In early December I along with Bethann Buskey, our Calf Supervisor, I attended Cornell Cooperative Extension’s annual “Calf and Heifer Congress” in East Syracuse, NY. The theme of the Congress was “Rising Above the Challenges.” In an industry where milk price is usually the biggest challenge we worry about, we often overlook the fact that raising calves can sometimes be a real struggle. In bringing up the next generation of the milking herd, as producers it sometimes seems like an uphill battle with starting calves on the right foot so that they grow up to be healthy, productive animals. That’s not always an easy thing to do when calves get sick. Danielle Mzyk North Carolina State University, discussed how disease impacts calves and how antibiotics work, as well as how to evaluate the efficacy of treatment.
The age of cattle significantly impacts body composition, immunology, physiology, and disease. Calves are physiologically different than cows, with a lower percentage of body fat and more extracellular water. Their immune system is undeveloped and they have lower neutrophil (white blood cell) function. The rumen is not developed yet, nor are all the metabolic pathways. Lastly, they are affected by different pathogens that cause disease and the ways that they are affected are different than mature cows.
Pharmacokinetics is a fancy way of saying what the body does to a drug. We give a drug, it gets absorbed, reaches a maximum concentration at a certain time, gets distributed, metabolized, and excreted. This can change based on the physiology of the calf and how the drug is given.
Disease can either increase or decrease the absorption of drugs, but very young calves usually have higher absorption because they have no developed rumen to break down the drug. Most absorption will occur through the enteral (intestinal) route. When a drug is ingested it will usually bypass the rumen and get absorbed in the abomasum and intestines. A lot of things affect enteral absorption, but some drugs will bind to milk proteins and won’t get absorbed, so you should consult your herd veterinarian if questioning mixing a calf’s drugs in her milk. Parenteral absorption happens when the calf is given drugs via subcutaneous or intramuscular injection. Hydration status greatly affects this process. If a calf is dehydrated there will be less blood fl ow to the site of the injection, therefore the drug will not be able to effectively and quickly get into the bloodstream to get distributed around the body. It’s a good idea to get a calf to a good hydration status with some Ringers before treating so whatever drug you are using is more effective.
Because calves have more extracellular water in their bodies, distribution of lipophilic (fat-loving) drugs decreases, while distribution of hydrophilic (waterloving) drugs increases. This is because the hydrophilic drugs have plenty of water to bind to and get carried around. Drugs also get distributed by binding to plasma proteins. This process can be affected by pathophysiological factors (the way the body changes when it’s diseased) or endogenous compounds that can alter the drugprotein binding interaction.
In calves the liver both metabolizes and plays a role in eliminating drugs, while the kidneys eliminate drugs based on glomerular filtration, tubular secretion, and reabsorption. As a cow ages she has a higher glomerular filtration rate (GFR) because she has a greater cardiac output and therefore renal blood and renal plasma flow. This means that young calves have a low GFR.
Certain considerations must be made when clinical disease symptoms affect drug use. The importance of rehydrating a dehydrated calf before treating her has already been mentioned. Another state of disease is sepsis/endotoxemia. This is when bacteria and/or their toxins infect the bloodstream. This affects the volume of distribution, and absorption. In this state the blood is directed only towards the vital organs, decreasing the absorption from subcutaneous and intramuscular injection, as well as the oral route. It also affects clearance of drugs from the body because the kidneys don’t work properly, so be aware of drug residues in this case.
Although it may sometimes be frustrating to deal with sick calves that are not responding to treatment, we can use what we know in terms of how their bodies work and how drug use is affected by their physical condition to make sure we are treating them effectively