Page 104 - August 2019

WHAT YOU NEED TO KNOW ABOUT EQUINE INFECTIOUS ANEMIA (EIA)
by Nancy S. Loving, DVM
“A horse diagnosed positive for
EIAV is either euthanized or placed under extremely strict quarantine conditions at least 200 yards away from other equids for the rest of its life.”
Nancy S. Loving, DVM
You’ve probably heard about the increasing inci- dence of equine infectious anemia virus (EIAV) in racing Quarter Horses. A positive diagnosis in a horse has serious repercussions. The virus is signifi- cant in that, much like its Lentiviral cousin the human immunodeficiency virus (HIV), there is no vaccine and there is no cure. A horse diagnosed positive for EIAV is either euthanized or placed under extremely strict quarantine conditions at least 200 yards away from other equids for the rest of its life.
HOW IS IT CONTRACTED?
Blood is the culprit in causing an infection from
one horse to another. Historically, natural transmission occurs when virus is transferred from an infected horse through biting flies, such as horseflies, deer flies, or stable flies. Unlike Flaviviruses such as West Nile virus or Zika virus, EIAV does not replicate in insect tissues and so the infective dose is limited by the amount of blood that is carried on the fly’s large mouthparts. EIAV survives for less than four hours on fly mouthparts.
A horsefly interrupted in its feeding will attack a new host, but the further away the potential new host, the more likely the fly is to return to its original victim. Studies show that if a new horse host is tethered at distances of 50 yards or more, it was predominantly (although not entirely) immune from attack, as the fly usually returns to the original host. Most authorities recommend larger distances, such as 200 yards, to err on the side of safety.
However, a different mode of transmission has become more commonplace in recent years. A discon- certing situation has developed in the USA: Between 2009-2013, of the 1.5-2 million horses tested yearly, an average of 40+ cases tested positive for EIAV. But, recently, there has been an uptick in positive cases. This is due to an increased number of iatrogenic cases, particularly in Quarter Horse racehorses and unsanc- tioned Bush Track racing. (Iatrogenic means that people and medical treatment are a likely source and cause of transmission.)
Prior to 2013, EIAV cases of iatrogenic transmission were not recognized as significant contributors to the number of positive horses. In 2013, about 25% of annual EIAV infections were caused by iatrogenic transmission. In 2014 alone, 54% of EIAV positives were traced back to an iatrogenic origin, which was preventable.
How does an iatrogenic infection occur? It comes through contaminated blood or blood products, through shared needles, or blood contamination of
Natural transmission of EIAV occurs when the virus is transferred from an infected horse through biting flies, but pales compared to human-related spread of the virus.
hands, dental equipment or other instruments that then contact another horse. Residual blood volume
in a used syringe and needle can be 5000-10,000 fold greater than that carried by even the largest horsefly, and the virus is viable for at least four days. Horsefly transmission pales to almost insignificance compared to human-related spread and perpetuation of EIAV. If it were not for iatrogenic transmission, EIAV would be of little significance today.
IS EIAV ON THE RISE?
While this is an insidious disease, it can be controlled as has been demonstrated by the declining number of cases in the USA over past decades due to surveillance. With the development of the Coggins test in the early 1970’s, approximately 4% of all equids tested were positive for antibodies to EIAV. Removal of these test-positive subjects from the population by the beginning of the 1980’s resulted in around 0.5% test-positives; the current incidence is less than 0.01%. From 534 EIA-positive horses identified in 2001, in 2015, only 69 horses of nearly 1.5 million tested were confirmed positive for EIAV. However, the number of cases is beginning to rise again.
Between 2012-2015, 39 racing Quarter Horses were confirmed positive for EIAV in California alone. Epi- demiologic investigations indicate that the majority of the positive horses participate in Quarter Horse racing and have potential exposure to high-risk practices such as sharing of needles and other medical equipment or the use of contaminated blood products.
102 SPEEDHORSE, August 2019
VETERINARY VIEWS