6.7.2: Cases
Up until this point we might have assumed that the term “case” means that a person for sure has a particular disease. Ideally, this is true - for infectious diseases this means a case is identified by the presence of that pathogen via a lab test. However, if an outbreak occurs, doctors may not always have the time to confirm each case by sending off a biological sample to a lab. Or, as with the influenza example above, people may not visit a doctor’s office and report their symptoms, or perhaps will report their symptoms after the fact - perhaps if interviewed with active surveillance. Often the definition of a case must be outlined by a local or national health department, so that epidemiologists can track suspected, probable, and definite cases more accurately. The most definite cases are those confirmed by a laboratory test for an infectious pathogen or toxic agent. However, a cluster of symptoms may also qualify as a probable case. For example, pertussis (whooping cough) is a bacterial infection with the pathogen Bordetella pertussis . Clinical criteria include paroxysms of coughing - a type of violent, uncontrollable coughing that can cause trouble breathing or vomiting afterward, or the characteristic heaving inhale (whoop) associated with this illness. Even without a laboratory test confirming the presence of the Bordetella pertussis bacteria in a sample, a doctor may report a probable case of whooping cough based on those symptoms - particularly if there is evidence of contact with a known case - they know they got it from someone with a lab-confirmed case (National Notifiable Diseases Surveillance System (NNDSS), 2022). This helps public health professionals be able to track cases without having to wait for laboratory confirmation - especially if labs may be overwhelmed with a backlog of samples in an epidemic.