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Equine piroplasmosis (EP), or babesiosis, is an infectious tick-borne disease that affects all equid species, including horses*, mules, donkeys and zebras. It is caused by protozoan parasites that are transferred to the horse via tick bite. [1][2]
The protozoan parasites, Theileria equi and Babesia caballi, infest the horse’s system and break down its red blood cells. This results in clinical symptoms, which can vary from mild to intense effects on animal health.
The impact of EP extends beyond the affected animal. Infected horses can serve as reservoirs (or carriers) for the parasites, facilitating disease transmission to other horses through tick vectors.
By implementing preventive measures, horse owners can minimize the risk of EP and protect the health and well-being of their horses. Early detection and prompt intervention are key to managing this disease effectively and preventing the spread to other horses.
*For clarity and ease of reading, the term “horses” will be used in this article to describe all equids.
Equine Piroplasmosis
Equine piroplasmosis is a parasitic infection with significant implications for the horse industry. The disease is caused by two protozoan parasites, which are transmitted to horses through the bite of infected ticks.
The two parasites responsible for piroplasmosis, or babesiosis, infections are:
- Babesia caballi
- Theileria equi (previously known as Babesia equi)
These parasites primarily target equid red blood cells, leading to hemolytic anemia. This is a condition where red blood cells (erythrocytes) are destroyed faster than they can be produced by the bone marrow, resulting in decreased oxygen delivery to tissues.
Equine piroplasmosis has been identified in many regions around the world. There have been isolated outbreaks in the U.S., but the disease has not been identified in Canadian horses. [3]
Many countries mandate that horses are tested and certified as free from EP before they can be imported or travel across borders.
Clinical Signs
Horses affected by EP can present a range of clinical signs. While some are asymptomatic or show only mild symptoms, others can become severely ill. The mortality rate among horses infected with EP is up to 50%. [4]
After a tick bite, it typically takes about 12-19 days for symptoms of T. equi infection to appear and 10-30 days for those of B. caballi. Clinical signs associated with EP include: [2][5][6]
- Acute or intermittent fever exceeding 40°C (often accompanied by sweating, congested mucous membranes and a fast heart rate, or tachycardia)
- Anemia
- Jaundice (yellowing of mucous membranes)
- Lethargy & exercise intolerance
- Weight loss or anorexia
- Labored breathing
- Pale mucous membranes
- Dark urine
- Enlarged spleen
- Gastrointestinal issues (constipation, diarrhea, colic)
- Organ dysfunction
If left untreated, EP can lead to the development of various complications. Affected horses can display intense anemia and fatigue, exhibit edema (fluid-induced swelling) in their lower limbs or suffer from organ damage. [5]
Laboratory Findings
Once the protozoan parasites linked to EP enter the horse’s bloodstream, they start breaking down the red blood cells. A blood test on horses with an EP infection might reveal the following laboratory findings: [2][7]
- Low erythrocyte (red blood cells) counts
- Low hemoglobin
- Low platelets
- High bilirubin
These laboratory results indicate hemolytic anemia in the horse, which can lead to a shortage of red blood cells. [2] These cells are crucial for delivering oxygen to the body’s tissues and their destruction can have wide-ranging impacts on equine health.
Chronic Piroplasmosis
Chronic equine piroplasmosis occurs when a horse is infected with protozoa but does not show clinical signs of disease. The affected horse may have recovered from an acute episode of piroplasmosis in the past, but parasites persist in the bloodstream. [6]
Horses with chronic infection remain in a carrier state, meaning they can transmit the parasites to other horses through tick bites. While they are usually asymptomatic, they may display a poor appetite, poor performance and weight loss. [5]
Carrier horses infected with B. caballi may clear the parasite from the bloodstream spontaneously within 12-42 months of initial infection, whereas T. equi does not spontaneously clear from the body. [6]
Transmission
Various tick species can carry and spread these protozoa, notably those from the Hyalomma, Rhipcephalus, and Dermacentor genera. While ticks from the Ixodes, Haemaphysalis, and Amblyomma genera are also suspected vectors, further research is needed to confirm this. [2][8]
When ticks carrying the disease bite horses, they introduce tiny protozoan parasites from their saliva into the horse’s bloodstream. These parasites travel to the horse’s liver and spleen, where they invade the red blood cells. [2][9]
Inside these red blood cells, the parasites reproduce and multiply, leading the cells to eventually burst. The released protozoa go on to infect other blood cells. [1]
Transplacental Transmission
Transplacental transmission occurs when a parasite is passed from a pregnant mare to the developing fetus through the placental barrier.
This mode of transmission has been reported in cases of EP, and may lead to the birth of a sick foal, stillbirth or abortion. [2][10]
Geographical Distribution
The parasites responsible for EP can be found in various regions around the world. This disease is prevalent in both tropical and temperate climates, including South and Central America, Europe, Africa and Asia. [2][5]
Equine piroplasmosis is not commonly found in the United States, Canada, Japan, or Iceland. While Australia used to be free of the disease, it is now present in some regions of the continent due to the import of infected horses. [4]
Environmental Influence
Environmental factors influence the life cycle, distribution, and abundance of ticks. These blood-sucking arachnids are active in temperatures of 4 to 44°C (39.2°F to 111.2°F), meaning they can be active in most seasons. [11]
As temperatures climb above 10°C (50°F) and humidity rises, ticks not only become more active but also multiply at a faster rate. However, when it gets excessively hot, ticks tend to detach from their hosts and seek shelter in the soil to escape the heat.
In regions with temperate climates, the occurrence of EP peaks during the warm and humid months of spring and summer. In tropical or subtropical areas, the disease might be seen consistently throughout the year, lacking a clear seasonal trend.
There’s a notable connection between increasing temperatures and heightened tick activity. Elevated warmth can speed up the lifecycle of ticks, boost their metabolism, and prolong the period they remain active. [11]
Such conditions favor the strong survival of tick populations, resulting in denser tick concentrations and a longer duration of activity. This, in turn, can increase the risk of tick-borne illnesses affecting both humans and animals. [1][12]
Diagnosis
To confirm a diagnosis of EP in a horse, a blood sample needs to be taken and sent for testing. Your veterinarian will diagnose EP based on the presence of protozoan parasites in the blood.
Blood Smear
Traditionally, protozoan parasites were detected using a Giemsa-stained blood smear. Under a microscope, this method reveals Babesia organisms within the infected red blood cells. [10]
However, this method can sometimes produce inaccurate results, particularly in carrier horses with low parasite loads. Protozoa concentrations in the bloodstream of an infected horse can vary and may not be detected on a single blood smear.
Polymerase Chain Reaction (PCR)
The Polymerase Chain Reaction (PCR) test is a method that multiplies a particular segment of DNA or RNA from a tiny sample, enabling the creation of millions of replicas of the desired sequence. It has a broad range of uses across different fields. [13][3]
Due to its high sensitivity, PCR can identify even minute amounts of parasite DNA or RNA in a horse’s blood. This makes it especially valuable for spotting early-stage or minor infections that might go unnoticed with traditional blood smear tests. [1][10]
Serology
Serological tests are used to detect antibodies to parasites produced by the horse’s immune system. These tests can help identify horses with active infections and horses that were previously exposed to the parasites. [10]
However, these tests are typically employed in epidemiological research and not used as diagnostic tools. This is because of their inability to distinguish between previous and ongoing infections and the absence of standardized procedures for diagnosing individual horses.
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