• EPIDEMIOLOGY (GENERAL)
• SEXUAL HEALTH
• CONTROL PROGRAMS

Outbreaks happen for a reason. Just like planes crash for a reason. And determining the reason why an outbreak occurred helps us prevent them in the future. This issue of the journal is devoted to outbreaks of STIs and includes articles that explore epidemics, clusters, changes in antibiotic resistance, changes in behaviours that increase the chance of outbreaks, as well as considering how we respond to and describe outbreaks (Shanka, Thomas, Li, Foster, Smith, Gilbart, Chisholm, Bourne, Oulfon, De Silva).

The first question is what is an outbreak? Last's Dictionary of Epidemiology describes outbreaks as ‘epidemics’ which ‘are events clearly in excess of normal expectancy’.1 Complex statistical techniques can be used to detect outbreaks, but often, as is shown in Figure 1 in the paper by Thomas et al and Figure 1a in the paper by Li, all that is required is the eye of an experienced public health professional (Thomas, Li).

The term outbreak gives a sense of immediacy to an event, but from a public health perspective gradual increases over time in endemic disease may be more important numerically than epidemics. The paper by Foster et al describes the latter scenario. An astute clinician noticed a change in the risk profile of clients with gonorrhoea, a potential outbreak that transformed to higher-level endemic disease unresponsive to acute outbreak measures (Foster). Surveillance systems can use techniques to improve the detection of outbreaks by increasing the specificity of the STI diagnoses or risk factors, or by incorporating geospatial techniques into analytical systems (cite Smith). For example, the rise in Shigella flexneri 3a was not obvious among all shigella cases nor was the rise in azithromycin-resistant gonorrhoea obvious among all gonorrhoea cases (Gilbart, Chisholm). Changes in the risk group acquiring infections can indicate an outbreak, as occurred in Northumberland when gonorrhoea appeared in heterosexuals rather than the usual risk group of men who have sex with men (MSM) (cite Foster).

The most important aim of outbreak management is to protect public health and implement control measures to prevent further spread or recurrence.2 The question why did it happen? figures high throughout an investigation and provides information that can be used to prevent future outbreaks. Given that many countries have common drivers of outbreaks and that international travel and sexual contact overseas are common, identifying the cause of outbreaks in one country will benefit humanity in all countries. There are a number of examples of common drivers; some new, some old. Two drivers that have appeared recently include chemsex and smart phone mobile applications (Smith, Bourne, Oulfon). But old ones remain as Shankar et al remind us when describing a cluster of hepatitis B among a group of self-identified heterosexual MSM (Shankar). There are a number of older examples where STI outbreak investigations have warned of early behavioural changes that were putting upward pressure on STI rates. For example, between 1986 and 1988, public health officials in San Francisco noticed a sharp rise in gonorrhoea among primarily black adolescents while overall rates of gonorrhoea had been falling.3 The investigation identified the primary driver as the exchange of drugs for sex as crack cocaine use rose rapidly, which alerted others to this new behavioural trend.3 Here, Li et al describe rises in gonorrhoea among sex workers that was temporarily associated with greater policing of sex work and a focus on using carriage of condoms as evidence of sex work (Li).

Studies presented in this issue report important changes in factors that may influence the reproductive rate of an infection and increase the likelihood of future outbreaks. Sometimes, these changes are gradual, such as the gradual rise in sexual risk over the 1960s and 1970s that saw the endemic level of STIs rise worldwide. Other changes can be more rapid such as the recent increase in ‘chemsex’ identified following interviews with 30 gay men from London (Bourne). They described chemsex causing intense sexual arousal and disinhibitory behaviour that facilitated men's ability to have higher numbers of partners without condoms and with prolonged episodes of sex (Bourne). This resulted in rectal and penile trauma and increased the risk of STIs and provided an entry point for newer STIs such as hepatitis C or the recurrence of older ones such as hepatitis A or lymphogranuloma venereum.

Other studies describe changes in endemic infection such as the outbreak of high-level azithromycin resistance in Neisseria gonorrhoeae in heterosexuals in Leeds (Chisholm). The authors noted changes in gonococcal resistance, which was being carefully monitored because of the real prospect of ceftriaxone failure in the future (Chisholm). They describe eight cases of azithromycin-resistant gonorrhoea that occurred in a 5-month period rather than the expected one per year. Outbreak measures were instituted, and although the outbreak in Leeds resolved further, connected cases of high-level resistant gonorrhoea have continued to appear elsewhere in the UK.4

A number of papers describe the introduction of outbreak control measures that did not result in sustained declines. This in part highlights how difficult human behaviour is to change despite very substantial resource allocation.5 Human behaviours from which we derive considerable pleasure, such as eating or sex, are more difficult to change than testing patterns, partner notification or improving clinical access. Consequently, an outbreak may be easier to reverse if it was caused by a change in public policy6 (Li), rather than it was associated with seeking biologically programmed pleasure (Bourne). An example of how a change in the health system could influence a rise in the reproductive rate occurred in Denver.6 A co-payment of between $15.00 and $65.00 introduced in an STD clinic was associated with fewer visits and diagnoses of chlamydia and gonorrhoea, and presumably a greater number of untreated cases circulating in the community. The decision was rapidly reversed. This observation provided an important message to public health professionals that STD clinics and their treatment need to be free at the point of care to encourage maximal attendance of those at greatest risk.6 Unexplained declines in infections also need to be investigated, such as the unexplained 25% reduction in chlamydia in one region of Sweden.7 ,8 This led to the discovery of a mutation in the target area of some laboratory assays that were not detecting the variant and changes to the diagnostic strategy resolved the problem.

Since the Comet air crashes of the early 1950s, airline safety has improved dramatically because of an iterative process of reporting all errors no matter how minor, understanding their significance and fixing any identified system errors. Exactly the same logic applies to STIs; report all outbreaks: investigate, communicate the findings and learn from them.