FeLV Vaccine Assessment: How Closely Does It Mimic Natural Infection?

Sponsored by Boehringer Ingelheim

Feline leukemia virus (FeLV) is a contagious, immunosuppressive, and oncogenic virus; pediatric patients are most likely to experience progressive, fatal disease secondary to infection with FeLV.¹

Infection may include a subclinical phase that can persist for years before appearance of clinical signs.¹ Older cats appear less susceptible to infection and some cats may not become clinically ill despite persistently harboring viral DNA.¹ Individual immune systems and FeLV strains vary¹ and may contribute to different outcomes in susceptible hosts.

Assessing the Efficacy of FeLV Vaccines

Various vaccines have been granted licensure by the USDA to aid in the prevention of disease due to feline leukemia virus, and include adjuvanted, inactivated, whole virus preparations, and a non-adjuvanted canarypox-vectored vaccine.^2-6

Vaccine efficacy is demonstrated by comparing protection afforded by a given vaccine following a virulent challenge to a group of vaccinated cats and a group of unvaccinated control cats. FeLV vaccine efficacy is usually demonstrated in cats vaccinated twice 2 to 4 weeks apart, with the initial vaccine typically being administered at 8-9 weeks of age.<sup2-6 sup> 

Interpretation of results in long-term immunity studies have been confounded by difficulties in infecting naturally resistant older cats.<sup7 sup> Additionally, induction of persistent viremia in unvaccinated juvenile cats may not always be achievable. In some models, the intraperitoneal route of administration was utilized as a way of increasing infectivity of the challenge strain.⁸ In other models, the severity of FeLV challenge was enhanced by methylprednisolone-induced immunosuppression,⁹ which may introduce bias in the conclusions of a study and does not reflect the common conditions of natural infection. Corticosteroid-induced immunosuppression more specifically affects T-cell-mediated immunity, by suppressing cellular (Th1) immunity yet promoting humoral (Th2) immunity.¹⁰ Thus, the impact of this immunosuppression may be different depending on the mode of action of the vaccine, with the potential for inactivated vaccines to appear artificially more efficacious.⁹

Duplicating natural conditions to assess vaccine efficacy

Veterinary vaccinologists strive to create challenge models that capture the variation that occurs in natural infection under field conditions.* Numerous studies have shown that persistent infection can be achieved in unvaccinated immunocompetent cats via oronasal challenge using a virulent FeLV challenge strain or by contact challenge, without concurrent immunosuppressive treatment.^11-15

Oronasal Challenge

A 2017 study compared the efficacy of 3 FeLV vaccines (a canarypox recombinant vaccine and two inactivated vaccines) and a placebo against an oronasal FeLV challenge in kittens. There were no significant differences between vaccine groups in ability to prevent persistent viremia.<sup11 sup>

Natural Contact

A 2015 study compared the efficacy of 2 FeLV vaccines (a canarypox recombinant vaccine and an inactivated vaccine) and a placebo in healthy kittens brought in close contact with persistently viremic kittens. Healthy kittens were either vaccinated or administered a placebo and were housed in close contact with persistently viremic FeLV kittens for 6 months. There was no significant difference in the fraction of kittens protected between the 2 vaccines.¹²

Protecting Patients

The AAFP highly recommends vaccination for all cats at or under 1 year of age because kittens are more susceptible to FeLV infection, and infection results in greater morbidity and mortality rates in kittens. In addition, a kitten’s ultimate status as an indoor or outdoor cat is more dynamic and subject to change.¹⁶ Adult cats at increased risk include:

• Outdoor cats

• Housemates of untested or FeLV-positive cats

• Cats exposed to transient populations (foster homes, shelters, clinic cats)¹⁷

Conclusion

In the effort to demonstrate efficacy of FeLV vaccination, some recently published studies have used protocols more closely mimicking natural infection than have other studies. Veterinarians can feel comfortable knowing that industry standards and these recent studies support their efforts to protect patients at risk for FeLV by using vaccines tested for safety and efficacy in situations that mimic the conditions their patients may face.

* Knight-Jones TJ, Edmond K, Gubbins S, Paton DJ. Veterinary and human vaccine evaluation methods. Proc Biol Sci. 2014; 281 (1784)