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Human and animal health are irrevocably linked. Often, infectious diseases are present in animals before they are passed into the human population. In recent years, many disease outbreaks have been the result of animal to human transmission; the West Nile virus outbreak in North America, avian flu, rabies, Middle East respiratory syndrome coronavirus (MERS-CoV), human immunodeficiency virus (HIV), severe acute respiratory syndrome (SARS), anthrax and Ebola1. The novel Covid-19 virus, a coronavirus, which spread across the world in early 2020, is another example of a zoonotic disease. Early evidence suggested that this virus was spread from its animal host (bats), to humans via an intermediate host (Malayan pangolins)2.
Farm animals, companion animals and wild animals can all be sources of disease that can be passed onto humans.
|Table 1. Examples of zoonotic diseases in farm, companion, and wild animals|
|Species||Disease in animal host||Disease in human host||Method of transmission from animals to humans|
|Cow (farm animal)||Bovine spongiform encephalopathy (BSE)||Creutzfeldt-Jakob disease (CJD)||Consuming infected animal products|
|Dog (companion animal)||Rabies||Rabies||A bite or scratch from an infected animal|
|Birds (wild animal)||Avian flu||Spanish flu||Genetic material from the avian flu was transferred to human infecting virus strains (reassortment)|
Farm, companion, and wild animals can carry zoonotic diseases. Images are creative commons. Left (Photo by Rebeccawithers, license CC BY-NHD 2.0), Middle (Photo by Bekathwia, license CC BY-SA 2.0) Right (Photo by ramendan, license CC BY-NHD 2.0)
Creutzfeldt-Jakob disease (CJD) is a rare, fatal neurodegenerative disease in humans. It is carried by cattle, where it causes Bovine spongiform encephalopathy (mad cow disease) and is passed to humans through contaminated animal products. Since the link between variant CJD and BSE was discovered in 1996, strict controls have proved very effective in preventing meat from infected cattle entering the food chain. Furthermore, the control measures have dramatically reduced the levels of BSE in cattle, meaning that the risk posed is extremely small.
Rabies kills thousands of people a year, mostly in Asia and Africa, and infections often occur from an infected dog bite. Vaccination of dogs to prevent rabies from infecting them also has a hugely positive affect on human health, eliminating the risk that rabies infected dogs pose to human health.
The 1918 influenza pandemic, which resulted in the deaths of an estimated 50 million people, is thought to have originated in birds before changes to the virus allowed it to infect humans3.
It is important that public health authorities, food safety authorities, doctors, vets and farmers, collaborate together to prevent the spread of diseases from animals to humans taking a true ‘One Health’ approach.
There are many preventative measures that farmers and pet owners take to prevent disease, vaccination, biosecurity, good animal husbandry and good nutrition4.
Biosecurity refers to measures that are designed to protect against the spread of diseases, such as fencing around cattle fields to prevent their interaction with wild animals.
Vaccines contain a dead or altered form of the pathogen, incapable of causing disease. This introduces the body’s immune system to a particular disease antigen. An antigen is a protein on the surface of a microorganism, which can stimulate a response by the immune system. The body learns how to recognise the antigen, respond effectively to fight off the infection, and prevent the microorganism from causing damage in the future.
Vaccines initiate a response from the primary immune system, which produces antibodies against the antigen. This allows the secondary immune system to spring into action more quickly if the animal comes into contact with the pathogen a second time, preventing infection. Learn about the activity of vaccines here.
Vaccines are an effective tool in preventing the spread of diseases in animal populations, maintaining animal health, and preventing transmission of zoonotic diseases to humans.
Animals that produce beef, dairy and lamb products are protected from a range of endemic (e.g. a disease naturally occurring in the UK) and exotic diseases by vaccinations. There is a constant threat to livestock from bacteria which live in soil, e.g. Clostridial diseases. An infection with one of these diseases is often fatal, so cattle thought to be at risk are vaccinated to protect them from infection. Other farm animals, such as pigs and chickens, can also be vaccinated to protect them from a range of deadly infections6. Vaccines also play a huge role in preventing many different types of respiratory diseases, e.g. influenza or “flu” in many different species.
Vaccines prevent disease spread and reduce the need for treatment with antibiotics as a result of bacterial infection. In this way, vaccines also help to reduce the use of prophylactic medicines. Prophylactic medicines are used to prevent disease – when vaccines can take this role instead of medicines, they are very effective, as they provide immunity to future infections.
Vaccines help prevent the spread of antimicrobial resistance by preventing opportunistic bacterial infections. Opportunistic infections are caused by pathogens that take advantage of a host with a weakened immune system, for example, if they were infected previously with a viral pathogen. These opportunistic bacterial infections would need to be treated with an antibiotic, but vaccines help to prevent this7.
Domestic animals such as cattle, sheep, dogs and goats can be hosts to many of the same virus types as humans, meaning these diseases may be able to infect humans if there are no control methods in place (e.g. vaccines or antibiotics)5. Salmonella are bacteria that can be present in some foodstuffs (e.g. undercooked chicken eggs), and infection can result in diarrhoea, vomiting, nausea, fever, and stomach pain. In 1993 there was an epidemic of Salmonella infections in the UK, but mass poultry vaccinations (amongst other control measures including strict hygiene improvements) have led to a dramatic fall in the number of cases6. This example is an excellent representation of how vaccines can protect and improve public health.
Animal diseases can impact human health indirectly. Vaccination played a central role in the successful worldwide eradication of a disease of cattle called rinderpest in 2011. The rinderpest virus does not cause human disease, yet it is still linked to human health. Before the eradication of rinderpest in 2011, the disease was responsible for a huge number of human deaths and loss of quality of life because of agricultural losses. This was especially the case in developing countries where subsistence agriculture is essential to human well-being. The disease would spread rapidly in a cattle herd and result in a devastating number of deaths of infected animals. Many people relied on these animals for food (milk or beef), and such huge losses resulted in widespread famine.
Cattle losses due to rinderpest infection in South Africa, 1896. Source: FAO.org, ‘Early warning as the weakest link in disease surveillance systems’, photo courtesy of G.R Thomson
All vaccines that are licensed to be used in the UK must meet strict regulatory requirements overseen by an independent regulatory authority, the Veterinary Medicines Directorate (VMD). Vaccines train the animal’s own immune system to fight off disease, and they pose no risk to human health4. The National Office for Animal Health has more information about the vaccination of farm animals, here.
Just as farm animals need vaccinating to protect them catching often fatal diseases, companion animals such as dogs and cats need vaccinations too. The development of companion animal vaccines has dramatically improved the health and welfare of pets, preventing serious life-threatening illnesses from infecting our pets.
|Table 2. Examples of diseases controlled by vaccines in companion animals in the UK7|
|Species||Diseases controlled by vaccines|
|Dogs||Distemper, infectious canine hepatitis, leptospirosis, parvovirus, kennel cough, rabies|
|Cats||Feline leukaemia, chlamydia, cat ‘flu’(feline herpes virus and feline calicivirus), rabies|
|Horses||Equine herpes virus 1, influenza, tetanus, viral arteritis, rabies|
|Rabbits||Myxomatosis, viral haemorrhagic disease|
It is thought that increasing human contact with wild animal populations is part of the current increase in the emergence of infectious diseases, such as Ebola and Covid-195. Wild animals that have adapted to live well alongside humans, such as rats, foxes, and primates, have been identified as host organisms for nearly 75% of viruses5. Wild species which typically have limited contact with human populations are also capable of transmitting diseases to humans, especially where natural habitats have been disrupted and reduced in size by human activity.
For vaccinations to protect every member of a community, herd immunity is important. Herd immunity occurs when a high percentage of the population is vaccinated and is resistant to the spread of a contagious disease because of this. In this instance, it is hard for an infectious disease to spread, because there are few susceptible host organisms for the disease to infect. This can help protect individuals that cannot be vaccinated, because they are too young, or allergic to a vaccine component (egg proteins in Influenza vaccine, for example) (figure 1, below).
Figure 1. The spread of an infectious disease in a population that has not been vaccinated (A) and where individuals have been vaccinated against the disease (B). Herd immunity protects individuals who cannot be protected through vaccines
Vaccines have been described as a ‘victim of their own success’. The success of vaccines in preventing fatal diseases has led to generations of people not experiencing the regular and sometimes devastating disease outbreaks common in previous centuries. When individuals do not experience disease outbreaks, myths surrounding vaccines can spread, and individuals are falsely led to believe that vaccines are more of a risk than contracting the disease the vaccine protects against. Vaccine hesitancy has been named by the World Health Organisation (WHO) as one of the top 10 health threats of 20198.
Opposition to vaccines has been around for centuries. In 1772, an English Reverend referred to vaccines as ‘diabolical operations’. Lack of knowledge and understanding around the causes of sickness meant that infectious diseases were often considered a punishment from God for sin, and vaccines were considered ‘an attempt to oppose God’s punishments upon man for his sins’9.
A quarantine poster warning that the house contained people infected with smallpox, sometime in the 1910s. Smallpox was a devastating disease of the 19th century which was spread by airborne particles and spread by sneezing, coughing or direct contact with bodily fluids. After a vaccine was developed, smallpox was eventually declared eradicated in 1979.
More recently, anti-vaccine actions included a paper published in 1998 by a now discredited British doctor and researcher. In this paper, which has since been removed from publication, the measles, mumps, and rubella (MMR) vaccine was incorrectly linked to the development of autism in young children9. Even with the removal of the paper from publication and other scientific studies disproving a link between this vaccine and autism, the damage had been done. This myth spread to many different parts of the world, resulting in a drop in the number of children getting the MMR vaccination. In the UK for example, vaccination rates dropped from 92% in 1996 to 84% in 2002 – as measles is a highly infectious disease, a 95% vaccination rate is necessary for herd immunity to protect those in our communities who cannot be vaccinated10. This has resulted in several breakouts of measles occurring throughout the world.
In 2019, 1282 cases of measles were reported in the United States, with 128 individuals having to be hospitalised and 61 reporting complications such as pneumonia (inflammation of the tissue in one or both lungs) or encephalitis (inflammation of brain tissue). This was the largest number of cases in the US since 1992. Most people infected had not been vaccinated11. Measles cases in the UK increased sharply in 2018 as a result of teenagers catching the disease who had not been vaccinated when they were younger.
Healthy animals are important for preventing spread of diseases to human populations and making sure there is enough food to feed our population. Vaccine hesitancy impacts the health of animal populations as well. As animal owners no longer see many of the preventable diseases in their animals, some may feel it is an unnecessary cost to protect their pet using vaccines. However, these vaccine preventable diseases have not been eradicated and still pose a serious risk to pets.
Vaccines protect people and animals from many serious and potentially deadly diseases, such as smallpox, polio, and tetanus, which used to kill or disable millions of people. It is necessary to have a high proportion of a population vaccinated to protect those who cannot be vaccinated against a certain disease. Vaccines are safety tested for years before they are authorised by independent scientific regulatory authorities, for use. Any side effects (e.g. swelling at the site of infection, mild fever, headache) typically pass quickly and are a result of the body’s immune system learning to fight off the infection, so it can respond more quickly if it encounters the live pathogen in the future12. Furthermore, the independent scientific regulatory authorities for both human and veterinary medicines monitor reports of any side effects and keep the safety of the products under continuous review and scrutiny, a process known as pharmacovigilance.