St. George’s University and Windward Islands
Research and Education Foundation,
Grenada, West Indies
Objectives: To determine the prevalence of Toxocara canis in puppies under 1 year of age and to understand the human-puppy relationship and risk behavior and also to determine the knowledge, attitudes and practices regarding its zoonotic potential amongst puppy owners, veterinarians, physicians and 6th term veterinary students
Methods: A cross-sectional prevalence study in puppies less than one year of age was conducted by collecting fecal samples from puppies during the period January through November 2017 from households in all 6 parishes throughout Grenada. These were tested for T. canis positivity. The owners of the puppies as well as veterinarians, physicians and veterinary students were surveyed using a questionnaire.
Results: A total of 306 fecal samples were collected. 147 (48%) puppies were positive for T. canis; of these 142 puppies were < 7 months of age and 5 between 7-10 months. 35 owners never treated puppies, 97% never spayed or neutered their adult dogs, 68% allowed their puppies to roam free and only 9% cleaned up after their puppies. 16.7% of physicians spoke with patients about zoonotic disease potential, none have ever diagnosed toxocariasis in adults or children in Grenada, and 60% cited ‘no concern’ regarding the zoonotic potential of T. canis to their patients. 6.5% of veterinary students ranked a ‘significant concern’ regarding the zoonotic potential of T. canis, 3.3% were aware of four clinical toxocariasis syndromes in humans and 64.5% correctly identified the main route for human transmission.
Conclusion: Puppies present a zoonotic threat to public health in Grenada. The zoonotic nature of T. canis is predominantly unknown to the public and there is a need for educating the general public regarding its public health importance.
Toxocara canis is a globally distributed gonochoristic helminth and cause of one of the most neglected zoonotic diseases in humans, toxocariasis [1-3]. T. canis reaches maturity in the intestine of its definitive host, Canis familiaris. Toxocara eggs are voided into the environment through dog feces where they become infectious to a wide range of paratenic hosts including humans . Through fecal-oral transmission, human can be accidental hosts introducing infectious T. canis eggs into their system. Once infected, the resulting larval migration inside the human circulatory system can result in any of the four clinical syndromes of toxocariasis; visceral larva migrans (VLM) or visceral toxocariasis (VT), ocular larvae migrans (OLM) or ocular toxocariasis (OT), covert toxocariasis (CT) and neurotoxocariasis (NT). Children are most at risk of developing toxocariasis due to their closer contact with contaminated soil in public parks, playgrounds and beaches  and their lack of proper hygiene . In the U.S., seventy individuals lose their sight every year as a result of OLM and most of them are children  (Epidemiology & Risk Factors, 2016). Climate is another risk factor for toxocariasis. Hot and humid environments such as Grenada, help to keep T. canis eggs viable in the soil and therefore increases toxocariasis prevalence in all its forms .
Four studies in the past eight years have explored the parasite prevalence in adult dogs in Grenada [8-11]. Each study determined the parasite load of dogs across Grenada and found evidence of a variety of helminths including: Ancylostoma caninum, Diofilaria immitis, Strongyloides stercoralis, Trichuris vulpis as well as T. canis. No prevalence study has yet to explore the prevalence of T. canis exclusively in Grenada nor how puppies play an especially important role in the transmission of related diseases such as toxocariasis. International prevalence studies on helminths, and on T. canis in particular, have included fecal samples from a range of dogs across different age brackets. When prevalence data has been stratified using the age of the dogs sampled, dogs under 12 months of age demonstrated the highest prevalence of T. canis. [10, 12-14]. It has been mentioned in the results section of many studies that puppies are disproportionately responsible for contaminating the environment with T. canis [8, 15-16]. One study determined that puppies can shed more than 294,000 T. canis eggs in a single stool . To our knowledge there has been no published study to focus on this age group of dogs exclusively and its direct connection to toxocariasis in the Caribbean.
With this background, the current study was undertaken to estimate the prevalence as well to elicit the knowledge regarding T. canis in Grenada.
A cross sectional study design was used and samples were collected from all 6 parishes in Grenada between January and November, 2017. Figure 1 delineates the exact locations of the samples taken.
Figure 1: Map of Data Collection Sites in Grenada
According to The United Nations Educational, Scientific and Cultural Organization, the average number of puppies that survive per litter in the tropics is 4. By multiplying our average numbers of litters by four, the sample size of n = 280 was calculated. This total was rounded up to allow for additional litters for a total sample size of n = 300. Criteria for sample collection included: (1) puppies must have been born in Grenada; (2) puppies must be less than 12 months of age; (3) puppies must not have had any deworming treatment leading up to the time of sample collection.
Collection of fecal samples included on the ground fecal samples of at least 2g were collected by the primary investigator with wooden spatulas. Samples were preferentially derived from the top of each fresh deposit to avoid ground contamination. The samples were then placed in plastic containers, submerged in 10% formalin, labelled with sample number and stored at 4C until examination. Fecal samples were analyzed using fecal floatation technique with centrifugation. All parasite eggs, including T. canis were identified by visualization and recorded. Questionnaires were distributed to puppy owners (n =35), veterinarians (n = 8), physicians (n = 35), and Term 6 students (n = 60) currently enrolled at St. George’s University. The results were used to assess the knowledge, attitudes, and practices surrounding zoonoses in Grenada and in particular, T. canis. Data were analyzed using Microsoft Excel.
A total of 306 fecal samples were collected during the study period. The age of puppies ranged from < 7 months old until 10 months of age. The age distribution of the samples is shown in Figure 2.
Figure 2: Age Distribution of Samples
Overall, 147 (48%) out of the total 306 puppies were positive for T. canis. Of these 147, 142 (96.6%) of puppies < 7 months of age and 5 (3.4%) were older than 7 months. The oldest puppies found infected were 10 months of age (Figure 2).
On a scatter plot comparing the proportions of positive T. canis samples by age of puppies, depicted in Figure 3, a smooth trend line starting at 2 weeks of age (50%), increases and peaks at 8 weeks of age (65%) and then steadily deceases.
Figure 3: Age prevalence of T. canis found in puppies in Grenada
The odds of a sample being positive for T. canis are 14.2 times higher in puppies < 7 months of age than puppies between 8-12 months of age (Odds ratio of 14.2; 95% confidence interval between 5.49 and 36.75, p < 0.05). Thus there was an overwhelming evidence that puppies under 30 weeks of age or younger will have a very high probability of testing positive.
Of the puppy owners interviewed, 100% of them never treated their puppies, 97% had not spayed or neutered their adult dogs, and 68% allow their puppies to roam free and only 9% clean up their puppies regularly. Of the physicians interviewed, 16.7% claimed to talk to their patients about zoonotic disease potential, none have ever diagnosed toxocariasis in adults or children in Grenada, and 60% cited ‘no concern’ regarding the zoonotic potential of T. canis to their patients. Of the 6th term students interviewed, 6.5% ranked a ‘significant concern’ regarding the zoonotic potential of T. canis, 3.3% were aware of the four clinical toxocariasis syndromes in humans and 64.5% correctly identified the main route of transmission to humans.
Toxocariasis, caused by Toxocara larvae, manifests as a variety of clinical presentations in humans. These include larva migrans, neurotoxocariasis and common toxocariasis. Toxocara canis is one of the most widespread public health and economically important zoonotic parasitic infections. In addition to humans vectors include dogs, cats and wild canines such as foxes. Toxocariasis has been shown to be especially prevalent among children from socio-economically disadvantaged populations both in the tropics and sub-tropics and as well as industrialized nations. Ingestion of embryonated eggs or larvae is leads to infection, although many patients are asymptomatic. Challenges to clinical diagnosis and management include expensive diagnostic tests such as serological, molecular and/or imaging tests, which are often of limited availability is the most affected communities. Treatment includes anthelmintic, including albendazole, thiabendazole and mebendazole together corticosteroids. 
This study is the first of its kind in Grenada and had some important findings related to the epidemiological characteristics of T. canis prevalence which can have definite public health implications.
The age of the puppies showing positivity of T. canis showed a clear trend starting at 2 weeks of age (50%), which increases and peaks at 8 weeks of age (65%) and then steadily deceases. The biological explanation of this trend is explained by the transmission routes of T. canis. As previously discussed, T. canis can be transmitted vertically from pregnant female to her offspring in utero via transplacental transmission or soon after parturition via trans-mammary transmission. Puppies are born infected with T. canis or they acquire the infection during lactation or via fecal oral transmission from the environment. Toxocara is successful at completing its life cycle in the dog and as is illustrated in Figure 3, does so predominantly in young puppies < 7 months of age. The declining trend can be explained by the life span of adult T. canis worms which is approximately 7 months of age. Without re-infection, the T. canis worms will die off naturally leading to a reduction and eventual elimination of egg excretion.
In terms of general parasite prevention strategies, a deworming treatment schedule conducted by a veterinary professional is the gold standard recommendation. However, due to the socioeconomic realities in Grenada, deworming is not a viable preventative option. Most community members do not have access to treatment or the financial means to call upon a veterinarian for deworming treatment. Most dog owners lack the resources to purchase treatment from a pharmacy to administer the treatment themselves. Alternative strategies need to be recommended.
Our recommendation therefore, is that dog owners be encouraged to pick up after their puppies and dispose of the feces appropriately. T. canis eggs need 2-3 weeks in the environment before they are able to mature into their infective L3 stage. Removing the feces immediately after excretion is an affordable and highly effective method of preventing environmental contamination. Educational efforts need to focus on this approach. As was seen in the questionnaires, only 9% of dog owners picked up after their dogs. Therefore, there is potential to make a big difference in this area.
In addition to research regarding the epidemiology of T. canis, new strategies for diagnosis and management are another important area of research.
Puppies infected with T. canis excrete many millions of eggs daily, and given the high prevalence coupled with prolonged shedding, present an enormous zoonotic threat to public health across Grenada. Knowledge of this threat is poorly appreciated by all groups interviewed and much can be gained by improving knowledge in these areas. Research on the prevalence of toxocariasis syndromes in humans in Grenada is required to complete the one health triad and gain an accurate picture of the public health threat in its entirety.
A number of countries have implemented reproductive control programs in owned and stray dogs to reduce the number of young dogs in the population. These programs would positively impact upon T. canis transmission since the parasite is most fecund and prevalent in puppies. Other control measures for T. canis include the regular and frequent anthelmintic treatment of dogs and cats, starting at an early age, education and enforcement of laws for the disposal of canine faeces, dog legislation and personal hygiene. The existence of wild definitive and paratenic hosts complicates the control of T. canis. Increasing human and dog populations, population movements and climate change will all serve to increase the importance of this zoonosis. This review examines the transmission, diagnosis and clinical syndromes of toxocariasis, its public health importance, epidemiology, control and current research needs.
To conclude, Toxocariasis is a poorly researched and insufficiently managed disease, particularly in poorer nation states. Although toxocariasis is endemic in many countries, due to its generally mild disease burden among otherwise healthy individuals it remains poorly addressed. Further research such as this in order to properly assess the degree of pervasiveness of T. canis and other Toxacara is an important technique for understanding the societal burden there is. Other areas of research which are necessary in order to effectively mitigate this disease includes determining effectiveness treatments and methods of eliminating the pathogen burden on populations, as well as an appropriate method for screening and maintenance of public health with respect to this disease. Some promising methods include puppy fecal screening, regular antihelminth treatments, and public awareness about this pathogen and its strong association particularly with young puppies in order to better protect people.
- Macpherson CNL. Human Behaviour and the Epidemiology of Parasitic Zoonoses. International Journal of Parasitology 2005; 35(11-12): 1319-1331
- Macpherson CNL, Meslin F-X, Wandeler AI. Dogs, Zoonoses and Public Health. CAB International, 2013;
- Macpherson CNL. Epidemiology and public health importance of toxocariasis: a global perspective. International Journal for Parasitology 2013; 43: 999-1008.
- Maizels RM, Tetteh KA, Loukas A. Toxocara canis: genes expressed by the arrested infective larval stage of a parasitic nematode. International Journal for Parasitology 2000; 30(4), 495–508
- Hotez PJ, Wilkins PP. Toxocariasis: America’s Most Common Neglected Infection of Poverty and a Helminthiasis of Global Importance? PLoS Negl Trop Dis 3(3): e400. https://doi.org/10.1371/journal.pntd.0000400
- Overgaauw PA, Nederland V. Aspects of Toxocara Epidemiology: Human Toxocarosis. Critical Reviews in Microbiology, 1997; 23(3): 215-231
- Epidemiology & Risk Factors of Toxocariasis. Available at URL: http://www.cdc.gov/parasites/toxocariasis/epi.html
- Coomansingh C. Helminth Parasites in Dogs in Grenada. 2008; Dissertation for MSc Veterinary Parasitology, St. George’s University, Grenada
- Pinckney RD, 2008, Emerging Parasites of Public Health Significance. Available at URL: http://helminto.inta.gob.ar/Congreso%20Brasil%202008/EMERGING%20 PARASITES%20OF%20PUBLIC%20HEALTH%20SIGNIFICANCE.pdf
- Tiwari K, Thomas JCD, De Allie C, Chikweto A, Hegamin-Younger C, Sharma RN. Prevalence of Gastro-Intestinal Parasites in stray dogs (Canis familiaris) from Grenada, West Indies. Journal of Animal Research 2006; 6(1): 1-5
- Neill KG. The Public Health Importance of Dogs, Grand Anse, Grenada. 2009; Dissertation for MSc Veterinary Parasitology, St. George’s University, Grenada
- Robertson ID, Irwin AJ, Lymbery RC, Thompson A. The role of companion animals in the emergence of parasitic zoonoses. International Journal for Parasitology, 2000; 30(12-13), 1369–1377
- Rubela D, Santillánb GZG, Wisniveskya C. (2003). Epidemiology of Toxocara canis in the dog population from two areas of different socioeconomic status, Greater Buenos Aires, Argentina. Veterinary Parasitology, 2003; 115(3): 275-286
- Akeredolu AB, Sowemimo OA. Prevalence, intensity and associated risk factor for Toxocara canis infection in Nigerian dogs. Journal of Parasitology and Vector Biology, 2014; 6(8), 111-116
- Vikrant S, Jaiswal A. Prevalence of endoparasitic infections of non-descript dogs in Mathura, Uttar Pradesh. Journal of Parasitic Diseases, 2013; 39(3), 491-494
- Martinez-Moreno FJ, Lopez-Cobos E, Becerra C, Acosta I, Martinez-Moreno A. Estimation of canine intestinal parasites in Cordoba (Spain) and their risk to public health. Veterinary Parasitology, 2007; 143(1), 7-13
- Thomblison P. Pets, worms, and little people. Contemporary Pediatrics 2003. Available at URL: https://www.contemporarypediatrics.com/pediatrics/pets-worms-and-little-people