“I got diamonds on the soles of my shoes…” Remember that Paul Simon song?
Well, if you have dogs you might have something a little less desirable than diamonds on your shoes – and so might your dogs – according to a new study published in the Journal of Veterinary Parasitology by Panova and Khrustalev (2018).
Read on to find out what…
HOW THE STUDY WAS DONE
The study compared 2 groups of individuals living in the same urban area of Moscow in Russia – a dog-owning/walking group and a non-dog-owning group. The dog-owning/walking group consisted of 8 dogs (Labradors, Labrador Retrievers and Airedale Terriers) and their owners who walked their dogs daily in local public recreational grassy areas. All dogs were healthy and lived with their humans in city apartments. The non-dog-owning group consisted of 6 employed adults that walked to work and back each day along tarmacked pedestrian pathways.
The study was carried out over a period of about 2 months.
For the dog-owning/walking group, immediately on their return home after a walk, the paws of each dog were washed in clean water and the washings collected in a large bowl. Likewise, the bottoms of the owners shoes were washed and the washings collected.
For the non-dog-owning group, the bottoms of their shoes were washed and washings collected on return home from work each day as for the dog-owning/walking group.
Over the 2-month period –
- 5 samples of the dog’s feet washings were collected.
- 7 samples of the dog-owners shoes were collected.
- 7 samples of the non-dog owners shoes were collected.
- Finally, a single faeces sample was collected from each dog and sealed in a plastic pot.
Each of the samples were sent off to a parasitology laboratory for analysis where they were examined within a few hours to avoid degradation of the contents.
WHAT THE STUDY FOUND
Key findings –
- Dogs faeces samples: None of the dogs in this study were infected with Toxocara roundworms.
- Dogs feet washing samples: Toxocara eggs were found in 19.4% (1 in 5) of the dogs.
- Dog-owners shoes washing samples: Toxocara eggs were found in 11.4% (1 in 10) of the shoes.
- None-dog-owners shoes washing samples: No Toxocara eggs were found in any of the shoe washings.
- The concentration of eggs on dogs feet were twice that found on their owners shoes.
- Most of the eggs found (67% Toxocara canis, 70% Toxocara cati) in the washings were viable and at different stages of embryonic development.
- In addition to Toxocara eggs, the eggs of other parasites were also found in both paw and shoe samples. These were Trichuris (whipworm), Ancylostoma (hookworm), Capillaria (dog bladder worm) and Demodex mites.
LIMITATIONS OF THE STUDY
There are a number of significant limitations of this study –
- The number of dogs and humans involved was small and restricted to a small urban area. A nationwide study across more geographically and demographically diverse urban communities would be more representative.
- There is no information provided about the cultural habits, customs and attitudes of the population with regard to cleaning up dog faeces by owners.
- There is no information about whether or not dogs were allowed off lead.
- There is no information about the estimated density of the dog population in the local area, nor how much space the dogs and owners have to walk in.
- It is not clear whether any of the 8 dogs in the study shared the same home, or represented 8 individual homes.
However, the findings are important and this study is easily duplicatable on a larger scale in countries near you that better reflect your particular area of concern.
RELEVANCE OF THE STUDY TO US
This study highlights a perhaps forgotten mode of transmission of Toxocara from dog-to-dog and – crucially – from dog-to-human.
The point here is that these dogs and their humans were not ‘covered in faeces’. Their owners will have been as careful as the rest of us to avoid stepping in a pile of poo, and when they did, they would have wiped it off as best they could to avoid taking it home with them.
Rather, the dogs paws and owners shoes carried home mud contaminated with traces of dog faeces containing Toxocara eggs.
NONE of the dogs in this study were themselves shedding Toxocara eggs in their faeces, probably because the owners were worming them regularly.
Infection of children “through doggie’s paws” and “through Mum’s and Dad’s shoes” in the safety of their own homes may be as important a mode of transmission of toxocariasis as infection through playing in a faeces-contaminated public grassy area.
MORE INFORMATION ABOUT TOXOCARA
Toxocara is a nematode worm and there are 2 species of importance to us – T canis and T. felis, where the primary hosts are dogs and cats respectively. The complexity of Toxocar’a life cycle make is a potentially serious threat to human and canine and feline health. The reason for this is it’s multi-modal means of transmission from one animal to another.
Let’s start with dogs.
Dogs can acquire T. canis in 4 different ways –
- By swallowing an egg passed in the faeces of another dog: Once in the dog the larvae emerge from the eggs, burrow their way through the small intestinal wall and into a blood vessel where they are then carried around the body in the circulation. In young puppies, the larvae hop out of the circulation whilst passing through the lungs where they are coughed up and swallowed. Once in the gastrointestinal tract, the larvae mature into worms and start shedding eggs of their own which are passed in the puppy’s faeces. In adult dogs and pregnant bitches, the larvae take a different path. They hop out of the circulation into other organs and tissues anywhere in the body where they form cysts and go dormant.
- By being infected by your mother in the womb: If a bitch has T. canis cysts lying dormant around her body, these can ‘wake up’ during gestation and find their way to the uterus via the placenta where they hop over into the unborn puppies. This is an important and common mode of infection in puppies and these worms mature and start shedding their own eggs in the pup’s faeces about 2 weeks after birth.
- By being infected by your mother’s milk: T. canis cysts in the bitch can also emerge and migrate into the bitches mammary glands where they are ingested by her suckling puppies. Waves of larvae can continue to emerge from cysts and infect the milk for up to 5 weeks post-partem.
- By eating another animal infected with T. canis: When animals such as mice or a rats, inadvertently swallow a T. canis egg, the larvae can migrate into tissues and organs all of the body, encyst themselves and become dormant. As the animal is digested in the dogs intestinal tract, the larvae emerge and mature into adult worms where they start shedding eggs that are passed in the dog’s faeces.
How about cats? Well, similar story as for dogs, but kittens do not get infected via the placenta in utero.
Once being passed out into the environment in the faeces of dogs or cats, the eggs take a few weeks to develop into embryos after which they are mature enough to infect another animal. They can remain alive and in this infectious state for years, depending on climate, soil conditions etc. The important point here is that you can’t become infected from fresh dog poo.
So, what’s the problem for humans?
POTENTIALLY, A BIG ONE! Here’s why –
Humans are not the natural hosts for Toxocara so if ingested, viable larvae never mature into adult worms. However, if they enter the circulation where they are carried all over the body, they can wreak havoc by burrowing into organs and tissues where they manage to get a foothold. This can cause serious damage – for example in the eyes, spinal cord, brain, liver and so on.
On the other hand, they may cause very little damage and you may not even know they were there. Or, you could have unexplained flu-like symptoms – aches and pains, fever, tiredness etc.
It’s a lottery.
It all depends on how many larvae there are and where they end up. T. canis is more commonly caught by humans than T. felis, but either way, the disease they cause is called toxocariasis, and you really don’t want to catch it.
OK, what’s the risk then?
Well, that depends on WHO you are, or more specifically, how old you are.
Here are the numbers –
- Older children and adults are at lower risk, and in this group infection is most likely through eating raw or undercooked, larvae-infected meat such as beef, lamb, chicken etc.
- Very young children up to 4 years old are at greatest risk because they explore the world predominantly with their hands and mouths so inadvertent ingestion of viable eggs in contaminated areas, such as grassy dog-walking parks, is the most common route of infection.
- Paws and shoes. This new study summarised above, shows that dogs paws and humans shoes are another potential risk factor in bringing the ‘grassy dog-walking park’, and potentially viable Toxocara eggs back into the home.
- Is a dog carrying Toxocara worms in its gut also a risk? The thought of having a dog in the house with Toxocara eggs stuck on the fur around its bottom after defaecating in the garden is not a pleasant one. And indeed, picking up viable eggs from dog’s fur has been shown to be a route of infection (Aydenizöz-Özkayhan et al., 2008). BUT, here’s the thing. First, Toxocara eggs are not viable when they are first passed in the dog’s faeces. They need to go through a period of maturation that can take 2 weeks to months, depending on the environment, before they become a risk. In addition, the eggs are really small and sticky so if stuck to a dog’s fur around its bottom, or anywhere else for that matter, they tend to stay there. A human has to ingest a LOT OF EGGS – thousands, tens of thousands perhaps – for an infection to become established. Remember that humans are an alien environment for this parasite, so even when viable larvae are ingested, most of them are killed off.
Yes, but what’s the real overall risk?
Well, that depends on WHERE you are, or more specifically, how poor you are.
Here are the numbers –
- Risk, and most importantly severity of toxocariasis depends on numbers of viable Toxocara eggs ingested. Worldwide, the population exposure to Toxocara infection is around 4%–31% in developed countries and up to 86% in third world tropical countries (Gillespie and Pearson, 2003). NOTE that these numbers reflect antibodies to Toxocara found in blood screening programs. Many of these infections do not cause disease. The presence of antibodies simply reflect an infection sometime in the past and that the body’s own immune system may well have killed off any viable larvae soon after ingestion. In short, the presence of antibodies are just a ‘fingerprint’ left at the scene of the crime. The criminal has long-gone – perhaps years before.
- The most serious form of infection occurs when thousands, perhaps tens of thousands of larvae are ingested and then many of them migrate into organs and tissues all over the body and cause a lot of damage. This is called visceral larva migrans (VLM). VLM is incredibly rare in countries like UK because dog owners pick up and bin their dog’s poo and they worm their dogs regularly. So the number of Toxocara eggs lurking around in recreational spaces is small. The situation is of course different in poor countries where there are many free-roaming dogs co-habiting the villages and towns where high-density populations of humans live, and where education programmes are not readily available.
- Toxocariasis can also occur through the ingestion of fewer numbers of viable Toxocara eggs. The most common reported example of this is ocular larva migrans (OLM) where 1 or more larvae migrate into the retina of one eye. There are few studies of this because the condition is rare. However, one relevant and well-conducted study has emerged from Ireland, and it reports an infection rate among school children of 13 cases per 1 million (Good et al., 2004).
- As mentioned above, toxocariasis can also be implicated in other miscellaneous forms of disease such as asthma (Li et al, 2014), unexplained chronic cough (Bede et al, 2008; Walsh, 2010) and cognitive dysfunction (Walsh and Haseeb, 2012). However, many studies like these ones, are limited by their design and the results should be interpreted with caution.
Humans, dogs and cats – and their worms – have lived closely together for tens of thousands of years. And you know what? We’re still here and we’re still standing! Fitter, healthier and thriving BECAUSE of our partnership with pets. So, my message is a simple one –
LOOK AFTER YOUR PETS AND LOVE THEM, AND THEY’LL LOVE YOU BACK.
© copyright Robert Falconer-Taylor, 2018
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Aydenizöz-Özkayhan, M., Yağcı, B.B. and Erat, S., 2008. The investigation of Toxocara canis eggs in coats of different dog breeds as a potential transmission route in human toxocariasis. Veterinary parasitology, 152(1-2), pp.94-100.
Bede, O., Szénási, Z., Danka, J., Gyurkovits, K. and Nagy, D., 2008. Toxocariasis associated with chronic cough in childhood: a longitudinal study in Hungary. Journal of helminthology, 82(4), pp.357-363.
Gillespie, S. and Pearson, R.D. eds., 2003. Principles and practice of clinical parasitology. 19a Toxocariasis. John Wiley & Sons.
Good, B., Holland, C.V., Taylor, M.R.H., Larragy, J., Moriarty, P. and O’regan, M., 2004. Ocular toxocariasis in schoolchildren. Clinical Infectious Diseases, 39(2), pp.173-178.
Li L, Gao W, Yang X et al (2014) Asthma and toxocariasis. Ann Allergy Asthma Immunol., 113(2): 187–92
Panova, O.A. and Khrustalev, A.V., 2018. Dog walking brings Toxocara eggs to people’s homes. Veterinary parasitology, 262, pp.16-19.
Walsh M, Haseeb M (2012) Reduced cognitive function in children with toxocariasis in a nationally representative sample of the United States. Int J Parasitol 42(13–14): 1159–63.