Virginiamycin was used as a production enhancer in animals; it is now only licensed for treating necrotic enteritis in chickens where nothing else is likely to work, and for preventing laminitis in horses. (It is supposed to alter gut flora, which results in less of whatever causes laminitis circulating to the feet.) Pristinomycin has been in human use in France for many years. It has to be given orally as it is not water soluble enough for parenteral use. Quinupristin & dalfopristin (RP59500, Synercid) is a newish drug used against methicillin resistant Staph aureus in people.
The nomenclature of this class of drugs is confusing as they are all synergistic mixtures of at least two different compounds. Thus virginiamycin is 75% virginiamycin M1 (a streptogramin A) and 5% virginiamycin S1 (a streptogramin B). The two drugs are structurally unrelated (streptogramin Bs are macrolides), bind to distinct sites of the 50 S ribosomal subunit, but cooperate to inhibit protein synthesis. The mechanism of inhibition is different for each component, however binding of Type A leads to a conformational change in the 50S subunit which potentiates the action of Type B streptogramin. Individually the molecules are only bacteriostatic, but together they act synergistically and are bactericidal.
Bactericidal against Gram positives.
A single gene confers cross resistance among the macrolides, lincosamides and streptogramin Bs, but the streptogramins as a combination are not usually affected. There is complete cross resistance between virginiamycin and quinupristin & dalfopristin.
Bacterial overgrowth leading to haemorrhagic diarrhoea has been reported. Rarely, lung oedema has been associated with virginiamycin.
Virginiamycin is practically insoluble and so is only given orally for its effect on gut flora, pristinomycin is available in injectable form. Virginiamycin has recently been banned in Europe.
Virginiamycin is licensed in NZ for prevention of laminitis in horses and necrotic enteritis in chickens where nothing else is likely to work. In view of the cross resistance with a drug of last resort in people, this is ethically dubious.
Synercid is one of the few drugs which work reliably (at the moment) against MRSA. Streptogramins should not be used in animals, and should be reserved for MRSA in people.
