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
- by Deborah McArthur, Class of 1997
- edited by EvanB.Janovitz,DVM,PhD
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