Toxocariasis: clinical aspects, epidemiology, medical ecology, and molecular aspects

Toxocariasis is caused by a series of related nematode species (ascarids) that routinely infect dogs and cats throughout the world. The eggs from these ascarids are common environmental contaminants of human habitation, due largely to the fact that many kinds of dogs and cats serve as pets, while countless others run wild throughout the streets of most urban centers. The eggs, present in dog and cat feces, become infectious within weeks after they are deposited in the local environment (e.g., sandboxes, city parks, and public beaches, etc.). Humans, particularly children, frequently ingest these eggs by accident and become infected. Infection in humans, in contrast to their definitive hosts, remains occult, often resulting in disease caused by the migrating larval stages. Visceral larva migrans (VLM) and ocular larva migrans (OLM) are two clinical manifestations that result in definable syndromes and present as serious health problems wherever they occur. Diagnosis and treatment of VLM and OLM are difficult. These issues are summarized in this review, with emphasis on the ecology of transmission and control of spread to both humans and animals through public health initiatives employing treatment of pets and environmental intervention strategies that limit the areas that dogs and cats are allowed within the confines of urban centers.

Human toxocariasis

Parasitic nematodes of the genus Toxocara are socioeconomically important zoonotic pathogens. These parasites are usually directly transmitted to the human host via the faecal-oral route and can cause toxocariasis and associated complications, including allergic and neurological disorders. Although tens of millions of people are estimated to be exposed to or infected with Toxocara spp, global epidemiological information on the relationship between seropositivity and toxocariasis is limited. Recent findings suggest that the effect of toxocariasis on human health is increasing in some countries. Here we review the salient background on Toxocara and biology, summarise key aspects of the pathogenesis, diagnosis, and treatment of toxocariasis, describe what is known about its geographic distribution and prevalence, and make some recommendations for future research towards the prevention and control of this important disease.

How common is human toxocariasis? Towards standardizing our knowledge

Our understanding of the global impact and cost of human toxocariasis is poor because there is insufficient clinical awareness and no clear repository for the efficacy of clinical, laboratory and treatment interventions. Uniform clinical and laboratory investigative approaches maximize disease diagnosis. International collaboration is required to develop web-based, professional educational support, surveillance questionnaires and standardized serodiagnostic criteria. Determining clinical benefits and treatment outcomes using less crossreactive antigens will enhance clinical and treatment interventions. Increased liaison will identify realistic occurrence and prevalence data and cost benefits of intervention. Web-based centres of excellence and repositories of current knowledge, which augment current veterinary and public health educational sites, should be supported. Expected outcomes should be capable of addressing the clinical and financial burdens of this treatable disease.

The expanded spectrum of toxocaral disease

Among 137 members of 30 families, 6% (and 8% of those aged under 15 years) were seropositive for toxocara antibodies. In these seropositive subjects and in 84 patients known to have raised toxocara titres the commonest clinical features were abdominal pain, hepatomegaly, anorexia, nausea, vomiting, lethargy, sleep and behaviour disturbances, pneumonia, cough, wheeze, pharyngitis, cervical adenitis, headache, limb pains, and fever. 61% of patients with raised toxocara titres had recurrent abdominal pain. Eosinophilia was in many cases associated with a raised toxocara titre, but 27% of patients with high titres had normal eosinophil counts. Toxocariasis is common, especially in children, and is associated with clinical features that are generally regarded as non-specific but together form a recognisable symptom complex. Toxocariasis should be considered in the differential diagnosis of such symptoms and especially in recurrent abdominal pain, which might otherwise be labelled as idiopathic. The absence of eosinophilia does not exclude toxocariasis.

Aspects of Toxocara epidemiology: toxocarosis in dogs and cats

Toxocara canis and Toxocara cati are common roundworms of dogs and cats. In this review the life cycles of these parasites are described, including the various routes of transmission, such as transplacental, transmammary infection, and infection through paratenic hosts. The somatic and tracheal migration in the body of the hosts after infection with Toxocara eggs or larvae is discussed, with special reference to age resistance and differences between dog and cat. The clinical symptoms and pathology in adult and young dogs and cats are given. Diagnosis of patent infections can be obtained by fecal examination, and treatment consists of the use of anthelmintics. Control of the infection and disease is achieved by prevention of contamination of the environment, anthelmintic treatment strategy, and education. Special attention is given to the efficacy of anthelmintics against adult worms and against somatic larvae. It is concluded that education on the life cycles of the parasites, hygiene, and anthelmintic treatment schedules is required because of the zoonotic risks of Toxocara spp. Deworming of pregnant dogs and cats is not recommended.

Ocular toxocariasis. A review

This review is an update on the subject of ocular toxocariasis. An introductory section is designed to familiarize the clinician with the epidemiology, characteristics of the organism, the life cycle in dogs and the systemic human disease known as visceral larval migrans ( VLM ). The more comprehensive second section considers the historical aspects of the ocular involvement, the clinical variations of ocular toxocariasis, pathology, differential diagnosis, diagnostic approaches, and treatment. Emphasis is placed upon new diagnostic techniques such as the enzyme-linked immunosorbent assay (ELISA) and cytologic diagnosis of intraocular aspirates. Newer therapeutic techniques such as vitrectomy are also considered. The review includes 200 references on systemic and ocular toxocariasis.

Immunodiagnosis of toxocarosis in humans: evaluation of a new enzyme-linked immunosorbent assay kit

Excretory/secretory (E/S) antigen derived from second-stage larvae of Toxocara canis maintained in defined medium in vitro has been well established worldwide for the immunodiagnosis of human toxocarosis by enzyme-linked immunosorbent assay. Such an enzyme-linked immunosorbent assay, based on the detection of human anti-T. canis (E/S antigen) serum immunoglobulin G, has recently been commercialized by Biokema-Affinity Products (Crissier-Lausanne, Switzerland). This commercial test kit was evaluated with regard to its application in a routine diagnostic laboratory and the reliability of the results. Of 78 patients with suspected clinical toxocarosis, 71 had anti-T. canis antibodies (positive serological result) corresponding to a diagnostic sensitivity of 91%; 14% of serum samples (n = 199) from patients with protozoan or with helminthic infections also showed positive reactions mainly related to infections with Trichinella, Strongyloides, and Fasciola species. An epidemiological study with 1,000 serum samples from randomly selected healthy blood donors and children in Switzerland demonstrated a seroprevalence of 2.7%. The test kit under evaluation had an overall diagnostic sensitivity of 91% and a relative specificity of 86%, the latter being related to some protozoan and helminthic infections. Because of the scarcity of such infections, potential cross-reactivity does not play a major role under the conditions found in the middle part of Europe. In conclusion, the application of the test kit provided for use in this study can be recommended for routine diagnostic use.

An estimation of Toxocara canis prevalence in dogs, environmental egg contamination and risk of human infection in the Marche region of Italy

The human risk of infection with larvae of Toxocara canis was estimated in people from the Marche region of Italy. This region includes both urban and rural areas and its inhabitants frequently keep dogs for company, hunting, as guardians or shepherds. T. canis infection was diagnosed in 33.6% out of 295 dogs examined. Nearly half of the dogs (48.4%) living in rural areas were found T. canis positive, compared to about one-quarter of the dogs (26.2%) from urban areas. Analysis by provenance and role revealed the highest infection rate in rural hunting dogs (64.7%) and the lowest in urban companion dogs (22.1%). According to questionnaire data, the peridomestic environment, i.e. gardens and dog pens, is the most important defecation site in both rural and urban areas. Since over 40% of the dogs who defecate in dog pens are infected and 24% of urban and 47% of rural dogs who leave their droppings in the house surroundings harbour the parasite, it is clear that these environments may constitute sites of zoonotic risk. Our analysis of soil samples from 60 farms confirmed the high contamination level, revealing positive soil samples in more than half of the farms. Substantial egg contamination was also found in urban areas, as 3/6 parks examined were Toxocara spp. positive. Finally, our serological findings indicate that human infection actually occurs in the area: 7 out of 428 adults examined (1.6%) had very high levels of antibodies to T. canis antigen, suggesting a previous contact with the Larva migrans of the nematode.

Seroprevalence estimates for toxocariasis in people worldwide: A systematic review and meta-analysis

Background

Human toxocariasis is an important neglected disease. We performed a systematic review and meta-analysis study to estimate the global and regional prevalence of anti-Toxocara serum antibodies (referred to as ‘T-seroprevalence’) in human populations around the world.

Methods

We searched five international databases (PubMed, EMBASE, Web of Science, SciELO and Scopus) for seroprevalence studies published from 1 January 1980 to 15 March 2019. We used random effect models to calculate the overall T-seroprevalence (with 95% CIs) in all six WHO regions and worldwide. We also conducted subgroup and linear meta-regression analyses to evaluate the impact of socio-demographic, geographical and climatic parameters on seroprevalence.

Results

We identified 250 eligible studies (253 datasets) comprising 265,327 participants in 71 countries for inclusion in the present meta-analysis. The estimated global T-seroprevalence rate was 19.0% (95%CI, 16.6–21.4%; 62,927/265,327); seroprevalence was highest in the African region (37.7%; 25.7–50.6%) and lowest in the Eastern Mediterranean region (8.2%; 5.1–12.0%). The pooled seroprevalence for other WHO regions was 34.1% (20.2–49.4%) in the South-East Asia; 24.2% (16.0–33.5%) in the Western Pacific; 22.8% (19.7–26.0%) in the American; and 10.5% (8.5–12.8%) in the European regions. A significantly higher T-seroprevalence was associated with a lower income level; lower human development index (HDI); lower latitude; higher humidity; higher temperature; and higher precipitation (P-value < 0.001). Potential risk factors associated with seropositivity to Toxocara included male gender; living in a rural area; young age; close contact with dogs, cats or soil; consumption of raw meat; and the drinking of untreated water.

Conclusions

The present findings indicate high levels of infection with, or exposure to Toxocara spp. in many countries, which calls for increased attention to human toxocariasis and improved measures to prevent adverse health risks of this disease.

There have been many epidemiological studies on the seroprevalence of Toxocara infection or toxocariasis in humans in individual countries, but there has been no systematic review/meta-analysis to estimate the extent of Toxocara infection or exposure in human populations worldwide. By reviewing 250 studies of 265,327 people from 71 countries, we provide the first comprehensive global estimates of T-seroprevalence in humans to date. The findings indicate that almost one fifth (19%) of the world’s human population is seropositive to Toxocara. The highest seroprevalence rates were found in Africa (37.7%) and the lowest in the Eastern Mediterranean region (8.2%). Individual countries with high HDIs and high income levels usually had low T-seroprevalence rates. Our findings also showed that T-seroprevalence varies markedly among countries, according to geographical location (coordinates) and climatic features. Evidence from this study indicates that human toxocariasis is a prevalent, neglected tropical disease and potentially one of major importance to global public health. Our study emphasizes the need for governmental authorities to pay increased attention to the risks and socioeconomic impact of HT in many countries around the world, and for them to design and implement HT prevention or control programs, considering risk predictors identified here.

Aspects of Toxocara epidemiology: human toxocarosis

Toxocarosis is the clinical disease in man caused by infection of zoonotic roundworms of dogs and cats, Toxocara canis and T. cati. In this review the mode of transmission to the human by oral ingestion of Toxocara eggs from the environment is discussed. T. cati seems to play a more important role than generally suggested. Direct contact with animals is not considered a potential risk because embryonation of excreted Toxocara ova requires a minimum of 2 weeks. For the same reason there is no relationship expected between infection and exposure to dogs and cats in the household. Children more frequently have clinical symptoms because of the closer contact with contaminated soil in yards and sandpits, the lack of hygiene, and because of eating dirt. Toxocara larval migration in the body can cause various clinical syndromes. Visceral larva migrans, ocular larva migrans, and covert toxocarosis are described. Serodiagnostic techniques are reliable tools to detect antibodies or antigens. Systemic treatment with anthelmintics is described but can result in hypersensitivity reactions caused by dying larvae. For ocular lesions, laser photocoagulation and corticosteroid therapy are described. Preventive measures consist of preventing contamination of the environment with Toxocara eggs and for education of pet owners and non-pet owners to increase awareness about potential zoonotic hazards. Veterinary practitioners, general practitioners, and public health agencies should therefore provide sufficient information and advice.

Prevalence, clinical features, and causes of vision loss among patients with ocular toxocariasis

BACKGROUND

To describe the prevalence, clinical features, and causes of vision loss among patients with ocular toxocariasis seen at a uveitis referral center.

METHODS

A review was completed of the charts of patients with ocular toxocariasis who were examined between 1977 and 1996 at the Francis I. Proctor Foundation of the University of California at San Francisco. The prevalence of ocular toxocariasis among all uveitis patients seen at the center was determined. Demographic features, symptoms, and signs in all patients were evaluated.

RESULTS

Ocular toxocariasis occurred in 22 (1.0%) of 2,185 uveitis patients. The mean patient age was 16.5 years. Inflammation was usually unilateral (90.9%). Toxocara uveitis presented as a granuloma in the peripheral retina in 50% of cases, as a granuloma in the macula in 25% of cases, and as a moderate to severe vitreous inflammation mimicking endophthalmitis in 25% cases. The primary causes of vision loss were vitritis (52.6%), cystoid macular edema (47.4%), and traction retinal detachment (36.8%).

CONCLUSIONS

Ocular toxocariasis is an uncommon cause of uveitis that mainly affects younger patients. Inflammation is typically unilateral and presents as either a granuloma in the peripheral or posterior retina or a moderate to severe vitreous inflammation mimicking endophthalmitis.

A critical look at the importance, prevalence and control of toxocariasis and the possibilities of immunological control

The visceral infection of humans with Toxocara canis is particularly prevalent in children and may cause a variety of symptoms that commonly persist for 6-24 months. The ocular infection usually causes permanent loss of visual acuity. Human infection is acquired by ingestion of embryonated T. canis eggs with contaminated dirt. Review of recent reports indicates that patent T. canis infection is widely prevalent in the general population of dogs all over the world (3-81%) and results in a substantial contamination of the ground (0.3-87%). The results of sensitive and specific serological tests suggest that about 7% of the clinically healthy human population of the United States, about 5% of that of Canada, and about 4% of that in Great Britain is infected with the parasite. Control of transmission of the parasite to man is often attempted by eliminating the infection in dogs, reducing the population of dogs and the environmental contamination with their feces, and educating the public about the zoonotic potential of toxocariasis. The evidence reviewed indicates that these methods are only marginally effective. Because T. canis relies on congenital and lactogenic transmission to persist in nature, only a procedure that effects the sustained killing of the reservoir larvae in the tissues of the bitch, or of newly-acquired parasites, is expected to be successful. Research with mice, rabbits and dogs demonstrated that prior infections of the host induce the development of protective immunity to reinfections. This procedure, however, leaves remnant populations of larvae from the immunizing infections that are resistant to anthelmintics and to the effect of prior irradiation. Hyperimmunization with partially-purified extracts of T. canis larvae induced 37% resistance to a challenge in mice when the extract was administered alone, and 76% resistance when administered with lipopolysaccharide adjuvant. Production of complete resistance, however, will probably require the prior control of the immunosuppression induced by the parasite. T. canis infections inhibit the production of homologous protective immunity and antibody responses to heterologous antigens, probably by interfering with the activity of helper T-cells, competing with protective antigens, and suppressing antibody synthesis. The evidence indicates, however, that an anti-T. canis vaccine to eliminate the parasite in dogs is feasible.

Human toxocariasis and direct contact with dogs

Toxocariasis in man is traditionally thought to be contracted through the ingestion of eggs from contaminated soil. The disease may manifest itself in different syndromes such as ocular larval migrans, visceral larval migrans and covert toxocariasis. This paper assesses the evidence for the soil contamination hypothesis and proposes that direct contact with dogs may provide a better explanation of the epidemiology of the disease. Hair was collected from 60 dogs from various places in Ireland and the UK and examined for the presence of Toxocara canis eggs. T. canis eggs were found in the hair of 25 per cent of the dogs; in total, 71 eggs were recovered, of which 4.2 per cent were embryonated and 23.9 per cent were embryonating. The maximum densities of the embryonating and embryonated eggs were 180 and 20 eggs per gram of hair, respectively, much higher than the densities reported for soil samples. It is suggested that dogs infected with T. canis may infect people by direct contact.

Ocular Toxocariasis in Schoolchildren

Ocular toxocariasis in humans is typically a unilateral disease caused by second-stage larvae of the Toxocara species. Serological evidence of widespread infection in humans provides little information on clinical disease. There is only a single previous estimate of the prevalence of ocular toxocariasis (from Alabama). The present survey examined the extent of consultant-diagnosed toxocaral eye disease among a population of schoolchildren. More than 120,000 participants were surveyed by questionnaire and follow-up. Two sets of control subjects from the same school and from the same county were compared with persons who had ocular toxocariasis. The prevalence of consultant-diagnosed toxocaral eye disease was 6.6 cases per 100,000 persons when only cases regarded as definite by the consultant ophthalmologist were included. This increased to 9.7 cases per 100,000 persons when both definite and strongly suspected cases were included. Geophagia and a history of convulsion were associated with toxocaral eye disease in both of the case-control studies.

Canine gastrointestinal parasitic zoonoses in India

Although well recognized and studied in developed countries, canine parasitic zoonoses pose a lowly prioritized public health problem in developing countries such as India, where conditions are conducive for transmission. A study of the most recent parasite survey determining prevalence and epidemiology of canine parasitic zoonoses among tea-growing communities of northeast India demonstrated the endemicity of the problem. This particular study serves as a model using conventional, as well as molecular parasitological, tools to provide novel insights into the role of dogs as mechanical transmitters of human parasites such as Ascaris and Trichuris, and discusses the risks dogs pose with regards to zoonotic transmission of hookworms and Giardia.

Neglected parasitic infections in the United States: toxocariasis

Toxocariasis is a preventable parasitic disease that is caused by the dog and cat roundworms Toxocara cani and T. cati, respectively. Humans become infected when they accidently ingest infectious Toxocara eggs commonly found in contaminated soil; children are most often affected. Clinical manifestations of Toxocara infection in humans include ocular toxocariasis and visceral toxocariasis. Although infection with Toxocara can cause devastating disease, the burden of toxocariasis in the United States population remains unknown. In addition, risk factors for acquiring infection need to be better defined, and research needs to be conducted to better understand the pathophysiology and clinical course of toxocariasis. Development of diagnostic tests would enable clinicians to detect active infection, and determination of optimal drug regiments would ensure patients were appropriately treated. Addressing these public health gaps is necessary to understand and address the impact of toxocariasis in the United States.

Toxocara seroprevalence in 5-year-old elementary schoolchildren: relation with allergic asthma

Toxocara seroprevalence and the relation between Toxocara seroprevalence and allergic asthma were investigated in Dutch schoolchildren aged 4-6 years. Data on Toxocara antibodies, allergen-specific immunoglobulin E, allergic manifestations, and risk factors (pets and playgrounds) were obtained from 235 children from The Hague and 477 from Rotterdam, the Netherlands. The surveys were carried out from September 1987 to January 1988 in The Hague and in March and April 1989 in Rotterdam. Logistic regression was used to evaluate putative relations. Toxocara seroprevalence was higher in The Hague (11%) than in Rotterdam (6%), but this difference was not quite significant. Seroprevalences varied widely among schools. No differences between socioeconomic categories or between the sexes were found. Occurrences of asthma/recurrent bronchitis and hospitalization due to asthma/recurrent bronchitis were significantly associated with seroprevalence. Furthermore, a marginally significant relation with eczema was found. Immunoglobulin E specific for inhaled allergens occurred significantly more often in the Toxocara-seropositive group. The risk factors investigated were not related to seroprevalence. It is suggested that Toxocara, among other environmental factors, may stimulate polyclonally immunoglobulin E production, including allergen-specific immunoglobulin E, and thus may contribute to the manifestation of allergic asthma and possibly of eczema in children predisposed to allergy.

Toxocariasis in North America: a systematic review

Toxocariasis is an important neglected tropical disease that can manifest as visceral or ocular larva migrans, or covert toxocariasis. All three forms pose a public health problem and cause significant morbidity in areas of high prevalence. To determine the burden of toxocariasis in North America, we conducted a systematic review of the literature following PRISMA guidelines. We found 18 articles with original prevalence, incidence, or case data for toxocariasis. Prevalence estimates ranged from 0.6% in a Canadian Inuit community to 30.8% in Mexican children with asthma. Commonly cited risk factors included: African-American race, poverty, male sex, and pet ownership or environmental contamination by animal feces. Increased prevalence of Toxocara spp. infection was linked in a group of case control studies conducted in Mexico to several high risk groups including waste pickers, asthmatic children, and inpatient psychiatry patients. Further research is needed to determine the true current burden of toxocariasis in North America; however the prevalence estimates gathered in this review suggest that the burden of disease is significant.

Sero-epidemiology of toxocariasis in school children

The seroprevalence of toxocariasis was investigated in 2129 school children ranging in age from 4 to 19 years. Infection, as measured by titre, was found to be high compared to values obtained from children in other countries, with 31% having a titre of > or = 1:50 and 3.1% having titres of > or = 1:800. Relationships between seroprevalence and host factors including age, sex and location, and risk factors including geophagia, thumbsucking and pet ownership were explored. Age was found to be a significant factor in relation to infection with Toxocara. Males were significantly more infected than females, as were the inhabitants of rural schools versus urban schools. The percentage of seropositive children varied widely from school to school. Geophagia and pet ownership were found to be significantly associated with toxocariasis. Logistic regression analysis was used to model the relative contribution of these factors to infection with Toxocara. While this analysis identified factors which were highly significantly associated with Toxocara titre, the odds ratios were generally low indicating the low specificity of the model.

Epidemiology of human toxocariasis in La Réunion

In La Réunion island (Indian Ocean) a seroepidemiological survey for toxocariasis was carried out among 387 subjects over 15 years old by Western blotting using Toxocara canis excretory-secretory larval antigens; 92.8% of the sera were positive. Statistical study, including logistic regression analysis, showed a significant correlation of Western blot results with sex, age, and absence of water supply. The final logistic model demonstrated that only the last 2 factors were significant multiplicative risk factors for toxocariasis. Sex was an independent risk factor. With this level of infection, we suggest that further surveys of ocular toxocariasis in children and teenagers of La Réunion would be advisable.

Seroprevalence of zoonotic toxocariasis in the United States: 1971-1973

Demographic characteristics associated with human Toxocara canis infection in children aged one to 11 years were investigated using data from the Health and Nutrition Examination Survey of 1971 to 1973. An enzyme-linked immunosorbent assay with larval stage antigen was used to measure the concentration of antibodies to T. canis in 1,409 available sera. From 4.6 to 7.3% of the children in different geographic regions of the United States have been infected, with serologic prevalence approaching 30% among black children of lower socioeconomic status aged six to 11 years. For both blacks and whites, higher seroprevalence was associated with a rural residence, increased age in children and number of persons in the household, and with decreased income, education, and number of rooms in the house. Multivariate logistic regression models indicated that blacks had higher infection rates than whites, even when socioeconomic factors were controlled. Certain critical variables, however, such as exposure to dogs or reliable pica histories, were not available for analysis.