Theileria in cattle and sheep: survey results and gaps
Theileria in cattle is common enough that the clinical cases are only the visible part of it. A countrywide molecular survey put 34.4 percent of 6,224 cattle and 11.5 percent of 1,467 sheep positive by PCR. Theileria orientalis reached all 17 surveyed provinces, and most carriers looked perfectly normal on the day of sampling.
Why subclinical infection is the real problem
Theileria is a genus, not a diagnosis. The parasites inside it differ in which ticks carry them, in which hosts they settle, and in how much damage they do once they are established. A herd that looks well will not reveal any of that.
Herd decisions usually run on what can be seen. Sick animals get attention and get removed. Animals that show nothing stay, stay productive, and keep the parasite in circulation for the next round of ticks. Theileria in cattle works this way more often than not, which is what makes culling on clinical signs such a poor filter.
Which species is involved adds another problem. Two positive animals can need completely different things from a control program. Genus-level testing settles whether Theileria is present. It does not settle the part that changes the decision.
How the survey was designed
The survey was cross-sectional, and it ran at molecular level. Blood was drawn from cattle and sheep across one Central Asian country. Every sample went through the same screening step first, and species-level work came afterward.
| Parameter | What was done |
|---|---|
| Design | Cross-sectional molecular survey |
| Cattle sampled | 6,224 animals drawn from 172 settlements |
| Sheep sampled | 1,467 animals drawn from 52 settlements |
| Geographic coverage | All 17 provinces for cattle; five southern provinces for sheep |
| Sampling window | July to August, across the 2022 and 2023 seasons |
| Screening test | Genus-level PCR targeting the 28S ribosomal RNA gene |
| Species identification | Bidirectional sequencing |
| Subtyping | MPSP genotyping of selectedT. orientalis-positive samples |
| Sampling frame | Not probabilistic |
The design has two consequences worth stating plainly. Genus-level PCR confirms that Theileria DNA is in the sample, and it does not say which species put it there. The sampling was also not probabilistic, so the animals were not drawn at random from a national list. Percentages from this dataset describe the animals that were tested, not the country.
How common Theileria in cattle and sheep turned out to be
Start with the two headline figures.
| Host | Animals tested | PCR-positive | Positivity |
|---|---|---|---|
| Cattle | 6,224 | 2,142 | 34.4 percent |
| Sheep | 1,467 | 169 | 11.5 percent |
Slightly more than one cattle in three was positive for at least one Theileria species. Among sheep the rate was about one in nine, on a far smaller sample.
Comparing those two numbers directly would be a mistake. Cattle were sampled in every one of the 17 provinces and in more than three times as many settlements. The sheep sample came from five provinces in the south. Tick-borne parasites travel with their vectors, and vectors travel with climate and habitat. A sample drawn from a narrower band of country will generally land lower and vary less.
Eleven and a half percent is still substantial on its own terms. About one sheep in nine was carrying a parasite that no routine examination would have caught, and this survey was the first molecular characterization of Theileria in sheep in that country.
Species matter more than the positive count
Once the positives were sequenced, they resolved into three species in cattle and three in sheep.
| Species | Host | Positivity |
|---|---|---|
| Theileria annulata | Cattle | 27.1 percent |
| Theileria orientalis | Cattle | 15.1 percent |
| Theileria sinensis | Cattle | 0.5 percent |
| Theileria ovis | Sheep | 5.5 percent |
| Theileria sp. genotype OT3 | Sheep | 4.4 percent |
| Theileria annulata | Sheep | 2.6 percent |
Theileria annulata came out on top in cattle at 27.1 percent. T. orientalis followed at 15.1 percent, and T. sinensis was well behind at 0.5 percent. Sheep produced a different order: T. ovis first at 5.5 percent, then Theileria sp. genotype OT3 at 4.4 percent, then T. annulata at 2.6 percent.
Those three figures per host do not sum to the overall positivity. They are not supposed to. Many animals were carrying more than one species at the same time. Of the 2,142 PCR-positive cattle, 508 animals, or 23.7 percent, had two or three species at once.
| Measure | Value |
|---|---|
| PCR-positive cattle | 2,142 |
| Cattle carrying two or three species at once | 508 |
| Share of PCR-positive cattle with mixed infection | 23.7 percent |
That number is the argument for species-level work. When a quarter of the positives are mixed, a genus-level result does not name the parasite that matters most inside the animal, and the parasite that matters most is the one that decides the control route.
How uneven the map really is
Theileria in cattle is usually discussed as a warm-climate problem with a boundary that holds fairly still. The province data here do not show a tidy boundary.
| Species | Host | Provinces where detected | Provinces surveyed |
|---|---|---|---|
| Theileria orientalis | Cattle | 17 | 17 |
| Theileria annulata | Cattle | 16 | 17 |
| Theileria sinensis | Cattle | 5 | 17 |
T. orientalis appeared in all 17 provinces that were surveyed. T. annulata appeared in 16. T. sinensis appeared in five. Two of the three species are effectively national, and the third is patchy.
Absence and presence do not carry the same weight. A province where a species was not detected is not a province where the species is absent. It may only mean that none of the animals sampled there happened to be infected. When sampling is not probabilistic, a blank is much weaker than a hit.
Reading the sheep data
Sheep carry more weight in this dataset than the raw percentage suggests, because this survey was the first molecular characterization of Theileria in sheep in that country. Three findings were logged as first detections for the country: Theileria orientalis genotype 2 (Ikeda), Theileria sinensis, and Theileria sp. genotype OT3.
MPSP genotyping of selected T. orientalis positives resolved three genotypes: genotype 1 (Chitose), genotype 2 (Ikeda), and genotype 3 (Buffeli). Three genotypes inside one national dataset means a single country can hold parasite populations that differ in the surface protein a host immune system learns to recognize.
For a control program, that detail sits above the prevalence figure in importance. A program built around one genotype does not automatically cover the others.
The counterintuitive finding
The surprising part of this dataset is not that infection is common. It is that the species with the widest reach was not the species with the highest burden.
T. orientalis covered all 17 provinces, a wider footprint than T. annulata at 16, and still T. annulata was roughly twice as frequent among the cattle that were tested: 27.1 percent against 15.1 percent. Reach and burden are two different measurements. A program tuned to one of them will not automatically handle the other.
Sheep supply a second surprise of the same kind. A lower percentage does not mean a less important result. The sample was smaller, it covered less of the country, and it came from a host that had never been characterized at this level there. Treat 11.5 percent as a floor rather than a ceiling.
The third point is arithmetic. With 23.7 percent of positive cattle carrying more than one species, the idea of one animal and one parasite fails about once in every four positives. Any program built on the single-species assumption is wrong at that rate.
What this means for parasite control programs
- Test first, and test to species level. A genus-level positive cannot tell you which control route is the relevant one.
- Treat every positive as information about the area as well as about the animal. If one animal in a settlement is positive, the parasite and its vector are both there.
- Test again instead of assuming. Theileria in cattle carriers give no outward sign, and last season’s clean result says nothing about this season.
- Plan for mixed infections. Paying for species-level sequencing on a defined subset makes sense when nearly one positive in four carries more than one species.
- Match the program to the species you found rather than the species you expected. Which drug class is relevant depends on which parasite is in the animal.
- Sample outside summer where the budget allows. A July to August window leaves the rest of the year unmeasured.
Limitations
This survey states its own boundaries clearly. Sampling was not probabilistic, so the positivity figures are not a national prevalence estimate. Two seasons were covered and both sat inside the July to August window, which leaves any seasonal pattern outside that window unmeasured.
Species-level sequencing ran on a subset of samples, and MPSP genotyping ran on selected T. orientalis positives rather than on all of them. Genotype frequencies therefore describe the genotyped subset, not the whole positive population.
A detection in a province is a firm statement. A non-detection is not. Under this sampling scheme, a province with no record is unmeasured rather than clean.
Bottom line
Theileria in cattle and sheep is widespread, unevenly spread, and close to invisible at the pen. This survey found 34.4 percent of 6,224 cattle and 11.5 percent of 1,467 sheep PCR-positive, put T. orientalis in all 17 surveyed provinces, and showed that nearly a quarter of the positive cattle carried more than one species.
Assume your way through that and most of it stays hidden. Test your way through it and there is something to act on.
FAQ
How common is Theileria in cattle
In this survey 34.4 percent of the 6,224 cattle tested were PCR-positive for at least one Theileria species. That is 2,142 animals carrying the parasite, and most of them showed no clinical signs when they were sampled. The figure describes the sampled cattle only and is not a national prevalence estimate.
What percentage of sheep tested positive for Theileria
Sheep positivity was 11.5 percent, with 169 of 1,467 animals positive by PCR. The sheep sample was smaller than the cattle sample and came from five southern provinces only, so it covers a narrower slice of the country. A narrower sampling frame usually returns a lower and less variable figure, which means the two host percentages are not directly comparable.
Which Theileria species infect cattle
Three species were identified in cattle: Theileria annulata at 27.1 percent, Theileria orientalis at 15.1 percent and Theileria sinensis at 0.5 percent. The three together account for the 34.4 percent overall positivity, and mixed infections explain why the individual percentages do not simply add up. Species assignment required sequencing after genus-level PCR.
Which Theileria species infect sheep
Three species were identified in sheep: Theileria ovis at 5.5 percent, Theileria sp. genotype OT3 at 4.4 percent and Theileria annulata at 2.6 percent. The sheep dataset also produced the first country detections of Theileria orientalis genotype 2 and Theileria sinensis. This was the first molecular characterization of Theileria in sheep in that country.
Can cattle carry more than one Theileria species at the same time
Yes. Among the 2,142 PCR-positive cattle, 508 animals carried two or three species at once, which is 23.7 percent of all positives. Mixed infection at that rate means a genus-level positive result does not identify the dominant parasite. Species-level sequencing is the only way to learn which species is driving the burden.
Why does species identity matter for parasite control
Species identity decides which control route is even relevant, because the parasite species shapes how the animal is managed. A program built for one species can be close to irrelevant against another. Genus-level positivity therefore answers a different question from the one a control program needs answered.
Is a PCR-positive animal always sick
No. Most PCR-positive animals in this survey were subclinical carriers, which is why the infection is so easy to miss at the pen. Detection reflects the presence of parasite DNA in blood rather than the severity of disease. A positive result is a distribution signal as much as an animal-level diagnosis.
How was the survey designed
It was a cross-sectional molecular survey of 6,224 cattle from 172 settlements and 1,467 sheep from 52 settlements. Every sample was screened by genus-level PCR targeting the 28S ribosomal RNA gene and positives were identified to species by bidirectional sequencing. Selected Theileria orientalis positives were then characterized by MPSP genotyping.
Which provinces had the widest Theileria detection
Theileria orientalis was detected in all 17 surveyed provinces, which made it the widest-ranging species in the dataset. Theileria annulata appeared in 16 provinces and Theileria sinensis in only five. Detection and non-detection are not symmetrical, so a province without a record is unmeasured rather than clean.
What is MPSP genotyping
MPSP genotyping reads the gene for the major piroplasm surface protein, a surface antigen that varies between strains. It sorts Theileria orientalis positives into defined genotypes rather than treating them as one uniform group. In this survey it identified genotype 1, genotype 2 and genotype 3 among selected positive samples.
What are the limitations of the survey
The main limitation is that sampling was not probabilistic, so the measured positivity is not a national prevalence estimate. Sampling covered only two seasons and both fell inside the July to August window. Species sequencing and MPSP genotyping were applied to subsets rather than to every positive sample.
When should a herd be tested for Theileria
Testing pays off whenever control decisions depend on knowing which species is present, which includes any herd with unexplained production losses. Summer sampling matches the window used in this survey and is the easiest season to compare against these figures. A negative result from one season should be repeated rather than treated as permanent.
