Vaccines are understood to be one of the greatest breakthroughs in modern medicine. Here, Health Europa explores how vaccines have benefitted not only humans but also animals, and limited the transmission of zoonotic diseases.
Treatment using vaccines is understood to be one of the greatest breakthroughs in modern medicine; no single medical intervention method has contributed more to the reduction of fatality and the improvement of quality of life. As a result of vaccinations, smallpox has been eradicated, whilst cases of polio are near eradication.
In a report from the World Health Organization, it states on the efficacy of vaccines: ‘Unless an environmental reservoir exists, an eradicated pathogen cannot re-emerge, unless accidentally or malevolently reintroduced by humans, allowing vaccination or other preventative methods to be discontinued.’
Although the efficacy of vaccination is high, diminished recognition of their vast importance poses a threat, whereby reduced vaccination rates could see the return of fatal diseases and viruses. In this article, Health Europa explores how vaccines have benefitted not only humans but also animals, whilst limiting the transmission of zoonotic diseases.
How are animal vaccines preventing the transmission of zoonotic diseases?
The vaccination of pets and farm animals is essential in order to maintain good animal health and welfare, whilst also reducing the disease burden in pets and livestock. As mutations of infection and disease develop, the role of vaccines in preventative treatment and disease control programmes is integral. With a long and successful history of preventing and controlling disease, the veterinary vaccines of today are symbolic of years of cutting-edge research but also represent the diseases faced by pets and livestock.
In order to prevent disease, animals are vaccinated to reduce suffering and the transmission of micro-organisms amongst the animal populous. Vaccination is also predominantly a more cost-efficient treatment pathway than treating sick animals. Whilst pets receive vaccines for infections such as rabies, parvovirus distemper and hepatitis, livestock — turkeys, chicken, cattle and pigs — is vaccinated against diseases such as rotavirus, E. coli, pinkeye and brucellosis. By vaccinating pets and livestock, people and herds can be kept healthy.
Another means of protecting livestock is that of herd immunity, whereby protection is provided to larger communities of animals (who may not all be vaccinated) in which a large majority are vaccinated, thus reducing the prevalence of a given disease and those susceptible within an area.
How do vaccines take effect?
Vaccinations take effect through stimulation of the animal’s immune system without causing the disease, enabling that animal to be prevented from catching the disease. Once an animal is vaccinated, its immune system responds and can, subsequently, remember the infectious agent which the animal is protected against and provide a sufficient level of protection against the disease, should the animal come into contact with that same agent.
Regardless of the vaccination provided, animals should be in a state of good health, as a properly functioning immune system is required in order to stimulate an effective immune response and to develop the necessary level of protection.
In the initial stages of the treatment process, a primary vaccination course is completed, but, depending on the vaccine type and species targeted, it may be necessary to have additional booster vaccinations at intervals in order to maintain protective immunity throughout an animal’s lifetime. As animals are exposed to a range of varying risks, related to age, lifestyle, disease threats and migration/travel, vaccination protocols are tailored by veterinarians for an individual animal or a group of animals.
What types of vaccine are there?
Today’s vaccines are categorised into:
- Modified-live (attenuated);
- Recombinant; and
Modified-live (attenuated) vaccines
Modified-live vaccinations are characterised by an intact, but weakened, pathogen contained inside the vaccine, which stimulates an immune response but does not cause clinical disease.
Inactivated (killed) vaccines
An inactivated vaccine contains an inactivated pathogen, meaning that it is no longer infectious. Such vaccines frequently contain an adjuvant – a compound added in order to strengthen the protective immune response.
Recombinant vaccines are produced through genetic engineering technology. They utilise genetic material from the desired pathogen in order to produce proteins which stimulate an immune response upon vaccination.
Toxoid vaccines encompass inactivated toxins which are produced by pathogens. As a result, these vaccines protect an animal against toxins through stimulation of immunity, which, in turn, protects the animal.
Stopping the transmission of zoonotic disease
But it is not only animals who are at risk when vaccinations are not made; public health amongst humans is also protected through vaccination of animals. Before entering, or returning, to the UK, animals such as cats, dogs and ferrets are required to show that they have a valid rabies vaccination. As one of the most prevalent and fatal zoonoses, vaccination is integral in protecting both animals and people in the UK from the threat of rabies.
Since the first course of rabies treatment was administered in the 19th Century by Louis Pasteur, rabies vaccines have been benefitted by developments in both production and control. Since then, vaccines for human use have seen a transition from vaccines being prepared from animal nerve tissue to embryonated eggs to cultures of human diploid cells (HDC) around 1960. This vaccine remains the reference in comparative studies of immunogenicity.
Refining the rabies vaccine for humans
In 1964, an inactivated rabies vaccine for human administration was prepared in cell culture, and in 1966 it was shown that the HDC strain W1-38 was an appropriate substrate for the propagation of the Pitman-Moore strain of fixed rabies virus. Since 1967, R&D on the vaccine has been led at the Mérieux Institute, Lyon, France.
Despite its safety and high immunogenicity, the low titre of virus production from these cells led to limitations on large-scale production, where a more cost-effective rabies vaccines of the same quality was available. The inactivated poliomyelitis vaccine was the first using this cell substrate and subsequently led to the revision of requirements for such a vaccine by the World Health Organization Expert Committee on Biological Standardization.
The technique was developed by Van Wezel and consisted of exploring the culture of cells on microcarriers, stimulating large-scale cultures of cells for human vaccine preparation. Following the production of an inactivated poliomyelitis vaccine in Vero cells, studies were conducted in an effort to develop a human rabies vaccine. As a result, the purified Vero cell rabies vaccine (PVRV) was produced, but it required a purification step in order to remove residual cellular DNA.
A purified chick-embryo cell vaccine for administration in humans is prepared using chick embryo cells and derived from specific pathogen-free (SPF) eggs. The vaccine is developed using freeze-dried preparation, which consists of purified and concentrated rabies virus antigen inactivated with B-propiolactone.
Reducing the threat of fatal disease
Owing to the introduction of a childhood vaccination programme by the NHS in the UK, children of the UK are now protected against many dangerous diseases, including: smallpox, polio, diphtheria and whooping cough.
In 18th Century Europe, smallpox was responsible for killing thousands; the disease was capable of killing around a third of victims, whilst leaving survivors scarred or blinded. However, in 1980, smallpox was successfully eradicated. Had a vaccine not been developed, smallpox could have caused an estimated two million death every year globally.
Meanwhile, polio has been eradicated from the majority of the world, including within the UK, Europe, the Western Pacific and the Americas. The disease is caused by a virus which destroys nerve cells and, at its peak, threatened millions of people worldwide. Furthermore, up to one in every 1,000 children and one in 75 adults who caught the infection were paralysed, affecting not only patients’ arms and legs but breathing muscles, leading to a rise in the risk of suffocation. As a result, the only way to alleviate the complications caused by polio-induced respiratory conditions was to place them in an iron lung, assisting them with breathing.
In 1940, diphtheria was responsible for more than 60,000 cases and around 3,000 deaths within the UK alone. By 2002, vaccines had almost eradicated this disease entirely, to the extent whereby the death toll of diphtheria was reduced to two between 1986 and 2002.
Since the introduction of a vaccine in the UK in 1999, meningitis C has been virtually eliminated. The first country to offer vaccination against meningitis C, the UK has seen a 99% reduction in cases of meningitis C in patients under 20 since the vaccination was developed and provided.
Owing to the provision of vaccines, such diseases are now either extremely rare or eradicated. Although these diseases may be rare currently, if children aren’t vaccinated, such diseases are able to return with the same threat to the population.
Progressing vaccination programmes
Cutting-edge research and development of safe, high-efficacy, high-quality vaccines means that pets and farm animals remain able to benefit from crucial medicines which may prevent or alleviate the clinical signs and stages of disease. R&I has led to the development of novel, highly sophisticated technologies, one example being marker vaccines. Traditional vaccination approaches which seek to protect animals evoke an immune response which is similar to that of a natural infection. Therefore, when testing, this causes complications in differentiating between those animals that have been infected and those which are vaccinated. The livestock marker vaccine for infectious bovine rhinotracheitis (IBR) – a highly contagious respiratory disease affecting cattle – is just one example of the cutting-edge outcomes made possible by RD&I.
Today, research continues to thrive, and more than 150 innovative vaccines are in the testing phase. It is anticipated that an improved pneumococcal vaccine will soon be available, offering an increased level of protection against more strains of pneumonia, whilst promising research continues on exploring vaccines for flu which remains active for extended periods of time.
This article appears in Health Europa Quarterly issue 6, which was published in August.