‘Tis the season for the obligatory “best of” lists for 2015. We thought we’d take a look back at some of this year’s notable diseases and outbreaks. Our list has been limited to the nice round number of ten. Does this list seem representative of 2015 to you?
In recent years, dengue has spread rapidly across the globe, but 2015 has been explosive in terms of scale and quantity of outbreaks. Dengue, a vectorborne viral infection, is also known as ‘breakbone fever’ due to its clinical presentation of widespread arthralgias, myalgias and fever . The disease was first recognized in Southeast Asia in the 1950s but as a result of exponential spread, over half of the world’s population is now at risk and the disease is endemic in over 100 countries . There is no specific treatment for dengue, but this year, Sanofi Pharmaceuticals released the first ever dengue vaccine - Dengvaxia. In the past month, Mexico, the Philippines and Brazil have all approved of the vaccine’s use .
Brazil, the most recent country to approve Sanofi’s dengue vaccine for use, has reported over 1.5 million cases this year . This case count is approximately three times higher than that in 2014, where approximately 575,000 cases were recorded across the country . However, Brazil has seen outbreaks of this scale before - two years ago in 2013, 1.4 million cases of dengue were reported . Brazil is one of a handful of countries in the Americas that has been host to all four Dengue strains, which can put individuals at increased risk for repeat infection and severe/hemorrhagic dengue .
Taiwan reported a record-breaking outbreak of Dengue this year. As of the 22 December Taiwan CDC press release, there stood a total of 42,572 indigenous cases reported across the country, approximately three times the tally for 2014, when a record-breaking 15,732 autochthonous cases were reported , . The outbreak has been primarily focused in two cities, Tainan and Kaohsiung, with 22,741 and 18,933 cases reported respectively . The outbreak is finally beginning to wane, with 23 December marking the first day since May with no new cases reported over a 24 hour period . The outbreak peaked in Taiwan on 24 September, when 709 cases were reported in Tainan City . The factors behind the two sequentially record-breaking dengue outbreaks remain unknown.
Sudan has also been experiencing a large-scale outbreak, with 571 suspected cases and 133 deaths reported since August . This outbreak is of note due to the country’s arid, non-tropical climate, which highlights the widespread prevalence of required vectors and ability of the disease to penetrate isolated or inland areas .
India’s National Capital Territory of Delhi 2015 dengue outbreak resulted in a near-collapse of its healthcare system . The Delhi dengue outbreak, the worst since 2006 - when over 10,200 cases were reported, rallied up to 15,730 cases . News outlets picked up on a story of two parents committing suicide after their son, who was suffering from severe dengue, was turned away from five hospitals and subsequently died as a result of the disease . The Indian government demanded an explanation from the hospitals, who claimed to be inundated and lacking beds, partly due to the intensity of the outbreak and partly due to poor healthcare infrastructure .
The United States has largely been able to escape and control the spread and reach of dengue. However, in September a cluster of dengue cases were discovered on Hawaii Island. The initial cases are believed to be traceable back to a popular campsite on the island, suggestive of an initial introduction to the vector population . As of 24 December, there were 180 cases linked to the outbreak .
2) West Africa Ebola Outbreak
The West African Ebola Virus Disease (EVD) outbreak, which began in March 2014, began to wane in 2015. As of 11 December 2015, there have been a total of 28,601 cases and 11,300 deaths in the outbreak, primarily concentrated across the countries of Guinea, Liberia, and Sierra Leone . In 2015, both Liberia and Sierra Leone were declared EVD-free – for Liberia, the end of the initial period of transmission was declared on 9 May; and for Sierra Leone, end of transmission was declared on 7 November [2,3]. However, declaration of the end of EVD transmission does not guarantee continuous EVD-free status. Ebola remerged again in Liberia on 29 June and 23 November; the country is counting down to EVD-free status for a third time [4,5]. In mid-November, after two years of battling Ebola, Guinea began their countdown to being EVD-free .
Further discoveries regarding EVD have emerged due to this outbreak’s unprecedented breadth and scale. In March 2015, it was suspected that a woman in Liberia who had had sexual contact with a male survivor, consequently contracted EVD . Prior to this event, survivors of EVD were advised to practice sexual abstinence or to utilize condoms for three months after recovery . However, preliminary reports studying the persistence of EVD within survivors has revealed that EVD RNA can still be detected in an individual’s semen for up to six months after onset . The persistence of EVD in semen implicates potential reemergence of EVD, particularly with potential increased frequency.
As EVD winds down in the region, we are only now appreciating the aftermath found in its wake. Survivors of EVD face little-known complications from the disease, appropriately dubbed ‘post-Ebola syndrome’ . Survivors have reported a variety of complications, including visual impairment issues, joint, muscle, and chest pains or aches, headaches, and severe fatigue . We are only beginning to understand the causes and possible treatments for this newly appreciated condition.
The measles outbreak that caught the media’s attention this year and reignited significant vaccination debate, was linked to the Disneyland resort in Anaheim, California. The outbreak began on approximately 20 December 2014, spiked in January, and was declared over on 17 April - 42 days after the last reported case [1-3]. The outbreak resulted in 147 cases from several states and Mexico, with a significant proportion of cases being un- or under-vaccinated children [2, 3]. The outbreak, which is unprecedented for the United States, where measles transmission was considered eliminated in 2000, has been linked to low vaccination rates among children [3, 4].
Measles is considered one of the most contagious diseases in the world . To give a sense of what that means, an individual in the contagious stage of measles infection would result in nine out of 10 unvaccinated/susceptible individuals becoming infected .
The outbreak that remained largely hidden from the spotlight, despite its unfathomable size and effects, is occurring in the Katanga province of southeast Democratic Republic of Congo (DRC) . The outbreak began in February but is attributed to a larger, ongoing epidemic starting in 2010 . This year’s outbreak has exploded to over 47,000 cases and resulted in over 500 deaths . Doctors Without Borders/Médecins Sans Frontières (MSF) has been on the ground, conducting vaccination campaigns, but there have been significant challenges . The vaccine must be kept refrigerated and requires multiple doses, weeks apart - which is particularly challenging for remote locations .
The 2014-2015 influenza season across the United States was declared an epidemic in early January 2015 . While increases in flu activity and hospitalizations during that period reflect a typical flu season’s cycle, the 2014-2015 flu season was particularly worse than other years.
On 4 December 2014, the CDC announced that the dominant strain circulating in the 2014-2015 flu season was influenza A, type H3N2, and seasons in which the H3N2 strain dominated have historically been worse than other flu years . To further exacerbate the situation, the influenza vaccine distributed and administered last season was found to be only 23% effective in protecting against H3N2 [2,3]. This discrepancy is accounted for by the flu virus’ antigenic drift, a process normal for influenza viruses, where slight changes in the genes of influenza viruses occur as a result of viral replication over time . Antigenic drifts happen annually and vaccines are often tailored to respond to the prediction for each season -- but, the drift in the H3N2 strain was not recognized until March 2014, missing the deadline to be incorporated into the season’s batch of vaccines .
On the other side of the globe, the H1N1 influenza strain ravaged India this past year, infecting approximately 29,938 people and resulting in 1,731 deaths, by mid-March . These figures surpassed the country’s 2009 H1N1 pandemic numbers, in which a total of 27,236 cases and 981 deaths were reported . According to researchers at Massachusetts Institute of Technology (MIT), samples of the H1N1 swine flu strain from this year’s outbreak in India indicated a mutation in which the strain became more infectious and pathogenic . Researchers, Kannan Tharakaraman and Ram Sasisekharanan, suggested that an evolution of the hemagglutinin (HA) protein -- which encompasses “receptor binding, fusion, and transmission properties” – took place, resulting in a more virulent strain of the pandemic H1N1 virus and increased disease severity among current flu cases in India . The mutations, alongside ideal environmental conditions such as high population density -- allowing for easier transmission -- permitted the virus to become entrenched within the population, resulting in a large-scale outbreak .
MERS, also known as Middle Eastern Respiratory Syndrome, emerged for the first time in South Korea in May 2015 . Discovered in 2012, MERS has mainly concentrated in the Arabian Peninsula region, with imported cases occurring occasionally in countries such as the United States . South Korea’s outbreak would become the largest outside of Saudi Arabia, resulting in 186 cases and 36 deaths .
The outbreak was suspected to have begun after a 68 year old man was diagnosed with MERS on 20 May 2015; he had recently returned from a trip to the Middle East . As the outbreak unfolded, the government of South Korea was criticized for its response and many believed that the government had vastly underestimated the gravity of the situation. After all, the government did not announce a list of affected hospitals until 7 June 2015 – weeks into the epidemic . After the country had gone infection free for 23 days, South Korea’s Prime Minister Hwang Kyo-ahn declared the outbreak to be ‘effectively over’ on 27 July 2015 .
Chikungunya, a viral vectorborne disease, was first described in 1952 in Tanzania . Infection results in flu-like illness with pronounced arthralgias, and is difficult to clinically distinguish from dengue infection . Since its discovery in 1952, Chikungunya virus (CHIKV) has resulted in outbreaks in Africa, Asia and Europe . On 5 December 2013, the first autochthonous transmission of CHIKV was confirmed in the Americas, on the Caribbean island of St. Martin . Since then, over 1.7 million cases have been reported in the region  with 607,961 autochthonous cases reported this year .
Within the Pan American Health Organization (PAHO) region, Colombia and Mexico have had notable CHIKV years. Colombia, which reported 91,092 confirmed or suspected autochthonous cases from introduction through the end of 2014, reported 356,254 confirmed or suspected cases in 2015 [4,5]. For Mexico, it was 13 months ago that the first autochthonous case was reported . So far this year, Mexico has reported 11,199 cases  from 28 of 31 states, including states bordering the southern United States .
7) Avian Influenza
Those of us in the United States most likely know about the avian influenza outbreak affecting large poultry producers that occurred in the Midwest states this year. The strain of avian influenza, H5N2, was first detected in wild birds and backyard poultry in Oregon, Washington and Idaho in November 2014 . The strain was later detected for the first time in the Mississippi flyway and in commercial poultry, on 4 March 2015 in Pope County, Minnesota . Since then, the highly pathogenic H5N2 strain has been detected 218 more times and has affected 48,082,293 birds . In Iowa, the country’s largest egg producer, the detection of the virus on one farm affected almost 1.5 million birds . In most cases, each time the virus was detected, the poultry population was culled to prevent the spread to surrounding populations. As a result of the outbreaks, there were significant price changes in egg and chicken/turkey meat throughout the country .
The H5N1 strain of avian influenza is considered highly pathogenic and deadly for poultry . It is known to cause sporadic infection in humans, and can result in severe disease and high mortality rates . Starting in October 2014, Egypt experienced a significant increase in H5N1 avian influenza outbreaks among poultry, with 435 outbreaks in total . The increased incidence in disease events is believed to be related to the substantial increase in human cases over the past year - 136 cases and 39 deaths . According to the CDC, this is “the highest number of human cases of H5N1 reported by any country in a similar time period” . It is surmised that recent mutations in H5N1 has led to a new, more pathogenic strain affecting both poultry and humans .
8) Antimicrobial Resistance
Antimicrobial resistance, the resistance of a microorganism to treatment by a drug once effective, continued to expand worldwide in 2015 . While the evolution of antimicrobial resistance is a natural phenomenon, its increased growth and spread implies a future where common infections may become untreatable.
In November 2015, Chinese scientists reported on the appearance of the mcr-1 gene, a gene mutation that relays resistance to colistin, found during routine surveillance of the flora of food animals . Colistin is considered to be an antibiotic of last-resort, it has been saved from regular use due to its nausea side effects in humans . Since the discovery by the Chinese scientists, the mcr-1 gene has been found in countries across the globe, from Algeria to France . Perhaps the most concerning factor in its discovery is the fact that mcr-1 is carried in plasmid, a piece of DNA, and can be readily exchanged from one bacterium to the other, easily passing on resistance across bacterium families .
Over the last twenty years, the rate of resistant infections has skyrocketed from 10-15% in the 1990’s to 60% of infections being resistant to one or more antimicrobials today . Pharmaceutical companies are also not investing in the development of antibiotics, with only 11 new antibiotics introduced between 1998 and 2014, and the last new class of antibiotics approved in 1987 . With natural selection in play for mcr-1, increased resistance of microorganisms to effective drugs is only going to continue, and development of new line antibiotics has yet to be carried out.
Plague, otherwise known as Yersinia Pestis infection, remains endemic in three countries: Madagascar, Peru and the DRC . However, 2015 was a comeback year for plague in the United States, with 15 human infections and four deaths . The United States averages seven cases of plague reported each year, but there have been other years with high case counts - in 2006 there were 17 human infections . The uptick in cases in the United States this year is suggested to be linked to El Nino and precipitation rates, lush vegetation and a burgeoning rodent population which hosts the Yersinia Pestis-transmitting fleas.
Between 17 and 30 August, Madagascar reported 14 cases and 10 deaths of the pneumonic version of the plague . This outbreak was significantly smaller than the outbreak that occurred in Madagascar from 2014-2015 where 335 cases and 79 deaths were reported . The 2014-2015 outbreak’s size has been attributed to population density of affected areas, weakness of the healthcare infrastructure, and flea resistance to the insecticide deltamethrin being observed in the country .
On Saturday, 28 November 2015, the Brazilian Ministry of Health confirmed the link between the country’s cases of microcephaly – a congenital disease resulting in head deformities in babies – and Zika virus, a mosquito-borne virus spread by Aedes species mosquitoes, the same mosquito species that carries dengue and chikungunya [1,2]. The link was first investigated when Zika virus was found to be present in a deceased baby with microcephaly and other genetic diseases . The connection also coincided with an increase in Zika virus infections across Brazil, with 739 suspected cases identified throughout the country as of November 29, 2015 . However, it is important to note that microcephaly and transmission of Zika virus from mother to child has yet to be fully determined. The CDC notes that mothers can pass Zika virus to their children during birth, though it remains a rare occurrence .
In November 2015, Zika virus was also found on the African island nation of Cape Verde for the first time . Seventeen people were confirmed positive with the virus through blood testing and cases have been mostly confined to the capital region of the country, Praia .
Zika has recently begun to spread rapidly into Central and North America. The countries reporting first autochthonous cases of Zika are as follows: Guatemala reported one case that became symptomatic on 11 November; El Salvador reported three cases on 24 November; Mexico reported two cases on 26 November; Panama reported three cases on 27 November; Honduras reported two cases on 16 December; and the Caribbean island of Martinique reported its first transmission on 28 December [4-9].