Equine veterinarians have a phrase they repeat too often in emergency situations: “if only the horse had been vaccinated.” It is not a cliché — it is a pattern. Tetanus kills horses that would have lived with a twenty-dollar annual vaccine. Internal parasites silently damage intestinal tissue for months before any visible symptom appears. Vaccination and strategic deworming are not bureaucratic checkboxes on a horse care list. They are the two most fundamental preventive health measures any horse owner must establish and maintain from day one.

Here is what to vaccinate against, when to vaccinate, how modern deworming actually works, and why the old calendar-based approach was causing more harm than good.

Core Vaccines: Non-Negotiable Regardless of Where You Live

The American Association of Equine Practitioners (AAEP) defines core vaccines as those that every horse should receive regardless of location, use, or lifestyle — because the diseases they prevent are severe, potentially fatal, and present in essentially all geographic regions.

Tetanus is the most critical. Caused by Clostridium tetani — a bacterium that lives in soil and manure — tetanus enters the body through wounds. Horses are among the most susceptible domestic species to tetanus toxin. Without vaccination, mortality in infected horses exceeds 80%. With annual vaccination, tetanus is nearly eliminated as a practical risk. After any deep wound, puncture injury, or surgical procedure, an additional tetanus booster is indicated even in vaccinated horses.

Eastern and Western Equine Encephalomyelitis (EEE and WEE): Mosquito-borne viral diseases causing fatal brain inflammation. Both are endemic across broad regions of North and South America. EEE is particularly lethal, with case fatality rates often exceeding 90% even with aggressive treatment. Annual vaccination before mosquito season is standard care.

West Nile Virus: Another mosquito-borne encephalitis, now documented in all contiguous US states. Annual vaccination is recommended; semi-annual dosing is appropriate in high-risk zones or where mosquito exposure continues year-round.

Equine Influenza: Highly contagious via respiratory droplets and airborne transmission. Any horse with regular contact with other horses — at boarding facilities, competitions, trail rides, or shared training — is at ongoing exposure risk. Semi-annual vaccination is standard for socially active horses.

Rabies: Uniformly fatal and transmissible to humans handling an affected horse. While less common in horses than in companion animals, annual vaccination is strongly recommended across the United States.

Risk-Based Vaccines: What to Discuss With Your Veterinarian

Beyond the core list, your veterinarian will recommend additional protection based on your horse’s region, use, reproductive status, and exposure profile.

Equine Herpesvirus (EHV-1 and EHV-4): EHV-1 causes respiratory disease, late-term abortion in mares, and — in its neurotropic form — a devastating paralytic disease (Equine Herpesvirus Myeloencephalopathy, or EHM) that is frequently fatal. EHV-4 primarily causes respiratory disease. Vaccination is particularly important for breeding mares (pregnant mares receive boosters at months 5, 7, and 9 of gestation) and for competition horses that travel and comingle regularly.

Strangles (Streptococcus equi): A bacterial upper respiratory disease causing severe lymph node abscesses, high fever, and copious nasal discharge. Highly contagious in group settings. Vaccination is appropriate for horses with frequent exposure to unknown horses — sales, multiday competitions, clinics.

Equine Viral Arteritis (EVA): Relevant primarily for breeding operations, particularly stallions and mares in active reproductive programs.

Botulism: Recommended in areas with high soil concentrations of Clostridium botulinum, or for horses fed fermented feeds (haylage, silage).

The Vaccination Schedule in Practice

For a healthy adult horse in routine management:

Annually: Tetanus, EEE/WEE, West Nile Virus, rabies

Semi-annually: Equine influenza (horses with regular social contact); EHV-1/4 (competition horses and breeding mares)

Foals require a separate primary vaccination series beginning at 3–6 months of age, depending on the dam’s vaccination history. Foals from vaccinated mares receive maternal antibodies via colostrum that provide early protection but also interfere with the foal’s own immune response — the timing of the primary series accounts for this window.

Pregnant mares receive pre-foaling boosters — tetanus, EEE/WEE, influenza, rhinopneumonitis — during the final 4–6 weeks of gestation. Higher antibody concentrations in the mare at foaling produce stronger colostral protection for the foal in its first weeks of life before it can be vaccinated directly.

Keep a vaccination record for every horse: date, product name, lot number, injection site, and administering practitioner. Many competition venues require documented proof of current influenza and herpesvirus vaccination, and inspection at interstate crossings may require a current health certificate with vaccination history.

How Modern Deworming Works

For decades, the standard deworming protocol was calendar-based: treat every horse every 8–12 weeks, rotating between drug classes to prevent resistance. The approach was simple, widely followed, and actively damaging.

Routine universal treatment was the primary driver behind the anthelmintic resistance now documented worldwide. Benzimidazoles (fenbendazole, oxibendazole) show significant resistance in cyathostomin populations in essentially every studied region. Pyrantel resistance is emerging. Even ivermectin, long considered broadly reliable, is showing reduced efficacy in some populations.

Current guidelines from the AAEP and equine parasitology researchers worldwide recommend targeted selective treatment based on fecal egg counts (FEC):

  • Low shedder (<200 EPG): No treatment needed; recheck in 6 months
  • Moderate shedder (200–500 EPG): Increased monitoring; treatment may or may not be indicated depending on age and clinical history
  • High shedder (>500 EPG): Treatment indicated

A Fecal Egg Count Reduction Test (FECRT) — performed 14 days after treatment — verifies whether the anthelmintic used was effective. If egg counts don’t drop by at least 95%, resistance in your horse’s parasite population is likely and should be communicated to your veterinarian before the next treatment decision.

The Parasites You Are Managing

Cyathostomins (small strongyles): The most prevalent and clinically significant internal parasites in adult horses. They encyst as larvae in the intestinal wall for months or years, emerging en masse — a syndrome called larval cyathostominosis — causing severe diarrhea, rapid weight loss, and protein-losing enteropathy that can be fatal. Adult cyathostomins show substantial benzimidazole resistance; moxidectin remains the most effective drug against encysted larvae.

Parascaris equorum: The equine roundworm, primarily dangerous in foals and young horses. Adult worms can cause intestinal obstruction. Notably, ivermectin efficacy against Parascaris has declined in foal populations in many regions; oxibendazole is often the more reliable choice for young horses.

Strongylus vulgaris (large strongyle): Arterial-migrating larvae that traverse mesenteric arteries, causing thrombosis and ischemic colic. Prevalence dropped dramatically with ivermectin use beginning in the 1980s, but the parasite has not been eliminated and requires ongoing monitoring.

Pinworms (Oxyuris equi): Cause intense perianal pruritus — the horse rubs its tail vigorously, damaging the tail head and dock area. Not life-threatening but chronically uncomfortable and immediately recognizable. Notably, standard fecal egg counts do not detect pinworm eggs (they are deposited around the anus, not passed in feces); sticky tape perianal testing is more diagnostic.

Bots (Gasterophilus spp.): Larvae of bot flies that attach to the stomach lining. Controlled effectively with ivermectin or moxidectin administered once after the first killing frost in fall — at that point, adult flies are no longer laying eggs and all larvae are accessible to treatment.

Anthelmintic Resistance: Why It Matters

Studies from North America, Europe, and Australia document significant benzimidazole resistance in cyathostomin populations at the majority of farms studied. Using the most potent available drug indiscriminately — treating horses that don’t need treatment — accelerates resistance development and will eventually leave the industry with drugs that no longer work.

Targeted selective treatment preserves anthelmintic efficacy for the horses that genuinely need it. It also saves money, reduces unnecessary chemical exposure, and aligns with current veterinary recommendations. If you are still treating every horse on a fixed 8-week calendar regardless of egg count, a conversation with your veterinarian about updating your protocol is overdue.

Vaccine Storage and Post-Vaccination Management

Vaccines are biological products that degrade with improper handling. Most equine vaccines require refrigeration at 35–46°F (2–8°C), protected from direct light, and must never be frozen. A vaccine transported without refrigeration for several hours — or left in a vehicle during a farm call on a warm day — may have lost all efficacy before administration.

Practical guidelines for vaccination day:

  • Check the expiration date and storage label before opening
  • Allow refrigerated vaccine to equilibrate briefly before injection
  • Record the product, lot number, date, and injection site
  • No intense exercise for 24–48 hours post-vaccination

Normal post-vaccination reactions include mild local soreness, slight lethargy, and reduced appetite lasting 12–24 hours. Contact your veterinarian promptly if you observe pronounced swelling worsening after 48 hours, generalized hives (urticaria), respiratory distress, persistent fever above 103°F (39.5°C), extreme weakness, or collapse within minutes of injection. Anaphylaxis is rare but possible; having your veterinarian’s number accessible on vaccination day is basic preparedness.

The calendar of vaccination and strategic deworming is not complex. It requires consistency, accurate documentation, and a veterinary relationship built before you need it for an emergency. Establish that relationship in the horse’s first weeks at your facility, and these protocols will feel routine within a year.