Equine Piroplasmosis

Lauren Lewis and Pete D. Teel

Department of Entomology

Texas A&M University

Introduction

Equine Piroplasmosis (EP) is a tick-borne disease of equids (horses, mules, donkeys, and zebras)

that was first reported in South Africa in the late 20th century. EP has also been referred to as biliary

fever, equine babesiosis, and equine malaria. EP has a geographic distribution throughout parts of

Africa, the Middle East, Asia, Central and South America, Mexico, the Caribbean and Europe. Countries

such as the US, Canada, Japan, Iceland, New Zealand, and Australia are not endemic and have strict

regulations to prevent the introduction of EP. The mortality rate varies between endemic and non-

endemic countries. Depending on the condition of the horse, treatment, and pathogen, there is a 5 to 10

percent mortality rate among horses native to endemic countries. Naïve mature horses introduced into

endemic countries have a 50 percent or greater mortality rate.

EP was considered eradicated from the US in 1988 and is now categorized as an exotic disease.

A recent increase in awareness and surveillance of EP in the US was due in part to the discovery of

infected horses on properties in Missouri, Texas and Florida. EP impacts breeding, working and

performance horses through travel restrictions, quarantine, and serological testing requirements that

vary from state to state. The recent increase in serologically positive horses has caused the

development of control strategies and guidelines for managing infected and exposed horses.

Seropositive horses are carriers of EP and although they exhibit no clinical symptoms, they still serve as a

source of infection. Since there are no drugs or vaccines available to treat infected animals and provide

protection, EP remains a major constraint to the movement of horses between states in the US and

between countries. In the US, EP is a reportable disease requiring veterinarians to report suspect cases

to state and federal authorities.

Transmission

EP is caused by the hemoprotozoan pathogens Theileria equi and Babesia caballi which are

transmitted by tick-vectors that tend to be region specific. Infections with T. equi , which are more

geographically widespread than with B. caballi, are transmitted by four Dermacentor species, four

Hyalomma species, and five Rhipicephalus species. In contrast, Babesia caballi is known to be

transmitted by seven Dermacentor species, six Hyalomma species, and two Rhipicephalus species.

Dermacentor (Anocenter) nitens (the tropical horse tick) is the only known natural vector of B. caballi in

the US and its distribution is limited to the southern parts of Florida and Texas. Experimental laboratory

transmission of B. caballi and T. equi has been achieved with D. albipictus (winter tick), D. variabilis

(American dog tick) and Rhipicephalus (Boophilus) microplus (southern cattle tick).There is recent

evidence from field collected ticks that Amblyomma cajennense and D. variabilis may be included as

natural vectors of T. equi. The US has suitable climates for foreign tick vectors, thus vigilance in tick

surveillance and animal testing are important elements of biosecurity.

In general, ticks become vectors of a disease when they ingest red blood cells while feeding on

an infected host. The pathogens B. caballi and T. equi can be transmitted from one tick generation to

the next via infection of the ovaries from the female tick, a process called transovarial transmission.

They can also be transmitted from one stage to the next through transtadial infection, such as larva to

nymph, or nymph to adult. An infected tick whose blood meal is interrupted by being groomed off of its

original host may attach to another host (without molting) and successfully transmit these pathogens.

Tick vectors can be classified by their life cycle differences. Some tick vectors complete their life

cycle by feeding as larva, nymph and adult on a single host animal over the course of 20-25 days and are

referred to as one-host ticks. Other types of ticks require a separate blood meal for each stage and are

classified as three-host ticks. After each blood meal these ticks return to the soil-vegetation

environment to molt or to lay eggs. In the three-host tick life cycle, weeks to months may pass between

blood meals taken on different host animals, thus sustaining a source of infection for extended periods.

Since the three-host tick requires more animals in its life cycle, it may spread infection more quickly than

the one-host tick if equines serve as the predominate hosts through multiple tick stages.

Both pathogens can also be transmitted via used needles, syringes, and blood transfusions.

Contaminated dental or tattoo equipment can also transmit EP. Horses that serve as blood donors

should be tested for EP. Poor biosecurity and risk management allows iatrogenic transmission of EP

between horses. Occasionally, humans can be infected from ticks.

Clinical Signs

The clinical signs of EP vary from mild to severe. They include fever, anemia, jaundice, blood

urine, and sudden death. Carriers of the infection in the chronic phase of the disease can appear normal.

T. equi can cause transplacental infection, abortion, stillbirths and neonatal EP. Infections with T. equi

are more severe than with B. caballi. Incubation period of EP is 12 to 19 days when caused by T. equi

and 10 to 30 days when it is caused by B. caballi. A level of active immunity from disease develops

following acute infection. Foals can acquire passive immunity through colostrums. Previously infected

horses do not usually develop EP if re-infected. Infected horses remain infected for life. EP is most

common in the summer and fall but cases can occur throughout the year, these seasonal changes are

often associated with seasonal activities of tick vectors.

Diagnostic Testing

EP diagnostic tests vary due to the different stages of the infection. Parasites can be visualized

with light microscopy of blood- smear preparations stained with Giemsa’s, Wright’s, or Diff-Quick stains.

These preparations must be thoroughly examined as the number of infected cells may be low, even in

acute cases. Babesia caballi exhibits large pyriform trophozoites, usually in pairs and joined at the

posterior end, while T. equi usually exhibits swollen pyriform trophozoites, usually in tetrads or in the

form of a Maltese Cross. Typically, from 1-10% of cells may be found parasitized in these preparations.

Serological tests include complement fixation (CF), indirect fluorescent antibody (IFA), cELISA, and

Western Blotting. The IFA test and the competitive ELISA are used for import testing. The IFA test can

differentiate between T. equi and B. caballi. Before collecting or sending samples the proper authorities

should be contacted, such as state or federal authorities. Texas Veterinary Medical Diagnostic

Laboratory offers testing for EP for intra-state and inter-state purposes. The National Veterinary Services

Laboratory handles testing for suspect cases and international export. Molecular tests for detecting the

presence of DNA specific to either B. caballi or T. equi have also been developed for research purposes.

These tests rely upon the amplification of the DNA by polymerase chain reaction (PCR) using specific

primers that cleave specific pieces of DNA unique to these pathogens, and subsequent identification by

various methods including DNA sequencing.

Prevention

The first step in prevention is to assess the risk of exposure based upon country and regional

status of EP case activity. In both endemic and EP-free countries, EP is controlled by pesticide use,

vegetation free zones, avoidance of tick infested areas and pre-purchase exams. Pre-purchase

examinations for ticks and testing for evidence of infection are imperative for controlling the movement

of EP. Regularly using acaricide/insecticide treatments will help control the tick population as well as

reducing animal contact with wildlife. It is necessary that animals are inspected after field exposure,

paying particular attention to preferred tick feeding sites such as nasal openings, ears, mane, tail,

perianal area, genital areas and auxiliaries. If ticks are found, safely remove and dispose of them. A

variety of acaricides and formulations are available for protecting horses from ticks

(http://tickapp.tamu.edu) and use of these materials should consider preferred feeding sites of ticks on

equines. Weed and brush management around barns and paddocks is beneficial to minimize tick

exposure by reducing tick habitat. With the proper steps, EP can be controlled.

Treatment

Approaches to treatment of EP depend on whether the country is endemic or non-endemic. In

endemic countries, the goal is often control by treatment of the disease. In contrast, the goal in EP-free

countries is to detect, confine and eliminate sources of infection. Imidocarb and diminazene are the

most commonly used treatments but are not legal in US and their efficacy is questionable. In EP-free

countries, infected animals are placed in quarantine and/or euthanized.

References:

“Iowa State Center for Food Security & Public Health Brochure”

http://www.cfsph.iastate.edu/Factsheets/pdfs/equine_piroplasmosis.pdf

“Gray Book on Foreign Animal Diseases”

p. 147 “Babesiosis”

http://www. aphis .usda.gov/emergency_response/downloads/nahems/fad.pdf

“USDA Website”

http://www.aphis.usda.gov/animal_health/animal_diseases/piroplasmosis/

“Equine Babesiosis-Review”

http://www.vet.uga.edu/VPP/clerk/edwards/index.php

“The TickApp”, a mobile smart phone app for ticks of Texas and the Southern Region

http://tickapp.tamu.edu

USDA suggestions for “Protecting your horses”

http://www.aphis.usda.gov/animal_health/animal_diseases/piroplasmosis/downloads/ep_protect_your_horses_en_sp.pdf

A Literature Review of Equine Piroplasmosis

http://www.aphis.usda.gov/animal_health/animal_diseases/piroplasmosis/downloads/ep_literature_review_september_2010.pdf

Equine Piroplasmosis and the World Equestrian Games

http://www.aphis.usda.gov/animal_health/animal_diseases/piroplasmosis/downloads/ep_2010_weg_wp.pdf

USDA, APHIS Factsheet

http://www.aphis.usda.gov/publications/animal_health/content/printable_version/fs_equine_piro.pdf