Theileriosis (also known as a theileria infection or scientifically known as theileriasis) affects cows and other animals like sheep and goats. It is caused by tiny parasites called theileria, which infect blood cells and are primarily spread by ticks.
Tick prevention and avoiding the introduction of infected animals into a herd or flock is a key part of herd health management.
Theileriosis is an immediately notifiable disease in Canada.
On this page
- Risk to human health
- Signs
- Where it is found
- Transmission and spread
- Inspecting animals for ticks
- Diagnosis
- Treatment
- Protecting Canadian livestock
- Our approach to managing theileriosis in Canada
- Additional information
Risk to human health
There is no evidence that animals that have theileriosis can transmit the disease to people. The disease does not spread from animals to humans and does not pose a risk to food safety.
Signs of the disease in animals
Clinical signs in infected cattle often appear as:
- fever
- swollen lymph nodes (lymphadenopathy)
- loss of appetite and weight loss
- nasal discharge
- diarrhea
- decreased milk production
- anaemia, jaundice, and bloody urine
- accumulation of fluid in the lungs and respiratory distress in terminal stages
- neurological signs such as "turning sickness" (circling or loss of coordination)
How sick the animal gets depends on the type of theileria parasite and the susceptibility of the animal host. In some cases, animals may get sick or die, especially if they've never had the disease before.
Where it is found
In cattle, theileria parasites have been reported in Europe, Asia, Africa, North and South America, Australia and New Zealand.
Theileriosis has been spreading in the United States and is currently reported in many eastern states.
Transmission and spread
With the rise in tick populations, diseases that are spread via ticks, such as theileriosis, are becoming increasingly common.
Theileriosis spreads mostly through the bites of certain ticks. Ticks carry the parasite and pass it to animals when they feed.
Other insects such as flies, mosquitoes and lice may also spread the disease but it's not clear how important they are in spreading the disease.
The disease can also spread through reused needles, surgical and medical equipment contaminated with blood and blood transfusions. Theileriosis does not spread by animals touching each other.
Inspecting animals for ticks
Regularly inspect cattle for ticks. Ticks are small, especially the Asian longhorned tick, and they can easily go unnoticed unless they are engorged from feeding. Tick larvae, nymphs, and adults may all be found at the same time on a single animal.
Ticks are often easiest to find on the animal's:
- head and neck
- back and sides
- groin
- under the tail
Cattle with low weight gain, who are lethargic or anemic, have patchy hair or generally look unthrifty, should always be inspected for ticks.
If you think your animal has theileriosis or is sick, call your veterinarian.
Diagnosis
An owner or veterinarian may suspect an animal has theileriosis based on clinical signs, but the diagnosis can be confirmed only with laboratory testing.
Treatment
There are no medications in Canada specifically approved to prevent theileriosis; management options for cattle include:
- measures to reduce tick burdens on animals and in the environment
- supportive care (fluids and anti-inflammatories)
There is limited vaccine availability, and no vaccines are currently approved for use in Canada.
Protecting Canadian livestock
The Canadian Food Inspection Agency (CFIA) first detected theileriosis in Canada in October 2025.
The CFIA enforces import controls and monitors animal health to prevent the introduction of foreign animal diseases. Surveillance and biosecurity measures help protect Canadian livestock. Despite these efforts, it is expected that additional cases of theileriosis will be identified as climate change creates more suitable conditions for the presence and persistence of Asian longhorned ticks.
Ways you can help reduce the introduction and spread of Theileria in your herd:
- reducing tick habitat
- avoiding the introduction of infected animals into a herd or flock
- avoiding the use of reused needles
- quarantining new animals
Producers should contact their veterinarian to discuss on-farm biosecurity measures to reduce the risk of theileriosis.
Our approach to managing theileriosis in Canada
The CFIA's approach recognizes the realities of disease spread in Canada's environment, where ticks and wildlife move freely across borders and regions. In this context, eradication through the depopulation of affected livestock is not considered feasible or effective. Instead, CFIA is prioritizing surveillance, science-based support, and collaboration to help manage theileriosis and protect animal health.
Our approach includes:
- supporting the provinces, territories and industry by providing laboratory diagnostics to confirm suspect cases
- notifying the World Organization of Animal Health (WOAH) and trading partners to meet international obligations
- communicating and creating awareness of the risk with stakeholders
- monitoring international and domestic risks and adjusting import and domestic programs as needed
- providing scientific guidance as needed to manage risks