Objective—To develop and characterize a cold-adapted
live attenuated equine-2 influenza virus effective as
an intranasal vaccine.
Animals—8 ponies approximately 18 months of age.
Procedures—A wild-type equine-2 virus, A/Equine/
Kentucky/1/91 (H3N8), was serially passaged in
embryonated chicken eggs at temperatures gradually
reduced in a stepwise manner from 34 C to 30 C to
28 C to 26 C. At different passages, infected allantoic
fluids were tested for the ability of progeny virus to
replicate in Madin-Darby canine kidney (MDCK) cells
at 34 C and 39.5 C. Virus clones that replicated at 26
C in eggs and at 34 C in MDCK cells, but not at 39.5
C in MDCK cells, were tested for stability of the coldadapted,
temperature-sensitive (ts), and protein synthesis
phenotypes. A stable clone, P821, was evaluated
for safety, ability to replicate, and immunogenicity
after intranasal administration in ponies.
Results—Randomly selected clones from the 49th
passage were all ts with plaquing efficiencies of < 10-6
(ratio of 39.5 C:34 C) and retained this phenotype after
5 serial passages at 34 C in either embryonated eggs
or MDCK cells. The clone selected as the vaccine candidate
(P821) had the desired degree of attenuation.
Administered intranasally to seronegative ponies, the
virus caused no adverse reactions or overt signs of clinical
disease, replicated in the upper portion of the respiratory
tract, and induced a strong serum antibody
Conclusion and Clinical Relevance—A candidate
live attenuated influenza vaccine virus was derived by
cold-adaptation of a wild-type equine-2 influenza
virus, A/Equine/Kentucky/1/91, in embryonated eggs.
(Am J Vet Res 2001;62:1290–1294)
Objective—To determine safety, efficacy, and
immunogenicity of an intranasal cold-adapted modified-
live equine influenza virus vaccine administered
to ponies following induction of exercise-induced
Animals—Fifteen 9- to 15-month old ponies that had
not had influenza.
Procedure—Five ponies were vaccinated after 5 days of strenuous exercise on a high-speed treadmill, 5
were vaccinated without undergoing exercise, and 5
were not vaccinated or exercised and served as controls.
Three months later, all ponies were challenged
by nebulization of homologous equine influenza virus.
Clinical and hematologic responses and viral shedding
were monitored, and serum and nasal secretions
were collected for determination of influenza-virus-specific
antibody isotype responses.
Results—Exercise caused immunosuppression, as
indicated by depression of lymphocyte proliferation in
response to pokeweed mitogen. Vaccination did not
result in adverse clinical effects, and none of the vaccinated
ponies developed clinical signs of infection following
challenge exposure. In contrast, challenge exposure
caused marked clinical signs of respiratory tract
disease in 4 control ponies. Vaccinated and control
ponies shed virus after challenge exposure. Antibody
responses to vaccination were restricted to serum
IgGa and IgGb responses in both vaccination groups.
After challenge exposure, ponies in all groups generated
serum IgGa and IgGb and nasal IgA responses.
Patterns of serum hemagglutination inhibition titers
were similar to patterns of IgGa and IgGb responses.
Conclusions and Clinical Relevance—Results suggested that administration of this MLV vaccine to
ponies with exercise-induced immunosuppression
was safe and that administration of a single dose to
ponies provided clinical protection 3 months later. (J
Am Vet Med Assoc 2001;218:900–906)