FORUM LETTERS 62-3 STEP INFECTION IN HUMANS
WHO BLAMES THE FAMILY DOG?
"What is your opinion about dogs being a source of Streptococcus A in humans?" writes KAY STOPFER, D.VM., Madison, Wisconsin. "I had always been under the impression it is a rare occurrence, especially if the dog is not or has not been ill. It seems as if physicians automatically blame the family dog if a human family?member gets Streptococcus infection more than once. If it is a legitimate concern, is it better just to treat the pet with antibiotics, if Streptococcus infection is suspected, or should the patient be tested first? How accurate in both sensitivity and specificity is the test?"
This quick review about group A streptococci infection in humans, dogs, and cats will most likely answer the questions you have asked. Humans are the primary natural reservoir hosts of group A streptococci, and most human infections are caused by this group. Dermatitis, pharyngitis, and scarlet and rheumatic fevers are the key syndromes caused by Streptococcus pyogenes. These orgarnisms uncommonly cause perianal cellulitis, vaginitis, and localized abscesses. Of the streptococcal groups, group A organisms have the greatest virulence for human adults, whereas organisms of the other groups, such as B, C, D, F, and G, cause the most severe manifestations in neonates. The sites of greatest carriage of group A streptococci in humans are the caudal aspects of the pharynx and tonsillar region. Group A streptococci can survive extremes in environmental temperature and humidity; however, most infections are associated with direct or close contact among susceptible individuals. As is true with many streptococcal infections, some individuals can harbor the infection for extended periods in the absence of clinical illness. Prevalence rates for group A streptococci are higher in young children, especially those in day-care or classroom situations. The prevalence of positive throat culture findings in such circumstances may approach 50 percent, even in the absence of an obvious epidemic. Symptomatic rather than carrier children are most likely to bring infection into the home. Under such circumstances, the isolation rate in other humans in the household approaches 25 to 50 percent. If the child is asymptomatic, the isolation rate is only nine percent. Dogs and cats have been suggested as possible reservoirs of reinfection of treated house hold members, but there is no convincing evidence that dogs and cats represent significant reservoirs of infection for humans. Whenever streptococcal typing has been conducted on the oral pharyngeal region of dogs and cats, groups G, C, L, and M have been present. However, screening for group A streptococcal colonization of the tonsils of dogs and cats from random households in urban environments has shown the apparent prevalence to range between one and 10 percent. Domestic pets that come into close contact with infected humans can sometimes apparently develop pharyngeal colonization with group A streptococci. Infected pets show no clinical illness or tonsillar enlargement. Because clinical symptomatology is absent in these animals, the main consideration is their potential public health risk. I f they were overlooked during treatment, they might serve as possible reservoirs for reinfection of family members. Pets, however, usually lose their infection within two to three weeks after they are removed from the household. Infected humans are carriers of group A streptococci for longer periods. It is not acceptable to consider culturing and treating the dog and cat in a household in which reinfection occurs without doing so for the human contacts. The recovery of group A streptococci may be affected by the method used in swabbing the throat, because overgrowth by indigenous microflora can result in the death of group A streptococci. Latex agglutination tests and enzyme-linked immunosorbent assays are avail able for rapid detection of group A streptococci in children. The value of these tests in detecting asymptomatic group A streptococci infections in dogs and cats is quite low. The antimicrobial spectrum for group A strepto coccal infection in pets is the same as for human strains. It is judicious to treat pets when they may be a source of recurrent infection of household members. Isolates of group A streptococci from dogs have shown the greatest susceptibility to penicillin, erythromycin, and chloramphenicol. Resistant strains can be treated with cephalosporins.
Johnny D. Hoskins, D.VM., Ph.D., Dipl. A.C.VI.M., Baton Rouge, Louisiana
Veterinary Forum 33 May 2001