Infectious and parasitic disease: what the evidence says about vaccines, dewormers and biosecurity

Last updated May 2026.

It is 6 a.m. and your barn manager texts you. A confirmed EHV case has popped up at the yard down the road. Your horse boarded there for a clinic three weeks ago. Your phone fills with questions you did not have ten minutes earlier. Is the EHV booster you paid for last spring actually doing anything? Should you call your vet now or wait? What does the test result mean if you ask for one?

Most owners decide about vaccines, dewormers and biosecurity from three sources: the schedule a vet practice circulates, what the yard requires, and a memory of how things were done when they first owned a horse. None of those are bad inputs. None of them are the recent research. The published science on routine equine infectious disease has shifted noticeably in the last five years, and in some places the schedule your barn ran in 2018 no longer matches what studies show.

This guide walks through the practical questions owners actually ask. It keeps a careful line between two very different ideas: "we do not yet have good studies" (absence of evidence) and "we have good studies and the intervention failed" (evidence of no effect). Both come up here. Topics requiring high-containment lab work are out of scope.

Is your dewormer still working?

Probably not as well as it used to. This is the strongest verdict in the whole guide.

For decades, the standard advice was to dose every horse on the yard every eight to twelve weeks. That schedule was built when ivermectin and moxidectin (the two main "wormers" most owners use) cleared cyathostomin eggs for two to four months at a time. Recent research shows that window has collapsed to four or five weeks in many populations. The eggs are reappearing far faster, and that is what resistance looks like in real life. A 2023 study tracked the shortened reappearance period across multiple herds [2].

The fix is selective treatment driven by an FEC (fecal egg count, a quick lab test on a dung sample). Instead of dosing on a calendar, you only treat the horses whose count is high. You add targeted cover for tapeworm and encysted larvae at defined times of year. The British Equine Veterinary Association published proper graded primary-care guidelines on this in 2024 [1].

Does it actually work in practice? Yes. A Swiss retrospective after the country shifted to selective treatment found the strategy holds up [3]. A Swedish study tracking parasite community structure found no resurgence of the dangerous species [4]. The catch is owner habit: Australian Thoroughbred surveys show many barns still calendar-dose despite the evidence pointing the other way [5][6][7].

Tapeworm is the soft spot. A standard FEC does not pick up tapeworm eggs reliably. A 2024 head-to-head compared three diagnostic approaches and found a saliva ELISA test the most useful in practice [8]. A sensible plan adds either a saliva ELISA once a year or a defined late-autumn treatment that targets tapeworm.

Worth it

FEC-based selective deworming, with targeted cover for tapeworm and encysted larvae. The evidence against blanket calendar dosing is strong enough to act on. Ask your vet for an FEC schedule before the next routine wormer.

Does the strangles vaccine actually prevent strangles?

For the first time, the answer is genuinely yes. A 2025 field study of Strangvac (a newer recombinant fusion-protein vaccine) reported zero clinical disease in vaccinated horses during a real outbreak on yards with documented exposure [9]. That is a striking result. Earlier sequence work also confirmed the vaccine targets are conserved across the strangles strains we have sequenced, so the protection is unlikely to be a quirk of one outbreak [10].

This bumps Strangvac from "promising" to a first-line option for boarding yards, sale barns, and any operation that takes in returning horses.

The reproduction rate matters here. A 2022 meta-analysis crunched outbreak reports and found strangles spreads fast once an infected horse mixes with a susceptible group [11]. Silent carriers can seed an outbreak before any signs appear. That is why screening new arrivals before they mix with the herd is not optional admin: it is the quantitative reason yards still have outbreaks.

Two studies on biosecurity gear are pointed. Endoscopes and twitches used in carrier diagnosis can stay contaminated with the strangles bacterium even after standard cleaning. A 2020 study and a 2023 follow-up both showed high-level disinfection is required to fully kill the organism [12][13]. If a visiting vet scopes a horse on your yard, it is fair to ask what their disinfection protocol is. A 2023 multiplex PCR now reliably tells the strangles bacterium apart from a much more common cousin (S. equi subspecies zooepidemicus), which matters because confusing them can mean needless quarantine or missed cases [14].

Worth it

For boarding yards and high-traffic operations: Strangvac on the manufacturer schedule, screening of new arrivals before mixing, and asking visiting vets about endoscope and twitch disinfection between yards. The research backs each of these specifically.

Should you panic about EHV?

Some panic is reasonable. Some is misdirected. EHV (equine herpesvirus) is the cleanest example in this guide of why "we have no proof it works" and "we have proof it does not work" need to be kept apart.

Two recent meta-analyses, one in 2022 and one in 2024, reached the same conclusion [15][16]. Current EHV-1 vaccines reduce respiratory disease and reduce viral shedding (how much virus your horse spreads when infected). There is moderate evidence they reduce abortion. There is no robust evidence any current vaccine prevents EHM (the neurological form of EHV that owners are afraid of, which can leave a horse wobbly, incontinent, or recumbent).

This last point is the one most often misstated. Vaccinating against EHV-1 is worth doing for what it actually does: it lowers the chance your horse becomes the spreader during a respiratory outbreak. EHM outbreaks continue to occur in fully vaccinated populations. The honest framing is that the EHV-1 vaccine is a public-health move (you protect the herd) rather than personal armour against EHM (you do not).

A couple of practical points. A 2024 Pusterla study showed nasal swab PCR is more sensitive than a blood PCR for detecting EHV-1 in clinical horses, and combining both raises detection further [17]. And a 2024 trial found that giving dexamethasone (a steroid) around the time of a combined flu/EHV booster blunts the antibody response [18]. If your horse is on a steroid course, ask whether to reschedule.

Worth it (with caveat)

EHV-1 and EHV-4 vaccination on the manufacturer schedule. Studies show reduction in respiratory disease, shedding, and likely abortion. Studies do not show prevention of the neurological form (EHM); do not treat the vaccine as personal protection against EHM.

What if the flu vaccine "fails"?

Equine influenza vaccines have a deep, decades-old evidence base. Annual or biannual boosters reduce both individual disease and outbreak risk. A 2020 cohort study in Thoroughbred racehorses confirmed the link between booster compliance and lower flu risk [19]. A 2025 challenge trial of a newer particle-based vaccine reported strong efficacy [20].

The mixed picture is not about whether to vaccinate. It is about how to read an outbreak that happens in a vaccinated population. The 2018 Argentinian outbreak and the 2019 epidemic in Great Britain both happened in well-vaccinated herds. Detailed investigation of the British outbreak [21] and the Argentinian one [22] found the same pattern. The virus had drifted antigenically (changed shape enough that older immunity worked less well) and a tail of under-vaccinated horses kept transmission going. The vaccine did not "stop working" wholesale.

The practical move during a known regional outbreak is to confirm booster timing, not to throw the vaccine out. The dexamethasone interaction noted above [18] applies here too.

Mixed evidence

Annual or biannual flu boosters work, and outbreaks in vaccinated populations consistently trace to antigenic drift plus lapsed coverage in part of the herd, not to whole-vaccine failure. Worth confirming booster timing during a regional outbreak rather than assuming the vaccine has stopped working.

What does a positive test actually mean?

A test result is not the same thing as the answer to your question. The diagnostic literature cuts across all these diseases.

For EHV-1, the intranasal modified-live vaccine produces measurable virus shedding and an antibody response after you give it. A 2023 study documented this clearly [23]. A nasal swab PCR taken shortly after intranasal vaccination can read positive without your horse being infected with field virus.

For strangles, the multiplex PCR validated in 2023 reliably separates the strangles bacterium from its much more common cousin [14]. Older single-target tests are still in use in some labs. Asking which test was actually run is a fair question.

For tapeworm, the 2024 comparison [8] shows no test is dominant. The saliva ELISA is the most useful in practice.

For tick-borne disease, a positive Lyme serology in a horse with no clinical signs is not Lyme disease. Seroprevalence data including a 2020 Ontario survey shows seropositivity runs far higher than actual clinical disease [24]. Many positive horses have been exposed and cleared rather than having an active infection.

Mixed evidence

Test choice and timing matter more than most owners realise. A positive PCR taken shortly after intranasal vaccination is not the same as a field infection. A positive Lyme serology is not the same as Lyme disease. Ask which test was run and what its known false-positive sources are.

What about ticks and Lyme?

Tick-borne disease has the thinnest evidence base of anything in this guide. Most published work is seroprevalence: a 2023 Brazilian survey for Theileria [25], the 2020 Ontario Lyme serosurvey [24], piroplasmosis screening of imported horses [26], and earlier Greek work [27].

What is missing is the clinical question owners actually need answered. When should you treat a seropositive horse with no signs? Do tick prevention products marketed for horses meaningfully reduce attachment and transmission? Are the long-tailed musculoskeletal complaints sometimes blamed on subclinical Lyme actually caused by the bacterium at all?

The honest answer is "we do not know yet". Research does not currently support treating a positive serology as a diagnosis or starting prolonged antibiotic courses on serology alone.

Insufficient evidence

The seroprevalence picture is documented; the clinical thresholds and the question of whether tick prevention works in horses are not. Treat positive serology as exposure history, not diagnosis, and ask the vet what specifically would change with treatment.

Should you vaccinate against West Nile?

Depends on where you live. The West Nile vaccine itself has solid efficacy and duration data going back to the 2007 Long study [28]. The disease is endemic across the United States and expanding through southern and central Europe. A 2026 pathogenicity study of Spanish isolates confirms virulence varies by viral lineage [29], a 2025 UK overview reviews the surveillance picture for a country where the threat is currently theoretical [30], and Spanish surveillance for 2023-2021 documents the northward expansion [31].

A single global verdict does not work for West Nile. Vaccinate in known endemic regions. Stay alert in expansion regions. Routine vaccination is not justified where there is no documented circulation. The right question is "what is the West Nile picture in my region this year", not "should I vaccinate".

Are post-surgery antibiotics doing anything?

Often, no. This is the rarest verdict shape in the guide and the most owner-relevant in routine surgical and post-injury care. The research is converging against extended antibiotic prophylaxis after most equine procedures.

A note on language. Antimicrobial stewardship means using antibiotics carefully so they keep working over the long run. Every unnecessary course makes resistance worse. A 2022 prescribing audit [32] and a 2025 point-prevalence study [33] both show wide variation in prescribing alongside rising resistance in clinically important bacteria. French isolate surveillance documents multidrug resistance [34]. A 2024 Dutch eye study shows the same narrowing of effective options in equine ulcerative keratitis [35].

In situations where antibiotics have historically been given for several days or weeks after the fact (clean surgical wounds, joint injections, post-castration), the evidence is moving toward shorter or targeted courses. In foal medicine, work on Rhodococcus equi (a pneumonia-causing bacterium) has tightened too: blanket screening of well-looking foals followed by treatment is now flagged as a resistance driver. A 2022 review [36] and a 2020 efficacy study [37] both push toward more targeted action.

The owner-side action is not to refuse antibiotics. It is to ask. "Is this course evidence-indicated or precautionary, and how long should it run?" is a fair, useful question.

Evidence of no effect

Extended antibiotic prophylaxis after most routine procedures has converging evidence against benefit and clear evidence of contribution to resistance. Ask your vet whether a prescribed course is evidence-based or precautionary, and how long it should run.

Does barn biosecurity actually work?

Specific practices, yes. The bundled "biosecurity protocol" idea, less so. The published research backs four things in particular:

• Screening new arrivals before mixing. The strangles reproduction-number estimate [11] is the quantitative reason. Two to three weeks of separation with at least one PCR or culture from a relevant sample is typical in evidence-based protocols.
• Disinfecting shared veterinary equipment between yards. Residual contamination of endoscopes and twitches after standard cleaning is documented in both [12] and [13]. Visiting vets should use disposable equipment or proper high-level disinfection between premises.
• Isolating febrile horses. Temperature checks of horses returning from competitions, sales, or transit are low-cost and high-yield, particularly during regional respiratory outbreaks.
• Vaccination compliance at the herd level. The 2020 flu cohort showed dose-response between booster timing and outbreak risk [19]. The equivalent argument for EHV-1 is in the meta-analyses [15][16].

What the research does not currently support is the "complete biosecurity protocol" tested end-to-end. Individual practices have been tested. Bundled protocols are mostly reasoned. A 2021 frontline-vet perspective from Great Britain is candid about that gap [38].

Mixed evidence

Specific practices (new-arrival screening, equipment disinfection, isolation of febrile horses, vaccination compliance) are evidence-supported. The bundled barn protocol concept is more reasoned than measured. Pick the components with evidence behind them.

What to actually do, in plain steps

The research supports a small number of concrete actions.

Parasite control. Ask your vet for a fecal egg count schedule (typically two to four counts a year depending on age class) before the next routine wormer. Add a saliva ELISA or a defined late-autumn tapeworm treatment. Stop dosing on a calendar.

Vaccination. Stay on the manufacturer schedule for flu, EHV-1/EHV-4, and tetanus, and confirm boosters before competition or transport. In endemic regions, add West Nile. For high-exposure populations, discuss Strangvac. Reschedule any booster that falls during a steroid course.

Diagnostics. When a positive result comes back, ask which test was run, when relative to any vaccination, and what the result would mean if the horse has no clinical signs.

Antibiotic stewardship. If a course is prescribed, ask whether it is evidence-indicated or precautionary, and how long it should run.

Biosecurity. New arrivals get a screening period before mixing. Visiting vets sterilise or use disposable equipment between yards. Febrile horses get isolated. Boosters are tracked at the yard level.

When to call your vet

These are the clear lines:

• Your horse goes off feed with a fever above 39°C (102°F) for more than twenty-four hours.
• Nasal discharge that is purulent or coming from both nostrils, especially with swelling under the jaw.
• A pregnant mare exposed to an EHV-1 case, or any abortion or stillbirth.
• Acute neurological signs (stumbling, urinary incontinence, hindlimb weakness) at any time.
• Any positive PCR for EHV-1, the strangles bacterium, or equine flu.
• A foal that becomes lethargic or develops respiratory signs in the first six months, particularly on a yard with a Rhodococcus equi history.

Bottom line

Test before you deworm. Vaccinate on schedule, but understand precisely what each vaccine prevents and what it does not. Ask whether antibiotics are evidence-indicated or precautionary. Treat new arrivals as a defined screening period rather than an unknown.

References

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See also

• How we evaluate evidence (methodology)
• Welfare and behaviour
• Reproduction and breeding