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As with humans, most common adjuvants in pet and other veterinary vaccines are alum (crystalline aluminium salts such as aluminum oxyhydroxide or aluminium hydroxyphosphate) and oil-water emulsions (either water-in-oil or oil-in-water) stabilized by surfactants (detergent). Chemical structure of oils used in such adjuvant emulsions influence both the efficacy and safety of the vaccine. Older veterinary vaccines used to contain mineral oil-based adjuvants. Mineral oils are powerful immune stimulators but also induce injection site reactions that could be severe (1). Metabolizable oils (e.g. squalene, a precursor of cholesterol) are less immunostimulatory compared to mineral oils, which means the formulated vaccine is safer (2).

Pets and Other Veterinary Vaccine Adjuvants
Safety regulations and registration costs are lower for veterinary vaccines. Thus, their regulatory and liability burden is lower. In turn, a greater variety of adjuvants are used in veterinary vaccines. Many of the adjuvants used in commercial veterinary vaccines are proprietary, i.e. their composition is not public knowledge. However, a few examples of pet and livestock adjuvants components are publicly available. The following examples are not meant to be comprehensive. Rather to illustrate two points, one, the range of adjuvants used in veterinary vaccines, and two, inclusion of adjuvants that wouldn’t be approved in humans, such as Quil-A and IFA (Incomplete Freund’s Adjuvant).

Leishmune, anti-Leishmania donovani vaccine, is licensed for use in Brazil. Leishmaniasis is a vector-borne (transmitted by the bite of an infected sand fly) protozoan parasite disease. It’s endemic in Brazil so they need to vaccinate dogs against it. This vaccine is adjuvanted with Quil-A, a plant-derived Saponin, a sugar with glycosidic bonds.

Here are the principal features of Quil-A.

As is clear, Quil-A not approved for human use due to excess toxicity. This suggests either that adjuvant toxicity is species-specific or that the bar for regulatory approval is lower for non-humans.

Pirodog and Nobivac Piro are vaccines licensed in Europe to protect against tick-borne parasitic diseases caused by Babesia canis and Babesia rossi. Like Leishmune, they are also adjuvanted with plant-derived saponin.

The Cat Adjuvants Controversy: Feline-Injection-Site Sarcomas (FISS)

  • In 1992, Dr. Mattie Hendrick, a veterinary pathologist at the University of Pennsylvania, published a study linking adjuvants to sarcomas in cats (3). This coincided with two things. One, a recent enactment of a Pennsylvania state law requiring rabies vaccination of cats. Two, the first Feline Leukemia vaccine (FeLV) adjuvanted with alum came out in 1985, and around the same time the rabies vaccine switched from modified live to alum-adjuvanted killed.
  • Occurring at the vaccination site, these sarcomas sometimes contained residual alum and appeared to be inflammatory in appearance (4).
  • However, these sarcomas were also found associated with injection of nonvaccine products (5, 6, 7, 8).
  • A 1993 study (6) suggested causal association between alum-adjuvanted vaccines (FeLV and rabies) and sarcomas. It and another study (9) also estimated sarcoma risk at around 2 per 10000 doses of vaccine administered.
  • Several other studies also implicated vaccine adjuvant-induced inflammation at the injection site (10, 11, 12).
  • In the US, the  Vaccine-Associated Feline Sarcoma Task Force was formed in 1996 to address this issue and to support research on this topic (13).
  • A later multicenter case-control study did not find higher sarcoma risk among cats vaccinated with adjuvanted vaccines compared to nonadjuvanted ones (14). The caveat to this study was that nonadjuvanted vaccines such as recombinant ones were used in small numbers of cats.
  • Later, other studies as well found no relationship between vaccine brand, type, and inactivated versus modified-live vaccines, and the risk of sarcoma formation (15, 16, 17).
  • Later and larger epidemiologic studies estimated risk at 1 per 10000 doses of vaccine, i.e. much lower (18, 19).
  • It’s not clear if a newer canarypox-vectored rabies vaccine reduces sarcoma risk since studies support both possibilities (15, 16). FISS was an important impetus for development of this vaccine.
  • A silver lining to this story is that in January 2002, the US Department of Agriculture (USDA) proposed a new rule that made it mandatory for manufacturers to keep a record of adverse effects and report them to the USDA (20).

Based on detailed review of the literature, the Vaccination Guidelines Group (VGG) of the World Small Animal Veterinary Association (WSAVA) published the following guidelines for the vaccination of dogs and cats (21):

Sites of Vaccination for Cats
Over the past 20 years it has become evident that one trigger for the feline injection site sarcoma (FISS) may be the administration of adjuvanted FeLV and rabies vaccines. Most subcutaneous injections (including of vaccines) have traditionally been given into the interscapular region of the cat and this is a common site for formation of a FISS. The infiltrative nature of these tumours has meant that often radical surgical resection was necessary to attempt removal of these lesions. In North America the response to this issue was the recommendation of a protocol whereby the two perceived high-risk adjuvanted vaccines would be administered into distinct anatomical sites that would be more amenable to surgical removal of any FISS that might develop. Accordingly the recommendation ‘left leg leukaemia, right leg rabies’ suggested that FeLV vaccine should be given as far distal as possible into the left hind limb, whilst rabies vaccine should be given as far distal as possible into the right hind limb. A recent study has evaluated the effect of this practice by comparing the anatomical distribution of FISS in cats before the recommendation was made (1990–1996) and after the practice was adopted (1997–2006). These data show a significant decrease in the prevalence of interscapular FISS and an increase in prevalence of tumours in the right (but not left) hind limb. More notably, there was also an increase in the number of tumours reported arising in the right and left lateral abdomen, and this was attributed to the difficultly of injecting into the distal hindlimb and these abdominal sites being accidentally injected (Shaw et al., 2009).
This practice has not been adopted outside of North America. Given these recent data, the VGG recommends the following approach to reducing the risk of FISS:
Non-adjuvanted vaccines should be administered to cats wherever possible.
Vaccines (particularly adjuvanted products) should not be administered into the interscapular region.
Vaccines (particularly adjuvanted products) should be administered into other subcutaneous (and not intramuscular) sites.
The most accessible sites, with acceptable safety for the vaccinator (i.e. to avoid accidental self-injection during difficult restraint of the animal), would appear to be the skin of the lateral thorax or abdomen. The skin of the lateral abdomen represents the best choice as FISS that might arise at this site may be more readily excised than those occurring in the interscapular or intercostal regions where more extensive surgical resection is required.
Vaccines should be administered into a different site on each occasion. This site should be recorded in the patient’s record or on the vaccination card by use of a diagram indicating which products were administered on any one occasion. The sites should be ‘rotated’ on each occasion. Alternatively, a practice might develop a group policy that all feline vaccinations are administered to a specific site during one calendar year and this site is then rotated during the following year.
The VGG encourages all cases of suspected FISS to be notified via the appropriate national reporting route for suspected adverse reactions.

Other useful references for FISS (22, 23, 24).

I’m including pig vaccine adjuvants whose composition is publicly available, again to illustrate the inclusion of adjuvants that wouldn’t be approved in humans, such as IFA (Incomplete Freund’s Adjuvant).

Porcine Circovirus Type 2 (PCV2) causes a wasting syndrome in piglets, a considerable economic cost in livestock agriculture. There are at least 4 commercial PCV2 vaccines, each formulated with a different adjuvant.

  • The mineral (paraffin) oil called IFA (Incomplete Freund’s Adjuvant) was long ago shown to have site injection site reactivity in humans. Circovac approved for use in Canada and Europe, Circumvent in Canada and USA, and Porcilis PCV2 in Europe and Korea, all include it.
  • Acrylic acid polymer listed as aqueous polymer in the table above is one of the oldest and safest adjuvants used in pigs and other livestock (25).
  • Its half-life is unknown (26).
  • Also called Carbopol, it cross-links to create a network, a shape that easily entraps a variety of antigenic structures, enabling optimal controlled release of small antigenic moieties (27).
  • Squalene, a cholesterol precursor, is an adjuvant with a historic lineage stretching back to the original discovery of adjuvants in the 1890s by Lydia Rabinowitsch-Kempner. Context? Butter, yes, butter!
  • In turn, squalene brings us full circle to some of the modern adjuvants we now use in human vaccines.

Human Vaccine Adjuvants: Recent Innovations and additions

  • For several decades, the only adjuvant approved for human use was alum.
  • Problem with alum is that it mainly promotes antibody production but not other types of immune responses.
  • This truncates its usefulness.
  • For example, activated cytotoxic T cells are crucial in control and elimination of viruses but alum is not efficient at activating them.
  • An attempt to circumvent such limitations of alum is to add Pattern recognition receptor (PRR) agonists to it.
  1. GlaxoSmithKline’s Hepatitis B vaccine, Engerix-B, has alum (aluminium hydroxide) plus a TLR4 (Toll-like Receptor) agonist (MPL®,Monophosphoryl lipid A, a proprietary derivative of gram-negative bacteria cell wall) named AS04.
  2. The same adjuvant AS04 is also present in the Cervarix® vaccine, approved by the US Food and Drug Administration (FDA) in 2009.

Virosomes are liposomal adjuvants, a combination of natural or synthetic phospholipids with viral envelope phospholipids, glycoproteins or other viral proteins. Their key attribute? Ability to fuse (fusogenic). Two virosome-based vaccine products are licensed in Europe. The Crucell company, Berna Biotech AG developed and patented the virosome-adjuvanted flu vaccine Inflexal®,and Hepatitis A vaccine, Epaxal®.

MF59, a new Oil-in-Water Emulsion approved for use in humans in Europe

  • Developed and licensed by Chiron-Novartis Vaccines.
  • Contains biodegradable squalene oil (4.3%) and non-ionic surfactants, Tween 80 and Span 85, in a citrate buffer.
  • Squalene is a natural precursor to cholesterol.
  • MF59 adjuvant activity was serendipitously discovered when it was being developed as a delivery system for another experimental adjuvant.
  • Used in the flu vaccine Fluad in Europe.
  • MF59-adjuvanted flu vaccine induces similar response against H5N1 compared to the alum-adjuvanted one.
  • A major advantage of MF59 is that it is very dose-sparing, i.e. induces similar immune response using much less dose of antigen so very cost-saving and much more optimal for pandemic flu vaccine production.
  • Unlike the alum-adjuvanted vaccine, the MF59-adjuvanted one induces both cellular immunity and antibody production.


  1. Oda K., Tsukahara F., Kubota S., Kida K., Kitajima T., Hashimoto S.: Emulsifier content and side effect of oil based adjuvant vaccine in swine. Research in Veterinary Science 81, 2006 51-57.
  2. Aucouturier J., Deville S., Perret C., Vallée I., Boireau P. Assessment of efficacy and safety of various adjuvant formulations with a total soluble extract of Trichinella spiralis. Parasite. 2001 Jun;8 (2 Suppl):S126-32.
  3. Hendrick MJ, Goldschmidt MH, Shofer F, et al: Postvaccinal sarcomas in the cat: Epidemiology and electron probe microanalytical identification of aluminum. Cancer Res 52:5391-5394, 1992.
  4. Hendrick MJ. Historical review and current knowledge of the risk factors involved in feline vaccine-associated sarcomas. J Am Vet Med Assoc 1998;213:1422–1423.
  5. Morrison WB, Start RM. Vaccine-Associated Feline Sarcoma Task Force. J Am Vet Med Assoc 2001;218:697–702.
  6. Kass PH, Barres WG Jr, Spangler WL, et al. Epidemiologic evidence for a causal relation between vaccination and fibrosarcoma tumorigenesis in cats. J Am Vet Med Assoc 1993;203:396–405.
  7. Macy DW. Vaccine adjuvants. Semin Vet Med Surg (Small Anim) 1997;12:206–211; Hendrick MJ. Historical review and current knowledge of the risk factors involved in feline vaccine-associated sarcomas. J Am Vet Med Assoc 1998;213:1422–1423.
  8. Couto CG, Macy DW. Review of treatment options for vaccineassociated feline sarcoma. J Am Vet Med Assoc 1998;213;1426–1427.
  9. Esplin DG, McGill LD, Meininger AC, et al. Postvaccination sarcomas in cats. J Am Vet Med Assoc 1993;202:1245–1247.
  10. Macy DW, Hendrick MJ. The potential role of inflammation in the development of postvaccinal sarcomas in cats. Vet Clin North Am Small Anim Pract 1996;26:103–109.
  11. Hendrick MJ. Feline vaccine-associated sarcomas: current studies on pathogenesis. J Am Vet Med Assoc 1998;213:1425–1426.
  12. Jelinek FE. Postinflammatory sarcoma in cats. Exp Toxicol Pathol 2003;55:167–172.
  13. Richards JR. Feline Sarcoma Task-Force meets. J Am Vet Med Assoc 1997;210:310–311.
  14. Kass PH, Spangler WL, Hendrick MJ, et al. Multicenter case-control study of risk factors associated with development of vaccine-associated sarcomas in cats. J Am Vet Med Assoc 2003; 223:1283–1292.
  15. Wilcock B, Wilcock A and Bottoms K. Feline postvaccinal sarcoma: 20 years later. Can Vet J 2012; 53: 430–434.
  16. Srivastav A, Kass PH, McGill LD, Farver TB and Kent MS. Comparative vaccine-specific and other injectable-specific risks of injection-site sarcomas in cats. J Am Vet Med Assoc 2012; 241: 595–602.
  17. Scherk, Margie A., et al. “2013 AAFP Feline Vaccination Advisory Panel Report.” Journal of feline medicine and surgery 15.9 (2013): 785-808. Page on felineasthma.org
  18. Moore GE, DeSantis-Kerr AC, Guptill LF, Glickman NW, Lewis HB and Glickman LT. Adverse events after vaccine administration in cats: 2560 cases (2002–2005). J Am Vet Med Assoc 2007; 231: 94–100.
  19. Gobar G and Kass P. World Wide Web-based survey of vaccination practices, postvaccinal reactions, and vaccine site-associated sarcomas in cats. J Am Vet Med Assoc 2002; 220: 1477–1482.
  20. Animal and Plant Health Inspection Service, USDA. 9 CFR Parts 101 and 116 [Docket No. 00-071-1]. Viruses, Serums, Toxins, and Analogous Products; Records and Reports. Fed Regist 2002;67: 1910–1913.
  21. Day, M. J., M. C. Horzinek, and R. D. Schultz. “WSAVA guidelines for the vaccination of dogs and cats.” Journal of Small Animal Practice 51.6 (2010): e1-e32. Page on wiley.com
  22. Spickler, Anna R., and James A. Roth. “Adjuvants in veterinary vaccines: modes of action and adverse effects.” Journal of Veterinary Internal Medicine 17.3 (2003): 273-281. Adjuvants in Veterinary Vaccines: Modes of Action and Adverse Effects
  23. Richards, James R., et al. “The 2006 American association of feline practitioners feline vaccine advisory panel report.” Journal of the American Veterinary Medical Association 229.9 (2006): 1405-1441.
  24. Heegaard, Peter MH, et al. “Adjuvants and delivery systems in veterinary vaccinology: current state and future developments.” Archives of virology156.2 (2011): 183-202. Page on researchgate.net
  25. Diamantstein, T., Wagner, B., Beyse, I., Odenwald, M.V., Schulz, G., 1971. Stimulation of humoral antibody formation by polyanions. I. The effect of polyacrylic acid on the primary immune response in mice immunized with sheep red blood cells. Eur. J. Immunol. 1, 335–340.
  26. Mair, K. H., et al. “Carbopol improves the early cellular immune responses induced by the modified-life vaccine Ingelvac PRRS® MLV.” Veterinary microbiology 176.3 (2015): 352-357. Carbopol improves the early cellular immune responses induced by the modified-life vaccine Ingelvac PRRS® MLV
  27. Singla, A.K., Chawla, M., Singh, A., 2000. Potential applications of carbomer in oral mucoadhesive controlled drug delivery system: a review. Drug Dev. Ind. Pharm. 26, 913–924.