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Abstract

The optimal vaccination protocol to induce immunity in kittens with maternal antibodies is unknown. The objective of this study was to determine the effects of maternally-derived antibody (MDA) on serologic responses to vaccination in kittens. Vaccination with a modified live virus (MLV) product was more effective than an inactivated (IA) product at inducing protective antibody titers (PAT) against feline panleukopenia virus (FPV). IA vaccination against feline herpesvirus-1 (FHV) and feline calicivirus (FCV) was more effective in the presence of low MDA than high MDA. Among kittens with low MDA, MLV vaccination against FCV was more effective than IA vaccination. A total of 15%, 44% and 4% of kittens had insufficient titers against FPV, FHV and FCV, respectively, at 17 weeks of age. Serologic response to vaccination of kittens varies based on vaccination type and MDA level. In most situations, MLV vaccination should be utilized and protocols continued beyond 14 weeks of age to optimize response by all kittens.

Introduction

Feline panleukopenia virus (FPV), feline herpesvirus-1 (FHV) and feline calicivirus (FCV) are present worldwide.^1–4 Owing to the widespread use of vaccines in the general pet cat population, FPV is an infrequent cause of life-threatening gastroenteritis in owned cats. FHV and FCV are more common, but usually result in mild-to-moderate transient upper respiratory disease. In contrast, these infections are a constant and serious threat to the health and welfare of cats residing in animal shelters, often resulting in death as a result of disease progression or euthanasia. Outbreaks of FPV in shelters have been reported across the country and are frequently managed by depopulation of both clinically diseased and healthy exposed cats.^5–7 The risk of upper respiratory disease and associated euthanasia increases with the length of shelter stay.^8,9 Because of these threats, a rapid response to vaccination is critical in the high-risk environment of an animal shelter.^10–12

By regulation, vaccine-licensing trials are carried out in seronegative laboratory cats. In the absence of interfering maternal antibodies, kittens generally develop sterilizing immunity or resistance to severe clinical disease after 1–3 vaccine doses.^5,13–18 In the field, the presence of maternally-derived antibodies (MDA) at the time of initial vaccination substantially reduces response to vaccination, such that a large proportion of kittens with MDA may fail to develop a protective antibody titer (PAT) by the end of the initial vaccination series.^19–21 For pet cats with little chance of exposure to viral infections, the risk associated with a delay in effective immunization is minimal. In contrast, kittens entering an animal shelter may face high-dose exposure to viral pathogens under stressful conditions. Rapid immunization may be life-saving in this environment. It is generally not practical to predict which kittens carry interfering MDA. Variability in MDA levels exists even among individuals from the same litter, resulting in a range of susceptibility to infection and the ability to respond to vaccination. Thus, vaccination protocols must be developed to successfully immunize kittens with all levels of MDA.

The optimal vaccination protocol to induce rapid immunity in kittens with MDA is unknown. Various authors have proposed that either inactivated (IA) or modified-live virus (MLV) vaccines are superior for overcoming MDA.^10,21 The purpose of the study reported here is to determine the effect of MDA on the serologic responses to vaccination with either IA or MLV vaccines.

Materials and methods

Animals

Thirteen specific pathogen-free domestic shorthair queens were bred to give birth to 16 litters of kittens between November 2004 and August 2005. Twenty-seven kittens were included in the study. All adult cats and kittens were free of feline leukemia virus (FeLV) antigen and feline immunodeficiency virus (FIV) antibodies, as determined by a point-of-care enzyme-linked immunosorbent assay (ELISA) kit (SNAP FIV/FeLV Combo test; Idexx Laboratories, Westbrook, ME, USA). Food (Iams Kitten Formula; Iams, Dayton, OH, USA) and water were offered ad libitum throughout the study. Kittens were weaned at 8 weeks of age and placed in group housing. Kittens were identified by use of radiofrequency identification microchips (MicroChip identification system, AVID, Norco, CA, USA). At the conclusion of the study, the animals were surgically sterilized and adopted. The research protocol was approved by the University of Florida Institutional Animal Care and Use Committee and was conducted in facilities accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International.

Vaccination

Queens were vaccinated against FPV, FHV, FCV, FeLV and rabies virus with IA vaccine products (Fel-O-Vax Lv-K III and Rabvac 3 TF; Fort Dodge Animal Health, Fort Dodge, IA, USA) and according to professional guidelines.²² All kittens were vaccinated at 8, 11 and 14 weeks of age against FPV, FHV, FCV and FeLV. The kittens were randomly selected using a computer-generated random numbers table for vaccination with either a MLV vaccine (Fel-O-Guard Plus 3 Lv-K; Fort Dodge Animal Health) (n = 16) or an IA vaccine (Fel-O-Vax Lv-K III; Fort Dodge Animal Health) (n = 11). Both products contained IA FeLV vaccine. The vaccines were administered subcutaneously in the left hind limb, distal to the stifle joint, as recommended by the American Association of Feline Practitioners for vaccines containing FeLV. At 14 weeks of age, an IA rabies virus vaccine (Rabvac 3 TF; Fort Dodge Animal Health) was administered subcutaneously in the right hind limb, distal to the stifle.

Serologic testing

Blood samples (3–5 ml) were obtained from kittens via jugular venepuncture at 8, 9, 11, 14 and 17 weeks of age. On vaccination days, blood was collected immediately prior to vaccination. Blood samples were collected into serum separator tubes, allowed to clot for 30 min and then centrifuged for 20 min. Serum was separated and stored at −20° C pending analysis. Serum antibody titers were measured for FPV (hemagglutination inhibition), FHV (virus neutralization), and FCV (virus neutralization) by a university-affiliated diagnostic laboratory (Animal Health Diagnostic Center, College of Veterinary Medicine, Cornell University, Ithaca, NY, USA). At the time of analysis, the laboratory in which assays were conducted recommended assessment for booster vaccination when titers <40, <16, and <32 are reported for FPV, FHV and FCV, respectively, based on literature reviews, including independent challenge studies.²³ As such, titers greater than or equal to these were considered protective (ie, PAT) for the purposes of this study and those less than these were considered insufficient. Kittens whose antibody titers at 8 weeks were ≥20, ≥4 and ≥8 for FPV, FHV and FCV, respectively, were considered to have high MDA levels. Kittens with titers ≤10, 0 and ≤6 for FPV, FHV and FCV, respectively, were considered to have low MDA levels. These titers were selected based on previous observations regarding responses to vaccination in the authors’ laboratory.^5,19

Statistical analysis

Proportions of kittens with PAT against FPV, FHV and FCV in each of the four groups (low vs high MDA and MLV vs IA vaccine) were compared using Fisher’s exact test for each time point. Values of P <0.05 were considered significant.

Results

MDA titer groups

Seventeen kittens were determined to have low MDA and 10 kittens were determined to have high MDA against FPV on the basis of 8-week pre-vaccination antibody titers. Seventeen kittens had low MDA and 10 kittens had high MDA against FHV. Sixteen kittens had low MDA and 11 kittens had high MDA against FCV (Table 1).

Table 1

Median (range) serum antibody titers against FPV, FHV and FCV in specific pathogen-free kittens with low and high MDA levels that were vaccinated at 8, 11 and 14 weeks of age with inactivated (IA) and modified live virus (MLV) vaccines

Antigen	MDA level	Vaccine	Number	Age at determination of serum antiviral antibody titers (weeks)
Antigen	MDA level	Vaccine	Number	8	9	11	14	17
FPV	Low	IA	8	0 (0–10)	10 (0–20)	80 (0–160)	640 (80–2560)	1280 (320–2560)
	Low	MLV	9	0 (0–10)	40 (0–320)	1280 (0–5120)	1280 (40–5120)	1280 (160–5120)
	High	IA	3	20 (20–40)	20 (10–20)	0 (0–10)	0 (0–0)	20 (10–40)
	High	MLV	7	80 (20–160)	40 (10–80)	40 (10–80)	40 (0–160)	160 (0–640)
FHV	Low	IA	8	0 (0–0)	0 (0–8)	6 (0–12)	40 (12–128)	48 (8–192)
	Low	MLV	9	0 (0–0)	0 (0–4)	0 (0–0)	12 (0–32)	16 (12–24)
	High	IA	3	24 (12–24)	12 (8–16)	6 (6–8)	6 (0–6)	0 (0–4)
	High	MLV	7	8 (4–12)	4 (0–12)	4 (0–8)	4 (0–12)	12 (4–48)
FCV	Low	IA	6	6 (0–6)	12 (0–16)	16 (6–32)	128 (16–256)	320 (32–768)
	Low	MLV	10	2 (0–4)	24 (12–64)	64 (16–512)	384 (96–1536)	448 (128–2048)
	High	IA	5	32 (8–2048)	8 (6–4096)	16 (6–2048)	192 (12–4096)	384 (16–2048)
	High	MLV	6	14 (8–3072)	56 (8–4096)	432 (12–6144)	2560 (32–4096)	1152 (64–8192)

Low MDA is defined by titers ≤10, 0 and ≤6 for FPV, FHV, and FCV, respectively. High MDA is defined by titers ≥20, ≥4 and ≥8 for FPV, FHV, and FCV, respectively

Serum anti-FPV antibody titers

Fourteen out of 27 kittens (52%) had detectable MDA against FPV at the time of the 8-week pre-vaccination antibody titers and 13 were seronegative. Of the 10 kittens with high MDA, a lower proportion of those that received IA vaccines had PAT at each subsequent time point than those receiving MLV vaccines, with a significant difference observed at 11 weeks of age [0/3 (0%) vs 6/7 (86%), P = 0.03] (Figure 1). Of the 17 kittens with low MDA, a lower proportion of those that received IA vaccines had PAT than those receiving MLV vaccines at 9 weeks of age [0/8 (0%) vs 6/9 (67%), P <0.01]. Of the 11 kittens that received IA vaccines, a lower proportion of kittens with high MDA had PAT than those with low MDA at 11 [0/3 (0%) vs 7/8 (88%), P = 0.02] and 14 [0/3 (0%) vs 8/8 (100%), P <0.01)] weeks of age. Of the kittens with high MDA, 4/10 (40%) did not achieve PAT against FPV by 17 weeks of age.

Figure 1

Proportion of specific pathogen-free kittens with PAT against FPV following vaccination at 8, 11 and 14 weeks of age with IA or MLV vaccines. (A) Kittens (n = 17) with low MDA at 8 weeks (range 0–10). (B) Kittens (n = 10) with high MDA at 8 weeks (range 20–160). *Indicates significant difference in proportion of kittens receiving MLV vs IA vaccines that develop PAT (P <0.05); **Indicates significant difference in proportion of kittens with low vs high MDA levels that develop PAT (P <0.05)

Serum anti-FHV antibody titers

Ten out of 27 kittens (37%) had detectable MDA against FHV at the time of the 8-week prevaccination antibody titers; the others were seronegative. Of the 11 kittens that received IA vaccines, a lower proportion of kittens with high MDA had PAT than those with low MDA at 14 [0/3 (0%) vs 7/8 (88%), P = 0.02] and 17 [0/3 (0%) vs 7/8 [88%], P = 0.02] weeks of age (Figure 2). Of the kittens with high MDA, 7/10 (70%) did not achieve PAT against FHV by 17 weeks of age. Of the kittens with low MDA, 5/17 (29%) did not achieve PAT against FHV by 17 weeks of age.

Figure 2

Proportion of specific pathogen-free kittens with PAT against FHV following vaccination at 8, 11 and 14 weeks with IA or MLV vaccines. (A) Kittens (n = 17) with low MDA at 8 weeks (range 0). (B) Kittens (n = 10) with high MDA at 8 weeks (range 4–24). **Indicates significant difference in proportion of kittens with low vs high MDA levels that develop PAT (P <0.05)

Serum anti-FCV antibody titers

Twenty-one of 27 kittens (78%) had detectable MDA against FCV at the time of the 8-week prevaccination titers and six were seronegative. Of the 16 kittens with low MDA, a lower proportion of kittens that received IA vaccines had PAT than those that received MLV vaccines, with a significant difference observed at 11 weeks of age [1/6 (17%) vs 9/10 (90%), P <0.01] (Figure 3). Of the kittens with high MDA, 1/11 (9%) did not achieve PAT against FCV by 17 weeks of age.

Figure 3

Proportion of specific pathogen-free kittens with PAT against FCV following vaccination at 8, 11 and 14 weeks with IA or MLV vaccines. (A) Kittens (n = 16) with low MDA at 8 weeks (range 0–6). (B) Kittens (n = 11) with high MDA at 8 weeks (range 8–3072). *Indicates significant difference in proportion of kittens receiving MLV vs IA vaccines that develop PAT (P <0.05)

Discussion

High MDA was associated with a delay in the induction of PAT for all three viruses, regardless of the type of vaccine administered. Following vaccination at 8, 11 and 14 weeks, 40%, 70% and 9% of these kittens had insufficient titers against FPV, FHV and FCV, respectively, at 17 weeks of age. A study of kittens from rescue shelters and breeding catteries with varying MDA levels vaccinated at 6, 9 and 12 weeks of age with an MLV vaccine had similar findings: those with no MDA (at 6 weeks of age) responded to vaccination sooner than those with MDA and between 67% and 100% of kittens demonstrated response to vaccination at the end of the study.²⁰ It is likely that the lack of protection in some kittens in both studies is a result of MDA interference.

The duration of MDA interference is related to the original level of MDA which is dependent upon the antibody titer of the queen, as well as the amount of immunoglobulin received in colostrum.^2,24 A wide variation exists with MDA interference reported up to 14–16 weeks of age for FPV,^19,20,25 2–10 weeks of age for FHV^3,14,19,20 and 10–14 weeks of age for FCV.^19,20,26 The current study evaluates MDA levels in kittens from queens vaccinated with an IA product, which are likely lower, and therefore result in a shorter duration of interference, than those vaccinated with an MLV product or naturally infected. In addition, vaccine products from a single manufacturer were utilized; the use of other products may have resulted in different results. In light of these limitations, the data suggest that interference can last >14 weeks for each virus. As individual titer measurement is rarely performed during a kitten’s initial vaccination series in practice, these data reinforce the need for vaccination with a MLV product at least until 16 weeks of age, as is currently recommended, although the optimal age for concluding the initial vaccine series cannot be determined from the results of this study.

MLV FPV vaccines were able to overcome MDA sooner than IA products, regardless of the MDA level for the study population. The majority of kittens vaccinated with low MDA achieved PAT against FPV within 1 week of vaccination with a MLV product and within 2 weeks of vaccination with an IA vaccine, which is consistent with previous findings.^5,13,17,27 These findings reinforce the need for vaccination against FPV with an MLV product, particularly in high-risk environments where a rapid response is desired. A delay in protection of only a few days can predispose to outbreaks of FPV in animal shelters.

Consistent with previous reports,^23,27,28 vaccination against FHV in the current study was less effective than other vaccine components, regardless of type of product. The IA product was only effective at inducing PAT against FHV in kittens with low MDA. This finding contrasts with those of an earlier study in which IA vaccines were able to induce PAT in the presence of MDA by 8 weeks of age, but titers rose at a slower rate in kittens with MDA than in those without MDA.¹⁴ In that study, neither product prevented infection following viral challenge and the severity of clinical signs in kittens receiving a MLV product did not differ from those that received an IA product.

Among kittens with low FCV MDA, the MLV vaccine was more effective than the IA vaccine for rapid induction of PAT against FCV. Previous studies have demonstrated similar findings, inducing PAT in kittens with low MDA by 9 weeks of age with the use of MLV vaccines.²⁰ A study evaluating the onset of immunity after two injections of an IA product induced protection against challenge by 12 weeks of age in kittens with no MDA.¹³ Neither of those studies compared responses between MLV and IA vaccinations.

The effect of continued vaccination after 14 weeks was not evaluated in this study, leaving the exact age at which PAT can be established in 100% of kittens unknown. Caution should be taken not to equate PAT with protection against infection (in the case of FPV) or disease (in the case of FHV and FCV) as universal laboratory reference standards for PAT have not been established. In addition, different commercial laboratories can report different results when evaluating the same sample so a single distinct threshold at which protection exists cannot be defined. However, combined with the variation in previous reports regarding onset of protection against FPV, FHV and FCV, the current findings emphasize that a single vaccination in kittens may not be sufficient to induce protection, particularly in vulnerable populations of animals and when assessment of MDA level is not practical. In the absence of individual titer measurement, vaccination is most likely to be effective at inducing PAT in a majority of kittens when a MLV product is administered at least until 16 weeks of age, in accordance with current professional guidelines for both pet and shelter cats.^10,12,21 It is possible that some kittens are especially slow responders, either because of very high levels of MDA or idiosyncrasies of their individual immune responses. In these cases, PAT may not develop until after the booster vaccination given a year after the kitten series is completed. It is also possible that some kittens are ‘non-responders’ to particular antigens and would not develop PAT, regardless of the timing of additional vaccinations.

Four percent to 44% of kittens failed to develop PAT by 17 weeks of age, with failure highest in kittens receiving the IA vaccine and in kittens with high MDA. As vaccination was discontinued at 14 weeks of age, the optimal age range for the kitten vaccination series remains unknown.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Conflict of interest

The authors declare that there is no conflict of interest.

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Effects of maternally-derived antibodies on serologic responses to vaccination in kittens

Abstract

Introduction

Materials and methods

Animals

Vaccination

Serologic testing

Statistical analysis

Results

MDA titer groups

Serum anti-FPV antibody titers

Serum anti-FHV antibody titers

Serum anti-FCV antibody titers

Discussion

Footnotes

Funding

Conflict of interest

References