Scours is caused by a wide variety of pathogens, but bacteria and viruses are particularly prevalent in western Canadian herds. Sound management can help prevent these infections, but few producers can avoid scours completely. To establish an effective treatment protocol for a calf with scours, it is important to understand how each infectious agent induces diarrhea.
The inside of the intestine is lined with millions of intestinal villi Ð minute finger-like projections that protrude from the surface. These structures increase the surface area of the intestinal wall, which enhances absorption of fluids and nutrients. The valleys between the villi, called crypts, are lined by cells that secrete fluid into the intestine.
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Viruses prefer to invade the mucosal cells that cover the villi. These cells multiply as they die, which reduces fluid absorption from the intestine into the bloodstream.
However, the viral attack does not affect the crypt cells, which means that while fluid secretion by the crypts continues unabated, fluid absorption is slowed. As long as the calf can still absorb as much fluid as it secretes, it has no symptoms of viral disease. However, once this balance is disrupted, diarrhea strikes and the calf soon becomes dehydrated, leading to depression, weakness, loss of the suckle reflex and death. A calf with viral enteritis dies from hypothermia coupled with electrolyte imbalances that cause heart failure.
Intestinal viruses are not all equal Ð they cause characteristic clinical signs in specific age groups of calves. Rotavirus typically infects younger calves about a week old, but it can attack newborns and those up to two weeks. It induces a yellow to green, profuse, watery diarrhea. Rotavirus also causes calves to lose interest in nursing, which worsens dehydration.
Coronavirus, in contrast, tends to strike older calves, even those up to one month old. Their feces initially resemble rotaviral diarrhea, but it soon changes character, becoming laden with mucous. Calves with coronavirus diarrhea are not as depressed as calves with rotavirus.
Because viruses have such an impact on fluid absorption, the key to successful treatment is to keep the calf hydrated. In the early stages of viral diarrhea, when villi are intact and fluid absorption is still possible, oral fluids are beneficial. However, once the calf is no longer able to absorb fluids from its intestine, intravenous fluids will be required.
It is important for producers to be proactive when treating calves with viral diarrhea. Fluid therapy should be initiated as soon as calves have a little looseness in their manure or when their abdomens first tuck up.
Viral diarrhea can be complicated if bacteria move in and attach to the damaged intestinal villi. Besides reducing fluid adsorption even further, these infectious agents release toxins that increase the crypt cells’ fluid secretion, thereby worsening the diarrhea and dehydration.
E.coli is the most common bacterium recovered from calves with diarrhea. This agent can complicate an ongoing viral infection, but it can also cause disease by itself. Some E.coli strains have a predilection for the intestinal tract, which are called enterotoxigenic, while others invade the body, often through the umbilicus, and localize in various sites, which are called septicemic. Swollen joints, pus in the eye or nervous signs caused by brain infections are common symptoms of septicemic E.coli organisms.
The most extensively studied enterotoxigenic E.coli strain is the K99 positive strain. This organism, which has antigens on its surface that allows it to bind to cells lining the intestines, can attack calves as young as a day old. The toxins it releases can cause a calf to die even before diarrhea strikes.
Like calves with viral disease, those with bacterial scours require fluid and electrolyte therapy. Antibiotics are also indicated to kill the organism.
Vaccination can protect calves against the major diarrhea-causing infectious organisms. Oral vaccines can be given to calves right after birth to boost their antibody production against coronavirus and rotavirus.
Alternatively, cows can be vaccinated to boost specific antibody production against coronavirus, rotavirus and E.coli.
This protection is passed on to their calves through their colostral antibodies.
Jeff Grognet is a veterinarian and writer practising in Qualicum Beach, B.C.
