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The following is a critique of the paper: Wolfe, A., Wright,
L. P. (2003), Human toxocariasis and direct contact with dogs.
Veterinary Record 152:419–422.
Mention in the paper, of the role that current understandings
of the topic might have in determining legislation, raises
the question of the motivations prompting the work. Is it
possible that the conclusions that might be drawn from the
results have already been identified as part of an argument
to influence future legislation? However that may be, the
paper should be welcomed on its intrinsic merits.
The authors' assertion that workers have previously not looked
for T. canis eggs in dogs' coats may well be correct.
Certainly in recent years there has been little research at
all in the epidemiology of human toxocariasis in the UK. There
has even been at least one attempt to disassociate it from
dogs. It should be acknowledged that the present paper reports
opportunistic research and the gathered data are situational.
The geographic locations of the dogs surveyed are not given,
but among those tabulated, three areas predominate. Thus care
should be taken not to regard the data as universally applicable.
The dogs in rescue homes for example, are atypical of the
dog population as a whole, both in regard to circumstance
and history. Numerous as they are, they are still a minority.
This may seem a small point, but the authors do go on to entertain
more global considerations, firstly by pointing out the unsurprising
observation that the density of T. canis eggs in the
dogs' coats was far greater than has been reported in soil
samples and secondly, by suggesting that this undermines soil
as a route. It is obvious that a dog's perianal region is
closer to the origin of any eggs than is 'the soil'.
Whether a quantitative comparison is meaningful is open
to question. Why, for most citizens, should a higher density
on dogs constitute a main route of infection or a greater
degree of risk? The authors do indeed caution against assuming
that the route is a direct one. The case for seroprevalence
in humans (owners and non-dog-owners alike) being independent
of the density of eggs in soil samples though, is well argued
from the literature and could be accepted irrespective of
the authors' work. Correlating seropositivity with dog ownership
and demonstrating the presence of T. canis on dogs'
coats does provide a compelling argument that for dog owners
at least, their dogs are the disease vector. It could be argued
that of the non-dog-owners who contract the disease, the route
is in any case, most likely to be indirect. If it is to be
believed that the likelihood of a dog owner contracting the
disease is occasioned predominantly by the fact of ownership,
exposure peripherally would make little difference to him
whereas for a non-dog-owner, exposure would increase the risk
considerably. To illustrate this, a dog owner daily exposed
to Toxocara eggs does not increase the risk much if
he travels in a taxi recently sat in by an infected dog, but
for someone with little contact with dogs it could represent
the sum total of his (albeit brief) exposure.
One possible reason why instances can be quoted of T.
canis egg densities in soil samples not correlating with
seroprevalence could be that in urban areas particularly,
contact with the eggs is more direct, by way of faeces on
pathways and on the soil surface.
A number of prestigious names are acknowledged as having
helped. One of these belongs to the University of Bristol's
Department of Clinical Veterinary Science, a Division of which
sells Hill's pet food.
Summary
Viable Toxocara eggs were collected from coats of dogs
at various locations in Ireland and in the UK, reinforcing
the notion that toxocariasis in humans is acquired from dogs
rather than from wildlife, the soil or the environment in
general.
A Press Reaction
Predictably, the canine press has already quoted the report.
In Dog World, 16 May 2003 p4, under 'Toxocara:
a timely reminder', Mike Findley writes: 'The research shows
that detection of Toxocara eggs in hair samples from
round the back end is a much more reliable test than checking
soil samples, the method by which "incidence figures"
were previously obtained.'
Spin-off
In an article in New Scientist, 26 April 2003 'Dogs
in the dock over debilitating worm' Andy Coglan writes:
'The evidence points to toxocara infection being a
dog ownership problem. It has probably got little to do with
dogs fouling public places.' The piece includes a picture
of a dog with children on playground apparatus. What more
could campaigners for the abolition of dog fouling laws wish
for?
More spin-off
An article in Nursing Standard, 18 June 2003 'Risk
of toxocariasis simply from handling pet dogs' is more restrained
with: 'Previously dog faeces were thought to be the only route
of infection.' But the piece does use a picture of a child
cuddling a dog.
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