Illness occurs when the body is overwhelmed by infection. A healthy animal with strong immunity is less likely to become sick. Immunity refers to the body’s ability to fight off bacteria or viruses, and this ability is developed in a complex process in which the body creates specific weapons for fighting specific invaders.
Antibodies
When viruses or bacteria enter the body, they start invading tissues and causing damage by multiplying and creating toxic products. This damage stimulates the body to create an antibody (a serum protein called an immunoglogulin) to react with the invading agent and neutralize it. Antibodies are carried through the body in blood and lymph systems. The main role of one type of lymphocyte (white blood cell) is to produce antibodies, the proteins that can neutralize certain infectious agents.
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If an animal already has antibodies against a specific disease organism, then whenever that particular organism invades the body again, an army of white blood cells (with their antibodies) converge on the site to kill the invader.
Vaccination can stimulate production of antibodies, since the vaccine serves as the antigen (like an invading pathogen). The body builds protective antibodies to fight the invader. Then when the animal later comes into contact later with the infectious agent, the antibody is present in the bloodstream to inactivate the pathogen. If enough antibodies are present to inactivate all the pathogens that invade the body, the animal will not get sick, and the invasion stimulates rapid production of more antibodies for future protection.
A cow in a natural environment may not be exposed to very many disease-causing organisms, but today most cattle are confined during some parts of the year (in corrals, small pens or pastures that have been contaminated by heavy cattle use) and come in close contact with other cattle, with more chance of disease spread. But with vaccination and natural exposure to various pathogens, the cow develops many antibodies and strong immunity. During the last part of pregnancy she puts these antibodies into the colostrum she produces, so her calf can have instant immunity after his first nursing. The antibodies in colostrum are very important to the newborn calf because he has very little disease resistance of his own.
Passive immunity
Young calves are vulnerable to diseases such as scours and pneumonia but Mother Nature has this loophole covered. To help protect calves during this precarious period, the necessary antibodies are provided in the cow’s colostrum, to give the calf a temporary (passive) immunity against the challenges he will soon face.
Antibodies in a cow’s bloodstream are unable to cross the placental barrier because these molecules are too large. A calf can receive the antibodies from his dam only from drinking her colostrum. During the last three weeks of pregnancy she accumulates antibodies from her bloodstream into her mammary glands; a well-fed, healthy cow produces an abundant amount of colostrum and a large volume of antibodies. You can maximize those antibodies by making sure she is not too thin (undernourished cows produce less total colostrum), and that her vaccinations are up to date so she will produce a high level of antibodies against those specific diseases.
At the time she gives birth, the concentration of antibodies in her milk reaches its highest peak, then drops rapidly. It is important that the calf nurse as soon as possible after birth, to get full benefit from her antibodies.
Absorbing antibodies
There are several types of antibodies present in the cow’s colostrum. Their absorption rate and role in disease prevention varies depending on the class of antibody (IgG, IgM, IgA). Some are designed to be absorbed immediately and directly through the calf’s intestinal wall, where they enter the lymph system and bloodstream to be ready to fight disease organisms, while others stay in the gut and attack any pathogens found there.
The newborn calf must nurse quickly (to absorb the immunoglobulins that go through the intestinal wall before it thickens) and continue to obtain colostrum during its next several nursings. This helps to keep the other antibodies in its gut to protect against scours bacteria hit may ingest from dirty teats or a dirty environment. The remaining antibodies in the mother’s dwindling supply of colostrum (as it becomes diluted with regular milk) continue to benefit the calf even though it can no longer absorb any through the gut lining.
The crucial antibodies the calf needs in its bloodstream are absorbed by a process called pinocytosis, which involves creation of a fluid pocket that aids in movement of antibodies through the wall of the intestine and into the lymph system. This works best when the calf is first born and its gut lining is thinnest, making it easier for big molecules to slip through. The lining begins to thicken after birth. The calf gets maximum antibody absorption if it nurses within the first 15 to 30 minutes after birth.
By the time a calf is four hours old it has lost 75 per cent of his ability to absorb antibodies, and absorption rate decreases rapidly after that. Any calf that has not been able to nurse in the first hour or two should be assisted, or given colostrum by bottle, stomach tube or esophageal feeder. A calf needs to receive enough colostrum equal to about five per cent of his body weight soon after birth (1.5 quarts for a 60-pound calf, two quarts for an 80-pound calf, or 2.5 quarts for a 100-pound calf), and the same amount again about six hours later.
Once the calf starts to nurse, gut closure is hastened. This is probably nature’s way to insure that nothing else slips through the intestinal lining (such as bacteria or viruses). Thus it is important that a calf have a full feeding of colostrum soon after birth. If a calf is cold and weak and only able to nurse a little, or if you feed him a small amount rather than a full feeding, this will speed up closure of his gut lining. The calf may not be able to absorb any more antibodies by its next nursing. After the gut closes the calf will only get the benefit of antibodies that fight pathogens in the gut itself.
Antibodies in the calf’s bloodstream obtained via colostrum can help it resist blood-borne infections caused by bacteria such as salmonella, pasteurella, streptococcus, but cannot directly prevent gut infections such as those caused by E. coli. But high levels of certain antibodies in the blood can help reduce severity of scours, and the antibodies that stay in the gut after the intestinal wall closes (colostrum ingested in subsequent nursings) can attack any scours-causing pathogens found there.
The number of colostrum antibodies that fight scours organisms such as E. coli can be increased by vaccinating the cow ahead of calving. Make sure the vaccine is given far enough ahead (at least two weeks), allowing the cow time to develop the antibodies. On the other hand don’t vaccinate too early (no more than 50 days) so that her immunity level is dropping. Some types of scours can be prevented by giving the calf a commercially prepared concentrated antibody source or oral vaccine soon after birth.
Heifers’ calves
Heifers’ calves are at greater risk of disease than calves born to older cows. Heifer colostrum does not contain as many antibodies and has less variety of antibodies because young cows have been exposed to fewer infectious organisms in their short lives. Heifers may also have less volume of colostrum. A heifer’s calf may not absorb as many antibodies as it should if it is slow to nurse after a difficult birth. The stress of a hard birth can also make the calf less able to absorb antibodies efficiently; stress may affect absorption even if you make sure it gets plenty of colostrum.
Active immunity
Calves lose their temporary immunity (protection gained from antibodies via colostrum) by seven to eight weeks or earlier, at which time their own immune system must take over. The time it takes for a calf’s immune system to gear up to ward off invaders will vary, depending partly on how strong its passive immunity was. If the calf had a high level of antibodies from colostrum, which effectively neutralized any invading organisms, its own defences are not stimulated to develop until that protection begins to wear off.
The antibodies gained through colostrum can also interfere with effectiveness of vaccinations. If a calf is vaccinated young, while it still has high levels of maternal antibodies in its blood, the calf’s own immune system won’t bother to respond to the antigens in the vaccine because they are being neutralized by the maternal antibodies. Many types of vaccination given to a calf when it is only two to three weeks old won’t give protection, since it will not stimulate immunity. Most vaccines should be given at eight weeks of age or older, and repeated with a booster shot two to six weeks later to ensure the calf’s immune system will be able to respond.
