The American College of Veterinary Internal Medicine (ACVIM) recently released its consensus statement on guidelines for treatment, control, and prevention of strangles, which is caused by Streptococcus equi. The 12-page paper took about a year to complete and reflects the input of researchers and veterinary clinicians across the country.
The document was published in the January issue of the Journal of Veterinary Internal Medicine and can be found online at http://www.acvim.org/wwwfp/ConsensusStmts/Strangles.pdf
Corinne Sweeney, DVM, professor of Medicine and vice chair of the Department of Clinical Studies at the University of Pennsylvania's New Bolton Center, was one of four veterinarians on the committee that assembled the guidelines. A draft of the guidelines was presented at the ACVIM Forum in June 2004, where input was solicited from large animal internists. Sweeney said there was a great deal of input at the forum and a modest amount after the draft was posted on the ACVIM web site for comment.
While the guidelines are extremely comprehensive and cover many facets of strangles, "Most interesting to horse owners would be the control of outbreaks--what to do when there's an outbreak in the barn," said Sweeney.
The section details management efforts such as stopping movement of horses on and off the premises, maintenance of strangles-affected horses, and clustering of cases on the premises according to diagnosis. Surveillance methods are described, including nasopharyngeal swabbing and endoscopy. Hygiene and disinfection measures for people and equipment are outlined.
In the epidemiology section, the authors highlight that, "S. equi may originate from outwardly healthy horses that are incubating the disease and go on to develop disease.”
The authors explained that recovering horses might continue to harbor the organism for several weeks after clinical signs have disappeared, so those horses might be potential sources of infection for at least six weeks after clinical signs have resolved. "Their introduction to herds may be a source of new outbreaks, even in well-managed groups of horses," wrote the authors.
The guttural pouch is the best-recognized site for prolonged harboring of S. equi; in some animals, guttural pouch empyema (accumulation of pus) with S. equi can persist asymptomatically for months or years. "About 50% of horses with guttural pouch empyema cough sporadically, and some may have an intermittent unilateral nasal discharge," said the authors.
However, there is important information on S. equi's lack of environmental persistence. "Some people think a farm that has had strangles is contaminated for life; it's not," said Sweeney. "That's the take-home message from that section--there's no proof that S. equi lasts very long, even in the ideal situation (the laboratory, where it was found that S. equi survived for 48-60 days in various temperatures on a variety of surfaces). "This is not a hardy organism," she added.
Under the section on treatment, the authors discussed the myth that giving antibiotics in the face of strangles will cause "bastard strangles" (formation of abscesses in areas other than the head and neck). Some have proposed this happens because killing the organism could indirectly affect the development of immunity and thereby increase the risk of infection. But Sweeney assures, "There's no experimental or clinical studies that suggest that."
The authors pointed out, however, "Immediate treatment with antibiotics of horses that show the earliest clinical signs of fever could be an effective way of controlling strangles outbreaks in racing stables or riding barns, although the disadvantages of treatment should be weighed." The downside is that since infection confers natural immunity, by treating the horse early, you've protected him from a full-blown case, but the next time strangles hits the barn, he won't have the advantage of natural protection.
The guidelines close with future directions for strangles research. "A serological test to identify subclinical carriers of S. equi would be an extremely useful tool in the control of strangles and prevention of new outbreaks," wrote the authors.
Sweeney stressed that these guidelines are current now, and they will be updated as new research is published.
Dana K. Vaughan, Ph.D.
Associate Professor of Biology
University of Wisconsin Oshkosh