While the leading causes of calf illness and death within the first few days after delivery are non-infectious physiological
disturbances, after that time, the majority of illness and death losses are attributable to infectious conditions. It is useful
to group these infectious conditions based on their underlying pathophysiology, rather than diagnosing them by specific disease
agents. Distinguishing between localized viral/protozoal enteritis, bacterial enteritis and septicemia can help focus attention
on appropriate sampling for diagnosis, establishment of accurate prognosis, effective treatment measures, and effective preventive
Viral or protozoal enteritis in calves is attributable to organisms that cause superficial damage to the mucosal lining of the intestinal tract. The
primary sign of disease is diarrhea (or "scours"), attributable to malabsorption, maldigestion, or hypersecretion. The disease
effects and eventual cause of death in affected calves are attributable to the loss of electrolyte-rich body fluids. Without
invasion beyond the gastrointestinal mucosa, the organisms involved do not cause significant pathology in other organ systems.
The dehydration, depression, weakness, inappetence, and metabolic acidosis that accompany this problem typically occur in
direct relation to the severity of the fluid loss. The magnitude of fluid loss and acid-base imbalance and ability of calves
to withstand insults are impressive.
The direct cause of calf scours is exposure to and infection by specific infectious agents. Indirect causes include the many
factors that increase calf exposure to the microorganisms or decrease calf resistance to infection. The microbes may act
individually to cause diarrhea but frequently when a scours outbreak is found in a herd, two or more of these agents can be
identified in affected individuals or in the group of scouring calves. Although each of these agents is unique, and they
can cause disease in several different ways, the end effect of the infection in the calf is strikingly similar for all of
the major scours-causing agents. The four agents that cause the majority of calf scours are rotavirus, coronavirus, cryptosporidia
and K-99 E. coli.
The four microbes just described are the leading causes of calf scours but they are not the only causes of diarrhea. An important
distinction needs to be made between calf scours and other types of infections that may produce diarrhea but also other organ
problems. The prevention, treatment and eventual prognosis of these diseases are quite different. In calf scours, the primary
problem is diarrhea. The diarrhea seen in calves with some other infections is only one of many problems caused by the infecting
organism and in such cases calves usually die because of damage to other tissues besides the gut. Calf scours primarily affects
calves within the first three weeks of life. The more aggressive diseases that affect multiple organ systems can occur early
in the calf's life but can and frequently do affect calves older than three weeks of age.
Focal bacterial infection can localize in almost any organ system of the calf, but most typically these problems occur in the gastrointestinal tract
or respiratory system. In contrast to superficial localized enteritis, these infections could be characterized as severely
inflammatory to the infected tissue. Gastrointestinal infection is most common in the neonate, while respiratory infections
are more typical of the calf beyond one month of age. Organisms commonly incriminated in gastrointestinal infection include
Clostridia and Salmonella spp. These organisms can not only damage tissue locally but also cause severe systemic inflammatory responses due to toxin
release. Appropriate treatment of affected calves includes aggressive antibiotic and anti-inflammatory treatment in addition
to supportive fluid therapy. Prognosis even with prompt and aggressive treatment is guarded. Calves affected with clostridial
enteritis usually die rapidly before the onset of diarrhea due to the profound effects of exotoxin on the vascular system.
Septicemia occurs when the infectious organism spreads beyond the site of invasion and is carried hematogenously so that multiple organs
show inflammatory damage concurrently. The prognosis for septicemia is grave unless it is recognized early and aggressive
medical therapy is instituted. Successful treatment must be intensive and usually includes supportive fluid therapy, antibiotics,
intravenous plasma, anti-inflammatory agents, and often intranasal oxygen delivery.
The invasive types of E. coli are capable of invading beyond the gastrointestinal tract to cause septicemia (microorganisms in the blood). In this instance,
the infectious bacteria can spread to any organ in the body, including lungs, brain tissue, kidneys, and joints and can cause
severe damage at these infected sites. The Salmonella species of bacteria have been associated with calf enteritis but like the septicemic E. coli, Salmonella has a strong tendency to spread beyond the gut and cause widespread disease.