Ongoing Diphtheria Crisis affects eight African countries.

Between 1 January and 2 November 2025, 20 412 cases of suspected Diphtheria were recorded across the African continent, with 1252 people believed to have died from the disease (a case fatality ratio of 6%) according to a press release issued by the World Health Organization on 21 November 2025. Eight countries have recorded ongoing epidemics of Diphtheria, Algeria, Chad, Guinea, Mali, Mauritania, Niger, Nigeria, and South Africa. Epidemics in some places have been ongoing since 2023, with the disease continuing to spread to new areas. 

In many areas, laboratory confirmation of cases remain the exception rather than the rule, due to a shortage of suitable testing facilities, as well as a shortage of diagnostic equipment. There is also currently a global shortage of Diphtheria Antitoxin, the most effective treatment for the disease, as well as a shortage of suitable facilities for administering the treatment in many areas. Many of the countries affected have extremely fragile healthcare systems, combined with large numbers of displaced persons and in many cases ongoing civil conflicts, leading to a strong possibility of the disease spreading further within the region. This has led the World Health Orgnization to categorise the crisis as a Grade 2 Emergency under the terms of the Emergency Response Framework.

Diphtheria is a highly contagious vaccine-preventable disease caused mainly by the Actinomycete Bacterium Corynebacterium diphtheria but also by the related Corynebacterium ulcerans. It spreads between people mainly by direct contact or through the air via respiratory droplets. The disease can affect all age groups; however, unimmunized children are most at risk.

Symptoms often come on gradually, beginning with a sore throat and fever. In severe cases, the Bacteria produce a poison (toxin) that causes a thick grey or white patch at the back of throat. This can block the airways, making it hard to breathe or swallow, and also creates a barking cough. The neck may swell in part due to enlarged lymph nodes.

Treatment involves administering Diphtheria antitoxin as well as antibiotics. Vaccination against Diphtheria has been effective in reducing the mortality and morbidity from Diphtheria dramatically. Diphtheria is fatal in 5-10% of cases, with a higher mortality rate in young children. However, in settings with poor access to Diphtheria antitoxin, the case fatality ratio can be as high as 40%.

Niger is suffering an ongoing Diphtheria epidemic which began in the Zinder Region in November 2022, and which has subsequently spread to the Adadez and Diffa regions, with 34 of the country's 72 districts reporting cases. In 2025, there have been 1926 suspected cases, with 122 deaths (a case fatality ratio of 6.3%), with 765 cases confirmed by laboratory testing. Attempts to combat the outbreak have been hampered by a severe humanitarian crisis, driven by civil conflict, climatic effects, economic breakdown, and a large number of displaced persons, with around 2.6 million people currently reliant upon humanitarian assistance. A vaccination program was initiated in September 2025, with uptake reported to be good, and a second round is being planned.

A woman receives a dose of Diphtheria vaccine in Niger. World Health Organization.

Nigeria has been suffering an ongoing Diphtheria epidemic since December 2022, and has the highest number of cases in Africa. In 2025 till November 2, a total of 12 150 cases were reported (8587 of which have been confirmed by laboratory testing), with 884 deaths (a case fatality ratio of 7.2%). Diphtheria has been reported this year from 240 local government areas in 30 of Nigeria's 36 states. This outbreak is disproportionately affecting children and adolescents, and is being exasperated by a low vaccine take-up rate, with around 71% of the population having received a first dose of vaccine and about 67% having received all three recommended doses. Nigeria is suffering from a shortage of vaccines, a limited ability to carry out laboratory testing, a poor public understanding of Diphtheria, and weak infection control practices.  

A Diphtheria outbreak has been ongoing in Chad since mid 2024, with 4462 cases and 47 deaths reported in 2025 to 2 November, a case fatality ratio of 1.1%. Most of the cases have been among children aged 3-13 years. Chad is currently at the centre of a humanitarian crisis triggered by conflicts in multiple neighbouring countries, as well as political instability within its own borders. It is currently home to 1.4 million refugees, including 870 000 from Sudan, as well as about 300 000 internally displaced people. This makes it hard to assess the vaccination status of much of the population in areas where the outbreak is ongoing.

A patient receiving treatment for Diphtheria at a hospital in Chad. Seigneur Yves Wilikoesse/Medecins Sans Frontieres.

Mali is suffering an ongoing Diphtheria epidemic, which began in the northern part of the country in September 2024, with 430 suspected cases and 29 deaths (a case fatality ratio of 6.7%) in 2025 till 2 November. The epidemic spread significantly during 2025, reaching seven of the country's eleven regions and 30 of 75 districts, including the capital city, Bamako. Mali is currently suffering a complex humanitarian crisis, driven by civil conflict and climate-related disruptions, with 6.4 million people currently reliant on humanitarian assistance. Vaccine uptake is low, with 91% of the population having received a first dose, and only 82% all three recommended doses. The country also has a shortage of Diphtheria Antitoxin, as well as a healthcare system which has been severely overstretched by the multiple crises in the country.

Mauritania is suffering a Diphtheria outbreak which began in September 2024, and has spread to 11 of the country's 53 departments, with 849 suspected cases and 33 deaths (a case fatality ratio of 4%). Mauritania has a fairly high vaccination rate for Diphtheria, with 95% of the population having received a first dose of the vaccine, and 86% having received all three doses, but it is also currently home to a population of 118 000 Malian refugees, with vaccine records only available for about 10% of this population. The epidemic also coincides with an outbreak of Rift Valley Fever in the same areas. Efforts to combat these outbreaks has been hampered by poor funding, logistical problems, and a shortage of skilled healthcare workers.

A child receiving treatment for Diphtheria at the Mbaerra Refugee Camp in Mauritania. World Health Organization.

A Diphtheria outbreak began in Guinea in June 2025, with 476 suspected cases and 123 deaths (a case fatality ratio of 25.8%, currently the highest of any affected country). Four of the country's 38 prefectures have been affected, although the outbreak is centred on the gold mining areas of Kankan, particularly in Siguiri District. Only 70 cases have been laboratory confirmed, largely due to a lack of facilities. Guinea also has a low vaccine uptake, with only 77% of the country having had a first vaccine dose, and 63% having received all three recommended doses. The country also has a chronic shortage of Diphtheria Antitoxin, as well as few facilities for treating patients.

Algeria is generally considered to be Diphtheria-free, with a high vaccination rate (98% of the population having received at least one vaccine dose, and 92% having received the recommended three doses). However, in recent years the southern part of the country has become host to a large number of displaced people, fleeing conflict zones in the Sahel Region. In 2024 this area suffered a Diphtheria outbreak in which over 900 people were infected and 119 people died (a case fatality ratio of 13%). A new outbreak was reported in southern Algeria in October 2025, with 13 people infected and two deaths. 

Uniquely among affected countries, the Diphtheria epidemic in South Africa is unrelated to the ongoing crisis in the Sahel Region. The country has a long history of sporadic outbreaks, although these are usually small and quickly contained. The current outbreak, however, first appeared in Western Cape in October 2023, and quickly spread to KwaZulu-Natal, since when it has proven hard to contain. In 2025 till November 2, a total of 106 cases of Diphtheria have been reported in South Africa, all of them laboratory confirmed - a total which includes 37 asymptomatic carriers. The epidemic is still centred on Western Cape, but cases have also been reported in Limpopo, Gauteng, KwaZulu-Natal, and Mpumalanga. Vaccination cover remains low in South Africa, with 76% of the population having received a first dose and 74% all three doses, and this is believed to be worsening. South Africa is also affected by the global shortage of Diphtheria Antitoxin, as well as a health service stretched by multiple other issues.

Geographical distribution of Diphtheria outbreaks in Africa, January 2025 until 2 November 2025. World Health Organization.

Diphtheria is a recurrent problem in many areas of Africa, with 18 789 cases reported between 2020 and 2022. However, this has become much worse since 2023, with 57 000 cases and about 2000 fatalities reported in 2023 and 2024, across  Algeria, Chad, Gabon, Guinea, Mali, Mauritania, Niger, Nigeria and South Africa, with Guinea, Niger, and Nigeria being the worst affected. The disease particularly infects children under 15, and females more than males. About 50% of those infected in 2020-24 are thought to have been unvaccinated. 

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Using a multivariate analysis of the teeth of Sahelanthropus tchadensis to assess its status as a Hominin.

During the twentieth century palaeoanthropologists searching for the origin of the Hominins became convinced that the group has an East African origin. However, in 2001 scientists from the working on Upper Miocene deposits in the Toros Menalla region of Chad uncovered a group of possible Hominin fossils which they assigned to a new species, Sahelanthropus tchadensis, and which included a nearly complete, is distorted, cranium as well as a mandible and some isolated teeth.

Cranium TM 266-01-060-1, nicknamed Toumai, the holotype of Sahelanthropus tchadensis. Smithsonian Human Origins Program.

If correctly interpreted as a Hominin, then Sahelanthropus tchadensis would be the oldest known member of the group. As such the discovery has been subjected to a considerable amount of scrutiny, particularly as the original specimens had been subject to considerable taphonomic (post-mortem) alteration. One of the problems with the original material was that it was had to interpret the position of the foramen magnum (the whole in the skull through which the brain is connected to the central nervous system) because of distortion of the skull. This is important, because in (upright) Hominins it is located roughly in the centre of the bottom of the skull, while in Apes it is typically towards the back of the skull. A virtual reconstruction of the cranium of Sahelanthropus tchadensis has suggested that the foramen magnum would have been on the base. 

Sahelanthropus tchadensis also has a shorter, more vertical face, reduced canines and a related lack of a honing complex (a gap between the lower canine and the first premolar, into which the upper canine fits, allowing the two canines to rub together and sharpen, or 'hone' one-another), and a downward facing lip on the nuchal crest at the rear of the skull, all of which are Hominin traits. However, it also has a small, Ape-like neurocranium, as well as having a size and number of tooth-roots consistent with an Ape, presenting a mosaic of features consistant with an very early Hominin, or possibly an Ape closely related to the earliest Hominins.

Different analyses of a femur and two ulnae found at the same location as the cranium have suggested both that Sahelanthropus tchadensis was either habitually bipedal, or not habitually bipedal, and that it probably spent at least some of its time in the trees. Since a cladistic analysis has recovered Sahelanthropus tchadensis as a Hominin, this raises at least the possibility that Hominins were habitually walking on two legs 7 million years ago.

One aspect of Sahelanthropus tchadensis which has not been studied extensively is its dentition, despite a number of teeth and tooth fragments being available. In a paper published in the South African Journal of Science on 31 July 2024, Walter Neves, Leticia Valota, and Clovis Monteiro of the Institute of Advanced Studies at the University of São Paulo, present the results of a morphometric analysis which compared the upper posterior dentition of Sahelanthropus tchadensis to that of living Apes and fossil Plio-Pleistocene Hominins.

The upper posterior teeth were chosen as these were the teeth for which the mesiodistal and buccolingual diameters could be determined in Sahelanthropus tchadensis. These were compared to a selection of teeth from Pan troglodytes (Chimpanzees), as well as the Hominins Orrorin tugenensis, Ardipithecus ramidus, Australopithecus afarensis, Australopithecus africanus, Paranthropus boisei, Paranthropus robustus, Homo habilis, and Homo erectus.

For each tooth the ratio between the mesiodistal and buccolingual diameters (length and width) was calculated, and these were plotted on a distribution map. The teeth fell into approximately three groups on this map, one comprising modern Chimpanzees, one comprising the 'Robust Autralopithecines' Paranthropus boisei and Paranthropus robustus, and one containing all other Hominins.

Distribution of the species and specimens included in the study along the morphospace defined by principal component 1 (PC1) x principal component 2 (PC2). Neves et al. (2024).

The teeth of Sahelanthropus tchadensis fell within the non-Robust Hominin cluster, plotting closest to Ardipithecus ramidus, a Miocene-Pliocene species with a large number of known specimens, which again has some Ape-like characteristics, but which is generally accepted as a Hominin by palaeoanthropologists today. Neves et al. interpret this as supporting the hypothesis that Sahelanthropus tchadensis is a Hominin rather than an Ape.

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Outbreak of Hepatitis E in Ouaddai Province, Chad.

An outbreak of Hepatitis E has resulted in seven deaths and over 2000 people becoming sick in Ouaddai Province, Chad, according to a press release issued by the World Health Organization on 8 May 2024. The outbreak was first detected on 2 January 2024 when to cases of Acute Jaundice Syndrome were reported by medics from Médecins Sans Frontières who were operating a clinic at a temporary high school at a refugee camp in the Adré Health District of Ouaddai Province.

Between 2 January and 15 February 2024 the number of reported cases of Acute Jaundice Syndrome in Adré Health District rose to 113, of which 28 were confirmed as being Hepatitis E using rapid diagnostic test kits. On 19 February a case was reported at the Allasha Refugee Camp in the Hadjer-Hadid Health District, also Ouaddai Province.

Between 1 and 19 March 2024 forty blood samples taken from Acute Jaundice Syndrome sufferers in Ouaddai Province were sent to the Institute Pasteur of Dakar, Sénégal, where they were tested for Hepatitis E, Yellow Fever, Dengue, West Nile Fever, Zika, Chikungunya, Rift Valley Fever and Crimean-Congo Haemorrhagic Fever. Of these, 36 tested positive for Hepatitis E.

A refugee camp in eastern Chad in January 2024. Médecins Sans Frontières.

Between 2 January and 28 April 2024 a total of 2092 suspected cases of Hepatitis E were reported in Ouaddai Province, with seven fatalities. One hundred and three of these cases were members of the local conflict, while the remaining 1989 were refugees from the conflict in neighbouring Sudan, with the Lycée d'Adré, Aboutengué, and Metché refugee camps being particularly badly hit. 

Hepatitis E is caused by a single-stranded, nonenveloped, RNA Virus, and is usually a self-limiting infection, causing fever, nausea, loss of appetite, vomiting, jaundice, abdominal and joint pain and discolouration of the urine and stool, which typically passes within 2-6 weeks. However, in some cases the disease can cause acute liver failure (hepatitis) which is often fatal. Pregnant women are considered to be at particular risk from this disease, with a fatality rate of about 30%, compared to about 1% for the general population.

The Hepatitis E Virus. Centers for Disease Control and Prevention/Wikipedia.

Hepatitis E is spread through faeces and contaminated water, and thrives in unsanitary and crowded conditions. This makes it a particular problem in settings like refugee camps, where large numbers of people fleeing conflict situations arrive over short periods of time, overwhelming local sanitation systems. 

A queue of water containers at the Metché Refugee Camp in Ouaddai Province, Chad, where Médecins Sans Frontières is providing clean water. Médecins Sans Frontières.

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Outbreak of Dengue Fever in Chad.

On 15 August 2023, a Dengue Fever outbreak was declared by the Ministry of Public Health and Prevention in Chad, according to a press release issued by the World Health Organization on 16 October 2023. As of 1 October, there have been 1342 suspected cases, including 41 confirmed cases reported across eight health districts in four provinces. Among the confirmed cases, one death was reported. Abéché Health District in Ouaddaï Province, in the east of the country, is the current epicentre of the outbreak. The Ministry of Public Health and Prevention has initiated a number of key response activities by implementing, in collaboration with the World Health Organization and other partners, the national contingency plan for Dengue preparedness and response. Dengue is a viral infection transmitted to Humans through the bite of infected Mosquitoes. Many dengue infections produce only mild flu-like illness and over 80% of cases are asymptomatic. There is no specific treatment for Dengue; however, timely detection of cases and appropriate case management are key elements of care to prevent severity and fatality of Dengue. This is the first Dengue outbreak ever reported in Chad, and the country has limited surveillance, clinical and laboratory capabilities. Given the favorable environmental conditions for Mosquito spread, an ongoing humanitarian crisis due to a massive influx of refugees and returnees from Sudan and limited response capacities, the World Health Organization assesses the risk posed by this outbreak as high at the national level.

The outbreak of Dengue declared in Abéché Health District, Ouaddaï Province, represents the first Dengue outbreak ever reported in Chad. The declaration was made after the confirmation of Dengue infection in eight out of 12 blood samples tested using real-time polymerase chain reaction at the national Biosafety and Epidemics Laboratory in N'Djamena. Subsequently, the samples were sent to the Institut Pasteur in Cameroon for confirmation, which was completed on 22 August by polymerase chain reaction and enzyme-linked immunosorbent assay, confirming the presence of Dengue. The Dengue serotype responsible for this outbreak remains unknown.

As of 1 October, there have been 1342 suspected cases, including 41 confirmed cases reported across eight health districts in four provinces. Among the confirmed cases, one death was reported (a Case Fatality Ratio among confirmed cases 2.4%). Eight districts in four provinces (NDjamena, Ouaddaï, Sila, and Wadi Fira) have reported confirmed Dengue cases. Notably, Ouaddaï, the epicentre of the outbreak, has reported the highest number of confirmed cases, accounting for 31 out of the total 41 confirmed cases (76% of confirmed cases). The age group most affected by this outbreak are those between 15 to 34 years old, representing 27% of the reported confirmed cases. 

Dengue is a Viral infection transmitted to humans through the bite of infected Mosquitoes and is found in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas. The primary vectors that transmit the disease are Aedes aegypti Mosquitoes and, to a lesser extent, Aedes albopictus.

Dengue Fever is caused by a Positive Single-strand RNA Virus of the Flaviviridae family and there are four distinct, but closely related, serotypes of the Virus that cause Dengue Fever (Dengue Fever Virus-1, Dengue Fever Virus-2, Dengue Fever Virus-3 and Dengue Fever Virus-4). Recovery from infection is believed to provide lifelong immunity against that serotype. However, cross-immunity to the other serotypes after recovery is only partial, and temporary. Subsequent infections (secondary infection) by other serotypes increase the risk of developing Severe Dengue Fever.

A transmission electron micrograph showing Dengue Virus virions (the cluster of dark dots near the centre). Centers for Disease Control and Prevention/Wikimedia Commons.

Although Chad has previously experienced outbreaks of Arboviruses (Arthropod-born Viruses) such as Chikungunya and Yellow Fever, this is the first Dengue outbreak ever reported in the country. Chad, including the Ouaddaï Province, experienced a Chikungunya outbreak in 2020, with a total of 34 052 cases recorded and one associated death.

The Ministry of Public Health and Prevention has initiated a number of key response activities, with the support of the World Health Organization and other partners, including mobilizing resources for the implementation of the national contingency plan for Dengue preparedness and response, strengthening surveillance and coordinating the response, including active case finding in healthcare facilities and the community and in-depth epidemiological investigations including regularly updating the case line list, increasing early case detection capacity by disseminating community alert definition of cases and procuring rapid diagnostic tests for health facilities, ensuring effective logistics and operational support, including the transportation of samples for confirmation, developing standard operating procedures for clinical management of suspected and confirmed Dengue cases, including severe Dengue and ensuring inventory of existing case management kits and address gaps, strengthening cross-border collaboration and implementing prevention and vector control measures in border areas, srengthening entomological surveillance, including aquatic and adult stages of the vectors, and characterizing vector bionomics, implementing effective vector control measures as part of integrated vector management, and strengthening community mobilization and commitment to disseminate key information on transmission and control to the population.

This is the first dengue outbreak reported in Chad, and the country lacks the necessary public health preparedness and response capacities. Community cases are likely underreported because Dengue is unknown to the general public and clinicians are not yet sensitised to its presentation, which is sometimes confused with those of other common febrile infections, making early diagnosis challenging, particularly in settings with lack of laboratory facilities for testing. There is a high risk of spread due to the presence of Mosquitoes in large, densely populated cities in eastern Chad near the Sudan border, with a tropical climate, and poor sanitation conditions suitable for Mosquito development.

The province of Ouaddaï, which borders Sudan, is the epicentre of the outbreak and is also the province most affected by the ongoing humanitarian crisis due to a massive influx of refugees and returnees from Sudan. According to the United Nations High Commissioner for Refugees, the number of refugees in the Ouaddaï Province is currently more than 400 000. The movement of returning Sudanese refugees and Chadian nationals has the potential to spread the outbreak to new provinces and across the border. Based on the information available for this event, the World Health Organization assesses the risk posed by this outbreak as high at the national level, moderate at regional level, and low at global level.

The proximity of Mosquito breeding sites to human habitation is a significant risk factor for Dengue Virus infection. The prevention and control of Dengue depend on effective vector control. Vector control activities should focus on all areas where there is a risk of human-vector contact (place of residence, workplaces, schools, and hospitals). The World Health Organization promotes a strategic approach known as Integrated Vector Management to control Aedes spp., the vector of Dengue. Integrated Vector Management should be enhanced to remove potential breeding sites, reduce vector populations, and minimize individual exposure. This should involve vector control strategies for larvae and adults (i.e. environmental management and source reduction), especially of water storage practices, and include covering, draining and cleaning household water storage containers on a weekly basis, applying larvicide in non-potable waters using World Health Organization-prequalified larvicides at correct dosages, distribution of insecticide-treated nets for fever/Dengue inpatients to contain spread of virus from health facilities, as well as strategies for protecting people and households. Indoor space spraying (fogging) is another approach for rapid containment of Dengue-infected Mosquitoes but may be challenging to deliver in densely populated areas of camps.

A health agent at work fumigating an area to prevent Mosquitoes. Sia Kambou/AFP.

Personal protective measures during outdoor activities include topical application of repellents to exposed skin or treatment of clothing, and wearing long-sleeved shirts and pants. Indoor protection can include household insecticide aerosol products, or Mosquito coils. Window and door screens can reduce the probability of Mosquitoes entering the house. Insecticide-treated nets offer good protection against mosquito bites while sleeping during the day. Since Aedes Mosquitoes (the primary vector for transmission) are active at dawn and dusk, personal protective measures are recommended, particularly at these times of day, both in residential areas and also at places of work and schools for children.

Entomological surveillance should be undertaken to assess the breeding potential of Aedes Mosquitoes in containers and monitor insecticide resistance to help select the most effective insecticide-based interventions. There is no specific treatment for Dengue infection, but early detection and access to appropriate healthcare for case management can reduce mortality. Case surveillance should continue to be enhanced in all affected areas and nationwide. Where feasible, resources should be allocated to strengthen a sample referral mechanism for the confirmation and sub-typing of the Dengue Virus.

Communities play a major role in the success and sustainability of vector control activities. While coordination among many stakeholders is required, vector control is critically dependent on ensuring that communities are aware of the risk of infection and know which measures to take to protect themselves. Community engagement and mobilization involve working with local residents to improve vector control and build resilience against future disease outbreaks. Where appropriate participatory community-based approaches are in place, communities are supported to take responsibility for and implement vector control. Participatory community-based approaches aim to ensure that healthy behaviours become part of the social fabric and that communities take ownership of vector control at both inside and outside households.

Based on the information available for this event, the World Health Organization does not recommend travel or trade restrictions be applied to Chad.

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How a changing climate is changing rainfall patterns and vegetation cover in the dry Sahel Region of Africa.

The Sahel Drylands spread across Africa to the south of the Sahara from Senegal in the west to Sudan in the east. The area suffered a series of severe draughts in the 1970s and 80s, but since then rainfall increased in both frequency and intensity, resulting in a steady greening of the area observed in satellite images. This increase in rainfall can be directly linked to rising global temperatures, with warmer air over the Atlantic Ocean leading to higher levels of evaporation there, and subsequently higher rainfall over Africa. However, the Sahel region is not a completely homogeneous environment, but rather is made up of a patchwork of dry grassland and woodland environments, which are likely to react to changing rainfall regimes in different ways, differences which may be beyond the capacity of satellites to detect without long-term terrestrial observation data for comparison, something which is absent in much of the region.

In a paper published in the journal Communications Biology on 23 April 2019, Martin Brandt of the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Pierre Hiernaux of the Pastoralisme Conseil, Kjeld Rasmussen, also of the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Compton Tucker of NASA's Goddard Space Flight Center, Jean-Pierre Wigneron of Interactions Sol Plante Atmosphère at the INRA Nouvelle-Aquitaine-Bordeaux Centre, Abdoul Aziz Diouf of the Centre de Suivi Ecologique, Stefanie Herrmann of Agricultural and Biosystems Engineering, at the University of Arizona, Wenmin Zhang, again of the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Laurent Kergoat of Geosciences Environnement Toulouse at the Observatoire Midi-Pyrénées, Cheikh Mbow of START International Inc., Christin Abel, again of the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Yves Auda also of Geosciences Environnement Toulouse at the Observatoire Midi-Pyrénées, and Rasmus Fensholt, once again of the Department of Geosciences and Natural Resource Management at the University of Copenhagen, present the results of a study of vegetation in the Sahel which used ground obsevation data from the fields sites run by the Centre de Suivi Ecologique in the Ferlo Region of north-central Senegal for comparison with satellite data.

The Ferlo Region covers much of northern Senegal, and has an extremely arid climate with a nine-month dry season when the climate is driven by dry winds from the Sahara and a rainy season that lasts from July to September, when moister air from the Gulf of Guinea brings irregular rainfall. The the Centre de Suivi Ecologique has routinely collected ecological data in the region since 1987, using a consistent methodology over more than three decades and measuring both woody and herbaceous vegetation properties over a series of 1 km transects.

A village in the Ferlo Region of Senegal. Hôtel Cap Saint-Louis.

Brandt et al. used field data from the Ferlo to derive simple and reproducible metrics of vegetation composition, which was divided into above-ground herbaceous mass and woody plant foliage mass, across the Sahel, which was then compared to rainfall data. Rainfall was divided into core rainfall, which fell during the usual rainy season, and early/late rainfall, which fell outside of it.

The largest increases in rainfall measured across most of the Sahel were in early/late rainfall (i.e. rainfall that fell outside the typical rainy season), and this corresponded in most places to an expansion in woody plant foliage mass. Brandt et al.theorise that perennial woody plants were better able to utilise this out-of-season rain as most of the herbaceous plants have a very short growing season, starting their life cycle with the onset of the rains and finishing it as the rains end, with little potential to increase the length of their growing season.

Trends for the Sahel (1992–2012). (a) Vegetation optical depth estimated herbaceous mass trends. (b) Vegetation optical depth estimated woody foliage (trends. Significant (p less than
0.05) trends are shown in dark green and red, insignificant trends (p greater than 0.05) are shown in light red and green. (c) Areas where the slope in rainfall (early and late rains) is larger than the slope in rainfall (core wet-season rains) are shown in blue colour. d Pearson correlation between annual Vegetation optical depth 90% and pixel-wise fitted sinusoidal term (1st harmonic). (e) Core wet-season rainfall  trend. (f) Early and late rainfall trend. Brandt et al. (2019).

Significant increases in above-ground herbaceous mass were mostly located in the Ferlo and the northern Sahel of Mali. Areas with a significant increase in woody plant foliage mass are observed across the entire Sahel; notably in Senegal, Chad, eastern Niger and large parts of Mali. Decreases in both above-ground herbaceous mass and woody plant foliage mass were observed in western Niger and around Lake Chad, with no obvious relationship with rainfall, pointing towards other, rainfall independent causes (for example land management). 

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