In this section, we're going to work together on crafting a case definition for Nipah virus surveillance in Bangladesh. So, how should we define a case of Nipah virus? Let's recall first the surveillance objectives, right. What we're looking for is really important for us as we think through what our case definition should be. So, the surveillance objectives were to identify outbreaks of Nipah virus and to describe year-to-year variation in incidence of Nipah virus. Right now, I want you to take 10 minutes and think about what you would suggest for a Nipah virus case definition. What clinical characteristics would you use, would you have laboratory testing as a part of the case definition? Are you going to go with a more sensitive or more specific case definition and why? Are you going to think about suspect cases as well as confirmed? Write down your case definition and then consider its strengths and limitations. When you're done with that, join us back to the lecture and we'll go over the case definitions that we currently use in that surveillance system. In Nipah virus surveillance in Bangladesh, we have a few different case definitions that we use. They involve clinical characteristics, they involve laboratory confirmation. First, we define a suspect case as any person admitted to a surveillance hospital with evidence of fever and brain pathology, which could be altered mental status or coma that I mentioned earlier during December through March. So, this gives you something about person. It tells you something about place, one of the surveillance hospitals, and it also gives you some information about time. We only look at December through March because that's the time of year when people most commonly drink date pumps out. So, that's where we think we're going to find cases. So, our surveillance definition includes a time component. Now, all suspect cases have biological specimens collected, including blood and cerebrospinal fluid. We also collect their clinical information, and we test specimens for evidence of infection with Nipah virus. A confirmed case is defined by any person who's tested through the surveillance system. So, they have the clinical characteristics of a suspect case, and they also have IgM antibodies to Nipah virus in their serum or cerebrospinal fluid. Now, we take action when we have a confirmed case, and we go out and identify all of their contacts and we start an outbreak investigation with a public health goal of stopping transmission. We also have a probable case definition. We use this less often, but these are cases who meet the suspect case definition and they have an epidemiological link to a confirmed case. So, maybe they drank date palm sap with the confirmed case, maybe they were a caregiver of a confirmed case, but we have no specimens collected from them. So, probable cases tend to be patients who are associated with an ongoing outbreak in a community. But perhaps they died before we could collect blood, or we have a blood sample only from the first day they were ill and they may not have developed antibodies yet. So, we call them a probable case. We think they are more likely to be a case than just the suspect cases, but we don't have that laboratory confirmation. I want to talk now for a bit about one of the major limitations of the case definition we use. You can compare what we did to the case definition that you came up with and see what limitations we have compared to what you have. But one of the main limitations that we struggle with is the time lag between when a patient seeks care at a hospital when they become ill and when we're able to confirm with a laboratory tests that they have Nipah. Most cases in fact, are not diagnosed until the day they die or even after-death, which severely limits our ability to provide them adequate treatment or even to intervene to prevent onward transmission. If you remember, that's the biggest public health concern for Nipah, is that patients are going to spread Nipah and infect many other people. So, we want to be able to intervene early. So, we asked ourselves if we can improve upon our surveillane case definition and what we use for that public health action, particularly around infection control. So, we thought maybe instead of waiting for laboratory results, patients who come in and meet a suspect case definition for Nipah, so they have the clinical signs and symptoms that we associate with Nipah from December through March, let's ask them about their recent consumption of date palm sap or contact with a sick patient. We know that these are two of the major ways that people get Nipah in Bangladesh. So, if someone presents with signs and symptoms compatible with Nipah and they report this recent exposure, we thought maybe this could be, well, at least a quicker way to try and diagnose patients and intervene. So, we did this study and we compared this exposure question to laboratory confirmation test, and found that this was actually a very sensitive case definition though it wasn't very specific. So, we captured 93 percent of the true cases using this in lieu of a laboratory test. It was pretty specific, but the positive predictive value was only 37 percent. So, what does that mean? That means that only 37 percent of patients who had signs and symptoms compatible with Nipah and who said that they recently drank date palm sap who had contact with a sick patient, actually had Nipah. Why might this be? Drinking date palm sap is very common and Nipah virus is still rare even among patients who have encephalitis at our surveillance hospitals during the Nipah season. But this did solve the problem for us in terms of at least the, having the ability to identify some patients early and trying to focus infection control interventions there.
