, the farm remains immune for 3 to 5 years. However, it is logical to apply vaccinations to avoid an outbreak that generates severe economic impacts.

Avian infectious bronchitis (IBV)

History and impact

It was the first coronavirus isolated from domestic birds around 1936 by the authors Beach and Schalm in North Dakota, United States. This virus causes important losses in poultry, affecting egg production where it causes up to 50% decrease in laying. It also affects meat production where daily weight gain decreases. In addition, it causes high mortality of up to 30% (due to nephropathogenic strains). For all the above, it has great repercussions on health and economic status on the affected farms. It is distributed worldwide.

Transmission

Transmission of the virus occurs mainly by air through aerosols expelled by sick birds when they cough or sneeze, releasing viruses into the environment. There is also transmission by oral and fecal routes, as the virus is eliminated in the fecal matter of infected birds, which contaminates water sources, tools, and food, thus infecting other susceptible birds.

Symptomatology

The IBV, as its name indicates, generates a clinical picture, mainly respiratory , with sneezing, runny nose, exudate in the trachea and air sacs, coughing, and decay. However,  renal-type and reproductive-type forms can also occur. In the renal form, there will be lesions in the kidneys and ureters, visible at necropsy and under the microscope. In the reproductive form, infection of the oviduct will occur, which will lead to a decrease in the production of eggs, which will have  soft and deformed shells.

Diagnosis and control

To make the diagnosis of avian infectious bronchitis, it is recommended to carry out a necropsy of sick birds, in which tissues such as trachea, tonsils, oviducts, and nasal sinuses should be sampled and sent to the laboratory for virus isolation. Diagnostic tests such as ELISA, RT-PCR and hemagglutination inhibition are also used as a complement to give more accuracy to the diagnosis. In addition, it is recommended to identify the serotype involved that is affecting the farm and/or the region to take the most appropriate prevention measures.

Regarding its control, a combination of good management and hygiene practices is recommended, such as total disinfection of the production system and repopulation with 1-day-old chicks; this must be added to the immunization of birds through vaccination, which is based on the identified serotype. The vaccine can be made from the attenuated virus and is administered in the water, by spray or directly in the conjunctiva of the birds. The mixture of these two measures, hygiene and vaccination, is the key to prevent the disease.

Bovine coronavirus (BCoV)

History and impact

Some outbreaks of diarrhea in calves were reported at the beginning of the 20^th century in the United States under the name of Neonatal Diarrheal Syndrome, although it was only identified and accurately classified as a coronavirus until 1973. It has been called winter dysentery because of generating diarrhea in cattle during the coldest months of the year. It affects both adults and young animals and is distributed throughout the world, causing significant economic losses.

Transmission

It is transmitted both by the air through aerosols and orally. Infected animals release large amounts of virus into the environment through nasal discharges, infecting susceptible animals.

Symptomatology

The BCoV has two main presentations:

• Enteropathogenic strain (EBCoV): generates yellow diarrhea in neonatal calves (1 to 3 weeks of age) that can last up to 5 days, leading to severe dehydration; in adults, bloody diarrhea is accompanied by anorexia, a drop in milk production, and depression.
• Respiratory strain (RBCoV): affects mainly calves between 3 and 9 months of age, which present nasal discharge, lacrimation, respiratory distress, and fever. Clinical signs can last 1 to 2 weeks.

In adult animals, the disease also has effects at an intestinal level, with diarrhea that can go from “pasty” to totally liquid, causing marked dehydration and weight loss. Furthermore, milk production can drastically decrease between 30 and 95%. Moreover, some respiratory signs that may appear are lacrimation, rhinitis, and nasal discharge. It is recognized that, in adult bovines, mortality is low, of only 1 to 2%, but with very high morbidity that ranges between 50 to 100%. Approximately two weeks after the outbreak starts, the cattle herd overcomes the disease; if thes herd is large, recovery can take up to 2 months.

Diagnosis and control

The diagnosis of bovine coronavirus is a challenge for veterinary medicine. An important list of infectious and parasitic agents can be the cause of diarrhea in a group of calves and are difficult to be differentiated from each other on a clinical level. Therefore, it is vital to carry out complementary and specific diagnostic tests such as indirect immunofluorescence or RT-PCR test to detect the virus.

For its prevention, there are modified live vaccines that generate immunity in the herd; immunized cows will transmit these defenses to calves through the colostrum. Also, good livestock practices and complete disinfection of multiple areas within the farm are recommended.

Turkey coronavirus (TCoV)

History and impact

It was first identified in 1951. It is closely related to the avian infectious bronchitis virus. It is also known as blue crest disease of turkeys. It is one of the viruses that most affects poultry production systems dedicated to turkeys and occasionally infects other production birds and even wild birds. Major outbreaks have been reported in North America.

Transmission

Transmission occurs through the oral and fecal routes, where infected birds shed the virus in their feces. They contaminate the environment and equipment, thus spreading the virus. Wild birds are important in its transmission.

Symptomatology

The symptoms are mainly gastrointestinal, where anorexia, weight loss, depression, sudden changes in temperature, and diarrhea are evident. In laying turkeys there will be a drastic decrease in their posture. Mortality is variable and morbidity can reach 100%. Younger birds are the most affected by the disease.

Diagnosis and control

For the diagnosis of this disease, intestinal tissues can be sent to the laboratory for immunohistochemical diagnostic test. Serum from sick animals can also be used to perform the indirect immunofluorescence test. Finally, the virus can be isolated by inoculating it into embryonated turkey eggs and confirming this method by RT-PCR.

For its control, total depopulation and thorough disinfection of poultry facilities are recommended. Also, it good results were obtained with a controlled exposure of birds that have overcome the disease to5 to 6 week-old birds to  develop herd immunity. There is no vaccine available for its prevention.

Torovirus

Toroviruses, which are part of the Coronaviridae family, are less frequent in production animals.

Bovine torovirus

This virus mainly affects calves. It comes in two ways:

• Bovine enteric torovirus: 1 to 3-day-old calves suffer from watery diarrhea and signs of dehydration. The crypts and Peyer’s patches of the intestine are so affected and it takes longer to recover normal intestinal absorption and functions than in other intestinal infections. It is transmitted orally.
• Bovine respiratory torovirus: affects 6 to 9-week-old calves, causing pneumonia. It is transmitted by aerosols.

Conclusions

The current pandemic due to a coronaviruses called COVID-19 has shown more than ever the importance of studying viruses and their relationship with animals. Among the coronaviruses, there are some that have been known since the 20^th century and, nowdays,they still seriously affect the world’s productive systems by generating intestinal and respiratory diseases in animals such as pigs, cattle, and birds. Understanding them is key to be able to identify them and prevent their appearance on farms. It is worth mentioning that a vaccine is a key tool that should be further explored.

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