uEscherichia coli (Colibacillosis)
Escherichia coli (E. coli) has been incriminated as a major cause of scours. Many times this is the only organism identified following routine bacteriologic culturing. Certain E. coli can cause diarrhea. Many different serotypes (kinds) of E. coli have been identified; some cause scours while others do not. E. coli is always present in the intestinal tract and is usually the agent that causes a secondary infection following viral agents or other intestinal irritants.
E. coli scours is characterized by diarrhea and progressive dehydration. Death may occur in a few hours before diarrhea develops. The color and consistency of the feces are of little value in making a diagnosis of any type of diarrhea. The course varies from 2 to 4 days, and severity depends on age of the calf when scours starts and on the particular serotype of E. coli.
Upon postmortem examination, lesions are nonspecific. However, the small intestine may be filled with fluid and the large intestine may contain yellowish feces.
Diagnosis depends on an accurate history, clinical signs, and culture of internal organs for bacteria and serotyping of the organism. The location at which the culture from the intestine was taken is also important. Control of E. coli scours can be difficult in a severe herd outbreak. All calves should receive colostrum as soon after birth as possible. This helps the calf resist E. coli infection. Early isolation and treatment of scours helps to prevent new cases. There are new E. coli cow vaccines now on the market. These vaccines contain the K99 antigen which should give immunity to many types of E. coli. The vaccine is administered 6 weeks and 3 weeks prior to calving. The new E. coli vaccine is also available in combination with the rota- and coronavirus vaccine. This vaccination builds high antibody levels in the colostrum, but the calf must get colostrum in the first few hours of life for the vaccine to be effective.
There are more than 1000 types of salmonella, all of which are potential disease producers. Salmonella produces a potent endotoxin (poison) within its own cells. Animals may be more severely depressed following treatment with antibiotics as treatment causes the salmonella organisms to release the endotoxin, producing shock. Therefore, treatment should be designed to combat endotoxic shock.
Calves are usually affected at 6 days of age or older. This age corresponds very closely to the age of the coronavirus infection. The source of salmonella infection in a herd can be from other cattle, birds, cats, rodents, the water supply, or a human carrier.
Clinical signs associated with salmonella infection include diarrhea, blood and fibrin in the feces, depression, and elevated temperature. The disease is more severe in young or debilitated calves. Finding a membrane-like coating in the intestine on necropsy is strong presumptive evidence that salmonella might be involved. Salmonella isolations should be checked by a bacteriologic sensitivity test to determine the antibiotics of choice.
Enterotoxemia can be highly fatal to young calves. It is caused by toxins produced by Clostridium perfringens organisms. There are 6 types of Clostridium perfringens that can produce toxins, of which types B, C, and D appear to be the most important in calves.
The disease has a sudden onset. Affected calves become listless, display uneasiness, and strain or kick at their abdomen. Bloody diarrhea may or may not occur. It is usually associated with a change in weather, a change in feed of the cows, or management practices that cause the calf to not nurse for a longer period of time than usual. The hungry calf may over-consume milk which establishes a media in the gut that is conducive to the growth and production of toxins by the clostridial organisms. In many cases, calves may die without clinical signs being observed.
Postmortem lesions may be a hemorrhagic intestinal tract; thus, the common name, "purple gut." In the small intestine, there may be large hemorrhagic or bloody, purplish areas where the tissue looks dead. This is usually attributed to type C. Types B and D may produce diarrhea without the usual postmortem lesions. Diagnosis of these toxins is by finding the toxin in the small intestine by laboratory methods. This toxin breaks down rather rapidly so the contents of the intestinal tract must be collected very soon after death and preserved by freezing. Finding lesions of hemorrhagic enteritis at postmortem in a calf that has died suddenly is basis for a tentative diagnosis.
This disease is best controlled by vaccinating the cows with Clostridium perfringens toxoid 60 and 30 days before calving. A single booster dose of toxoid should be given annually thereafter before calving. If this problem is diagnosed in calves from nonimmunized cows, antitoxin can be given to the calf. Administration of antitoxin and oral antibiotics is the only treatment that is effective.