Equine Babesiosis (Piroplasmosis)—A Barrier in the International Movement of Horses

Equine babesiosis (piroplasmosis), a disease of equids in many regions of the world, is caused by the tick-borne hemoprotozoans Babesia equi and Babesia caballi. The central concern is the risk and consequences of entry of these parasites through international movement of horses into the United States, where equine piroplasmosis is limited to Florida and apparently not established. This concern continues to have dramatic impacts, such as those occurring during the 1996 Olympics. The horse population within the United States is presumed to be entirely susceptible to infection; therefore, management safeguards against the entry and/or dissemination of piroplasmosis continues. For owners whose horses become infected while residing in countries with endemic piroplasmosis, the main consequence of infection is that their horses are restricted from re-entry into the United States. Regardless of on which side of the issue you stand, the central question remains: is the regulatory impact on the international movement of horses justified? Below are discussions of the key factors, which must be understood to adequately address this question. The first and foremost factor concerns the tick population within the United States and its ability to transmit these parasites. Since ticks are biological vectors of B. equi and B. caballi, it is assumed that without ticks capable of transmission, infection cannot occur.

In the United States, B. caballi is known to be experimentally transmissible by three native tick species: Anocentor nitens, Dermacentor albipictus, and D. variabilis. In a recently completed study, five North American tick species—Amblyomma americannun, Boophilus microplus, Dermacentor andersoni, D. occidentalis and D. variabilis—were tested for their ability to transmit B. equi. Intrastadial transmission was demonstrated by D. variabilis males and transstadial transmission by B. microplus adults. These collective data indicate that the possibility exists for natural transmission of equine piroplasmosis within the United States. All of these ticks are native to the United States, although B. microplus has been prevented, through acaracide use, from becoming re-established north of Mexico. Obviously, vector control by use of acaracides is a viable part of a piroplasmosis control program; however, there are concerns of the development of acaracide resistance. While the control of potential tick vectors is one method to prevent transmission of equine piroplasmosis, the United States has adopted the strategy of preventing infected horses from entering the country based on finding anti-B. caballi and/or anti-B. equi antibodies in horses presented for importation. One of the re-occurring problems of this strategy is that the majority of horses that are found to be serologically positive for one or both of these parasites are in other measurements clinically normal. Stated more clearly, the majority of infected horses do not have indications of decreased performance.

This collective information indicates that the potential for tick transmission of equine piroplasmosis exists within the United States. Is the regulatory impact of B. equi and B. caballi infection on the international movement of horses justified? There are also many other factors that influence whether infection and clinical disease caused by these parasites could become endemic in the United States. We have minimal understanding concerning influences of a number of factors that determine whether or not an infected horse shows clinical disease. Immunogenetics of the horse, the virulence of the infecting strain, the tick burden, the tick infection rate and the challenge dose are factors impacting disease expression and possibly transmission.

Therefore, since there are tick vectors within the United States that have been shown experimentally to transmit B. caballi and B. equi, and considering our lack of knowledge concerning these other factors that influence the establishment of endemicity and disease expression, it must be concluded that the restricted movement into the United States of horses infected with one or both of these parasites must continue. An exciting current area of research is examining the development of these parasites within their tick vectors with the goal of developing vaccines that would block transmission. This endeavor may one day lead to a vaccine that would aid in alleviating the burden of equine piroplasmosis on the international movement of horses.

Don Knowles, dknowles@vetmed.wsu.edu
Animal Disease Research Unit, ARS-USDA, Washington State University.