Understanding Immunity and Vaccination
Published: September 22, 2008
By: Dr. Terry Whiting, Veterinary Disease Control and Epidemiology, Livestock Knowledge Centre - Manitoba Agriculture, Food and Rural Initiatives
The Immune System
In the normal healthy animal, the skin, the mucous membranes of the eyes, the respiratory tract, and the digestive tract form natural protective barriers to the entry of bacteria, viruses, or other disease-causing micro-organisms into the body. Some animals also have a natural immunity to certain diseases. Cattle, for example, do not get swine fever.
In addition, animals can acquire immunity either passively or actively through the production of antibodies. Antibodies are specially produced bits of protein produced by the immune system of the body to coat invading bacteria or viruses. This makes them easier to ingest and be destroyed by the white blood cells circulating in the body.
It is important to understand the term passive immunity. The normal calf is born without having any antibodies to protect itself from infectious diseases. Passive immunity is the transfer of antibodies from one animal to another either via a blood transfusion or, in the case of the newborn, from antibodies secreted into the cow's first milk (colostrom). These antibodies are absorbed intact without digestion through the calf's gut into its bloodstream after ingesting high quality colostrom within the first few hours after birth. Without the antibodies provided by the colostrum, the calf is virtually unable to survive. Passive immunity provides the calf with resistance to many diseases. Antibodies transferred through passive immunity to the newborn gradually disappear within 2-3 months and are not replaced unless the calf is exposed to a disease organism or is vaccinated.
Active immunity occurs when an animal's immune system is stimulated to develop antibodies, which are able to help the animal to resist disease. Antibodies are produced either in response to stimulation by natural infection assuming the animal lives long enough to develop an immune response, or as a vaccination response. If the animal produces a sufficient amount of its own antibodies, then it is protected or immune to that particular disease. This is referred to as active immunity; however, over a period of several months to a year, the level of antibodies circulating in the body fluids gradually declines. Therefore, timely revaccinations are needed to boost this active immunity.
Immunity can be overwhelmed under certain conditions. In the presence of a large number of organisms or factors, which affect the integrity of the protective barriers, these organisms will enter the body. Their entry and subsequent replication (referred to as an infection or disease) may overwhelm the animal. This will result in either the animal's death or causing it to call upon other protective mechanisms which hopefully will rid the body of the causative organisms.
Very often an infection or disease will, if it does not kill the animal, cause chronic weakness or poor production. A common example is a calf that has been chronically infected with pneumonia during its early life. Sometimes these diseases are referred to as "production limiting diseases".
Factors Affecting Immune Response During Disease or Vaccination
Stress
Stress can be defined as anything that causes change to the body. The two most important stressors which negatively affect vaccine response are calving and extreme climatic conditions. Vaccination of the recently fresh cow is of little value due to the steroid release that takes place at calving. This suppresses her immune response. In temperatures greater than 78ºF (25.5ºC), Holsteins will show mild heat stress. Vaccinations should be done in the cool of the morning.
Age
The immune system in youngstock does not normally reach full maturity until 5-6 months of age. This doesn't mean a young calf cannot respond to antigens but the response will be weaker (see definition of antigens in next section under VACCINES EXPLAINED) . In mature animals over 7-8 years, the immune response declines with age.
Colostrum
If high quality colostrum is consumed in sufficient quantity and immediately following birth, colostral antibodies will be rapidly absorbed from the newborn calf's gut into its bloodstream to provide passive immunity for about two months. It is generally believed that maternal colostral antibodies block the stimulation of immune responses when young calves are vaccinated. More recent studies indicate that some beneficial response may occur even in the presence of maternal antibodies. Many producers are vaccinating calves as early as one month of age to prevent BRSV.
Nutrition
Rapid growth of replacement heifers, increased nutritional demands of the fetus late in gestation, and high milk production are times when the nutrition status may be deficient. This is especially true with respect to micronutrients such as Copper, Iron, Zinc, Selenium, and Vitamins A and E. In such conditions, the animal's immune system may be unable to fully and properly respond to vaccination. To ensure a strong response to vaccination, maintain a well-balanced nutrition program.
Vaccines Explained
To understand what a vaccine is there is need to first understand what an antigen is. An antigen is a substance that induces the formation of antibodies because it is recognized by the immune system as a threat to health. Many substances can cause an antigenic response such as pollen, toxins, bacteria and viruses.
Vaccines are suspensions of antigens. These antigens can be disease-causing organisms or parts of these organisms, which have been altered in some way so as to not cause disease.
Vaccination is the deliberate introduction of antigens into the animal with the objective that the animal's body will be stimulated to produce special protective proteins called antibodies.
Types of Vaccines
There are two basic types of vaccines: killed and modified live vaccines. With killed vaccines the actual disease-causing organisms have been made non-viable but they will stimulate an immune response. With modified live vaccines, the organisms can still proliferate in the body but have been modified so as to not cause disease; however, direct exposure of the fetus by using some modified live virus vaccines on a pregnant cow is of concern and can potentially cause abortion.
Killed Versus Modified Live Vaccines
Killed Vaccines
Advantages
•No risk of reverting to virulent form
•Little risk of causing abortion
•More stable in storage
•No on-farm mixing, therefore less risk of contamination
Disadvantages
•More likely to cause allergic reactions
•Two initial doses required
•May not produce as strong or as long a lasting immunity
Modified Live Vaccines
Advantages
•One initial dose is usually sufficient (at least two exceptions are noted in this paper)
•More rapid protection
•Usually less expensive
•Less likely to cause allergic reactions
Disadvantages
•Potential to revert to virulent forms
•Could exacerbate disease in immuno-suppressed animals
•Some risk of causing abortion or delaying estrus.
•Must be handled and mixed more carefully
Reactions to Vaccines
Allergic reactions can occur following the administration of vaccines, drugs or any chemical substance but these reactions are usually rare. This reaction may take the form of sudden cardiopulmonary arrest or may include signs such as swelling of the eyelids, bloat, restlessness or respiratory difficulty. If any of these signs occur within several hours after administration, they must be treated with a drug called epinephrine. All producers who vaccinate or treat livestock should keep epinephrine on hand and know how to use it properly.
Handling and Storage of Vaccines
1. All vaccines need to be stored at refrigerator temperature. Use a cooler with ice packs to
transport them home from the veterinary clinic, and at chute side; however, do not freeze
vaccines.
2. If reconstitution of vials is necessary, always use a sterile transfer needle and only prepare
sufficient vaccine that will be used up quickly (1 to 2 hours). Keep the mixed vial in the cooler.
If there is any delay between drawing up the vaccine and using it, put the syringe back in the
cooler between animals as well.
3. Use entire contents of the vial when first opened. A reconstituted modified live vaccine is
useless the next day.
4. Sterilize syringes in boiling water since disinfectants may kill a modified live vaccine or adversely
react to substances in a killed vaccine. Change needles frequently. Dull needles cause injection
site damage and may allow more leakage. As well they become more and more contaminated
after each use causing infection at the site. Producers with a Bovine Leukosis Virus (BLV) free
herd must use a sterile needle for each cow to prevent transmission of this virus.
5. Find a clean injection site, preferably in the neck region. Subcutaneous injections may also be
given over the rib cage.
6. Read the label instructions carefully before each use and pay particular attention to information
about proper injection site, dosage, type of animals to be used on, and withdrawal time to
slaughter.
7. Administer only to healthy animals.
8. Dispose of used needles and vials in hard plastic storage containers so that they can be safely
transported to the nearest landfill site.
Immune Response From Vaccinations
It is important to follow label directions for administering vaccines. The first time a killed vaccine is administered, a short-lived, weak primary response occurs. The second or booster vaccination, which is usually recommended to be given 2-4 weeks after the first dose, causes a response, which is much stronger and longer-lived. If this second dose is given too early, the secondary response does not occur, and if too much time elapses between the initial dose and the booster, it acts as a primary and not a secondary stimulus.
With modified live vaccines, the primary vaccination usually stimulates a good initial response without needing a booster right away because the virus is replicating in the body. An exception to this is the use of modified live BRSV vaccine, which requires a booster 2-4 weeks after the initial vaccination to stimulate sufficient immunity. Most vaccines then call for annual revaccination.
Efficiency of Vaccine
A large variety of vaccines are available. Do the antigens included in a particular vaccine adequately represent the field strains of the disease in question? Do the antigens stimulate the desired protective response? The use of sub-unit and DNA recombinant technology to produce genetically engineered vaccines has the potential to greatly improve the efficacy of vaccines. Currently available vaccines, if used properly, will provide good protection to prevent outbreaks of the following diseases.
A Description of Common Infections Diseases With Available Vaccines
Infectious diseases of the dairy herd which may be controlled by proper vaccination and which will be briefly discussed. Although it is not the purpose of this factsheet to discuss in detail the etiology, clinical signs, and treatment of these diseases, some of the pertinent reasons for vaccination are mentioned. The timing of initial and booster vaccinations is set out in the protocol chart near the end of this factsheet.
Clostridial Diseases
Organisms of the clostridia family cause diseases such as blackleg, malignant edema, tetanus, necrotic hepatitis, enterotoxemia, and bacillary hemoglobinuria. These are predominantly soil-borne organisms that produce disease mainly through the toxins they release. Vaccination is necessary if pasturing young animals or if feeding high grain levels to lactating cows. Good immune protection is provided at a very low cost through vaccination. All replacements should be vaccinated starting as early as 3-4 months of age.
Infectious Bovine Rhinotracheitis (IBR)
IBR is a highly contagious virus disease and may cause several different clinical symptoms which include:
1. Respiratory disease of varying severity
2. Abortion
3. Nervous form
4. Genital infections
5. Conjunctivitis (eye inflammation)
6. Infection of newborn calves
In adults, IBR may cause only mild clinical signs or reduced milk production; however, it often results in abortions. These abortions can occur up to 2-3 months after the initial infection making the diagnosis somewhat difficult. Animals less than one year of age that become IBR infected commonly have secondary bacterial pneumonia. Studies have shown that the virus can persist in cattle without causing clinical signs for up to 17 months. Shedding of the virus may be induced periodically by stress from inclement weather, transportation and overcrowding of animals.
Both modified live and killed vaccines are available.
Bovine Virus Diarrhea (BVD)
A number of recent on-farm outbreaks of a more severe form of BVD caused by BVD virus type II has prompted producers and veterinarians to place higher priority on prevention of this disease in cattle herds. The introduction of the type II virus to a susceptible herd will result in abortions (regardless of gestational age), high fevers, severe depression, and respiratory distress. Infected animals are not responsive to treatment and a large number die. The BVD virus type I, considered to be a milder form, can circulate in the herd almost unknowingly; however, it may cause abortions, render younger cattle more susceptible to respiratory disease and may infect the developing fetus. Some fetuses infected in utero may be born alive and may survive for 1-2 years, but they never develop immunity to BVD. These persistently infected animals usually die at a young age due to a syndrome called mucosal disease; however, while in the herd these persistently infected calves intermittently shed large amounts of the virus.
Both modified live BVD and killed vaccines are readily available. Some modified live vaccines are not licensed for use in pregnant cows. Killed virus vaccines must be administered twice within a 2-4 week interval with the booster preferably being given 2-3 weeks prior to breeding.
The objective is to have the best immune protection possible throughout the pregnancy.
Bovine Respiratory Syncytial Virus (BRSV)
BRSV may cause an interstitial pneumonia in all ages of cattle, but can be especially devastating if it spreads through the milking herd causing severe drops in milk production and mortalities. Mortality can be especially high in young replacements when complicated with Influenza, IBR, BVD or pasteurella pneumonia. A booster at 2-4 weeks is necessary even if a modified live vaccine is used. Some reports indicate that killed BRSV vaccines are not very effective.
Parainfluenza-3 (PI3)
PI3 is a contributing virus to the enzootic pneumonia syndrome of young calves 2-5 months of age. Environmental factors such as housing, air quality, and humidity level play a significant role in the incidence and severity of pneumonia in replacements and its impact on growth rate and body weight at maturity. Antigens for the PI3 virus are included in most multi-valent vaccines.
Leptospirosis
Leptospirosis is a bacterial disease that is relatively uncommon in Manitoba cattle but of some significance because it can affect a wide range of species on the farm including humans. Rodents are the predominant carriers. Large numbers of organisms can be shed in the urine of an infected animal thus contaminating pastures, drinking water, and feed. A major drop in milk production may occur at initial infection, and abortion is a common sequel several weeks later. There are up to five serovars or strains of Leptospirosis organisms in some killed vaccine products.
Mastitis
An acute syndrome of endotoxic shock and sometimes death can occur in lactating animals right at or just after calving. This is caused by an E. coli infection of the udder. A vaccine is available to use during the dry period, with the booster given 3-4 weeks before calving. This vaccination does not protect against other mastitis pathogens. It may be helpful when used in herds experiencing a high incidence of coliform mastitis.
Neonatal Diarrhea Caused by E. coli, Rotavirus, Coronavirus or Clostridium perfringens Type C
These organisms cause severe diarrhea in calves from 2-30 days of age. It can be an ongoing labor intensive job to treat affected calves and mortality rate can be high. Cows can be vaccinated during the dry period and transfer protection to the calves via passive immunity. Also, a rotavirus/coronavirus vaccine may be given to newborn calves before they nurse. The pro's and con's of this option should be discussed with your local veterinarian.
Ringworm
Ringworm is a fungal disease of the skin, which, while generally self-limiting, may be a problem on some farms. The fungus survives for long periods on the planks of barn pens, which are difficult to disinfect. A modified live vaccine is available to use on calves as young as two weeks of age; however, even though this is a modified live vaccine, a booster vaccination is required. To be effective the vaccine must be administered prior to exposure. Immunity will not develop in animals incubating the disease.
Table 1. Dairy cattle vaccination protocol
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Conclusion
The minimum vaccination program should be built around the four major viral diseases (BVD, IBR, PI3 and BRSV) and the Clostridial diseases. Other vaccinations should be considered on a herd to herd basis. Always remember that vaccination is just one of a number of management factors which helps to minimize the drastic effects that a disease outbreak could have. The chart above summarizes a typical vaccination protocol. Also remember that conditions may vary somewhat from area to area and that the protocol must be tailored to meet the individual needs of each producer.
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23 de septiembre de 2008
It is an excellent article which provides a lots of basic information especially for those farmers who are not technically very sound, and will help to understand the need for vaccinations to young calves.
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