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For transmission of the disease to occur, there must be an infectious source, a means of transmission and a susceptible new host.
The EIAV is a blood-borne virus, tests show that it could be present in other bodily fluids from an animal suffering acutely from the disease, but the virus must still enter the bloodstream of a susceptible horse for transmission to occur.
It does not become airborne nor is it transmitted through casual social contact the way other viruses such as EHV can be.
Basically, infected blood has to be transferred from one horse to another within a limited time frame. Historically, the most notable occurrences of the disease proved to have been transferred by people using hypodermic needles, improperly disinfected surgical equipment etc. on more than one horse.
Although it is considered possible for an unborn foal to become infected if its mother suffers an acute episode of the disease during pregnancy and the possibility exists of an infection being transmitted by an acutely ill stallion if the mare suffers a tear during breeding, these possibilities are slight enough to be of little concern to the average horse owner.
Large biting insects, such as horse flies, are capable under the right conditions to transfer enough blood for an infection to be transmitted.
Horses with the acute form of the disease are considered to be the most likely donors of the virus for mechanical transmission by biting flies (Issel and others, 1988). Infected horses need to carry at least 106 infective doses of the virus per ml of their blood in order for biting flies to successfully infect other horses (Issel and others, 1990). Persistently infected (asymptomatic) horses only have 1/250th of this dose level, but horses in the acute phase of the disease may exceed the required infective dose level (Issel and others, 1990).
We don’t deny the possibility that horse flies etc. can contribute to the spread of infection but one must bear in mind that, according to Issel and others (1990) the likelihood that a susceptible horse will become infected with EIA would depend on:
a) The proximity to an infected horse,
b) The quantity of the virus in the blood of the infected horse,
c) The possibility of vector feeding on an infected horses being interrupted and subsequently feeding on an uninfected horse,
d) The volume of infected blood mechanically transmitted by the vector to an uninfected horse,
e) The quantity of the virus which remains infective after the time interval before transmission.
Issel and Foil (1984) extrapolated that approximately 99% of horse flies would be expected to return to their original host to feed again after interruption of feeding if they were released when alternative hosts were at a distance of up to 160 feet (49m). Therefore, a 200 yard (183m) distance between infected and susceptible horses is considered to adequately reduce the potential for transmission of EIA virus by horseflies.
Incidentally, the current information being circulated by the USDA contains statements such as "In general terms, there is a low risk of acquiring EIA from an inapparent carrier of EIAV. In fact, the perceived threat of transmission from this source often exceeds the actual risk by several orders of magnitude." As well as the fact that only 1 (one) horsefly in 6,000,000 (six million) is likely to spread the disease from an asymptomatic (the most common form of the disease) horse. We applaud the USDA for finally publicizing what the researchers have been telling us for years.
In the Real World
The state of Florida allows Positive horses to live in the open air
The following is an excerpt from a letter dated Mar 13, 1996 from Mr. Ralph Weeks in Florida.
“I have 6 mares and 1 stallion that have tested positive.
All of the horses are very healthy and after being away from their mothers for 60 days the colts test negative. Each mare has a colt each year.”
(These horses are in a separate pasture from his other horses with approximately ¼ mile between).
“Of course, other than for breeding purposes having positives is not good. Mine are restricted to the pasture I have them in and the state checks once a year to make sure they are there.”
Note: And yet in Canada’s colder climate and shorter vector season a positive horse is practically not allowed to see the light of day, never mind being allowed to pass on it’s valuable blood line.
The Friends Ranch
F.R.I.E.N.D.S. Florida Research Institute for Equine Nurturing, Development and Safety, Inc.
In the early 1970’s a family in Florida set up a quarantine ranch for Coggins–positive horses. The ranch has housed as many as 100 head of horses at times. The following was condensed from a letter we received in 2001 from Debra Beye-Barwick, Chairman of the Board of F.R.I.E.N.D.S. Inc.
“In the 1970's, the facility had 2 stallions; 1 Coggins positive and 1 Coggins negative. They were bred frequently to the many registered mares of various breeds. All of the foals became Coggins negative after the maternal antibody waned. Many were sold, shown and never returned nor became Coggins positive.
Over the last 30 years, 2 acute EIA cases were admitted to the ranch, in the late 1970's, that were very sick and died. All cases admitted since that time have been asymptomatic. These horses have experienced febrile episodes, but have been diagnosed with other conditions such as respiratory disease, colic or low grade infections. F.R.I.E.N.D.S. has never had a veterinarian attribute the cause of a death in their horse herd to EIA, other than the 2 mentioned acute cases.
Most of the people that have donated horses to the ranch claim their horses weren't sick and found it hard to believe that anything was wrong with them.
The horses are ridden regularly and are housed across the road from the everglades (wetlands preservation property). A large horsefly population exists at the ranch yet none of their negative horses have ever tested positive. Eight horses testing negative in the last 9 years have never tested positive.
Coggins tests are pulled on the negatives monthly. Each of these horses were different breeds and ages, some were ponies.
The ranch currently houses 59 horses (soon to be 60), 2 miniature donkeys, 2 goats & 4 potbelly pigs. Of the 59 horses, 2 are negative "test horses," one of which originally tested positive on 12/22/93 and is now still testing negative; last test was done on 1/30/01. The ranch still has 2 horses from 1973 and 1974, now aged 33 yrs and 34 yrs. Many horse breeds are represented. The horse residing at the ranch the longest died at the age of 34 and he was still negative even after being among the herd for more than 15 years.
The ranch had 2 horses sent to them that were test positive, both were freeze branded and among the herd for years before their retest was done. Of five horses tested that day, two were known to be Coggins negative and the other three test horses were expected to come back positive.
Surprisingly, 2 of the 3 positives came back negative. They have been continually tested since 1999 and they still test negative. One has died of old age, but the other is still alive.”
Note: How dangerous do you think an asymptomatic positive horse really is? Does this sound deadly and devastating, the way it has often been portrayed? Keep in mind, the Friends people didn’t hand pick these horses, they took what was offered. Check out the Friends Ranch at www.eiahorses.org you can see their pictures etc. Do they look sick to you?
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