The Activity of Tetracycline on Feline Pneumonitis Virus Infection of Chick Embryos

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Abstract
Tetracycline and oxytetracycline show equal but higher activity than chlortetracycline in protecting chick embryos infected with feline pneumonitis virus. The greater instability of chlortetracycline in vivo may account for this difference. Tetracycline and oxytetracycline are as effective at 0.031 mg as at much higher concentrations (1 mg). The effectiveness of tetracycline within wide limits is independent of the size of the virus inoculum employed. Tetracycline possesses activity in protecting embryos infected with feline pneumonitis virus even when there is a considerable delay between time of infection and administration of the drug. Its effectiveness decreases gradually with a delay in administration. When given to infected embryos 12-24 hours before their expected death, the drug still shows slight activity. Tetracycline is without activity on feline pneumonitis virus in vitro. In vivo tetracycline (0.25-2 mg) has chiefly a virustatic rather than a virucidal action. Initial bodies and plaques are formed in its presence but subsequent multiplication of these bodies is inhibited to a considerable extent. Virus, once released from the presence of the antibiotic, is able to multiply freely once again and bring about the death of a susceptible host.