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Bovine Coronvirus Infection: Clinical Syndromes in Adult Cattle and Calves

Bovine coronavirus(BCV) is recognized as the primary etiology of Winter Dysentery (WD) of adult cows and coronaviral diarrhea in young (3 to 21 days-old) calves.   WD is a clinically and economically important disease in many countries including the United States. It is characterized by high morbidity (50-100%), but low mortality (1-2%), and occurs primarily in adult dairy cows. The highest incidence ofWD is between November and April when cattle are kept in close confinement.    BCV survives well at low temperatures and low levels of ultraviolet light. Clinically, WD is characterized by severe watery diarrhea (sometimes with blood and mucus), decreased milk production, listlessness, depression, anorexia, weight loss, and sometimes cough and/or nasolacrimal discharge. Cows that recover from WD are apparently immune from disease for 1 to 5 years, but carrier cows can be a source of repeated outbreaks in a herd. Microscopic lesions of WD are most evident in the spiral colon where crypt epithelium   exhibits   varying   degrees   of degeneration and necrosis. Grossly, hemorrhage (including petechiae) can be found in the mucosa of both the small and large intestine.

Coronaviral diarrhea in young calves is characterized by profuse watery or hemorrhagic diarrhea, that in surviving calves can last 2 to 6 days,   listlessness,   anorexia,   pyrexia,   and dehydration. In some calves, feces contain flecks of blood. Morbidity and mortality are high and calves with bloody diarrhea can die of hypovolemiawithin a few hours of the onset of clinical signs. Infection is established by ingestion or inhalation of BCV which replicates in "rapidly dividing" cells such as those that line intestinal villi. Microscopic lesions of BCV enteric infection in young calves can often be observed in both the small intestine and colon. In the small intestine, villi may be atrophic and lined by attenuated epithelium. This lesion results in a diminished absorptive and digestive function, and, perhaps, an increased secretory function, all of which lead to severe diarrhea.  Fluid and electrolyte loss results in dehydration,acidosis, and hypoglycemia. In calves that recover from the acute stages of infection, the intestinal crypt epithelium can regenerate lost mucosa and eventually regain normal function.

BCV can also cause a "respiratory syndrome" that is usually quite mild or even "sub-clinical" Clinical signs are usually seen in calves 2 to 16 weeks-old and include sneezing (from rhinitis) and coughing (from tracheitis).  Because BCV can infect respiratory epithelium, it can predispose calves to opportunistic bacterial infections. Calves with respiratory BCV infection can shed high numbers of virus in their nasal mucus and this virus can cause enteric and/or respiratory disease in other cattle.

In calves with BCV enteric infection, viral particles can be detected (e.g., by electron microscopy) in the feces 1 to 2 days before the onset of diarrhea and for several days after the diarrhea has resolved. BCV can also be found in nasal secretions of calves with BCV diarrhea. Recovered calves that are apparently immune to disease can still shed BCV in their nasal secretions or feces.

Because mucosal immunity is important in preventing or recovering from BCV infection, calves that do not receive colostrum (or receive colostrum from "naive", primiparous cows) are especially susceptible to BCV enteritis. Vaccina­tion of herds can provide added protection.

A presumptive diagnosis of BCV infection can be accomplished with a variety of techniques. Serology can detect sero-conversion or a 4-fold increase in serum immunoglobin levels. Electron microscopy can detect coronavirus particles in feces or intestinal contents. Immunofluorescentmicroscopy can detect coronavirus antigens in infected tissues (e.g., small intestine and spiral colon of calves, spiral colon of cows, or trachea and lungs of calves with "respiratory syndrome"). BCV antigen can also be detected by ELISA in feces or gut.

A two-year study of BCV-associated disease (including neonatal diarrhea,WD, and respiratory disease) in beef and dairy cattle demonstrated the use of these diagnostic techniques (Carman, 1992). ELISA was especially useful for diagnosing BCV diarrhea in calves. Virus neutralization assay of serum (detecting a four-fold increase in antibody liters) was valuable for diagnosing WD and BCV-associated respiratory disease.  Cattle with WD often showed serologic evidence of concurrent infection with Bovine Virus Diarrhea Virus (BVDV), Infectious Bovine Rhinotracheitis(IBR), or Bovine Parvovirus(BPV).    Cattle with respiratory disease often showed serologic evidence of concurrent infection to Bovine Respiratory Syncytial Virus (BRSV),parainfluenza-3(PI3), BVDV, IBR, or Bovine Adenovirus(BAV). This study indicates that BCV should be considered as a complicating factor in both enteric and respiratory disease syndromes of cattle even when other potential viral pathogens are identified.

In conclusion, BCV can cause clinically significant diarrhea in both calves and adult cows. Respiratory signs often accompany or precede enteric disease. Winter when cattle are confined in "close quarters" is the prime time for BCV infection.

- by Deborah McArthur, Class of 1997

- edited by EvanB.Janovitz,DVM,PhD


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