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Outbreaks of VL

How to identify an outbreak and recognise how to respond? In this step, we cover the principles of outbreaks with recent examples.
Dr Koert Ritmeijer: The definition of an outbreak depends on the epidemiological context. In context where VL is endemic, then an outbreak can be defined as an unusual increase of number of cases. There is no consensus of how much this increase should be, but as a rule of thumb, you could say if the number of cases are more than twice as high compared to previous years for that particular place and time, then you could call it an outbreak. In a context where VL is not endemic, then one or a few cases of VL could already mean an outbreak.
Also in a context where VL has been eliminated and there haven’t been cases for a few years, then one or a few cases that are emerging again could indeed again mean an outbreak. An outbreak is an unusual occurrence of cases. An epidemic means usually a large scale outbreak with a large number of patients affected over a larger period. The interval between the waves is explained by the fact that during an outbreak most of the population will be infected and either go through an episode of disease or develop a natural immunity.
And then it takes a number of years before you have a sufficient number of new susceptible subjects, children that have never been infected before, and which then have become a large enough group to carry an outbreak. Even over the recent history, there have been plenty occurrences of outbreaks, especially in East Africa, where you normally see an epidemiological wave pattern with outbreaks or epidemics occurring every six to 10 years. But on top of that, there might be unexpected outbreaks related to migration or displacement due to conflict, which we often see in East African context, which might result in unexpected outbreaks.
In the South Asia context, we see less of these unexpected outbreaks, but more you see the epidemiological waves which seem to occur every 15 years. The VL situation in Latin America is quite different, because that is zoonotic VL. So it means that humans are not a reservoir, but dogs or other mammals are the reservoir of the disease. So that is a more complex transmission dynamic with the dog or the mammal in between the vector and humans. That means that also the risk of outbreaks is much less than if there is a direct anthroponotic transmission from human to human. The occurrence of outbreaks is very rare. And if there are outbreaks, they’re not very big.
But nevertheless, you can see a gradual increase in the incidence of VL in Latin America, which is mainly due to the urbanisation of previously endemic rural areas where you have an abundance of the sand-fly vectors. And then if men with their dogs are colonising those rural areas, then you have your transmission cycle. A good example of unexpected outbreaks is the one that occurred in 2014 in South Sudan in the area of Lankien, which was clearly explained by the conflict in South Sudan, which emerged in December 2013 when in the Malakal area tens of thousands of civilians were displaced because of the violence. Most of them, they took refuge in the Acacia forests during the transmission season.
So when the sand-fly density is high, where they sought refuge and intense transmission took place during those periods. Then later they settled in the village of Lankien where MSF has a small field hospital. And then as of June, July 2014, we saw an immense upsurge of admissions of kala-azar cases, which peaked in September 2014 when we had almost 250 new admissions per week. Another example of an unexplained recent outbreak occurred in Ethiopia about 10 years ago when VL was introduced in a new area by migrant workers who were seasonal migrant workers. They work in the lowlands in Northeastern Ethiopia, where they work during the agricultural season on big industrial farms, producing sesame mainly.
And after the harvest, they go back to their home areas and many of them have been infected with VL. But if the sand-fly is present in their home area, they introduce the parasite that can give rise to an active transmission. And that’s what we saw in Libo Kemkem in Ethiopia, where there was a big outbreak of VL in a new area.

The importance of data collection and surveillance becomes even more pronounced in the context of an outbreak. A large outbreak can have a rapid impact, threatening the lives of many and becoming hard to control. How to identify an outbreak and recognise how to respond?

In this step, we will cover the principles of outbreaks and recent examples of unexplained outbreaks. In Step 4.8, we will look into outbreak management in practice, through an interview with Dr Koert Ritmeijer, Lead Neglected Tropical Diseases/Research Technical Support for Médecins Sans Frontières.

Learning objectives:

  • Define an outbreak and an epidemic
  • Describe what an epidemiological wave pattern of VL is
  • Understand contextual differences – geography, risk factors
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