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To determine pharmacokinetics of single and multiple doses of rimantadine hydrochloride in horses and to evaluate prophylactic efficacy of rimantadine in influenza virus-infected horses.


5 clinically normal horses and 8 horses seronegative to influenza A.


Horses were given rimantadine (7 mg/kg of body weight, IV, once; 15 mg/kg, PO, once; 30 mg/kg, PO, once; and 30 mg/kg, PO, q 12 h for 4 days) to determine disposition kinetics. Efficacy in induced infections was determined in horses seronegative to influenza virus A2. Rimantadine was administered (30 mg/kg, PO, q 12 h for 7 days) beginning 12 hours before challenge-exposure to the virus.


Estimated mean peak plasma concentration of rimantadine after IV administration was 2.0 µg/ml, volume of distribution (mean ± SD) at steady-state (VdSS) was 7.1 ± 1.7 L/kg, plasma clearance after IV administration was 51 ± 7 ml/min/kg, and β-phase halflife was 2.0 ± 0.4 hours. Oral administration of 15 mg of rimantadine/kg yielded peak plasma concentrations of < 50 ng/ml after 3 hours; a single oral administration of 30 mg/kg yielded mean peak plasma concentrations of 500 ng/ml with mean bioavailability (F) of 25%, β-phase half-life of 2.2 ± 0.3 hours, and clearance of 340 ± 255 ml/min/kg. Multiple doses of rimantadine provided steady-state concentrations in plasma with peak and trough concentrations (mean ± SEM) of 811 ± 97 and 161 ± 12 ng/ml, respectively. Rimantadine used prophylactically for induced influenza virus A2 infection was associated with significant decreases in rectal temperature and lung sounds.

Conclusions and Clinical Relevance

Oral administration of rimantadine to horses can safely ameliorate clinical signs of influenza virus infection. (Am J Vet Med 1999:60:888–894)

Free access
in American Journal of Veterinary Research


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 immunosuppression.

Design—Prospective study.

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)

Full access
in Journal of the American Veterinary Medical Association