Prevention and treatment cascades are the new way to make sense of epidemics and how they might be better controlled.
Cascade analyses have, for example, shown that the biggest public health issue for controlling some common and challenging sexually transmitted infections (STIs), such as Chlamydia trachomatis and gonorrhea, is identifying infected people who are asymptomatic, said STI expert Dr. King K. Holmes, of the University of Washington, Seattle.
"Enormous attention is being given to prevention and treatment cascades" right now, Dr. Holmes said at the STI & AIDS World Congress 2013 in Vienna. This trend only began a couple of years ago.
Courtesy Wikimedia Commons/Nicholas A. Tonelli/Creative Commons license
Though occasional iterations of the cascade concept go back to at least 1969, the concept went high profile in 2011 with an analysis (Clin. Inf. Dis. 2011:52:793-800) of HIV diagnosis and treatment by a team of U.S. epidemiologists led by Dr. Edward M. Gardner, of the Denver Department of Public Health and the University of Colorado Denver, Aurora. Although the 2011 report never even used the word cascade, it spelled out the linked sequence of events required to successfully treat HIV-infected patients: diagnosis, getting patients to places where HIV management occurs, and finishing with treatment and adherence.
Even before Dr. Holmes cited the Gardner paper in his talk a few weeks ago, other experts recognized how groundbreaking and important the Gardner thesis was. Blogging a year ago, Dr. Ronald Valdiserri, director of the U.S. Office of HIV/AIDS and Infectious Disease Policy, wrote, "The HIV/AIDS treatment cascade provides a way to examine critical questions including: How many individuals living with HIV are getting tested and diagnosed? Of those, how many are linked to medical care? Of those, how many are retained in care? Of those, how many receive antiretroviral therapy? Of those, how many are able to adhere to their treatment plan and achieve viral suppression?"
By a year ago, "at the Federal level government agencies used the treatment cascade to inform discussions about how best to prioritize and target resources," Dr. Valdiserri wrote.
Infectious disease groups that recently applied prevention and treatment cascades to their favorite diseases include the GAVI Campaign for childhood immunization, Roll Back Malaria the Stop TB Partnership, and the Integrated Management of Childhood Illness (IMCI) initiative of the World Health Organization, said Dr. Holmes.
He then challenged every person at his talk by asking the audience, "How are you using cascade models in your work?"
He also presented an example of cascade analysis applied to his own study, Peru PREVEN, which last year showed no effect from four interventions designed to cut STI incidence in Peru (Lancet 2012;379:1120-8). This controlled study randomized 10 cities that tried the interventions and 10 cities that continued with standard practice. The study’s primary result showed no significant change in infection rates linked to the interventions. But the cascade analysis identified the major choke point for improving outcomes with intervention: identifying patients with an STI who lack symptoms. For example, in the Peru PREVEN study, 3% of the nearly 6,400 men in the study had chlamydial infections, but only 9% of infected men had symptoms. Even though nearly half of the symptomatic men sought treatment, the absolute number of infected men seeking treatment was minuscule because more than 90% were symptom free. Cascade analysis identified why the Peru PREVEN interventions failed to produce a statistically significant difference in outcomes.
Of course, the next step will not be easy: coming up with good, new ways to identify asymptomatic people to eradicate more infections and stop their spread.
–Mitchel L. Zoler
On Twitter @mitchelzoler