Approximately 140 vaccines are registered for use in livestock and companion animals in Australia. Many more animals are vaccinated each year than humans.
Vaccines are used in farm animals:
to protect livestock against endemic diseases
to modify reproductive performance (for instance by preventing sexual maturity in young males)
to improve food quality (for instance to reduce boar taint in pork)
to reduce the risk of transmission of diseases such as Hendra virus from animals to humans
to produce diagnostic reagents for use in pathology services
to produce therapeutic products for use in human and veterinary medicine.
Most decisions to vaccinate farm animals are made by livestock owners on a commercial basis. They balance the cost of vaccination against the risks of disease, reduced growth rates and compromised animal welfare.
An important benefit of vaccination – both for the farmer and more broadly for the community – is reduced reliance on antibiotics for treating infections in farm animals.
Adaptive immunity – learning from the environment
All animals are subjected to attack by microbes and parasites. In return, animals have well developed molecular and cellular defence mechanisms to fight off and kill infectious agents.
Within the time span of each animal’s life, it undergoes non-genetic (phenotypic) adaption to its local environment. Living in the environment leads to changes in physiology, behaviour and immune functions that enable the animal to fine tune its ability to cope and thrive.
Environmental conditions are learnt and remembered by the physiological, behavioural and immune systems of the animal. For the immune system, the lessons learnt from infection by a disease agent are remembered primarily by lymphocytes and are recalled when the animal is again exposed to the same disease-causing agent.
The recalled immune response is faster and more effective at clearing the infection. The lessons learnt from some infections such as orf virus (“scabby mouth”) in sheep are usually remembered for life, with a single infection inducing lifelong immunity to re-infection by the same disease agent. In contrast, some infections induce no effective immunity. In other instances immunity can wane over a matter of months.
Vaccines aim to induce protective immunity by controlled exposure to fragments of disease-causing organisms without exposure to the disease itself.
Passive immunity – animal vaccines helping humans
Offspring receive a cultural inheritance of acquired knowledge about local disease threats from their mothers in the form of antibodies. Depending on the species, these are acquired via the placenta, egg yolk, colostrum or milk. Maternal antibodies provide passive immunity to offspring for the first few weeks of post-natal life. Some vaccines can be used during pregnancy in animals to enhance antibody transfer to offspring.
Antibodies from animals can also protect humans. Indeed, the first Nobel Prize in Physiology or Medicine was awarded to Emil von Behring in 1901 for his development of serum therapy. Von Behring used blood serum from sheep and horses immunised with Corynebacterium diphtheriae to treat patients suffering from diphtheria.
Following his example, the use of antisera raised in animals to treat humans for systemic diseases such as tetanus has been commonplace for many decades. In Australia, horses continue to be vaccinated to generate anti-toxins to tetanus, snake venoms and other toxins. Sheep are vaccinated to produce antivenin against rattle snake venom for use in America.
In the 1970s, it was found that oral ingestion of antibodies isolated from colostrum of cows immunised with human gut pathogens can protect humans from a range of gut infections. Products containing antibodies isolated from colostrum of immunised cows protect humans from rotavirus infections, traveler’s diarrhoea and dental caries. Similar products are also used in animals.
As the efficacy of antibiotics for control of bacterial infections has diminished, there has been a resurgence of interest in “passive immunisation”. This uses antisera produced in animals for prophylaxis and treatment of disease in humans and farm animals.
For instance, the prevalence and severity of diarrhoeal disease in humans can be reduced by daily ingestion of colostrum-based products from ruminants immunised with the disease-causing agent (and possibly also by consumption of fresh unpasteurized milk from the same animals. There is a very large potential to implement this technology in developing countries to help control diarrhoeal diseases.
Can vaccination and alternative farming mix?
Vaccination is usually used as part of an integrated disease control strategy in animals. Eradication and quarantine are the most effective strategies; however eradication is rarely achievable. Vaccination played an important role in eradication of equine influenza from Australia in 2008. Indeed Australia is the only country to have successfully eradicated this disease. Selecting breeding stock for resistance to disease is also a very important disease control strategy in farm animals.
As with all foods, medicines and therapies used in humans or animals, there are divergent views on the merits of using vaccines in animals. Some agricultural production philosophies, such as organic farming, discourage use of vaccines.
However, when mandated by regulatory authorities or in the face of an adverse disease history or when recommended by a veterinarian, the use of vaccines can be authorised by the organic certification entities Australian Organic and Organic Growers to aid in disease control on organic farms.
This approach provides neither an argument against organic farming nor against vaccination. A diversity of farming practices and production philosophies is likely to strengthen food security in the face of changing environmental threats and consumer preferences.