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There are several livestock diseases that can lead to sudden mortality and major losses for livestock enterprises. These include diseases caused by clostridium bacteria and those with such a rapid onset, that there is no time to treat animals with antibiotics (e.g. blackleg in cattle).
Antibiotics have no effect against diseases caused by a virus, as it cannot kill these pathogens. Pre-vaccination is the only way to keep these diseases at bay. Bluetongue in sheep, lumpy skin disease in cattle, and Rift Valley fever (RVF) in both species are examples of viral diseases that only a vaccine can prevent.
Certain diseases, such as bovine viral diarrhoea (BVD) and vibriosis, which have a specific effect on in-calf cows, are in most cases not treatable either. Vaccinating animals is the only way of keeping these diseases out of the herd.
Other organisms multiply so slowly that they can infect the animal without stimulating the immune system. When symptoms do appear and the immune system finally reacts, it is already too late for the animal to launch an attack. Heartwater, anaplasmosis (tick-borne gallsickness) and redwater are examples of such diseases, and vaccination is essential on farms that experience repeated outbreaks.
How does the immune system work?
To understand what vaccination entails, the livestock producer must have a basic understanding of the animal’s immune system. The immune system reacts to any foreign matter (antigen) that is not part of the animal’s normal genetic makeup. The antigens described in this article refer specifically to disease-causing organisms in animals.
Large, specialised immune system cells recognise and envelop foreign organisms that enter the animal’s body. Inside these cells, the organism is broken down into smaller pieces, after which these pieces are pushed out so that part of them protrudes through the cell wall. This protrusion contains the ‘code’ that the immune system requires in order to launch an immune response against that specific antigen.
The cell that has broken down the organism produces a substance that attracts specific types of white blood cells (B and T cells). These cells will make a copy of the specific ‘code’. In the process they are prompted to start dividing. Each cell that emerges from this division is programmed to recognise only this specific ‘code’.
B cells eventually form antibodies that carry this ‘code’ and are transported via the blood and other body fluids throughout the body. This is called humoral immunity. When these antibodies encounter the organism against which they are programmed, they will recognise the ‘code’ and attach to the organism, thus preventing the antigen from entering body cells.
They also act as ‘handles’ for the T cells to envelop and destroy the organism more easily. T cells divide to form activated cells which destroy the body cells already infected by the organism, or they will secrete substances that prevent the organism from dividing further. This function of the T cells is called cell-mediated immunity.
This process takes around two weeks; animals will therefore only be protected against the specific disease after this period. Once the immune response has been launched and the organisms destroyed or controlled, the immune response will ‘switch off’. However, a number of ‘memory cells’ that carry the specific ‘code’ will remain. If the animal is exposed to the specific antigen once more, the memory cells will be reactivated very quickly.
Each immune response launched is therefore very specific. For an animal to be able to offer resistance against various disease-causing organisms, it must be exposed to each of the antigens in order to develop acquired immunity or resistance.
How do vaccinations work?
Vaccination imitates this process. Here, cultured organisms that are either inactivated or weak are injected into a healthy animal in order to expose the immune system to the specific antigens. This elicits a reaction similar to being infected with the actual organism. The level of reaction is determined by the type of vaccine, as well as the age and health status of the animal.
The vaccine reaction then provides active protection for a specific period via the high levels of antibodies in the blood. After that, the level of protection will weaken, but in most cases memory cells will still be present.
If diseases that develop very rapidly are to be controlled, the level of active protection in animals must be maintained. Animals must therefore be regularly vaccinated against these diseases (e.g. cattle must be vaccinated against blackleg every 12 months).
Guidelines for a vaccination plan
When developing a vaccination plan, the herd needs to be divided into three groups: calves up to weaning, replacement heifers, and adult animals.
1. Calves up to weaning age
The immune system of the newborn calf is fully developed, but completely immature. The calf has never been exposed to any antigens and the various cells of the immune system have therefore not yet been programmed to recognise specific foreign organisms. During the first few months of its life, the calf depends mainly on the immunity it received from its mother via the colostrum.
The calf is unfortunately exposed to environmental antigens, which enter its body through its mouth, nose, other mucous membranes or its skin, from day one. The young calf’s immune system does respond to exposure to disease-causing organisms, but the response is slower, weaker and easier to counteract. The calf’s immune system reaches maturity at around six months of age because of this exposure.
At three months of age the colostrum-induced immunity is already low, and calves can then be exposed to vaccines for the first time. Implementing a vaccination programme before three months of age is only necessary in specific cases (e.g. heartwater and paratyphoid vaccinations) and upon the recommendation of the veterinarian.
Diseases caused by the clostridium family are among the most important diseases against which calves must be vaccinated from three months of age. Clostridial diseases such as swollen head, malignant oedema, dysentery and redgut are seen more frequently these days.
It is essential to confirm whether any of these diseases are present on the farm. If a young animal dies suddenly, without having presented any recognisable symptoms, the veterinarian must perform a post-mortem. There are vaccines against most clostridial diseases – consult the herd veterinarian in this regard.
Producers have a legal obligation to vaccinate all animals against anthrax annually – this includes calves. Similarly, all heifers between the ages of three and eight months must be vaccinated once with Brucella S19 against bovine brucellosis.
Other key organisms to vaccinate against are viral diseases that can affect the mucous membranes of calves (gastrointestinal tract and respiratory tract) and can suppress the calf’s entire immune system. The most important vaccinations are those against BVD and infectious bovine rhinotracheitis (IBR). Vaccinations should take place as recommended by the veterinarian.
Blood vaccines should be considered if mortality due to redwater, gallsickness and heartwater are an annual problem on the specific farm. It is vital to handle and administer the vaccine correctly. Calves must then be exposed to the disease within a period of three to six months after vaccination.
2. Replacement heifers
Replacement heifers usually remain in the herd for an extended period, and it is therefore necessary to develop a complete vaccination plan for heifers to expose them to all the necessary disease-causing organisms. The period after weaning until they are mated for the first time is ideal, as they do not experience much stress at this time and are not yet in calf.
Possible vaccines to consider include a broad-spectrum clostridial vaccine, vaccines against lumpy skin disease, three-day stiffsickness, RVF, botulism, anthrax, the Pasteurella leukotoxin vaccine, the Brucella RB51 vaccine (with a follow-up programme for infected farms), as well as vaccines against vibriosis, trichomonosis, leptospirosis, redwater (African and Asiatic strains) and anaplasmosis.
As the development and growth phase of post-wean replacement heifers lasts for more than a year, the vaccines can be repeated, which in certain cases will provide good long-term protection.
When preparing for the first breeding season, animals can be vaccinated one month before the start of the breeding season against those diseases that can cause poor conception or abortions. This includes vibriosis, BVD, IBR and leptospirosis.
3. Mature animals
If the mature animals were properly vaccinated as calves and heifers, then vaccination against botulism, blackleg and anthrax must be repeated once a year. The need for other vaccines will depend on specific problems encountered in the herd or the specific environment.
Once the necessary vaccinations have been determined for each group of animals, the complete vaccination plan is drawn up and presented to all the herd managers during the next farm visit. It is vital that herd managers have a firm grasp of the ins and outs of vaccination in order for them to have a meaningful discussion about the vaccination programme. – Dr Danie Odendaal, Veterinarian Network
For more information, send an email to Dr Danie Odendaal at email@example.com.