| Description:
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Rotaviruses and coronaviruses
are the most common causes of viral diarrhea in calves.
Rotavirus infections occur earlier in life and produce
milder lesions than do coronaviral infections. Both
viruses cause varying degrees of dehydration, acidosis,
and electrolyte derangements (hyponatremia, hypochloremia,
hyperkalemia). Three groups of rotavirus, A, B, and
C (rare), have been identified in cattle. Group A bovine
rotaviruses are recognized as a significant cause of
calf diarrhea and are the most common source of infection,
whereas group B rotaviral diarrheas are viewed as an
emerging and increasingly important disease. Rotavirus
infection is characterized by the rapid onset of diarrhea
in calves, from 1 day of age up to about 3 weeks of
age, with most cases occuring in the first week of life.
Clinical signs of infection include: yellow watery diarrhea,
mild depression (worsens as fluid, acid-base and electrolyte
disturbances worsen), inappetence, and reluctance to
stand. Uncomplicated cases are self-limiting and symptomatic
for only 1-2 days. However, secondary infections occur
commonly, and will influence the ultimate clinical course
of the illness. Calves are mainly infected through an
orofecal route. The organism is very hardy in the environment,
and is resistant to inactivation by most disinfectants.
The presence of colostral antibodies in the bowel lumen
is protective initially, but once the antibody level
in the milk declines after a few days, the calf is susceptible
to infection.
Rotaviruses invade the tall columnar cells at the tips
of the small intestinal villi. The infected cells are
desquamated, and the villar tip atrophies. The absorptive
and digestive functions (lactase secretion, etc.) of
the villar tips are impaired, yet secretion by the crypt
cells continues in an uninterrupted fashion. Bacterial
fermentation renders undigested, unabsorbed nutrients
into osmotically active small acids that draw more fluid
into the digestive tract. As a result, the feces from
animals with pure rotaviral infections will be acidic.
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| Diagnosis: |
Bovine rotaviral infections
are non-specific and therefore cannot be identified
solely on clinical findings. The most accurate method
of detection for group A rotaviruses is by PCR. Using
PCR, specific viral genomic sequences are targeted,
amplified and detected, providing veterinarians with
a definitive method of detection. PCR is so sensitive
for the detection of group A rotaviruses that it is
the method of choice for detecting these viruses in
food samples intended for human consumption.
In the past, detection of group B rotaviruses proved
difficult since this virus cannot be grown in cell culture.
In addition, only low numbers of group B rotaviral particles
are shed in feces. Electropherotype analysis requires
a large amount of virus, and this technique does not
allow a conclusive diagnosis of group B rotavirus. Fortunately,
new generation DNA based tests provide a highly sensitive
and specific means of detection for group B rotaviral
particles in a sample. These tests use PCR to target
unique rotaviral genetic sequences, thereby offering
veterinarians a definitive method of diagnosis for this
disease.
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