Green Prairie Animal Hospital · Large Animal & Equine Care · Mason County, IL
You’re getting ready to haul a horse to a show or sale and someone asks for the Coggins. You know you need it — but do you actually know what it’s testing for and why it matters beyond the paperwork?
Equine Infectious Anemia is one of the most serious viral diseases affecting horses in Illinois. There’s no treatment and no vaccine. Understanding what it is, how it spreads, and what a positive test means for your operation is something every horse owner in central Illinois should have straight before they need it.
What EIA Actually Is
Equine Infectious Anemia — EIA, sometimes called swamp fever — is a viral disease caused by a lentivirus, a type of slow-moving retrovirus that attacks the immune system. Once a horse is infected, the virus stays in the animal’s body for life. There is no cure. There is no vaccine. Management and testing are the only tools available.
The virus targets red blood cells and the immune system, causing episodes of fever, anemia — a drop in red blood cell count that reduces the body’s ability to carry oxygen — and weakness. Some horses show acute severe symptoms. Others become chronic carriers that look completely healthy but can still transmit the disease to other horses.
That last part is what makes EIA particularly dangerous from a herd management standpoint. A horse that looks fine can be actively spreading the virus. You cannot identify a carrier by looking at them. That’s exactly why Coggins testing exists.
How EIA Spreads
EIA is transmitted primarily through blood-to-blood contact. The most common route is large biting insects — horseflies and deerflies in particular — that feed on an infected horse and then immediately feed on another. These insects don’t travel far between meals, which is why horses in close proximity to an infected animal are at the highest risk.
Other transmission routes include contaminated needles, surgical equipment, or blood products. This was a more significant historical concern before single-use needle practices became standard, but it’s still worth noting for operations that do their own injections.
Mare to foal transmission can also occur — either across the placenta during pregnancy or through colostrum, the antibody-rich first milk, after birth. Foals born to EIA-positive mares should be tested.
The virus does not spread through casual contact — shared water sources, fence-line contact, or handlers moving between horses are not significant transmission routes. The blood-to-blood requirement is specific, but biting insects in central Illinois make that route very real during summer months.
Symptoms to Watch For
EIA presents differently depending on the stage of infection and the individual horse. Three clinical patterns are recognized.
Acute EIA hits fast and hard. High fever — often above 104°F — severe depression, rapid weight loss, swelling of the lower legs and abdomen, and a sharp drop in red blood cell count. Some horses die during an acute episode. Others survive and move into the chronic or inapparent carrier phase.
Chronic EIA involves recurring episodes of fever and anemia, often triggered by stress — heavy work, shipping, illness, or other physical demands. Between episodes the horse may appear normal. Weight loss that keeps coming back despite adequate feed, recurring fever with no obvious cause, and progressive weakness are the signals that warrant testing even without a known exposure.
Inapparent carriers show no clinical signs at all. They’re the most common form of EIA-positive horse encountered through routine Coggins testing. These horses look healthy, perform normally, and would never be identified without a blood test. They are still infectious to other horses through biting insects.
If you’re seeing unexplained recurring fever, anemia, or weight loss in a horse — call your large animal veterinarian before assuming it’s something else. EIA is uncommon enough that it’s not the first thing on most people’s minds, but it’s serious enough that ruling it out matters.
The Coggins Test — What It Is and When You Need It
The Coggins test — named for Dr. Leroy Coggins, who developed it in the 1970s — is a blood test that detects antibodies to the EIA virus. A blood sample is drawn and sent to a certified laboratory. Results typically come back within a few days.
A negative Coggins means no EIA antibodies were detected. A positive Coggins means the horse has been exposed to EIA and is infected — there are no false positives on a confirmed Coggins result.
In Illinois, a current negative Coggins is required by law for:
Horse transport across state lines. Sale or transfer of ownership. Entry to shows, races, exhibitions, and trail rides on public or managed lands. Many boarding facilities also require a current negative Coggins for new horses on the property.
The standard Coggins is valid for 12 months in Illinois. Some events and jurisdictions require a test within six months — check the specific requirements before you haul.
Don’t wait until the day before a show to get this done. Lab turnaround takes time, and a last-minute scramble over paperwork is avoidable. Build Coggins testing into your annual spring veterinary schedule and it’s never a problem.
What Happens If a Horse Tests Positive
A confirmed positive EIA test is a reportable finding — meaning your veterinarian is required by law to notify the Illinois Department of Agriculture. This is not optional and it is not a judgment call.
Once reported, the state will work with you on next steps. In Illinois, EIA-positive horses have three options: euthanasia, permanent quarantine on the property of origin, or transfer to a federally approved research facility. There is no option for continued normal use or transport. A quarantined horse must be kept at least 200 yards from other horses — far enough to break the biting insect transmission chain.
This is a hard outcome. But it’s the reality of a disease with no treatment and a permanent carrier state. The goal of the Coggins testing program is to identify positive horses before they expose an entire operation — which is why routine testing matters even when your horses appear healthy.
Green Prairie Animal Hospital’s San Jose Clinic Serves Mason County Horse Owners
For horse owners in Mason County and across central Illinois, Green Prairie Animal Hospital’s San Jose location is your large animal veterinary resource. The San Jose team handles all equine care — wellness, Coggins testing, farm calls, diagnostics, and emergency response.
Coggins testing is straightforward to schedule as part of your annual spring visit. The San Jose team does farm calls across Mason County, so if hauling isn’t practical, we can come to you. As an AAHA-accredited practice, the diagnostic protocols behind every test and every farm call meet the highest standards in veterinary medicine.
Don’t wait until you’re standing at a gate being turned away for missing paperwork. Get your Coggins scheduled now.
Call San Jose for Equine Care in Central Illinois
Green Prairie Animal Hospital’s San Jose location is our large animal and equine center for Mason County and central Illinois. Our team handles horses, cattle, livestock, and farm animal care — and we do farm calls.
