Co-infection is Frequent in Patients Hospitalized with Influenza

According to a recent article published online in Intensive Care Medicine, co-infection occurs commonly in patients with influenza-related infections that require intensive care unit (ICU) admission.

“Although primary viral pneumonia may evolve towards acute respiratory distress syndrome and death, bacterial co-infection is frequently described in these cases, may contribute to the development of ARDS and respiratory failure, and is clearly associated with higher mortality,” write Charles-Edouard Luyt, MD, PhD, from Université Paris 06, France, and Todd W. Rice, MD, from Vanderbilt University School of Medicine, Nashville, Tennessee, United States.

Influenza infection causes widespread epidemics each year. Bacterial co-infections also occur commonly in these cases, representing a major cause of the high morbidity and mortality associated with influenza epidemics. Indeed, bacterial co-infection complicated most of the deaths that occurred in the 1918 influenza pandemic.

Dr. Luyt and Dr. Rice highlight data from a recent meta-analysis, showing that co-infection rates range between 2 and 65%. They note that different sampling methods and the timing of obtaining samples could account for this disparity. Factors such as prehospital antibiotic administration, and whether a bacterial co-infection was microbiologically confirmed, could also contribute.

The authors discuss the results of a Spanish study in patients with influenza infection who were hospitalized. Of 2,901 patients, 16.6% had a community-acquired co-infection. The researchers found that Streptococcus pneumoniae was most commonly involved, followed by Pseudomonas aeruginosa, and methicillin-susceptible Staphylococcus aureus (MSSA). Interestingly, the rate of P. aeruginosa co-infection was 14.1%—much higher than what has been previously reported for co-infections with this organism, which have included rates of 1.3% in patients with community-acquired pneumonia, and 8.3% in patients with healthcare-associated pneumonia (HCAP). The rate of Aspergillus co-infection was also high (7.2%). This is also unexpected, the authors say, because although invasive pulmonary aspergillosis can occur in patients with influenza pneumonia, it typically arises as a secondary fungal infection, rather than a community-acquired co-infection.

Data from this study also indicated an increased rate of co-infection over time—from 11.4% in 2009 to 23.4% in 2015. However, the researchers could not explain the cause of this increase.

A US study involving 683 influenza patients found that 30.3% had a bacterial co-infection. According to the researchers, S. aureus—and predominantly methicillin-resistant S. aureus (MRSA)—was most commonly isolated.

Dr. Luyt and Dr. Rice also emphasize that co-infection in patients with influenza is associated with a higher mortality rate than that with primary viral infection. This could be due to either the bacterial infection itself or to an association of virulence factors from both virus and bacteria, they add.

On the basis of these findings, clinicians should therefore initiate early, empiric antibiotic therapy in patients with influenza who are admitted to the ICU, the authors say. In the United States, clinicians should also consider the high incidence of MRSA as they make their initial choice of antibiotics.

“If P. aeruginosa incidence is increasing over time (which remains to be confirmed in further studies), it may also need empiric antimicrobial coverage since it may have an impact on overall mortality,” Dr. Luyt and Dr. Rice conclude.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.