Semi-domesticated Eurasian tundra reindeer across Finland and northern Norway carry antibodies against the zoonotic parasite Toxoplasma gondii at a rate of 5.5 percent, and the burden climbs steadily the further south the animals graze, according to a cross-sectional serological survey of 635 reindeer slaughtered in autumn 2015. None of the animals tested positive for antibodies against a second coccidian parasite, Neospora caninum, giving an apparent seroprevalence of zero with a 95 percent confidence interval stretching only to 0.47 percent. The findings, published in Acta Veterinaria Scandinavica, come from blood plasma collected at nine reindeer slaughterhouses in Finland and one in Finnmark County, Norway, between 30 September and 2 November 2015.
The research team, led by Pikka Jokelainen and including Kayla J. Buhler, Eva Marie Breines, Morten Tryland and Sauli Laaksonen, screened each sample with commercial indirect enzyme-linked immunosorbent assays designed for multiple host species. Altogether 35 of the 635 reindeer were seropositive for T. gondii, an apparent seroprevalence of 5.51 percent with a 95 percent confidence interval of 3.93 to 7.50. Six of the 600 samples interpreted as negative had tested repeatedly doubtful and were treated as negative in the statistical analyses. For N. caninum, not a single animal crossed the positivity threshold, and no samples fell into the doubtful category twice.
Because the sensitivity and specificity of these multi-species ELISA kits have never been validated in reindeer, the authors report their results as apparent seroprevalence rather than true prevalence. Samples were run in single wells, with results expressed relative to a positive control as S/P percent. A result of 50 or above counted as positive, 40 or below as negative, and values in between were retested, with repeatedly doubtful samples classified as negative. The kit controls were porcine sera for the toxoplasmosis assay and bovine sera for the neosporosis assay, since species-specific control sera for reindeer are not available.
Age emerged as a strong predictor of Toxoplasma exposure. Adult reindeer, defined as animals over 18 months old, had 4.14 times the odds of being seropositive compared with calves aged six to seven months, based on an univariable logistic regression model with a 95 percent confidence interval of 1.95 to 8.77 and a P value below 0.001. Roughly one in ten adult reindeer carried antibodies. The authors note that this pattern fits what is seen in many other host species, where most T. gondii infections are chronic and subclinical and older animals have simply had more time to encounter the parasite. They also caution that the sample included no very young calves, so the overall estimate could be an overestimation, while parasite-related mortality among exposed animals before slaughter could conversely make it an underestimation.
Geography told an equally striking story. Region 1, the southernmost part of the Finnish reindeer herding area, recorded seroprevalence above 10 percent, the highest in the study, while more northern regions showed progressively lower rates. An univariable model treating region as a continuous variable suggested that the odds of being seropositive almost doubled, increasing by a factor of 1.94, with each step from north to south. When age group was added to that model, the regional factor rose to 2.14 and adults carried 5.08 times the odds of calves, with the model showing good predictive power as measured by an area under the ROC curve of 0.76. In the final model, reindeer from Regions 3 and 4 had significantly lower odds of seropositivity than those from Region 1, with odds ratios of 0.18 and 0.15 respectively in the univariable analysis, and the authors report that the geographical differences were not explained by differing age distributions across regions.
The study population was overwhelmingly Finnish. Of the 635 samples, 597, or 94 percent, came from nine reindeer herding cooperatives in northern Finland, while 39, or 6 percent, came from Finnmark County in northern Norway. Calves made up 60 percent of the sample and adults 40 percent. All animals were apparently healthy, having been judged fit for slaughter for human consumption, and no reindeer were killed specifically for the study; blood was drawn after stunning, during bleeding, at routine slaughter.
Toxoplasma gondii is a globally distributed apicomplexan parasite capable of infecting virtually all warm-blooded animals, including humans. Felids are its definitive hosts, shedding unsporulated oocysts in their faeces that sporulate in the environment and remain infective. Herbivorous animals such as reindeer are thought to acquire infection mainly by ingesting these oocysts from contaminated pasture, which means that detecting antibodies in herbivores can serve as an indicator of environmental oocyst contamination. The authors suggest that the north-to-south gradient in seroprevalence may reflect exactly such a gradient in environmental contamination, plausibly linked to the distribution of domestic cats and their owners, which is concentrated in southern Finland, where feline T. gondii seroprevalence estimates range from 35.6 percent in shelter cats to 49.7 percent in purebred pet cats, and where wild Eurasian lynx show a seroprevalence of 86.1 percent. Climatic conditions that affect oocyst survival may also contribute, raising the possibility that a warming climate could increase exposure in the future.
The 5.5 percent figure is notably higher than previous estimates from Fennoscandia, which had all been below 2 percent. An earlier study based mostly on Finnish samples with relatively few from Finnmark reported that 0.9 percent of 2,577 reindeer had T. gondii antibodies. The authors attribute the difference to several possible factors, including different age distributions in the convenience samples, the switch from a direct agglutination test to an iELISA, changing environmental conditions, and evolving herd management. Reindeer herding has moved toward more intensive practices and supplementary winter feeding in corrals, and since T. gondii seropositivity has previously been positively associated with the degree of domestication and corral feeding, the higher seroprevalence might reflect these management changes. Comparisons elsewhere reinforce the pattern: free-ranging caribou in Alaska and Canada showed seroprevalences of 0.4 percent and 1.8 percent respectively, Swedish reindeer tested in the same year with the same commercial kit showed 2.3 percent in adults and 1.9 percent in calves, while captive reindeer in Germany reached 52.1 percent.
The complete absence of Neospora caninum antibodies stands in sharp contrast to findings elsewhere. This parasite, whose definitive hosts are canids, causes abortions in cattle worldwide and may be capable of causing abortions and neurological disease in reindeer and caribou. Captive reindeer in Germany showed a seroprevalence of 4.2 percent using the same test kit, and free-ranging caribou in Alaska and Canada showed 11.5 percent and 26.7 percent respectively, though those North American studies used different serological methods. The authors speculate that the higher North American figures may reflect a greater density of wild canid hosts such as gray wolves and coyotes, which are abundant at northern latitudes there but scarcer in Fennoscandia. Domestic dogs, kept in the region as pets, sledge dogs, herding dogs and hunting dogs, could also shed oocysts if infected, particularly if fed raw meat. The absence of any dual seropositivity in the dataset suggests that major cross-reactivity between the two assays is unlikely, although a weak to moderate statistical correlation between the serology results, with a Spearman coefficient of 0.400, was observed and attributed partly to the tenfold higher scale of the T. gondii results.
The authors emphasise that the findings should be interpreted with caution. Slaughterhouse sampling excludes breeding animals and those that died in the field, though the use of multiple slaughterhouses improved representativeness, and there is no known biological mechanism by which serostatus for these parasites would systematically influence which animals are sent to slaughter. The antibody-based methodology does not directly demonstrate the presence of infective parasites in the meat, but it does show that the animals were exposed. Because the reindeer were slaughtered for human consumption, the results carry food safety and One Health relevance, since eating undercooked meat of infected animals is a recognised route of human T. gondii infection, and routine meat inspection at reindeer slaughterhouses does not attempt to detect the parasite. Thorough cooking destroys tissue cysts, which are eliminated by heating meat to 67 degrees Celsius. The team calls for continued monitoring of exposure in reindeer, noting that shifts in herd management, climate and environment may create further opportunities for transmission of both parasites.
Subject of Research: Seroprevalence of Toxoplasma gondii and Neospora caninum antibodies in semi-domesticated Eurasian tundra reindeer in Finland and northern Norway.
Article Title: North-to-south increasing gradient in Toxoplasma gondii seroprevalence and no serological evidence of exposure to Neospora caninum in semi-domesticated Eurasian tundra reindeer (Rangifer tarandus tarandus) in Finland and northern Norway in 2015
Article References: Jokelainen, P., Buhler, K. J., Breines, E. M., Tryland, M., & Laaksonen, S. (2026). North-to-south increasing gradient in Toxoplasma gondii seroprevalence and no serological evidence of exposure to Neospora caninum in semi-domesticated Eurasian tundra reindeer (Rangifer tarandus tarandus) in Finland and northern Norway in 2015. Acta Veterinaria Scandinavica, 68(1), Article 34. https://doi.org/10.1186/s13028-026-00869-2
Image Credits: AI Generated
DOI: 10.1186/s13028-026-00869-2
Keywords: Toxoplasma gondii, Neospora caninum, reindeer, seroprevalence, Finland, Norway, Fennoscandia, ELISA, zoonotic parasite, environmental contamination, reindeer herding, One Health
News Source: Kristina Jarvis. (November 4, 2026). Reindeer reveal a north-to-south rise in Toxoplasma exposure across Finland and northern Norway. Scienmag.
