Toxocara Canis Survey

The incidence of Toxocara Canis Eggs in a survey of dog faeces in regularly dewormed dogs

George Leslie BVMS MRCVS

Summary

This is an account of a survey to check for the presence of Toxocara canis eggs in 256 faeces samples obtained from dogs belonging to owners who had dewormed their dogs at least twice in the previous year.

No attempt was made to insist on any particular anthelmintic. Owners were asked to choose their own dewormer provided it was a recognised proprietary brand purchased either from their veterinary surgeon, chemist, pet shop or supermarket.

All samples were checked by the University of Glasgow Department of Veterinary Parasitology under the direction of Professor James Duncan.

In this survey one sample contained Toxocara canis eggs at 50 eggs per gramme of faeces. This gave an overall incidence of 0.39% which is considerably lower than other reported surveys.

Introduction

Toxocariasis has been recognised as an important zoonosis associated with the dog since the early 1950's when Visceral Larva Migrans was first described by Beaver and others (1952) with Toxocariasis being reported by Nicholls (1956). The first human cases in the U.K. were recorded by Ashton (1960).

The main source of human infection is recognised as arising through contact with contaminated soil. Toxocara canis eggs passed in dog faeces can remain viable in the soil for over a year under the usual environmental conditions which prevail in the U.K. (Girdwood 1986). At temperatures of between 10 and 35 degrees Celsius and in conditions of relatively high humidity eggs become infective in two to three weeks (Girdwood 1986). 

Numerous attempts have been made to assess the potential risk to humans by analysing soil samples from parks and by analysing samples of dog faeces. 27% of soil samples were found to be positive for Toxocara eggs in a survey from London parks (Borg & Woodruff 1973). 5.2% of samples from parks in north west London were positive (Pegg 1975). Quinn, and others in 1980 found 12% of samples from Glasgow parks and 7.3% of other parks in Strathclyde to be positive. 

In 1987 another survey of parks in east London found 66% positive for Toxocara eggs (Snow, and others 1987). Only 21 % of these positive samples however, appeared to contain viable eggs and half the samples had fewer than 2.1 eggs per 5g of soil believed to be the density required for human infection (Woodruff and others 1981).

Surveys of dog faeces produced between 3.6 and 7.5% positive samples in dogs of over one year old, and 19.7 and 34% of positives for dogs less than one year old (Jacobs & Pegg 1976, Turner & Pegg 1977). Else and others (1977) surveying the faeces of 272 dogs from veterinary practices in East Anglia found 30% of dogs under 1 year positive and 6.8% of dogs over 1 year. Surveying only dogs over 6 months of age found incidences of 4.5% to 9.1 % (Jacobs, and others 1977).

Norval and Scott (1976) in an unpublished survey of dog faeces from Edinburgh parks found 10% positive, while Girdwood (1978) found 21% of randomly collected samples of dog faeces in Glasgow to be positive. More recently (1990) a survey carried out by Edinburgh District CouncilEnvironmental Health Department found 4.09% positive samples from dog and fox faeces collected at random form Edinburgh parks. The same survey also checked pet dogs in boarding kennels and stray dogs from the Dog and Cat Home and found both groups of adult dogs showed 6.6% of positive samples.

It was against this background that the current survey was undertaken to check on the efficacy of routine deworming of pet dogs.

Materials & Methods

Samples were sought from dog owners nominated by local veterinary surgeons, dog training clubs and members of the Canine Concern Scotland Trust (an organisation established to promote responsible dog ownership) who were sponsoring the current survey.

It was decided to exclude puppies under the age of 4 months since responsible dog owners were less likely to be exercising them in public places. Dog owners were asked to declare that they had dewormed their adult dog at least twice in the year before the sample was collected, or if their dog was less than one year old that they had dewormed it every 14 days as a puppy and at least once more since it became six months old.

Owners were asked to fill in a questionnaire to give details of breed, age, sex, neutering, pregnancy, phantom pregnancy, recent health problems and deworming. Owners were also asked to use their normal deworming remedy provided it was a recognised proprietary remedy purchased from veterinary surgeon, chemist, pet shop or supermarket. All faeces samples were sent to the University of Glasgow Department of Veterinary Parasitology for examination.

Details of Survey

256 Samples were obtained as follows:
Area: West Central Scotland 240 / East Scotland and NE England 16 
Source: Veterinary Practices 141 ~ Dog Training Clubs 71 ~ Members of CCST 44
Sex: Male 113 (25 Neutered) / Female 143 (85 Neutered)
Age: Average age - 4.09 years. (4 months to 16 years) 24 samples from dogs less than 1 year old

Deworming Routine

Average number of dewormings: 3.2 per year (all dogs) - 4.8 per year (under 1 year old)
Average time from last deworming to sampling: 2.8 months
Products used: Piperazine - 209 
Praziquantel/Pyrantel Embonate/Febantel - 17
Fenbendazole - 15
Nitroscanate - 9
Unspecified - 6

One dog in the survey recorded the presence of Toxocara canis eggs in it's sample at a rate of 50 eggs per gramme of faeces. This dog was a year old neutered female which had been dewormed with nitroscanate 8 months before sampling. The incidence of the presence of Toxocara canis eggs in the faeces of the dogs sampled in this survey was 0.39%

Discussion

The faeces samples which were included in this survey did not come from a typical cross section of dogs. Cross-bred dogs at 14% were under-represented while among the others Border Collie/Working Sheep dog 14.8%, Labrador 10.5%, Golden Retriever and German Shepherd both 7%, and Rottweiller 4.3% seemed to be overrepresented. To some extent the high representation of these breeds may have been influenced by the fact that 27.7% of samples were collected from dog training clubs.

The most significant point to be noted about the dog owners who co-operated in this survey was that they were to a large extent more interested, caring and responsible than the average dog-owner. 17% of samples came from dogs belonging to owners who were members of the Canine Concern Scotland Trust (an organisation devoted to promoting responsible dog ownership); 27.7% from dog owners who were active members of dog training clubs; and 55% were drawn from clients nominated by 3 veterinary practices. The majority of those drawn from the clientele of veterinary practices were attending their veterinary surgeon for annual booster vaccinations and routine check-ups while 21 of those samples were collected from clients attending a practice meeting.

From the point of view of the aim of this survey which was to assess the incidence of Toxocara canis eggs in the faeces of regularly dewormed dogs the dog owners who co-operated achieved an average of 3.2 dewormings in the year prior to sampling.

The one example in this survey which was positive for Toxocara provided 50 eggs per gramme of faeces. This does not suggest a heavy infestation. The Edinburgh District Council survey which gave details of egg counts produced a range of from 1 to 3379 eggs per gramme of faeces with an average of 319.

The incidence of 1 positive sample in 256 is 0.39% which compares with the average of 9% in the other reported surveys mentioned in the introduction. Even the most recent comparative survey (the Edinburgh survey) showed 4.09% which is an incidence more than 10 times greater than this present survey.

This survey included only 24 dogs of less than 1 year old (9.4% of the total). The age range of this sample was from 4 to 11 months with an average age of 7.5 months. The average number of dewormings in this group was 4.8 times. No positive samples were found in this group, although previous surveys had suggested that the incidence in this age group would be 3 to 4 times higher than average.

It is impossible in this type of survey to make any comments on the efficacy of the different anthelmintics used. If one examines the intensity of the anthelmintic regimes used, the one positive sample showed a gap of 8 months between last deworming and sampling (compared to the survey average of 2.8 months) and had only been dewormed twice in the previous year (compared to the survey average of 3.2). The only other point of interest about the different anthelmintics used is that 26% of the dog owners opted for a palatable remedy and those who did so dewormed their dogs on an average of 5 times per year. This could become significant if attempts are to be made to increase oral deworming of pet dogs.

The main conclusion to be drawn from this survey is that Toxocariasis as a zoonosis could be virtually eliminated by regular and repeated use of the presently available anthelmintics. The following recommendations could be made.

1) Increase the education of dog owners about the necessity of controlling dog worms.

2) Make it widely known that two dewormings per year is the absolute minimum which should be undertaken by all dog owners, with a recommendation that four or more dewormings per year would be desirable. This recommendation should apply to all available oral anthelmintics used in the dog.

3) Make oral anthelmintics for dogs (preferably palatable) as widely available as possible, and backed by advertising campaigns at least as effective as those for dog food or vaccinations.