TO THE EDITOR:
We would like to comment on the threshold for declaring a “clinical outbreak” of influenza presented in Lee and Murrell: 1 “Three or more residents within the same nursing unit present with a temperature > 100°F orally with symptoms consistent with influenza within a 3-day period.”1 Such a clustering of cases in time and space infers that a virulent and highly infectious agent is being transmitted within the facility. Based on our experience, we believe that this is too high a threshold. By the time such a cluster of cases was identified with confirmation of influenza by rapid diagnostic techniques, such an explosive infectious disease would probably be incubating in many other residents.

A Centers for Disease Control and Prevention (CDC) publication available online (last revised 12/23/05) entitled, “Infection Control Measures for Preventing and Controlling Influenza Transmission in Long-Term Care Facilities,” states that one case of confirmed influenza by any testing method in a long-term care facility resident is an outbreak.2 Outbreak control measures should also be implemented if “more than 1 resident in a unit develops acute, febrile, respiratory illness during a 1-week period.”2 The outbreak threshold of a single nursing home–acquired, laboratory-confirmed case is based on the likelihood that the single case represents the tip of an iceberg and the fact that influenza is usually extremely transmissible. Public health officials, therefore, recommend special control measures when there is evidence of transmission within the facility, rather than waiting for a full-blown “outbreak.”

The standard clinical case definition for influenza lacks proven sensitivity in vaccinated residents of long-term care facilities.3 We previously reviewed the charts of 147 culture-positive residents with influenza and found that 30% of them never had a temperature > 100°F during the entire course of their illness.4

The Wisconsin Veterans Home is a skilled nursing facility with an average daily census of 721, with four residential buildings within a two-block radius. Intense prospective surveillance, as part of funded prospective studies of influenza prophylaxis, was conducted over six influenza seasons.5 The first case of influenza in the last building affected occurred 27 to 64 days after the first case in this four-building facility. There was a common seasonal pattern of building attack rates. If a season begins with a severe outbreak in a single building, there will probably be similar attack rates in buildings affected later in that season. Delays between initial cases in buildings or facilities could allow clinicians to adjust defensive strategies after sampling attack rates and virulence in facilities affected earlier in the season.5

The decision to initiate influenza prophylaxis in a nursing building should optimally be made prior to the development of a full-blown clinical outbreak.5 Factors to consider include:

• Level of community influenza activity. A high level increases the likelihood of introduction by staff or visitors in multiple locations.
• Quality of the match between the circulating and vaccine strain. The most serious outbreak in our facility occurred during a year with a poor vaccine match (1997-98).5
• Severity of outbreaks in nursing homes affected earlier in the season (as discussed above).5
• Pattern of new clinical respiratory illnesses within the facility. (Clustered, severe cases demand prophylaxis.)
• Number and clustering of laboratory-confirmed cases. (Clustering implies transmission within the facility.)

We initiate prophylaxis on the day influenza is confirmed by rapid test if influenza is prevalent in the surrounding community, especially if there is a poor vaccine match or if we are aware of severe outbreaks in facilities affected earlier in the season.

Sincerely,
Paul J. Drinka, MD, CMD
Medical Director
Wisconsin Veterans Home
King, WI
Clinical Professor, Internal
Medicine/Geriatrics
University of Wisconsin -
Madison and
Medical College of Wisconsin -
Milwaukee

Peggy F. Krause RN, CCRC
Senior Research Nurse Clinician
Wisconsin Veterans Home
King, WI




References
1. Lee KW, Murrell E. Influenza prevention and treatment strategies in the elderly. Annals of Long-Term Care: Clinical Care and Aging 2006;14(9):20-27.

2. Centers for Disease Control and Prevention. Guidelines and Recommendations: Infection Control Measures for Preventing and Controlling Influenza Transmission in Long-Term Care Facilities. Revision draft 12/23/05; available at http:// www.cdc.gov/flu/professionals/infectioncontrol/longtermcare.htm Accessed 11/29/06.

3. Bradley SF, LTC Committee of SHEA. Prevention of influenza in long-term care facilities. Infect Control Hosp Epidemiol 1999;20:629-637.

4. Drinka PJ, Krause P, Nest L, et al. The effect of culture-positive influenza type A on resource utilization and adverse events in nursing home residents. J Am Geriatr Soc 2002;50:1416-1420.

5. Drinka P, Krause P, Nest L, Gravenstein S, et al. Delays in the application of outbreak control prophylaxis for influenza A in a nursing home. Infect Control Hosp Epidemiol 2002;23:600-603.



RESPONSE FROM DR. LEE
Thank you for your comment with regard to our manuscript and for sharing your clinical experience. The challenge in “defining” a clinical influenza outbreak for means of providing guidance to other clinicians is a difficult task when available data are so limited and highly variable. More important, “identifying” an actual clinical outbreak for means of initiating influenza prophylaxis is a larger challenge, due to the variable presentations of the infection in our elderly patients and the inevitable considerations for cost control in light of optimal patient care. Severity of outbreaks is highly dependent on not only the circulating viral strain for a particular year, but also on the characteristics of the facility itself that you have alluded to in your review of outbreak management. In addition to recognizing the listed factors to consider prior to initiating influenza prophylaxis, it is equally important to document staff observations of subtle clinical signs and symptoms of a possible influenza infection such as anorexia, decreased activities of daily living, change in respiratory status, etc, in addition to a fever. Early intervention is vital; a conservative approach is most desirable, when possible.

Respectfully,
Karen W. Lee, PharmD, CDM
Commonwealth Medicine at the University of Massachusetts
Medical School
Boston,