epidemiology

The World Health Organization (WHO) has declared March 24, 2016 as World Tuberculosis (TB) Day [1]. Over the month of March, the Global Health Office will be publishing a series of blog posts about TB in hopes of raising awareness of its worldwide impact. WHO’s “End TB Strategy” has a goal of ending the TB epidemic by 2030, by targeting poverty, improving testing and treatment, ending stigma and discrimination, and driving research and innovation. Put simply, TB is not yet a disease of the past, but if we unite and focus our efforts, we can make TB history.

Tuberculosis (TB), caused by the bacillus Mycobacterium tuberculosis, is an airborne disease that primarily affects the lungs (pulmonary TB) [2]. This means that when people with pulmonary TB cough, sneeze or spit, TB bacteria are propelled into the air, giving them the potential to spread from person to person via inhalation. TB can affect other sites as well, including the brain, the kidneys and the bones [3]. An ancient disease, TB has affected humans for over 4,000 years [2].

In many cases, the immune system will fight off the TB bacteria, and no infection occurs. If the immune system is not able to completely fight off the bacteria, some inactive bacteria may stay in the body, leading to latent TB infection (no symptoms). If the immune system does not respond sufficiently, active TB infection occurs (TB disease), and symptoms begin to manifest [4]. People with latent TB infection cannot spread TB bacteria to others, but can go on to develop active TB over time. WHO estimates that about one-third of the world’s population is infected with latent TB [5]. Symptoms of TB disease include a persistent cough of 3 weeks or longer, pain in the chest, coughing up blood or sputum, weakness or fatigue, weight loss, loss of appetite, chills, fever and night sweats [6].

The burden of TB is mainly concentrated in low and middle income countries, where 95% of TB deaths occur. In 2014, there were 1.5 million TB deaths, and 9.6 million people developed TB disease [5]. TB is one of the leading causes of deaths from infectious diseases worldwide [7]. The map below, produced by WHO, shows estimated TB incidence rates, by country, in 2014.

Map of the world showing estimated TB incidence rates, by country, in 2014 (Source: World Health Organization).

This map provides a visual representation of the worldwide burden of TB, with the lowest rates in Western Europe, Canada, the United States of America, Australia and New Zealand. This has led to a common misperception in higher income countries that TB is a disease of the past. However, in many countries, including Lesotho, South Africa, Swaziland and Mozambique, rates are still above 500 cases per 100,000 population [7].

Keep reading our series of blog posts on TB to learn more about risk factors, diagnosis, treatment and much more.

The Aedes mosquito.

The most prominent global health issue in the media in 2016 (so far) has, without a doubt, been Zika virus. On February 1, the World Health Organization (WHO) labeled Zika virus a public health emergency of international concern [1]. This has led to increased media coverage, particularly around the potential link between Zika virus and microcephaly. You may be wondering: what is Zika virus, where did it come from, how is it transmitted, and should you be concerned? Fortunately for you, we answer these questions, and many more, in this blog post!

Zika virus was first identified in rhesus monkeys through a sylvatic yellow fever monitoring network in Uganda in 1947. The first cases in humans were discovered in 1952, in Uganda and the United Republic of Tanzania [2]. Zika virus is vector-borne, meaning that an infected living organism transmits the infectious disease between animals and humans, usually through its bite. The Aedes mosquito is the vector in the case of Zika, the same mosquito that spreads chikungunya, malaria and yellow fever [3]. Other forms of transmission include from mother to child, through sexual contact and through blood transfusion [4]. Approximately 1 in 5 people who are infected with Zika through these channels go on to become ill. The most common symptoms are fever, rash, joint pain and conjunctivitis (red eyes). The time from exposure to experiencing symptoms (the incubation period) is not yet known; however, the CDC estimates that it is likely to be a few days to a week [5]. These symptoms are acute, common to many other illnesses, and rarely severe, meaning that many people who have Zika may not realize they have been infected.

Zika virus has been around for more than 60 years, so why are we suddenly so concerned in 2016? There have been outbreaks in the past in Africa, Southeast Asia and the Pacific Islands, but Zika virus gained international attention in the media due to events in Brazil in the second half of 2015. The ministry of health in Brazil first reported that Zika was circulating within the country on May 15 2015. Seven months later on December 24, the country declared a state of emergency due to a dramatic increase in cases of microcephaly (abnormal smallness of the head which can lead to neurological defects) in newborn babies. There were 150 cases recorded in all of 2014 in Brazil, compared to 2700 cases recorded from January to October 2015. Between October 2015 and January 20 2016, there were an additional 3893 cases of suspected microcephaly reported [6]. Brazil has also reported an increased number of cases of Guillain-Barré syndrome (GBS) in the same time period. GBS is a rare illness in which a person’s own immune system damages the nerve cells, causing muscle weakness, and sometimes, paralysis [7]. A total of eight countries/territories (Brazil, French Polynesia, El Salvador, Venezuela, Colombia, Martinique, Honduras and Suriname) have reported a temporal association between the emergence of Zika virus and microcephaly and/or GBS [8]. Although a causal association between Zika and these neurological disorders has not yet been proven, a growing body of both clinical and epidemiological data suggest this is the case. Further studies are currently underway in order to confirm (or refute) the link [9].

Stay tuned for Part II, which will examine risk factors, further implications, prevention and control measures and myths around Zika virus.

World Tuberculosis Day Part I – What is TB?

Current Topics in Global Health: Zika Virus (Part I)

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