Levamisole as vaccine adjuvant: dose, schedule and FMD data
Levamisole as vaccine adjuvant works as a separate oral course, not as an ingredient inside the shot. In a two-species study, oral levamisole given alongside an intramuscular foot-and-mouth disease vaccine lifted antibody and neutralizing responses in both mice and swine. Survival after challenge in mice went from 2 of 5 to 5 of 5. The injectable vaccine was never reformulated.
Why levamisole as vaccine adjuvant is a scheduling question
Foot-and-mouth disease is one of the costliest infections of cloven-hoofed livestock. An outbreak stops animal movement, cuts productivity, and closes export markets. Inactivated vaccination stays the backbone of control. Producers want more protection out of the vaccines they already own. What they do not want is to wait for a new formulation to clear registration.
Changing a registered vaccine is slow work. Changing how you dose an existing immunomodulator around that vaccine is not. Levamisole is an anthelmintic with a long history of immunomodulatory use. An earlier approach put it inside the injected vaccine itself. This study tested the opposite arrangement. Levamisole went in by mouth, on its own schedule, next to a conventional intramuscular vaccine. If the benefit survives that separation, a farm can add the adjunct without touching the vaccine formulation at all.
How the two-species study was set up
Two species. Mice supplied the challenge data, because protection can be measured directly there under containment. Swine supplied the target-species data, because pigs are the animals that actually receive FMD vaccination.
| Design element | Specification |
|---|---|
| Mouse model | Female C57BL/6N mice, 6 to 7 weeks old, 5 per group, mean body weight about 19 g |
| Swine model | FMD antibody-negative pigs, 8 to 9 weeks old, 5 per group, about 15 to 20 kg at first dose |
| Vaccine | Bivalent inactivated FMD vaccine, serotypes O and A, intramuscular, booster at day 28 |
| Adjunct | Levamisole hydrochloride, oral |
| Mouse dose | 100 micrograms per dose, about 5.2 mg/kg, by oral gavage |
| Swine dose | 5 mg per dose by oral dosing syringe, about 0.25 to 0.33 mg/kg at the start |
| Dosing schedule | Daily from day 0 to day 28, then weekly from day 28 to day 56 |
| Control groups | Phosphate-buffered saline and vaccine-only groups received vehicle alone, same route and schedule |
| Sampling days | 0, 7, 14, 21, 28, 35, 42, 56, 70 and 84 after vaccination |
| Antibody assays | Structural protein ELISA for serotypes O and A, plus virus neutralization |
| Mucosal readout | Secretory IgA in mouse serum and saliva and in swine serum |
| Gene expression | IL-2, IL-4, IL-12p40, IL-17A, IL-18, IL-23p19, IL-23R and interferon gamma in porcine PBMCs |
| Challenge | Mice only, serotype O field isolate at day 84, intraperitoneal, 100 times the median lethal dose |
| Statistics | Kaplan-Meier with log-rank Mantel-Cox test, and one-way ANOVA with Tukey correction |
The design is what the term route-decoupled describes. The vaccine went into muscle on day 0 and again on day 28. The levamisole went into the mouth every day for four weeks, then once a week for another four weeks. Nothing was mixed. No injection schedule changed.
Group sizes were five animals per arm in both species. That number came from precedent in earlier vaccine-adjunct work with comparable immune endpoints, plus the practical limits of biosafety level 3 containment and large-animal work.
Survival after challenge in mice
This is the headline result, and it is the only protective endpoint in the study. Vaccinated mice were challenged at day 84 with a serotype O field isolate, then watched for seven days.
| Group | Survivors | Survival rate | 95 percent confidence interval |
|---|---|---|---|
| Phosphate-buffered saline control | 0 of 5 | 0 percent | 0 to 45.1 |
| Vaccine only | 2 of 5 | 40 percent | 5.3 to 85.3 |
| Vaccine plus oral levamisole | 5 of 5 | 100 percent | 47.8 to 100 |
The difference across the three groups was significant on a log-rank Mantel-Cox test, with a chi-square of 11.04 at 2 degrees of freedom and a p value of 0.0040. Mice that received levamisole also lost less body weight after challenge than mice that received the vaccine alone. That is the second sign the challenge was better controlled in the treated group.
Read the confidence intervals before you read the point estimates. The 95 percent interval for the vaccine-only group runs from 5.3 to 85.3 percent. The interval for the levamisole group runs from 47.8 to 100 percent. Those bands overlap. Five animals per arm cannot pin down the true survival rate, so the honest claim is that levamisole as vaccine adjuvant was associated with better post-challenge outcomes. It is not a claim that the strategy delivers a precise 100 percent protection figure.
Antibody responses in both species
Serology moved in the same direction as survival, in both species, across the whole sampling schedule.
In mice, structural protein antibodies against serotype O and serotype A were higher in the levamisole group than in the vaccine-only group throughout the observation period. The virus neutralization assay agreed. Neutralizing activity against both the O and A test strains was higher in treated animals.
In swine, levamisole as vaccine adjuvant produced the same pattern: structural protein responses against both serotypes were higher than with vaccination alone, and neutralizing titers against both strains were higher at multiple sampling points. Duration is the part that matters here. Oral levamisole produced antibody responses that were higher and more sustained. They held up across the sampling days instead of spiking and falling away. That is the antibody response in swine a producer would want from a primary course plus a booster.
Secretory IgA after oral levamisole
Foot-and-mouth disease virus enters and replicates at mucosa-associated sites. So the study measured secretory IgA rather than stopping at serum antibody.
Oral levamisole was associated with increased secretory IgA responses in mouse serum. The same direction of change appeared in the serum of treated pigs. The adjunct went in by mouth, which makes a mucosal readout the natural place to look for an effect an injected adjuvant would not produce.
Cytokine gene expression in porcine PBMCs
Peripheral blood mononuclear cells from the pigs were profiled by quantitative RT-PCR for eight immune genes. Expression was normalized to a housekeeping gene and reported as fold change against the appropriate comparator group.
Transcription of IL-2, IL-4, IL-12p40, IL-17A, IL-18, IL-23p19, IL-23R and interferon gamma was increased relative to vaccination alone. The pattern splits into three readable groups. Raised IL-12p40, IL-18 and interferon gamma point to engagement of Th1-associated antiviral pathways. Raised IL-2 suggests broader lymphocyte activation and more helper T-cell activity during the post-vaccination window. The concurrent rise in IL-17A, IL-23p19 and IL-23R points to pathways involved in barrier-site immune coordination and mucosa-associated immunity. IL-4 moved least of the set, which argues for a broad helper-cell environment rather than a hard Th2 shift.
Treat the cytokine side of the levamisole as vaccine adjuvant story with the right level of confidence. The data are gene transcription, not protein. Transcription can rise without producing a sustained increase in circulating cytokine protein. What the findings describe is immune pathway engagement, not measured systemic cytokine output.
Tolerability and serum biochemistry in pigs
Repeated oral dosing was monitored clinically and by serum biochemistry at days 0, 28, 56 and 84. No overt treatment-related abnormalities appeared during the experimental period.
| Parameter | Group | Day 0 | Day 28 | Day 56 | Day 84 |
|---|---|---|---|---|---|
| ALT (U/L) | PBS | 41.20 plus or minus 3.68 | 56.80 plus or minus 4.03 | 49.60 plus or minus 1.85 | 47.00 plus or minus 1.74 |
| ALT (U/L) | Vaccine | 37.20 plus or minus 1.73 | 48.40 plus or minus 1.85 | 53.80 plus or minus 3.33 | 43.60 plus or minus 2.79 |
| ALT (U/L) | Vaccine plus levamisole | 41.60 plus or minus 3.67 | 55.00 plus or minus 6.83 | 46.60 plus or minus 3.61 | 39.00 plus or minus 1.92 |
| AST (U/L) | PBS | 40.00 plus or minus 2.06 | 46.33 plus or minus 12.25 | 43.20 plus or minus 13.28 | 46.00 plus or minus 14.04 |
| AST (U/L) | Vaccine | 32.60 plus or minus 0.61 | 45.50 plus or minus 3.32 | 48.60 plus or minus 9.20 | 47.75 plus or minus 5.86 |
| AST (U/L) | Vaccine plus levamisole | 37.60 plus or minus 2.94 | 46.60 plus or minus 5.62 | 42.40 plus or minus 4.46 | 40.60 plus or minus 2.56 |
| BUN (mg/dL) | PBS | 5.90 plus or minus 0.37 | 9.56 plus or minus 1.08 | 10.50 plus or minus 1.21 | 17.26 plus or minus 1.47 |
| BUN (mg/dL) | Vaccine | 7.50 plus or minus 1.43 | 9.30 plus or minus 0.67 | 13.98 plus or minus 0.95 | 16.78 plus or minus 1.60 |
| BUN (mg/dL) | Vaccine plus levamisole | 5.48 plus or minus 0.28 | 6.40 plus or minus 0.42 | 12.42 plus or minus 2.41 | 15.94 plus or minus 1.46 |
| Creatinine (mg/dL) | PBS | 0.75 plus or minus 0.02 | 1.01 plus or minus 0.05 | 1.15 plus or minus 0.02 | 1.46 plus or minus 0.05 |
| Creatinine (mg/dL) | Vaccine | 0.68 plus or minus 0.02 | 0.93 plus or minus 0.02 | 1.16 plus or minus 0.05 | 1.30 plus or minus 0.07 |
| Creatinine (mg/dL) | Vaccine plus levamisole | 0.78 plus or minus 0.03 | 0.98 plus or minus 0.03 | 1.17 plus or minus 0.05 | 1.38 plus or minus 0.06 |
Liver and kidney markers moved with age and growth rather than with treatment. Alanine aminotransferase, aspartate aminotransferase, blood urea nitrogen and creatinine all sat in the same band across the three groups at every timepoint. The pigs on levamisole were not the outliers at any of them. Reference ranges depend on the laboratory and the pig population, so the comparison that carries weight is group against group within this study.
The counter-intuitive finding: the route matters more than the formulation
Levamisole as vaccine adjuvant is normally assumed to mean a component inside the formulation. The logic is that the immune stimulant has to arrive at the same place, at the same time, as the antigen it is supposed to help.
This study breaks that assumption. The levamisole never met the vaccine. Different route, different schedule, different tissue, and the antibody, neutralizing, mucosal and transcriptional responses all still improved in two species. The practical consequence is large. A farm does not have to reformulate, re-register, or replace its injectable vaccine to add an immunomodulatory course. It only has to add a dosing schedule.
There is a second surprise buried in the dosing table. The levamisole dose in pigs was fixed at 5 mg per animal, so the milligram-per-kilogram exposure fell as the pigs grew. At 15 to 20 kg the animals received roughly 0.25 to 0.33 mg/kg. By day 56, at 60 to 75 kg, the same 5 mg worked out to about 0.07 to 0.08 mg/kg. That is a fall of more than threefold. The immune response did not fade with the falling dose per kilogram. It strengthened and persisted. Whatever drives the effect, it is not a simple per-kilogram threshold.
Practical recommendations
- Treat the adjunct as a separate course, not as a formulation change. The benefit in this study came without touching the vaccine.
- Dose daily through the primary response window, then extend to weekly. That was the schedule behind the antibody and survival results here.
- Expect the response to build and persist rather than peak early. Sustained titers, not a single high reading, are the signal to look for.
- Monitor late in the course as well as early. Sampling here ran from day 0 to day 84, and the differences widened rather than narrowed.
- Check that the adjunct is delivering before assuming it works in your herd. A levamisole-only arm was missing from this design, so antigen-independent effects cannot be excluded.
- Read survival differences with their confidence intervals. Five animals per group gives wide bands and overlapping ranges between groups.
- Do not transfer the mouse protection numbers to pigs. Protective challenge data exist for mice only in this study.
- Re-check liver and kidney panels if you extend dosing past eight weeks. Prolonged immune modulation was not evaluated here.
Limitations
The most important limitation is stated plainly in the study itself. There was no levamisole-only control group. Without one, the FMDV-specific responses reported here cannot be cleanly separated into enhancement of vaccine-induced immunity on one side and antigen-independent immune stimulation on the other. A future design with a levamisole-only arm is needed to settle that.
The challenge model has its own caveat. The vaccine strain and the challenge strain are different isolates within the same serotype, so the mouse challenge is an intratypic heterologous exposure rather than a strictly matched one. Serotype O was chosen because it is the epidemiologically dominant serotype in the region where the work was done. The choice is defensible, but it is not neutral.
Protective data come from mice alone. Mucosal immune responses were measured at the antibody level, not at the tissue level. Cytokines were measured as gene transcription rather than protein. The observation window is long at 84 days, and still finite. Group sizes of five per arm were set for feasibility under containment rather than by a formal power calculation.
The tolerability finding has a boundary too. No overt abnormalities and no clinically relevant liver or kidney changes appeared under these conditions. That does not rule out subtler effects of prolonged immune activation. Excessive or extended immune stimulation could in principle affect inflammatory homeostasis, growth, feed efficiency or stress responses in production settings. Longer field studies with performance measures are needed before levamisole as vaccine adjuvant can be recommended for routine use.
Bottom line
Levamisole as vaccine adjuvant does not have to mean mixing it into the vaccine. Given by mouth as a separate course around an intramuscular foot-and-mouth disease vaccination, it raised and sustained antibody and neutralizing responses in mice and swine. It improved post-challenge survival in mice from 40 percent to 100 percent. It left liver and kidney markers unchanged. The protection data rest on five mice per group and need a levamisole-only arm to be fully interpretable. Read this as proof of concept for a scheduling strategy, not a ready protocol.
FAQ
What is levamisole used for as a vaccine adjuvant?
Levamisole is an anthelmintic that also modulates immune function, and this study used it to strengthen the response to an inactivated foot-and-mouth disease vaccine. Given orally and separately from the injection, it raised structural protein antibody responses, virus neutralizing titers, secretory IgA responses and cytokine gene transcription in mice and swine. The vaccine itself was unchanged.
Can levamisole be given with the FMD vaccine on the same day?
The design here deliberately did not do that. Levamisole was given by mouth daily from day 0 to day 28 and then weekly from day 28 to day 56, while the vaccine went into muscle on day 0 and again on day 28. The two were kept apart in route, timing and site throughout. Nothing in the results speaks to same-day co-administration, so it should not be assumed equivalent.
How much levamisole was used in this study?
Mice received 100 micrograms per dose by oral gavage, which works out to about 5.2 mg/kg given a mean body weight near 19 g. Pigs received 5 mg per dose by oral dosing syringe, roughly 0.25 to 0.33 mg/kg at 15 to 20 kg. Because the pig dose was fixed per animal, exposure per kilogram fell to about 0.07 to 0.08 mg/kg by day 56.
Does oral levamisole improve survival after FMD challenge?
In this study it was associated with better survival in mice. Survival was 0 of 5 in the phosphate-buffered saline control, 2 of 5 with vaccine alone, and 5 of 5 with vaccine plus oral levamisole, with a log-rank Mantel-Cox p value of 0.0040. Treated mice also lost less body weight after challenge. The confidence intervals overlap, so treat the exact percentages with caution.
Why does oral levamisole work when it is not in the vaccine?
Because immune stimulation and antigen delivery do not have to happen in the same place. Adjuvants are conventionally treated as formulation components, but here the levamisole never met the vaccine and the responses still improved in two species. The mucosal route may also explain the rise in secretory IgA responses, which an injected adjuvant would be less likely to produce.
How long does the antibody boost last after oral levamisole?
Longer than with vaccination alone in this study. Antibody responses in pigs were described as both higher and more sustained, with virus neutralizing titers higher at multiple sampling points across a schedule that ran to day 84. Sampling occurred on days 0, 7, 14, 21, 28, 35, 42, 56, 70 and 84, so the persistence claim rests on repeated measurements rather than a single end point.
Is levamisole safe for pigs at these doses?
Under the conditions tested, yes. Pigs received oral levamisole daily for 28 days and then weekly to day 56, with no overt treatment-related abnormalities. Serum biochemistry at days 0, 28, 56 and 84 showed alanine aminotransferase, aspartate aminotransferase, blood urea nitrogen and creatinine in the same band across treated and control groups. Prolonged immune activation was not evaluated.
What immune pathways does oral levamisole activate?
Transcription of eight genes was profiled in pig blood mononuclear cells. Levels of IL-2, IL-4, IL-12p40, IL-17A, IL-18, IL-23p19, IL-23R and interferon gamma were all increased relative to vaccination alone. Raised IL-12p40, IL-18 and interferon gamma suggest Th1-associated antiviral engagement, while IL-17A, IL-23p19 and IL-23R point to barrier-site immune coordination.
Does levamisole help against both FMDV serotypes?
Yes, in this study. Antibody responses against serotype O and serotype A were both higher with oral levamisole in mice and in swine, and neutralizing activity was higher against both test strains in both species. The vaccine was bivalent, so a serotype-restricted effect would have been visible. The challenge study, however, used only a serotype O isolate.
Can this approach be used in cattle and sheep?
Not established here. The study used mice for challenge protection and pigs for target-species evaluation, so cattle and sheep were not tested. The immunological rationale is not species-specific, and oral dosing around an injectable vaccine is straightforward in ruminants, but the dose, schedule and duration would need their own evaluation before any field use.
What did the study not test?
Four things stand out. There was no levamisole-only control group, so antigen-independent immune stimulation cannot be separated from vaccine enhancement. Mucosal responses were measured as antibody rather than tissue-level data. Cytokines were measured as gene transcription, not protein. And protective challenge data come only from mice, never from the target species.
Is levamisole as vaccine adjuvant a replacement for a booster?
No. The vaccinated pigs in this study received a booster at day 28, and the levamisole course ran alongside that two-dose schedule rather than replacing it. The adjunct improved the responses generated by the primary course and the booster together. Nothing in the design suggests that oral levamisole could substitute for the second vaccination.
