Illness occurs when an animal's body is overwhelmed by infection. A healthy animal with strong immunities is less likely to become sick than an animal with poor immunity. 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 a virus or bacteria enters the body, it starts 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. The antibodies are carried throughout the body in the bloodstream. The main role of one type of lymphocyte (white blood cell) is to produce antibodiesthe proteins that can neutralize certain infectious agents.
If an animal already has antibodies against a specific disease organism, then any time that particular organism invades the body again an army of white blood cells (with their antibodies) converge on the site to kill the invader. Exposure to one strain of an organism may result in immunity to that specific strain, but not to other strains of the same organism. Antibody immunity depends on the level of exposure, and stresses on the animal and its current health. A severe outbreak of disease in a herd may eventually break down a healthy animal's immunities, and will overwhelm a stressed animal's defenses even sooner.
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 comes into contact later with the actual infectious agent, the antibody is present in the bloodstream and able to inactivate the pathogen. If enough antibodies are present to inactivate all the agents 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 become exposed to very many disease-causing organisms, but today most cattle are confined some parts of the year (in corrals, small pens or pastures that have been very contaminated by heavy cattle use) and comes in close contact with other cattle-with much more chance of disease spread. But with vaccination and natural exposure to various pathogens, the cow develops many antibodies and strong immunities. And during the last part of pregnancy she puts these antibodies into the colostrum she produces, so that her calf can have some instant immunities right after he has his first nursing.
The antibodies in colostrum are very important to the new born calf because he has very little disease resistance of his own. The fetus can begin to produce antibodies against certain pathogens at various stages of development (for instance a fetus can start to make antibodies to BVD and IBR as early as 90 days into gestation, and against leptospirosis bacteria after 180 days), since some of these invaders can pass through the placental barrier from the mother's bloodstream if she becomes infected. This is why a fetus can become infected with BVD or other diseases or die from lepto or Bangs and be aborted. Certain infections in the cow can kill the fetus or cause it to be born diseased, or cause it to start making some antibodies of its own.
But this fetal immunity doesn't do the calf much good when he is born. This is because the immunity of the fetus decreases at the time of birth. For a few weeks before calving and for a short while afterward, the calf's immune system is hindered by the high cortisol levels present in both the cow and calf-part of the hormone activity that helps stimulate birth. Cortisone interferes with the activities of the immune system-one reason to never keep an animal on steroid-type drugs for more than just a few days.
Passive Immunity � For about 10 to 14 days after birth, a calf cannot begin to build any immune responses against infectious agents. This is why young calves are very vulnerable to diseases such as scours and pneumonia. But mother nature has this loophole covered.
To help protect calves during this precarious period, the antibodies needed are provided in the cow's first milk, to give the calf a temporary (passive) immunity against the many challenges he will soon face.
Passive immunity can be obtained from some source outside the body, such as with use of antiserum, or ingestion and absorption of colostrum. 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 therefore a large volume of antibodies. You can maximize her colostrul 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 have produced 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 his lymph system and bloodstream to be ready to fight disease organisms, while others stay in the gut and attack any pathogens found there-such as E. coli bacteria.
This is why it is important a calf nurse quickly-to be able to absorb the immunoglobulins that must go through the intestinal wall before it thickens and continue to have some colostrum during his next several nursings to keep the other type of antibodies in his gut to protect against scours bacteria he may ingest from the cow's dirty teats or a dirty environment. The remaining antibodies in his mother's dwindling supply of colostrum (as it becomes diluted with regular milk) can continue to be of benefit to him even though he can no longer absorb them through the gut lining.
The crucial antibodies he needs in his 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 his gut lining is thinnest, making it easier for the big molecules to slip through. The lining begins to thicken right after birth and the rate of pinocytosis decreases the older the calf gets. He gets maximum antibody absorption if he nurses within the first 15 to 30 minutes after birth.
Once he starts to nurse, the gut closure process is hastened even more. This is probably mother nature's way to insure that nothing else will slip through the intestinal lining (such as bacteria or viruses) after he is up and around and sampling his new world. Thus it is important that he have a full feeding of colostrum very soon after birth and not just a small amount. If a calf is cold and weak and only able to nurse a little bit at first, or if you feed him just a small amount rather than a full feeding, this will speed up the closure of his gut lining and he may not be able to absorb any more antibodies by his next nursing. After his gut closes he will only get the benefit of antibodies that fight a few pathogens in the gut itself.
Antibodies in the calf's bloodstream obtained via colostrum can help him fight off blood-borne infections caused by bacteria such as salmonella, pasteurella and streptococcus, but they cannot directly prevent gut infections such as those caused by E. coli. But high levels of certain antibodies in the blood can help reduce the severity of scours, and the antibodies that stay in the gut after the intestinal wall closes (from the colostrum ingested by the calf in subsequent nursings) can attack any scours-causing pathogens found there.
The number of colostral antibodies that fight scours organisms such as E. coli can be increased by vaccinating the cow ahead of calving, making sure the vaccine is given far enough ahead (at least two weeks) that she can develop the necessary antibodies, and not so far ahead (no more than 50 days) 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 (such as the oral viral vaccine against rotavirus and coronavirus, which works best if given within 4 to 6 hours of birth).
Heifers' Calves � Calves that have an adequate amount of high-quality colostrum immediately after birth have a much better chance of fighting off the diseases they will encounter. Heifers' calves are at greater risk than calves from older cows, however, since heifers' colostrum does not contain as many antibodies and also 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, due to less udder development than an older cow. And a heifer's calf may not be able to absorb as many antibodies as he should since he may be slow to nurse if the birth was difficult. And the stress of a hard birth makes him less able to absorb antibodies efficiently; stress may affect his absorption even if you make sure he gets a large amount of colostrum.
Prevent Failure Of Passive Immunity � The main reason calves get sick in the first few weeks of life is inadequate passive immunity, due to not enough antibodies absorbed immediately after birth. If you make sure calves nurse promptly, this problem can be greatly reduced. By the time a calf is four hours old he has lost 75 percent of his ability to absorb antibodies, and absorption rate decreases rapidly after that. Any calf that has not been able to nurse on its own in the first hour or two after birth should be assisted, or given colostrum by bottle, stomach tube or esophageal feeder. A calf needs to have about five percent of his body weight soon after birth (1.5 quarts for a 60-pound calf, 2 quarts for an 80-pound calf, 2.5 quarts for a 100-pound calf), and the same amount again about 6 hours later.
Active Immunity � Calves lose their temporary immunities (the protection gained from antibodies via colostrum) by seven or eight weeks of age, or even earlier. So at that time their own immune system must take over.
The time it takes for a calf's immune system to gear up so it can ward off invaders will vary, depending partly on how strong his passive immunity was. If he had a high level of antibodies from his dam's colostrum, which effectively neutralized any invading organisms, his own defenses 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 he still has high levels of maternal antibodies in his blood, his own immune system will not bother to respond to the antigens in the vaccine because they are being neutralized by the maternal antibodies. A vaccination given to a calf when he is only two or three weeks old will not give him any 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 make sure the calf's immune system will be able to respond.
