Respiratory disease and diarrhea are the two most common illnesses that result in drug use in pre-weaned dairy calves. Respiratory disease is commonly caused by both viruses and bacteria and usually requires antibiotic treatment to address the bacterial component of the disease complex. Diarrhea in pre-weaned calves is more common than respiratory disease and is most commonly caused by viruses (Rota and Corona) and protozoa (such as Cryptosporidium) that are not killed by antibiotics. Occasionally, diarrhea can also be caused by bacteria such as E. coli K-99 and Salmonella, which require antibiotic treatment. Determining if a calf with diarrhea needs treatment with antibiotics can be challenging when relying only on loose feces as a clue. For example, calves infected with Salmonella or E. coli K-99 will typically have yellow or white loose feces; however, yellow loose feces is also caused by coronavirus infections. The purpose of this publication is to assist dairy producers and calf caretakers with treatment decisions for diarrhea in pre-weaned dairy calves.
Consequences of Diarrhea
When a calf has diarrhea, there are several consequences:
- The calf loses fluids in the feces leading to dehydration.
- The calf loses strong cations such as sodium (Na+) that causes an imbalance of strong cations and strong anions (such as Cl-), creating metabolic acidosis (excess of acid in the body).
- The calf may develop electrolyte imbalances because of losses of electrolytes in the feces.
- The calf loses weight due to reduced appetite or not absorbing nutrients from the gut.
- There is the potential for overgrowth of gram-negative bacteria (like E. coli) in the small intestine.
- There is an increased chance for development of bacteremia (bacteria in the blood), particularly in calves with failure of passive transfer of immunity (those that may not have or did not receive enough quality colostrum) or calves with severe diarrhea.
Use of Antibiotics and Antimicrobial Resistance
Why should antibiotics be reserved for special cases of diarrhea? One consequence of non-selective use of a drug during diarrhea is the effect on calf health: a potential increase in the number of days of diarrhea. A study by Berge et al. (2009) observed that calves treated with antimicrobials for any case of diarrhea had 70% more total days with diarrhea than calves treated with antimicrobials only in cases of fever or depressed attitude. This can happen because antibiotics do not affect only disease-causing bacteria; they can also kill the normal gut microbes (which may protect against unwanted bacteria) or cause overgrowth of other bacteria, including drug-resistant, disease-causing ones.
Beyond therapeutic uses of antibiotics, feeding antibiotics to dairy calves in the milk is a practice that has been and is still used by some for treatment and prevention of diarrhea in pre-weaned calves. Currently, feeding antibiotics in the milk or milk replacer is restricted to the first two weeks of life in the US. However, this practice can result in an increased selection of drug-resistant bacteria and more days of diarrhea (Berge et al. 2009; Pereira et al. 2014) and the actual effectiveness of in-feed antibiotics for calves is questionable.
A recent review of all the labeled drugs for prevention and treatment of diarrhea in calves revealed that none had consistently been shown fully effective (Smith 2015). Because of that, treatment of calves with diarrhea is allowable if drugs are used in a legal extralabel manner (see the What is Extralabel Drug Use? sidebar). All dairy producers are highly encouraged to participate in a veterinarian-client-patient relationship to ensure calf treatment plans are effective and follow legal regulations. According to the Animal Medicinal Drug Use Clarification Act (AMDUCA), extralabel drug use in animals means the actual or intended use of a drug in an animal in a manner not in accordance with the approved labeling.