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. 
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 Background
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. 
Many countries mandate that horses are tested and certified as free from EP before they can be imported or travel across borders.
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%. 
- Acute or intermittent fever exceeding 40°C (often accompanied by sweating, congested mucous membranes and a fast heart rate, or tachycardia)
- 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. 
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: 
- 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.  These cells are crucial for delivering oxygen to the body’s tissues and their destruction can have wide-ranging impacts on equine health.
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. 
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. 
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. 
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. 
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. 
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. 
Transplacental transmission occurs when a parasite is passed from a pregnant mare to the developing fetus through the placental barrier.
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. 
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. 
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. 
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. 
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. 
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.
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. 
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. 
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. 
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. 
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.
- Complement fixation test (CFT): While this diagnostic test is highly specific, it can sometimes yield false positives or false negatives, and fail to detect infections in the incubation period. 
- Indirect immunofluorescent antibody test (IFAT): Compared to the CFT test, this test is more sensitive and can detect the disease even in chronic stages when protozoa levels in the blood are minimal. However, the interpretation of results is subjective and not standardized.
- Enzyme-linked immunosorbent assay (ELISA): Several ELISA tests can detect EP infection from T. equi and B. caballi, but there is a lack of standardization in the interpretation of results.
Effective treatment of EP aims to control symptoms, reduce parasite loads in the horse’s body, and prevent transmission of the disease. Achieving complete eradication of the parasites is challenging, and there is no definitive cure for EP.
In endemic regions where the parasites that cause EP are common, treatment goals include alleviation of clinical signs and prevention of fatalities. In non-endemic regions, such as North America, the goal of treatment is to fully clear the infection to avoid the spread of parasites to other horses. 
Imidocarb dipropionate (ID) is an antiprotozoal drug, or medication used to treat infections caused by protozoa. This drug should only be administered under the direct supervision of a veterinarian.
ID is the most widely used and effective treatment for horses with EP. However, EP infections attributed to B. caballi parasites are usually more responsive to this medication than EP infections associated with T. equi. 
B. caballi infections are treated with two intramuscular (IM) injections of ID, administered 24 hours apart. The typical dosage range is 2 or 2.5 mg/kg of bodyweight.
Imidocarb dipropionate is considered safe for use in horses, but higher dosages can lead to adverse effects. Horses treated with this drug may exhibit the following side effects: 
- Increased perspiration
- Gastrointestinal symptoms, such as colic or diarrhea
Horses may be given a non-steroidal anti-inflammatory drug (NSAID) alongside this medication to reduce inflammation or pain.
Achieving complete parasite clearance may not be possible with one treatment cycle, so several rounds of ID treatments may be required. Some parasites are resistant to this medication.
Supportive treatment for horses with EP aim to manage symptoms, prevent complications and help the horse’s immune system fight the infection. Components of supportive treatment include: 
- Rest and isolation: Infected horses should be isolated from healthy horses to prevent disease transmission. Rest and limiting physical activity can help to conserve the horse’s energy and promote recovery.
- Fluid Therapy: Intravenous or oral fluid administration helps to maintain hydration status and manage dehydration caused by fever and anemia.
- NSAIDs: Non-steroidal anti-inflammatory drugs (NSAIDs) may be prescribed to reduce fever, inflammation, and pain associated with EP.
- Blood Transfusions: Severe anemia resulting from the destruction of red blood cells may necessitate blood transfusions to improve oxygen-carrying capacity.
Prevention of EP is important due to the health implications and potential for spread of this infectious disease within horse populations.
The primary preventive measure is controlling the tick vectors responsible for transmitting the disease. In regions where EP is prevalent, regular screening of horse populations can aid in early detection and containment of the disease.
Tick control is critical to the prevention of EP, as ticks are the primary vectors responsible for transmitting the disease. Effective tick control measures can help limit parasite transmission and prevent further spread of this disease.
Ticks typically feed for an extended period of time before they transmit pathogens, such as bacteria or protozoa, to their host. Horses should be checked for ticks regularly, because early detection is key to limiting disease transmission. 
Ticks tend to attach to areas of the horse’s body where the skin is thin, which include the horse’s chest, underbelly, inner flank, ears, mane and tail. Check your horse’s whole body for ticks daily, especially if they are kept outdoors on pasture.
Small ticks, or ones that match the color of your horse’s coat, may be difficult to see. Run your hand over your horse’s skin and coat, and gently scratch the skin to feel for bumps, can help to detect ticks.
If you find a tick, remove it immediately. Using tweezers or a tick-removal tool, grasp it by the head (as close to the skin’s surface as possible) and pull it away from the body with steady pressure.
Ensure that you have removed all of the tick from your horse’s skin. When a tick attaches to the skin, it buries its mouthparts, into the host to feed on blood. If not removed properly, these mouthparts can break off and remain embedded in the skin, causing irritation, infection, or an inflammatory response.
Topical acaricides are insecticide products formulated to control and treat external parasites like ticks. They are applied directly to the horse’s skin or coat and come in various formulations, including sprays, powders, wipes or shampoos. 
The active ingredients in topical acaricides include pyrethrins, pyrethroids, cypermethrin, or other insecticides. These chemicals act on the nervous system of the parasites, leading to paralysis and death.
Be aware that products containing Amitraz (Mitaban®) are toxic for horses and should not be used for pest control. Always adhere to the instructions on the product label and seek advice from a veterinarian to choose the best topical acaricide for managing ticks in your horses.
Reducing the number of ticks in pastures where horses graze is crucial for preventing tick infestations and reducing the risk of tick-borne diseases in horses.
Strategies to control tick populations in pastures include:
- Pasture maintenance: Maintain your pastures by regularly mowing grass and keeping vegetation trimmed. Ticks prefer tall grass and dense vegetation. 
- Fencing: Install fencing around the pasture perimeter to prevent the entry of wildlife that may be carrying ticks.
- Leaf litter: Clear leaf litter and debris from the pasture regularly. Ticks seek shelter in leaf piles, so removing them can disrupt their habitat. 
- Limiting exposure to new horses that may be harboring parasites
- Avoiding sharing tack and other equipment, which can spread diseases between horses
- Training caretakers and handlers about the importance of hygiene and best practices in horse management
- Ensuring that medical instruments are thoroughly cleaned and disinfected between uses
- Preventing re-use of needles, which can transmit blood-borne parasites and pathogens between horses
- Regularly screening horses to detect and address any potential infections early on
Important: All needles used for medical procedures, including vaccinations, blood tests and intravenous injections, must be sterile and used only once.
Quarantine measures are important to prevent the spread of EP to healthy horse, especially in non-endemic regions (areas where the disease is not common). If piroplasmosis is detected, affected horses must be quarantined immediately. 
Horses that have recently arrived to a new barn should undergo a quarantine period, keeping them isolated from other horses to minimize the risk of disease spread. During this time, their vital signs should be regularly checked. 
Restricting insect activity is vital for an effective quarantine. Using topical acaricides can help reduce the risk of insect-transmitted disease outbreaks within a group.
Mortality rates for horses infected with EP can be as high as 50%, and completely eradicating the infection is difficult. The prognosis for EP is guarded, hinging upon early detection, timely intervention, and effective treatment. 
The changing climate and globalization of the equine industry present significant challenges in controlling tick-borne infections, such as EP. 
Preventive measures, including tick control and biosecurity practices, play a vital role in preventing the spread of EP and protecting the equine population. Veterinary care is important for horses with EP to mitigate potential complications.
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