Stuart Fitzgerald
Dr Stuart Fitzgerald (MVB MANZCVS, University College Dublin)
Chihuahua receiving his injections

The subject of much debate in recent years, vaccination has saved countless lives, of pets and humans alike. While some human diseases, for example smallpox, have been eradicated by global vaccination protocols, the same cannot be said for any of the major infectious diseases of dogs.

Canine parvovirus, in particular, continues to kill many unvaccinated pets in sporadic outbreaks. With growing public concern regarding the overuse of vaccines, the focus of modern vaccinology is to establish vaccination regimes which are tailored to the efficacy of the product being administered, as well as taking account of the immune status of the dog being treated.

What Is Vaccination?

The term vaccine is derived from the name of the vaccinia virus, a member of the poxvirus family that was the active constituent in the smallpox vaccine developed in the nineteenth century. Physicians researching this deadly condition, which was responsible for millions of deaths annually, discovered that introducing this innocuous, low-level infection to individuals not previously exposed to the smallpox virus resulted in immunity to disease. Various methods of inoculation were attempted, with some being as crude as having a patient with a small vaccinia pox lesion rub against the skin of an ‘unvaccinated’ patient. Thankfully, however, laboratory and advances have long since resulted in the development of purified and refined vaccines, which may be administered by sterile injection.

The principle underlying vaccination is that stimulation of the immune system by what is termed an antigen results in an adaptive immune response. The low-level infection introduced by the vaccine results in the production of antibodies that target sites on the antigen’s surface. These antibodies fit to the antigen in a ‘lock and key’ type arrangement, allowing other white blood cells to attach to and destroy the foreign agent. Having eliminated the vaccine antigen, this initial response abates; however, a particular type of white blood cell – the memory T lymphocyte – is retained in small numbers. As the name suggests, these cells ‘remember’ and recognise antigens to which the animal has previously been exposed. Should the individual encounter the same antigen again, the ensuing immune response is more rapid, and more effective, than on the first exposure.

In this way, dogs that have previously been vaccinated with less aggressive, or even killed, vaccine strains are far less susceptible to illness when exposed to natural infection. However, as the reader may appreciate, this immunity is the result of the dog’s response to the vaccination, rather than the vaccine itself. Dogs that are incapable of an adequate response, for example those with other severe illnesses or experiencing major stress at the time of vaccination, may not mount the response expected. These cases account for many of the uncommon vaccination failure which are seen by veterinary surgeons.

What Diseases Are Dogs Vaccinated Against?

Vaccinations can prevent your dog from getting serious diseases

While the exact make-up of a vaccination protocol should be based on an assessment of an individual dog’s risk factors, certain ‘core’ diseases are recognised that should be included in every vaccine schedule. The viruses and bacteria included in these vaccines are presented in killed or inactive forms, and thus, while capable of inducing an immune response, are not capable of inducing clinical illness.

Canine Parvovirus

Probably the first disease to spring to mind in this discussion is parvovirus infection. Although the parvovirus family includes strains which affect species as diverse as humans and starfish, parvo infection was unrecognised until the 1970s, when a massive global epidemic occurred. While the outbreak’s origin remained a mystery for a long time, it is now recognised that the new canine parvovirus had developed as a mutation of parvo infection in cats, which causes feline panleukopaenia.

Parvo infection can cause severe inflammation and damage to the musculature of the heart, resulting in signs of heart failure. More commonly, however, the virus infects the rapidly dividing cells of the gastrointestinal tract, leading to severe bloody vomiting and diarrhoea. The severity of the disease depends on a number of factors – most importantly the age of the dog. Young pups are most commonly affected, and mortality rates average around 50%. Prompt diagnosis and treatment is vital to the prognosis. Parvovirus remains a major concern despite widespread vaccination, as viral mutations and unvaccinated dogs continue to facilitate occasional outbreaks.

Canine Distemper

Far less common is canine distemper virus, a disease which was a major problem in pet dogs until the introduction of vaccination in the 1950s. Distemper affects epithelial cells, which line the respiratory and gastrointestinal tracts. Signs of disease therefore include breathing difficulties, coughing, nasal discharge, vomiting, diarrhoea and loss of appetite. The nervous system may also be affected, with signs of depression or seizure activity. Its common name of ‘hard pad disease’ refers to the characteristic skin changes which may be seen on the pads of the feet. Mortality rates following infection range from 50 to 80%.

Infectious Canine Hepatitis

Caused by infection with a canine adenovirus, signs of hepatitis arise within a week of exposure. Signs can be quite diverse, with some of the more obvious being vomiting, diarrhoea, jaundice (an accumulation of bile pigment causing yellow discolouration of the eyes and gums), and corneal oedema (fluid swelling at the front of the eye causing a ‘blue eye’). Inflammation of the liver can also cause neurological signs, such as confusion and depression. Clotting disorders may also arise, leading to uncontrolled bleeding from any site. While the acute illness is severe, most affected dogs recover with appropriate supportive treatment, such as intravenous fluid and antibiotic therapy.


Known as Weil’s disease in humans, infection by leptospires may be caused by contact with urine from other infected animals – for example rats or cattle. Many different leptospire strains exist, causing diverse signs including jaundice, clotting abnormalities, vomiting, diarrhoea, and even sudden death. The severity of illness varies significantly between strains. It is important to note that this is a zoonotic illness, meaning humans can also be affected. Infected dogs must be isolated and handled with extreme care.

Infectious Tracheobronchitis

This infectious disease goes by several different names, the most-recognised being ‘kennel cough’. This is a contagious bacterial infection of the airways, caused by Bordetella bronchiseptica, which in most cases leads to a self-limiting, though very irritating, cough. Many owners of dogs with kennel cough think their pet may have something caught in their throat, so harsh and persistent is the cough.

While infection does not usually cause severe illness, very young or immunosuppressed dogs may go on to develop pneumonia. Its common name reflects the fact that infection is most likely to occur in crowded conditions; however, this is not solely an illness that develops in kennels, and dogs are just as likely to contract it when in contact with other dogs on their daily walk. Whether or not vaccination against kennel cough is a ‘core’ requirement is a matter for debate, and depends on the dog’s largely on the dog’s lifestyle.

As another bacterial infection, this disease also represents a zoonotic hazard. Although human infection with canine bordetellosis is rare, it can present a real risk to immunocompromised owners, in particular those receiving anti-cancer chemotherapy.

Other Infectious Diseases

For those of us living in the UK and Ireland, rabies vaccination is not essential unless travelling abroad with our pets. Thankfully, rabies is not endemic in our pet population: this may change in future with a relaxation of vaccination requirements for travel within the European Union.

Parainfluenza virus is often included as a component of combination vaccine products, as infection results in signs that overlap with those of kennel cough. Severe symptoms are rare, though may become significant in high-stress environments, such as rescue centres.

Likewise, coronavirus does not usually cause significant illness, although in combination with parvovirus, it can lead to very high mortality rates in young puppies. For this reason, owners of many breeding establishments may elect to include this in their list of core vaccinations.

Primary Vaccination Course

Puppies with their mother

While annual vaccination remains essential (see below), of prime importance is the manner in which the first course of vaccines is administered to puppies. All of the illnesses discussed above are most severe in young animals. Therefore, it is essential that we provide the earliest and most effective protection possible to pups.

In the first 24 to 48 hours of life, newborn puppies swallow and absorb antibodies from their mother’s milk. This ability to derive immunity from the mother is very short-lived, as the gastrointestinal tract becomes impervious to these antibodies after this time. Assuming the mother has been vaccinated herself, sufficient antibodies should be taken in to protect the pups against, for example, parvovirus for the first six weeks of life. After this time, maternally derived antibody levels drop, leaving the pup susceptible to infection. Clearly, at this point, we would like to supplement the pup’s immune response with vaccination. However, a difficulty exists in that the circulating maternal antibodies will ‘mop up’ any vaccine administered too early, protecting the pup, but also preventing it from developing its own immune response.

For this reason, first vaccinations are usually delayed until 6–8 weeks of age, at which point maternally derived antibody levels should have dropped. Having elicited an initial immune response, a second vaccine administered 2–4 weeks later is needed to ‘boost’ the response and ensure lasting immunity.

Vaccination failure can occur if the vaccination course is completed too early, or in occasional puppies in whom maternally derived immunity is unusually long-lived. This is a common problem in certain breeds, such as Dobermans and Rottweilers. Many veterinarians will therefore advise a third vaccination for puppies of these breeds.

Vaccination Frequency

One potentially positive aspect of the MMR misinformation debacle has been the increased public awareness of vaccination. Although this has led many to be unnecessarily fearful of vaccines, it has also placed a huge onus on veterinarians and the vaccine industry to establish the efficacy and safety of their products. Rather than focusing on getting pets vaccinated as frequently as their owners would allow, the industry has long since concentrated on establishing maximum vaccination intervals. Put another way, pharmaceutical companies now compete with each other to develop products that require the least frequent administration.

Most viral components of modern vaccines now provide a duration of immunity of at least three years, meaning full, broad-spectrum vaccine regimes are not required annually. However, immunity from leptospirosis remains short-lived, with even the most modern vaccines providing only 12 months of protection. As leptospirosis remains an illness to which pet dogs can be exposed in almost any environment, annual vaccination will still be a necessity into the foreseeable future.

Risks of Vaccination

Adverse effects of vaccination are exceptionally rare, but it does the veterinary profession no favours to deny that they can occur. However, it is many times more common to see unexpected adverse reactions to commonly prescribed medications, for example antibiotics, painkillers, anti-seizure medication, treatments for hormone disorders, and many more. Although most members of the public will understand that they may experience a reaction to a course of treatment prescribed by their doctor, there remains a widespread suspicion and fear of the effects of vaccines.

Transient, mild discomfort at the injection site may occur, especially following the primary vaccination course. Severe reactions are thought to arise after approximately 1 in 10,000 vaccine courses. At the extreme end of this spectrum are anaphylactic events, in which a massive allergic response can cause signs of severe shock. While this can be a disastrous event, it must be stressed by vets, and understood by owners, that the risk of an unvaccinated dog contracting one of the severe illnesses described above is hundreds of times higher than that of experiencing a vaccine reaction.

Homeopathic Vaccination

This is an area which has gained some traction in recent years, due in large part to the same adverse publicity surround MMR. Unfortunately, there is no scientific evidence that these homeopathic preparations induce a protective immune response. As owners of pets that have been ‘vaccinated’ by homeopathy may feel a false sense of security, and allow their dogs to mingle with other at-risk animals, this treatment regime is, to my mind, worse than no vaccination at all.