Cryptosporidiosis surveillance -- United States, 2011-2012.

Problem/condition: Cryptosporidiosis is a nationally notifiable gastrointestinal illness caused by extremely chlorine-tolerant protozoa of the genus Cryptosporidium.

Reporting period: 2011-2012.

Description of system: Fifty state and two metropolitan public health agencies voluntarily report cases of cryptosporidiosis through CDC's National Notifiable Diseases Surveillance System.

Results: For 2011, a total of 9,313 cryptosporidiosis cases (confirmed and nonconfirmed) were reported; for 2012, a total of 8,008 cases were reported; 5.8% and 5.3%, respectively, were associated with a detected outbreak. The rates of reported nonconfirmed cases were 1.0 and 0.9 per 100,000 population in 2011 and 2012, respectively, compared with an average of 0.0 during 1995-2004, and 0.3 during 2005-2010. The highest overall reporting rates were observed in the Midwest; 10 states reported >3.5 cases per 100,000 population in 2011 and in 2012. During 2011-2012, reported cases were highest among children aged 1-4 years (6.6 per 100,000 population), followed for the first time by elderly adults aged ≥80 years (3.4), and 75-79 years (3.3). Overall, cryptosporidiosis rates were higher among females than males during both years. For specific age groups, rates were higher among males than females aged <15 years and higher among females than males aged ≥15 years. Cryptosporidiosis symptom onset increased 4.4 fold during late summer.

Interpretation: Cryptosporidiosis incidence rates remain elevated nationally, and rates of nonconfirmed cases have increased. Rates remain highest in young children, although rates among elderly adults are increasing. Transmission of Cryptosporidium occurs throughout the United States, with increased reporting occurring in Midwestern states. Seasonal onset peaks coincide with the summer recreational water season and might reflect increased use of communal swimming venues.

Public health action: Future research is needed to address the evolving epidemiology of cryptosporidiosis cases, with a specific focus on the increase in nonconfirmed cases and increasing incidence rates among elderly adults. National systematic genotyping and subtyping of Cryptosporidium isolates could also help elucidate Cryptosporidium transmission and thus cryptosporidiosis epidemiology in the United States.