Extreme Weather Events and Pathogen Pollution Fuel Infectious Diseases: The 2024 Flood-Related Leptospirosis Outbreak in Southern Brazil and Other Red Lights

Abstract

The emergence of infectious disease outbreaks and ‘superbugs’ related to pollution combined with climate change is a current problem, not just a future threat. In May 2024, an extreme flood hit the Rio Grando Sul State, southern Brazil, triggering an important leptospirosis outbreak in urban settings with deficient sanitation systems. This and other cases discussed in this article exemplify how extreme weather events exacerbate the consequences of environmental pollution by multiple classes of pathogens in the global scenario of increasing anthropogenic pressures on the environment. A combination of actions to combat climate change and improvements in sanitation systems is essential to mitigate this problem.

1. Introduction

The participation of the environment as an intermediary piece in the puzzle of zoonotic spillover events has been increasingly recognized, with pathogens derived from a reservoir host being released into the environment and then infecting a new host or population [1]. Mounting evidence has indeed shown that pathogens, including antibiotic-resistant bacteria, viruses and parasites derived from humans and other animals, pervasively contaminate the environment, where they can become ‘superbugs’ or spread in new host populations. This can trigger deleterious and unexpected consequences, including potentially untreatable infections and diseases outbreaks [2,3,4]. It is noteworthy that environmental stressors such as pollutants and extreme heat may cause immunosuppression and disrupt the microbiota of reservoir hosts, potentially increasing the shedding of pathogens into the environment [5,6,7]. Also, climate change and related extreme weather events facilitate biodiversity loss and induce changes in the dynamics of disease vectors, which are both factors linked to the increased risk of infectious disease emergence and spread [8,9].

During extreme floods and other water-related weather events (e.g., hurricanes, heavy rainfalls, severe storms), pollution facilitates outbreaks of diseases caused by several pathogens, including viruses (e.g., adenovirus, enterovirus, hepatitis viruses, norovirus, and rotavirus), bacteria (e.g., Vibrio cholerae, Leptospira spp., Escherichia coli, and Salmonella spp.), protozoa/parasites (e.g., Cryptosporidium spp., Giardia lamblia, Schistosoma mansoni, and Leishmania spp.), and fungi (e.g., Aspergillus fumigatus and Scedosporium spp.) [10,11,12,13].

Although the scientific community already recognizes the climate emergency as an urgent and current health problem [14,15,16], many countries, communities, and industry sectors are still slow to apply solutions to curb climate change, suggesting that the consequences of climate change are problems to be faced in a distant future. However, remaining oblivious to such problems is a dangerous behavior. Furthermore, pollution caused by pathogens released into the environment through untreated sewage (among other routes), as well as pollution by chemicals that can accelerate the selection of multi-resistant microorganisms, are problems with medical and ecological importance. In this article, we briefly discuss some points regarding the connections between climate change, pollution, and emerging infectious diseases, highlighting that more than it being a problem to be faced in the future, this is a major ongoing process requiring urgent and bold action.

2. Methodological Notes

In order to carry out a brief review of infectious disease outbreaks and spread of pathogens caused by flood events around the world, searches were performed in the PubMed [17] and Google Scholar [18] databases (in August 2024) to obtain and select examples of flood-related human disease outbreaks (as will be discussed in Section 3). In order to obtain examples of different groups of pathogens, searches were performed using the terms “flood” + “outbreak” in association with “bacteria” or “fungi” or “parasite” or “protozoa” or “virus”. Articles in English were considered eligible, with no limitation regarding the year of publication. Works cited in the reference list of selected articles were also considered eligible. Three examples of each group of pathogens were selected for discussion in this work, valuing the studies that showed robust evidence of an association between the disease outbreak/pathogen spread and the flood event.

This article also compiled information on the 2024 flood-related leptospirosis outbreak observed in Rio Grande do Sul State, southern Brazil (as will be discussed in Section 4.1). Consolidated data on cases of leptospirosis in Rio Grande do Sul in the previous ten years were obtained from the epidemiological situation bulletins made available by the Brazilian Ministry of Health (Confirmed cases and deaths due to leptospirosis in Brazil and Major Regions of Brazil—2000 to 2024) [19].

Data on leptospirosis cases recorded in the Rio Grande do Sul in 2024 were obtained from epidemiological reports made available by the Secretary of Health of the Government of the Rio Grande do Sul State [20]. This is the body responsible for monitoring and disseminating data regarding cases of leptospirosis in Rio Grande do Sul in ‘real time’ (data are updated frequently as case numbers change significantly), including reports on cases related to the 2024 flooding event.

Finally, we highlight that much information about the 2024 extreme flood observed in southern Brazil and the leptospirosis outbreak described in this article express what the authors of this article observed in loco during the flood.

3. Flood-Related Infectious Disease Outbreaks in Different Countries: A Brief Review

Extreme weather events tend to intensify around the world due to climate change, increasing the risk of major floods and inundations in various world regions [21]. Other anthropogenic pressures on the environment exacerbate this problem. The deforestation of riverbeds increases the intensity of floods, as the loss of vegetation favors the water flow and facilitates soil loss [22]. Lack of flood risk management and insufficient urban planning increase the material damage caused by floods, also resulting in more people being affected [23]. Combined, these effects and conditions create serious environmental and public health problems, ranging from psychosocial problems to infectious diseases [24].

Even countries that are not traditionally affected by major floods and downpours are now experiencing these problems more frequently, as evidenced by severe floods observed recently in Germany [25] as well as in other European countries in the last decades [26]. This indicates that the health effects derived from floods will increasingly affect unexpected regions, not just populations in low- and middle-income countries from tropical regions [21]. Understanding the direct and indirect health impacts of floods is essential for establishing risk control and damage mitigation strategies.

Table 1. Examples of infectious disease outbreaks and spread of pathogens caused by flood/water-related events in different countries.

Pathogen Group	Pathogen	Country	Potential Risk Factors *	Reference
Bacteria	Leptospira kirschneri	Sri Lanka	Water retention in soil; use of buffalo in agriculture; peridomestic animal farming in rural areas	Agampodi et al. [27]
	Campylobacter spp.	Hungary	Contamination of water source due to extreme rainfall events	Dura et al. [28]
	Vibrio cholerae	India	Overcrowding; nutritional deficiency; lack of personal and domestic hygiene; lack of proper sanitation	Sur et al. [29]
Fungi	Aspegillus niger	Tanzania	Damp homes affected by floods (prone to mold growth)	Tibuhwa [30]
	Mucormycetes	United States	Widespread environmental exposure resulted in the infection of susceptible individuals	Davies et al. [31]
	Invasive mold infections	United States	Environment suitable for mold growth	Toda et al. [32]
Parasites (protozoa)	Cryptosporidium hominis	Germany	Exposure of children in areas affected by flood waters (e.g., playgrounds, river beach)	Gertler et al. [33]
	Plasmodium spp.	Sudan	Changes in habitat caused by floods resulting in the formation of new breeding sites for Anopheles mosquitoes	Elsanousi et al. [34]
	Acanthamoeba	United States	Keratitis epidemic caused by contact with contaminated flood water	Meier et al. [35]
Viruses	West Nile virus	United States	Increased human exposure to mosquitoes and emergence of new habitats suitable for mosquito development	Caillouët et al. [36]
	Hepatitis A virus	India	Overcrowded rescue camp; poor hygienic condition; contaminated water source	Pal et al. [37]
	Norovirus	United States	Lack of proper sanitation; crowded shelters	Yee et al. [38]

* Potential risk factors that contributed to the outbreak/pathogen spread in combination with the flood/water-related event.

shows a brief review of infectious disease outbreaks and spread of pathogens caused by flood events around the world [27,28,29,30,31,32,33,34,35,36,37,38]. These examples demonstrate that a variety of pathogens affect human populations during and after floods, causing diseases ranging from mild gastrointestinal problems to more serious conditions that lead to hospitalization and even risk of death [39,40,41].

Infections caused during floods may be directly due to contact with contaminated water (e.g., leptospirosis, hepatitis) or, alternatively, some infections occur as a consequence of the environmental and sanitary conditions caused by the flood. For example, homes that remain damp for long periods are susceptible to the proliferation of pathogenic fungi [42], and environmental conditions created by floods can facilitate the dissemination of mosquito-borne diseases [43]. In addition, large-scale floods can impair or lead to the collapse of healthcare services, such as vaccination centers, hospitals, and other types of medical facilities [44], also contributing to the spread of diseases.

As exemplified in Table 1, several risk factors contribute to the spread of infectious diseases/pathogens during and after floods. Poor sanitation, especially deficiencies in sewage collection and treatment systems, and the intense presence of animals that transmit zoonoses in the region affected by the flood are relevant risk factors because they create favorable conditions for contamination of flood and drinking water with pathogens from domestic sewage as well as urine and feces of animals [12,45,46]. During floods, direct contact with flood water, especially of mucous membranes and skin with lacerated wounds, significantly increases the risk of infection [46]. After floods, contact with flood sludge and management of debris contaminated by flood water without personal protective equipment while cleaning houses and streets also creates conditions conducive to infections [47].

During floods, many people lose their homes and are taken in by public shelters. The crowding of many people in these places favors the spread of respiratory diseases, among others [12]. Finally, it is important to highlight that large-scale floods cause biodiversity loss and changes in ecological networks, both factors that act as drivers of zoonotic diseases [1,8]. In other words, in addition to known diseases, as shown in the examples described in Table 1, floods can create favorable conditions for the emergence of new diseases in the human population.

4. Climate Change and Extreme Weather Events Already Favor Multiple Pathogens and ‘Superbugs’

4.1. The 2024 Flood-Related Leptospirosis Outbreak in Rio Grande Do Sul State

Extreme rainfall linked to a combination of atmospheric factors and anthropogenic climate change has recently been observed in southern Brazil [48,49]. This extreme weather event triggered a major flood that affected 2,398,255 people in 478 municipalities in the state of Rio Grande do Sul [50], including Porto Alegre City (Figure 1), the state capital. The rains began at the end of April 2024, but the peak of the flood occurred in May 2024. Until the last data update (10 July 2024), 182 confirmed deaths and 806 injured people were recorded, and 29 people remained missing [50].

The infrastructure of Rio Grande do Sul was severely damaged, with many highways and bridges being destroyed, making access to several cities difficult or blocked. In many cases, entry to or exit from many cities was only possible with the help of rescue boats or helicopters, which were often used to transport injured or hospitalized people. The monetary losses caused by the crisis in Rio Grande do Sul was estimated at BRL 62 billion (Brazilian Real), but losses could reach more than BRL 100 billion [51].

The health system was significantly affected in many cities, causing serious problems in the distribution of medicines, performing surgeries and compromising the treatment of chronic patients [52]. Thousands of animals were also affected, including dogs, cats, horses, livestock animals, and native fauna [53].

In addition to various types of trash and debris, the flood water was also polluted by zoonotic pathogens. Contamination of the flood water by urine from animals associated with other risk factors (e.g., poor waste management and direct contact with contaminated water) [45,46,54,55] has caused an outbreak of human leptospirosis in Porto Alegre and other cities in Rio Grande do Sul State [20].

To date (14 August 2024), 725 cases of leptospirosis have been confirmed, with 26 deaths recorded (25 of them directly related to the flood) (

Table 2. Epidemiological situation of leptospirosis in Rio Grande do Sul State in 2024 * (flood year) and in the previous ten years **.

Year	Cases under Investigation	Confirmed Cases	Registered Deaths
2024	2844	788	26 ***
2023	-	477	25
2022	-	280	16
2021	-	194	15
2020	-	214	7
2019	-	698	28
2018	-	448	20
2017	-	493	20
2016	-	409	17
2015	-	523	33
2014	-	481	25

* 2024 data updated on 15 August 2024 [20]. Unlike the data from previous years (full years), the data for 2024 represent only seven and a half months. ** Data from 2023 to 2023 updated on 29 May 2024 [19]. *** Only one case was not related to the 2024 flood, and an additional four deaths remain under investigation [20].

). Another 2844 cases remain under investigation (Table 2) [20]. The number of cases, and possibly deaths, will certainly be higher, as this is an outbreak under investigation. For comparison purposes, in 2024 (in which data represent only seven and a half months), a 65%, 181%, and 306% increase in the number of leptospirosis cases has already been seen, compared to 2023, 2022, and 2021, respectively (Table 2) [19]. Notably, the significant and rapid increase in the number of leptospirosis cases linked to the 2024 flood was widely reported by various news outlets [56,57,58,59,60].

The classic risk factors for leptospirosis are contact with water contaminated by bacteria of the Leptospira genus, which usually occurs in flood situations in places with the presence of livestock, zoonotic, and domestic animals (e.g., rodents and dogs) associated with poor sanitation. Of note, exposure to contaminated water with skin wounds is an important risk factor for Leptospira infection [45,46,54,55].

During the 2024 flood, a large portion of the population of Rio Grande do Sul came into extensive contact with flood water; this included both people whose homes were invaded by water and people involved in rescue activities and assistance to the affected human and animal populations [61,62]. In addition, many cities in Rio Grande do Sul have deficiencies in sanitation systems, which deteriorates the water quality of Rio Grande do Sul, especially in the Guaíba Lake basin [63]. Only 26.6% of sewage is collected and properly treated in Rio Grande do Sul, according to data from 2022 [64]. In Porto Alegre, the capital of the state, only 55.42% of sewage receives adequate treatment, according to data from 2024 [65]. The epidemiological data presented in Table 2 associated with the risk factors observed in Rio Grande do Sul during the 2024 flood strongly indicate the relationship between this extreme climate event and the disease outbreak. This flood-related leptospirosis outbreak provides an additional example of how climate change facilitates environment-mediated infections in sanitation-deficient settings [66].

4.2. Other Examples of Climate Change-Related Health Problems

Other red lights come from southern Brazil. A study performed in Porto Alegre City by our group indicated that heat waves can have a deleterious effect on non-pathogenic soil nematodes, without affecting populations of soil-transmitted pathogens (e.g., Ascaris spp., Trichuris spp., and hookworms) as intensely [67]. These findings are in line with a series of evidence indicating that anthropogenic disturbances in the environment, including climate change and biodiversity loss, affect the delicate equilibrium among species communities and the intricate web of interactions between humans, non-human animals, and their environments. Such disturbances disrupt ecological chains, creating conditions favorable to the dissemination of pathogenic microorganisms and disease vectors adapted to atrophic environments, thereby facilitating the emergence of infectious disease outbreaks [8].

Climate change also favors the adaptation of microorganisms to new ecological niches and hosts. The emergence of the multi-resistant fungus Candida auris as a human pathogen was potentially facilitated by climate change. Higher temperatures in humid regions would have acted as a pressure for the selection of variants that are thermotolerant to the human temperature. Wild and domestic animals would then have acted as intermediate hosts for the transmission of C. auris from the environment to the human population [68,69,70]. Other fungi species (e.g., Batrachochytrium dendrobatidis, Cryptococcus deuterogattii, and Puccinia striiformis) may have also obtained some benefit from climate change, with detrimental impacts on humans and biodiversity [70]. This phenomenon is intensified when climate change is combined with the release of antifungals for medical and veterinary use into the environment, a pollution problem of increasing importance globally [71].

5. Control of ‘Pathogen Pollution’ Is Urgent

Environmental contamination by domestic, industrial and hospital sewage, as well as residues from agriculture and livestock production, contribute to ‘pathogen pollution’, an expression used both (I) to describe the widespread presence of pathogens in the environment and (II) to refer to the introduction of pathogens into a new host/human population [72] (this second case is also called a ‘spillover event’ [1]). Eventually, ‘spillback events’ can also occur, with the transmission of pathogens from humans to the environment or non-human animal species [1,73]. In any case, pathogen pollution usually occurs due to anthropogenic forces [72].

The widespread release of microorganisms into the environment (from human and other animal populations) indeed facilitates the reinfection of humans and animals [4], especially in urban and rural environments with precarious socio-environmental conditions and poor sanitation infrastructure [72]. This problem is intensified by the widespread discharge of antibiotics into the environment derived from human and veterinary use, which contributes to the selection of multi-resistant bacteria [4,74,75]. Furthermore, we stress that the presence of varied microorganisms in the environment allows for the exchange of genetic information between them, subsequently allowing for the emergence of new pathogen variants [1,76,77]. Climate change and related warmer temperatures add more ‘energy’ to this system, potentially accelerating and intensifying the processes mentioned above.

6. Key Message

Climate change and pathogen pollution are related problems that, if not mitigated, will have increasingly harmful consequences for human and environmental health, facilitating the emergence of new diseases and degrading ecosystems (Figure 2). Examples from Brazil (Figure 1) and other countries make this problem increasingly palpable. To control climate change, countries need to follow international agreements to reduce the release of greenhouse gases into the atmosphere, especially through an energy transition that limits the use of fossil fuels. Improvements in sanitation systems and reducing the use of antimicrobial agents are fundamental measures to control pathogen pollution and, consequently, emerging pathogens. Finally, robust and continuous investments in traditional (e.g., classical epidemiology and entomological surveillance) and next-generation strategies (e.g., AI-assisted tools and genomic vigilance) for pathogen surveillance at human–environment–animal interfaces are needed.