Abstract

Effective vaccination regimes are of critical importance to protect mustelids and other highly susceptible species against lethal canine distemper virus (CDV) infection. An experimental killed vaccine (KV), when available, has been used to protect endangered black-footed ferrets (BFF) and other high-risk species, yet live virus challenge studies have never been done for this vaccine to prove its value in species where it has been used. In addition, a readily available modified live vaccine (Galaxy D, MLV) should potentially provide a more robust and longer-lasting immunity if safe boosting of KV vaccinates is possible. The goals of this study were to determine (1) if boosting with a commercial modified live virus (MLV) vaccine (Galaxy D®) was safe and effective in ferrets having received the killed vaccine previously, and (2) if previous KV use protects against virulent CDV challenge.

Fourteen non-releasable non-reproductive black-footed ferrets were identified and designated by the BFF SSP for these studies. The 14 KV protected (seven male, seven female) black-footed ferrets (Mustela nigripes), 4–6 years of age, were stratified by CDV SN serotiters into two groups before study. The first group was vaccinated with the Galaxy D vaccine, and the second group went unvaccinated. The results of this first part of the study indicated that the survival rate was 85.7% (6/7) following MLV vaccination of previous KV black-footed ferret recipients. The animal that developed typical CDV signs had the lowest pre-vaccination SN titer of the group (pre-study titer 1:8). One animal from the unvaccinated group in a separate bank of cages in the same room developed CDV-specific signs 30 days after MLV vaccination of the first group (pre-study titer 1:64).

In the second part of the study, all black-footed ferrets from both groups were challenged with a virulent defined CDV strain intranasally/intraorally using Synder Hill strain CDV (NVSL #90-18) at a dose four times the previously established 100% lethal dose (LD) in Siberian polecats. CDV was verified by typical clinical sign development or at necropsy. In this part of the study, black-footed ferrets vaccinated with MLV vaccine had a survival rate of 66.7% (4/6). Black-footed ferrets previously vaccinated only with KV and challenged with the virulent strain of CDV had a 50% (3/6) survival, although one death appeared to result from sepsis with E. faecalis and may have been indirectly related to CDV infection-induced immunosuppression.

Based on the survival data, we concluded that serologic evidence of having received KV previously was at least partially protective against MLV vaccination and during CDV challenge, whereas boosting with MLV vaccine appeared of marginal value and could in fact be detrimental. A partial explanation may have been that previous KV titers blocked the protective titer increases expected from MLV vaccination. Therefore, MLV vaccine boosting as reported here added minimal protection to black-footed ferrets already vaccinated repeatedly with KV against CDV, and Galaxy D by itself might induce clinical distemper disease in susceptible animals, including black-footed ferrets. Our results suggest that the potential benefits of boosting susceptible animals with a MLV CDV product needs to be carefully weighed against likely risks.