Do pets and their owners swap diseases? Sarah Caddy finds out...
‘There's no psychiatrist in the world like a puppy licking your face’. As Bern Williams once suggested, there is a very unique relationship between people and dogs. Dogs are often considered honorary family members, and their close contact with people can extend to face licking. Just as you might catch a cold from your child, is it possible to become infected with bacteria, viruses, fungi or parasites from your dog?
Diseases contracted from animals are called ‘zoonoses’. Some well known zoonoses such as avian influenza can be transmitted by direct contact with livestock, but the majority of the public are never in this situation. However, 1 in 4 households in the UK has a dog, and infections can jump between the species.
If I get diarrhoea, is the dog to blame?
Diarrhoea in humans can occur for a multitude of reasons. Acute diarrhoea is often the result of a food toxicity, and a proportion of these cases are caused by ingesting food contaminated with bacteria or viruses. As any dog owner will testify, dogs frequently get diarrhoea too. Most of the time this is because dogs are often fond of eating rubbish in the park, but infections can cause diarrhoea as well. The common bacteria that are able to infect humans and dogs include E.coli, Salmonella and Campylobacter. Theoretically, these bacteria can all be zoonotic. Humans are much more likely to become infected by these agents from an undercooked dinner as opposed to a pet dog, but good hygiene around dog poo is always a sensible idea.
A couple of diarrhoea-inducing viruses have also been suggested to be zoonotic pathogens. Rotavirus is a common cause of childhood diarrhoea, although the recent introduction of a rotavirus vaccine has considerably reduced the number of infected infants. Analysis of the sequences of human rotaviruses has shown that some are similar to canine rotavirus strains. There are very rare case reports of children being infected by suspect canine rotavirus strains, but the emphasis must be on ‘very rare’. Another recent paper raised the question about dogs and human norovirus. An owner was found to be infected with the same norovirus strain that was infecting their dog. It is yet to be established if norovirus is anything to worry about in dogs, but given that most human infections occur in locations where dogs are generally forbidden, e.g. hospitals and cruise-ships, it isn’t time to start holding your dog responsible yet…
What about rashes?
One of most common skin problems transmitted from dogs to humans is fleas. Dog fleas feed on dog blood, drop off and lay eggs nearby, then when these hatch they will feed on the nearest warm-blooded creature. Fleas cannot complete their life cycle on humans alone, but will certainly leave a series of insect bites behind if they come into contact with human skin. Fleas are easily treatable with the correct products for dogs and their environment, and should never be a significant problem.
Alongside fleas, mites are another zoonotic skin parasite that can cause disease in humans. Sarcoptes and Cheyletiella mites are generally uncommon in dogs, but worth considering if routine flea treatment isn’t stopping your dog itching.
Dermatophytosis or ‘ringworm’ is a fungal skin disease that can spread from dogs (and other animals) to humans. Circular, scaly patches of skin with broken hairs characterize the disease, although some dogs can be asymptomatic carriers. It is always advisable to get any unusual skin lesions on your dog examined by a vet. If you develop similar skin problems, it could be important to tell your own doctor about any contact with animals.
A much rarer human skin disease that can be spread by dogs is leishmaniasis. Leishmania parasites are transmitted by sandflies, which feed on dogs and can then pass the parasite onto humans. Leishmania parasites are only found where the sandfly vector can survive, which currently is in countries with tropical/subtropical climates and southern Europe. Affected humans with the cutaneous form of disease develop skin nodules which may progress to ulcers. With the visceral form of disease, the spleen, liver and bone marrow is affected. Infected dogs can develop skin lesions, weight loss and general malaise, although a proportion do not show any clinical signs.
What can I get from dog bites?
Injuries from dog bites, accidental or otherwise are a very common presentation to A&E. Aside from physical and mental trauma, infections are a major problem with animal bites. Dogs carry a wide variety of bacteria in their mouths, which will be transferred into the injured tissue. Bartonella henselae is a common bite-wound bacteria that typically causes fever and malaise, but can spread to the brain (meningitis) and heart (endocarditis). Pasteurella multocida and Staphylococcus species can also be a problem.
Rabies is a virus that is well known for being spread by crazed or ‘rabid’ biting dogs. Rabies universally causes death once clinical signs develop. Thankfully, rabies was last reported in dogs in the UK almost 100 years ago. It is still present in many countries across the world, although excellent vaccination campaigns have brought the virus under control in many areas. It always makes sense to be wary of strange dogs in rabies-affected countries.
Can I get worms?
Worms are common amongst dog populations, but usually they are only a ‘cosmetic’ problem and rarely cause disease. The simple roundworm Toxocara canis lives quietly in the guts of dogs, but if it gets into humans it can cause a systemic disease known as visceral larval migrans. The migrating worm larvae can spread to the eye and cause blindness. Tapeworms, such as Echinococcus species can also cause systemic disease in humans. Prevention of human worm disease is simple; treat your dog regularly with anti-parasitic drugs, and as worms are found in faeces, ensure no direct contact with stools.
Is dog urine infectious?
The only significant pathogen that can be secreted in urine is the bacterium Leptospira. This bacterium causes liver and kidney disease in dogs, although most dogs are vaccinated against this pathogen, so disease is rare. Infection in humans is typified by a febrile, self-limiting illness, and only 50-60 cases are diagnosed in the UK each year. It is important to note that most of these infections are actually contracted from livestock or contaminated water, and only very rarely dogs.
What if I’m immunosuppressed?
Humans with suppressed immune systems are at greater risk of contracting infectious diseases from dogs. All the above infections are often stopped in their tracks by a functioning immune system, but immunosuppressed individuals are more likely to become infected by the infectious agents that dogs secrete in their stools, urine, saliva or carry on their skin. In addition, immunosuppressed people can become susceptible to zoonotic infections that do not usually cause disease in humans. An example of this is Bordetella bronchiseptica, the bacterial component of the canine respiratory disease ‘kennel cough’. Dog owners on immunuppressive therapy can develop pneumonia if their dogs become infected by B. bronchiseptica. Strict hygiene is therefore essential when dogs are in contact with immunosuppressed people, and contact with obviously ill dogs should be avoided if at all possible. If these practices are followed, then there is no reason why dogs cannot be allowed to brighten up the lives of ill individuals.
Can I spread infections to my dog?
Reverse zoonoses are infections that are transmitted from humans to animals. Though these are relatively uncommon, bacteria and viruses can be inadvertently spread from humans to dogs. One of the most common reverse zoonoses is believed to be methicillin-resistant Staphylococcus aureus, better known as MRSA. Staphylococcus is normally an innocuous skin bacterium that only becomes a problem if skin barriers are breached, such as through surgery, and if no-one knows it is there; that is, incorrect antibiotics are given. A recent study from the US found 2.5% of 487 dogs tested were positive for human MRSA. This is clearly only a small proportion, but it is certainly worth considering the risk to your pet dog if any household members are diagnosed with MRSA.
Influenza is another common human infection that has been proven to spread to dogs. Following the 2009 swine flu epidemic, dogs in China with respiratory disease were found to be infected with the human H1N1 pandemic strain. Given the high incidence of influenza in people and the rare cases in dogs, reverse zoonotic flu infections are clearly the exception and not the rule. In addition, influenza typically causes only mild disease in dogs and thankfully does not spread easily from dog to dog.
The infection benefits from dog ownership
Whereas there are many theoretical infection risks from allowing a dog to share your home, an Australian study has shown an overall decreased risk of infection in dog-owning families. When examining the incidence of diarrhoea amongst 965 children, living with a dog significantly decreased the risk of disease.
In summary, despite the potential infection risks and possible benefits that might arise from sharing your home with a dog, I will always agree with author Bob Barker, who said that "a person who has never owned a dog has missed a wonderful part of life..."