Toxocara Parasite DNA Found in Eyes and Livers of Birds in Northeastern Iran

Molecular screening of 100 birds collected opportunistically in Razavi Khorasan Province, northeastern Iran, has detected DNA of the roundworms Toxocara canis and Toxocara cati in the brain, eye and liver tissues of more than a third of the animals examined. The cross-sectional survey, conducted in 2022 and published in Veterinary Medicine and Science, found that 35 of 100 birds (35.0%; 95% CI: 24.6–44.7) carried Toxocara DNA in at least one tissue, while 51 of 300 individual tissue samples (17.0%; 95% CI: 12.7–21.3) tested positive overall. Sparrows emerged as the most frequently affected hosts, with 46.7% of the 60 examined individuals positive at the bird level, compared with 21.1% of chickens and 14.3% of pigeons. The researchers describe the results as molecular evidence that free-ranging birds in the province act as paratenic hosts—animals that harbour the parasite’s larvae without supporting their development to reproductive adulthood—for these zoonotic ascarid nematodes.

The study relied on a nested multiplex PCR assay targeting the ITS1/ITS2 ribosomal DNA region, a technique that amplifies species-specific fragments of 208 base pairs for T. canis and 325 base pairs for T. cati. This molecular approach allowed the team to distinguish between the two species directly from tissue samples, something conventional parasitological methods cannot reliably achieve because larval morphology alone does not permit consistent differentiation. Six randomly selected positive PCR products were sequenced and compared against reference sequences in the GenBank database to confirm species identity. Among the 51 PCR-positive samples, T. canis predominated at 68.6% (35/51), followed by mixed detections of both species at 17.7% (9/51) and T. cati alone at 13.7% (7/51). The authors interpret the predominance of T. canis as suggesting that canine-associated environmental contamination is the primary driver of avian exposure in the region, while the presence of T. cati and mixed detections points to a complex transmission dynamic involving both dogs and cats as definitive hosts.

Tissue-level patterns varied significantly across the three organ types examined. Detection rates were highest in the eye (23/100, 23.0%) and liver (20/100, 20.0%), with the brain showing the lowest frequency at 8.0% (8/100). In sparrows specifically, the eye was positive in 31.7% of birds (19/60) and the liver in 26.7% (16/60), while the brain registered 11.7% (7/60)—a statistically significant difference among tissues within this species (p = 0.026). By contrast, pigeons and chickens showed no statistically significant differences between tissue types. In the multivariable mixed-effects logistic regression model, which accounted for the clustering of three tissue samples within each bird, the odds of detecting Toxocara DNA in the brain were significantly lower than in the liver (adjusted OR = 0.31, 95% CI: 0.12–0.78, p = 0.013), while the eye and liver did not differ significantly (adjusted OR = 1.34, 95% CI: 0.48–3.73, p = 0.581).

Bird species itself was a significant predictor of PCR positivity. Compared with chickens as the reference category, sparrows had significantly higher odds of infection (adjusted OR = 3.32, 95% CI: 1.02–10.79, p = 0.045), whereas pigeons did not differ significantly from chickens (adjusted OR = 0.73, 95% CI: 0.18–2.94, p = 0.654). The random intercept variance for individual birds was estimated at 1.168 (p = 0.021), indicating substantial variability in infection probability among birds beyond the fixed effects in the model. The authors attribute the higher burden in sparrows to their ground-foraging behaviour and synanthropic proximity to human dwellings, which increases contact with soil contaminated by embryonated Toxocara eggs. On this basis, they propose that sparrows may serve as effective ecological sentinels for monitoring environmental contamination in the region.

The relatively low detection rate in avian brain tissue stands out against the marked neurotropism commonly described in mammalian paratenic hosts such as rodents, where larvae preferentially migrate to the central nervous system during the chronic phase of infection. The study authors suggest several possible explanations: physiological differences in the avian blood-brain barrier, stronger immune-mediated protection within neural tissues, or the elevated core body temperature of birds, which could influence larval migration patterns. They note that the high detection rate in the liver is biologically plausible, since the liver is the first major organ reached by larvae through the portal circulation after they penetrate the intestinal wall. The substantial rate of detection in the eye—reaching 31.7% in sparrows—suggests that ocular tropism may also be important in avian hosts, though the authors caution that PCR positivity indicates the presence of parasite DNA rather than confirmed viable larvae.

From a public health standpoint, the authors highlight the liver findings as particularly relevant, since consumption of raw or undercooked avian giblets may provide a potential route of transmission to humans, citing earlier work on this pathway. They also note that ocular and visceral involvement may reduce host fitness, thereby increasing the likelihood that infected birds are preyed upon by dogs or cats—the definitive hosts in which the worms reach adulthood and shed eggs. This predation or scavenging route, the study argues, may help maintain the parasite’s transmission cycle in peri-domestic ecosystems where synanthropic birds, stray or owned carnivores and contaminated soil coexist. The findings complement soil-based surveys in Iran by showing that free-ranging birds may not only reflect environmental contamination but may also participate indirectly in sustaining transmission.

Sampling for the study was opportunistic and geographically stratified. The province was divided into five zones—north, south, east, west and central—based on administrative and ecological characteristics, with two counties randomly selected from each zone. Bird carcasses were collected based on availability in urban, peri-urban and rural environments; no live birds were captured, handled or euthanised for research purposes. Sparrows and pigeons came from field sampling, while chicken samples were obtained after routine slaughter for human consumption. During necropsy, brain, liver and eye tissues were aseptically collected from each bird, yielding 300 tissue samples that were processed individually to prevent cross-contamination. Genomic DNA was extracted using a commercial tissue kit, and PCR setup, extraction and post-PCR analysis were conducted in separate laboratory areas.

Spatial analysis added a geographic dimension to the findings. Kernel density estimation maps visualised the intensity of positive samples and revealed localised hotspots in the southern and southeastern regions of the province. However, global spatial autocorrelation analysis using Moran’s I yielded a value of 0.019 with a z-score of 1.382 (p = 0.166), indicating that the distribution of Toxocara DNA across the province does not follow a significant global clustering pattern and is statistically consistent with a random spatial distribution. The authors interpret the visual hotspots as likely reflecting local concentrations of positive samples rather than a strong regional spatial structure, and they acknowledge that the non-significant Moran’s I result may be partly related to the limited number of positive observations and the scale of analysis. The spatial analyses are described as exploratory: kernel density reflected the density of detected positive samples rather than true infection risk, since sampling effort was not incorporated, and ecological variables such as dog and cat density, soil contamination, seasonality and climatic factors were not included.

The distribution of Toxocara species did not differ significantly across the three avian hosts (Fisher’s exact test, p = 0.671), although descriptive proportions varied: sparrows and pigeons were primarily infected with T. canis (71.4% and 75.0% of their positive samples, respectively), while chickens showed a higher descriptive proportion of mixed infections (40.0%). Mixed infection was defined as co-detection of both species’ DNA in the same tissue specimen. The authors state that this pattern reflects a shared environmental interface in which birds, carnivores and contaminated soil overlap, consistent with previous evidence of widespread environmental contamination with Toxocara eggs in different parts of Iran, including northeastern regions.

Several limitations frame the interpretation of these results. PCR positivity does not confirm larval viability, so the findings should be read as molecular evidence of exposure and tissue invasion rather than definitive proof of infective larvae. The qualitative assay did not allow estimation of parasite DNA load or larval burden, and infection intensity at the individual-bird level could not be assessed. The opportunistic design may have introduced selection bias, and the small, uneven sample sizes across host groups limit generalisability and statistical power for interspecies comparisons. Differences in specimen sources—field carcasses for sparrows and pigeons versus post-slaughter sampling for chickens—may also affect direct comparability among hosts. Only liver, brain and eye tissues were examined, so larval distribution in other organs, particularly skeletal muscles, may have been missed. The authors call for future studies incorporating systematic sampling, larval recovery, histopathology, quantitative molecular approaches, soil contamination assessment and more detailed spatial ecological analyses to clarify the epidemiological role of avian paratenic hosts in Toxocara transmission, and they frame the work as supporting a One Health approach to surveillance and control in northeastern Iran.

Subject of Research: Molecular detection and spatial distribution of Toxocara canis and Toxocara cati in avian paratenic hosts in northeastern Iran

Article Title: Molecular Epidemiology and Spatial Distribution of Toxocara canis and Toxocara cati in Avian Paratenic Hosts: A One Health Perspective From Northeastern Iran

Article References: Rezaiemanesh, M.-R., Alimi, R., Gholizadeh, A., Hejazi, S.-H., & Mohaghegh, M.-A. (2026). Molecular Epidemiology and Spatial Distribution of Toxocara canis and Toxocara cati in Avian Paratenic Hosts: A One Health Perspective From Northeastern Iran. Veterinary Medicine and Science, 12(6), Article e71310. https://doi.org/10.1002/vms3.71310

Image Credits: AI Generated

DOI: 10.1002/vms3.71310

Keywords: Toxocara canis, Toxocara cati, avian paratenic hosts, nested multiplex PCR, Razavi Khorasan Province, zoonotic helminths, tissue tropism, sparrows, spatial analysis, One Health, environmental contamination, Iran