An ongoing dialogue on HIV/AIDS, infectious diseases,
October 29th, 2017
Cellulitis, Lyme, VZV, MRSA, TB, Tdap: Great Questions from ID in Primary Care
We’ve just finished our annual course Infectious Diseases in Primary Care, and once again our attendees — all busy clinicians — asked some excellent questions.
Below, a small sample:
- What is the drug of choice for cellulitis in outpatients who are allergic to penicillin? Importantly, this is about cellulitis — not abscesses — which means most are caused by beta strep (group A, B, C, G). If you can find a history of safe use of a cephalosporin, go with that. (Many of us ID doctors prefer cefadroxil to cephalexin, since the former is twice-daily.) If cephalosporins aren’t an option, one commonly cited alternative is clindamycin, though there’s increasing resistance to the latter among certain strep in the community, especially group B. Some would say that TMP/SMX is an option, but I’m not a fan. Levofloxacin is active versus most beta strep, so this also should be considered, even though it never appears in guidelines. The price of linezolid has come down since it went generic, but it’s still expensive, and not always well tolerated. But whatever you choose, remember that more than 95% of people with penicillin allergy aren’t actually allergic — have them get a skin test, and remove that penicillin allergy from their chart! So gratifying to do that.
- How long does the Lyme serology stay positive after treatment? Although Lyme serologies gradually decline in most patients, they can in individual patients persist for years, making follow-up serologies as a “test of cure” a particularly confusing practice — so let’s stop doing it! Importantly, this applies to both IgM and IgG antibody responses, which makes IgM antibodies an unreliable marker of recent infection unless they are correlated with recent-onset symptoms and exposure. Because there are no predictors of who will and will not have persistent Lyme antibodies, most of the time the diagnosis of reinfection relies heavily on clinical presentation. I hope someone out there is working on a reliable PCR or antigen test for Lyme — we really need it!
- One of my patients received the chicken pox vaccine, and now wants to get pregnant. Her varicella serology is negative. Should I revaccinate? Unfortunately, the varicella titers commonly used in clinical practice only reliably check for natural immunity, not vaccine-induced immunity. As a result, they are not recommended as a marker of varicella vaccine efficacy. But the bottom line is that if you have a clear record she received her vaccine, there is no need to revaccinate your patient — or check titers.
- How do you manage recurrent MRSA? Once these poor suffering people find their way to ID doctors, they’ve usually had multiple episodes occurring over months, sometimes even years. So these recommendations aren’t for the people with a few minor boils, or a couple of recurrences. Assuming that underlying conditions such as hidradenitis suppurativa are excluded, we go for the “try everything” approach, summarized at right. To enforce the effort, we pass out this information sheet. An important caveat is that clinical trials haven’t confirmed the efficacy of these strategies, and hence guidelines do not formally endorse them. Good luck!
- If recurrent zoster is so rare, why is the zoster vaccine recommended for people who have had shingles? It’s true that in immunocompetent hosts, recurrent zoster is rare — you can read more about recurrent zoster (both the real thing and fake-outs) in this post. However, a small fraction of people who get shingles remain at risk for another episode, presumably because the first outbreak did not sufficiently boost their antibody responses, or because their immune system is not intact. Furthermore, as anyone who does clinical practice knows, the motivation for zoster immunization is particularly high for people who have experienced a case themselves! While the full details of the ACIP recommendations for the just-approved zoster subunit vaccine are not yet available, presumably they will also recommend this vaccine for patients with a history of zoster, just as they did for the live virus zoster vaccine. Notably, 92 patients with a history of prior physician-diagnosed zoster received the subunit vaccine in a prospective study, with the primary endpoint being antibody responses; encouragingly, 90% of those immunized had a four-fold increase in their VZV titers.
- In TB screening, do you have a preference for using either an IGRA or a skin test? In general, the IGRA (interferon gamma release assay) is better — less operator dependent, no second visit required, no false positives from BCG. While the IGRA is more expensive, its advantages over tuberculin skin testing were acknowledged in the latest guidelines, which stated that the IGRA is generally preferred when available. See the summary table at right (click on image to see it larger).
- I take care of a nurse who got Tdap last year, and was recently exposed to pertussis. Should she get preventive antibiotics? It would be great if we knew the degree of pertussis protection afforded by the Tdap vaccine after a direct exposure, but unfortunately those data are limited; in addition, secondary cases of pertussis have occurred within households even when immunizations are up to date. As such, it’s recommended that most healthcare workers receive antibiotic prophylaxis after exposure to a case, as they may come in close contact with people at high risk of severe illness from pertussis.
- You ID doctors seem pretty smart. Where do you get all this information? Don’t be fooled by our apparent omniscience; it’s an illusion we create to compensate for the fact that we don’t do invasive procedures. Mostly, we just know where to look stuff up.
- What’s made you laugh out loud recently? How about this recent CDC report?
— Paul Sax (@PaulSaxMD) October 27, 2017
I know, I know — we have a peculiar sense of humor in this field of ours.