Over the last four decades, ~43% of amphibian species have become threatened with extinction worldwide (1, 2). Habitat destruction (3), natural environment pollution (4), climate change (5) and emerging infectious diseases (6) are all potential causes of morbidity and mortality. Chytridiomycosis is the disease caused by Batrachochytrium dendrobatidis (Bd) and has become a major menace to amphibians, leading to the decline and extinction of amphibians worldwide (7, 8).

Bd invades epidermal cells and grows the outer keratinized layers, causing skin thickening, erosions and ulcerations, disrupting the transport of water, oxygen and ions, and eventually leading to death (9). This pathogen has been detected in ~700 amphibian species (10) and occurs in different regions around the world, including Europe, Oceania, Africa, North America and South America (11, 12), particularly in the Neotropics and Australia (13, 14). Bd may be one of the most seriously infectious diseases currently threatening biodiversity (8) and is now listed as an important epidemic disease by the World Organization for Animal Health (15). Currently, there are several methods to control the spread of Bd, such as use of fungicides, bioaugmentation (microorganisms that inhibit the growth of Bd) and vaccination. The first two methods may be considered risky for other species due to their broad spectrum, and the effectiveness and safety of vaccination still need be further validation for controlling chytridiomycosis in the field (16, 17).

Bd is regarded as a generalist pathogen (18) and is distributed across different ecosystems (19, 20). The prevalence and infection intensity of Bd are associated with abiotic factors (21) and may vary according to host species, age and individual susceptibility (22). Currently, there are several Bd lineages, mainly divided into enzootic and epizootic lineages, where the enzootic lineages has a lower virulence probably due to its smaller zoosporangium size, and more limited geographic distributions vs. epizootic lineages (23–25). Meanwhile, the inherent characteristics of amphibians (behavior and immunity) and environmental features (vegetation coverage, temperature, rainfall, seasonality, and geographical conditions) can predict dynamic changes of diseases in natural populations (26, 27).

Amphibians are a critical links in ecosystem food chains, as they are able to prey on pests, effectively protect crops and play an important role in maintaining the ecosystem balance (18, 28). In addition, some amphibians are edible while others have high medicinal and scientific research value (29, 30). Considering the serious impact of chytridiomycosis on the health of amphibians, there are still limited articles on the overall assessment of the factors that potentially influence the prevalence of Bd. Thus, we conducted a global systematic review and meta-analysis on Bd infection over the last 21 years. The sampling year, geographic location, disease dynamic, detection method, season, host, age, International Union for the Conservation of Nature (IUCN) classification, sampling source and other potential risk factors along with several geographic factors were collected and analyzed to determine which were related to Bd prevalence in amphibians. This study could support ongoing conservation efforts for amphibian chytridiomycosis.

Chytridiomycosis is a worldwide infectious disease of amphibians, causing a decrease in biodiversity and a loss of economic benefits (42). Detailed knowledge of the epidemiological status of chytridiomycosis in amphibians provides a foundation for implementing efficient protective measures.

Therefore, we conducted a systematic review and meta-analysis of global Bd infection. The overall prevalence of Bd in amphibians worldwide from 2000 to 2021 was 18.54% (95% CI: 13.76–20.52; Supplementary Table 3).

In 2008, Office International des Epizooties (OIE) listed Batrachochytrium dendrobatidis as notifiable pathogen, because the international community recognized the impact of the global trade in amphibians on the spreading of chytridiomycosis (43). This legislation enabled countries connected to the World Trade Organization to specifically detect and restrict the trade of infected amphibians. Therefore, we chose 2008 as the entry point for our analysis of Bd infection in amphibians worldwide. We found that the Bd prevalence (18.91%) decreased after this policy was enacted (Table 1). Implementing similar policies might help to reduce Bd infection. Therefore, we suggest that more relevant legislation be implemented to protect amphibian species.

In our study, the Bd prevalence in South America and Oceania was basically the same and significantly higher than that in Asia (P < 0.05; Table 1). In view of our geographic subgroup analysis, we found that the highest Bd prevalence was in the latitude range 20–40°S and the longitude range 100–160°W (Table 2), which is also mainly concentrated in Oceania, Africa, South America and North America. Although Bd infection is mostly global, the distribution of fungus causing amphibian deaths has so far been limited to a few regions, mainly in Oceania, South America and North America (13, 44). Similarly, we found a high Bd prevalence in Australia, Brazil, Peru, Venezuela, Mexico, and the United States (21–49%; Supplementary Table 3). However, there were some areas where the prevalence of Bd was zero, mostly in Africa and Asia (Supplementary Table 3), which may be under-explored studies on the dynamics of amphibian chytridiomycosis (45). Or maybe frog populations in Africa have co-evolved with Bd to show resistance, even if the Bd was originated Africa (11, 46). When the elevation was between 1,700 and 5,000 m, the prevalence was significantly higher than that in the 0–100, 100–500, and 500–1,700 m groups (P < 0.05; Table 2). The joint analysis showed that regions explained 30.53% of heterogeneity in elevation subgroup (Table 2). These results may be due to the fact that different continents have different elevation characteristics. At high elevation areas, the prevalence of Bd increased and the survival rate of infected amphibians have been shown to be reduced (47, 48). In the entire Neotropical region, environmental conditions not only promote the abundance and diversity of amphibians, but also contribute to the growth and survival of Bd, especially in the cool and humid environments of many uplands (13). In central America, most amphibian declines occurred at elevations above 500 m (49). Since Bd is dependent on aquatic habitats (50) and intolerant to desiccation (51), higher levels of precipitation favor moist microhabitats and promote its growth and development (52, 53). The prevalence and infection intensity of chytridiomycosis are significantly correlated with temperature and rainfall in eastern Australia (54, 55). High environmental humidity may have promoted the growth of Bd, which is associated with high disease severity (56). In order to prevent the spread of Bd between various regions, we suggest that the domestic and international trade in amphibians should follow the veterinary supervision and quarantine guidelines issued by the IUCN and the OIE (43). According to the environmental conditions of specific locations and habitats, comprehensively evaluate the distribution, prevalence and infection intensity of Bd, and formulate more effective disease management strategies. In high-prevalence areas of Bd (more at risk compared to those in low-prevalence areas), more precautionary measures should be taken. These measures could include 1) closely stricter monitoring (increased intensity of supervisory personnel) the health of amphibians entering and exiting these areas and 2) continuously check the prevalence of Bd in amphibian habitats in these areas. If Bd is identified, fungicides could be used to control its spread (57).

For the seasonal subgroup, amphibians had the highest prevalence in spring (Table 1). The occurrence of chytridiomycosis has obvious seasonality (58). The fungal prevalence and infection intensity decreased from spring to summer within a population of frogs, which can clear fungal infections later in summer and at higher temperatures (59, 60). Bd is usually not randomly distributed in amphibian communities (47, 61), but rather is closely related to changes in microclimatic conditions. Therefore, we analyzed the most suitable conditions for Bd growth in combination with temperature and rainfall subgroups. Our results showed that amphibians were more susceptible to Bd infections at a temperature of 17–25°C (P < 0.05) and in rainfall of >150 mm (Table 2). On large geographic scales, temperatures and rainfall are the most fatal environmental factors affecting the occurrence of fungi, increasing the possibility of infection and transmission (10, 62). In view of the environmental scope of the above areas, we recommend multiple methods to control the spread of diseases such as Bd. Chemical methods are used in small epidemic areas, such as antifungal itraconazole, as well as environmental disinfectants (16, 63). Bio-augmentation methods have been proposed for more complex ecological environment, such as enhancing the amphibian cutaneous probiotic microorganisms (which can inhibit the growth of Bd) and vaccination to prevent fungal invasion (64).

In the 111 articles surveyed here, real-time PCR methods had a higher Bd prevalence (Table 1). Real-time PCR is more sensitive than traditional PCR and histopathology examination, with a higher and faster detection rate, making the results more objective rather than subjective (65). It can accurately detect Bd infection intensity in degraded samples or the low levels of organisms, increasing the detection of prevalence, and it is suitable for wildlife, laboratory animals, and the food trade as well as environmental samples (66). Therefore, most researchers prefer to use the real-time PCR method for the detection of Bd as early as possible. In addition, there are articles using nested PCR and Loop-mediated isothermal amplification (LAMP). Nested PCR and LAMP hhhimprove the sensitivity of traditional PCR, but are more prone to contamination (67, 68). Consequently, they have been rarely widely used. In fact, the use of appropriate detection methods is vital for helping relevant personnel obtain accurate testing data and better understand the epidemic status of a disease.

Among Bd infections in amphibians of different ages, the point estimate of juveniles was higher than the other three age ranges, but this was not significant (P = 0.0528; Table 1). The susceptibility to Bd was shown to vary at the different stages of life (61). The results have shown that the Bd prevalence in younger life stages was higher than in older life stages (larvae: 18.7%, juveniles: 40%, sub-adults: 5.5%; adults: 6.6%), and that juvenile stages had the highest prevalence of Bd (69). As frogs get older, Bd infection can be cleared (70). Fungi have mobile zoospores that have the ability to spread rapidly in water, which might promote tadpole infections (71), but tadpoles rarely died from these infections (51, 72). Thus, tadpoles might be the reservoirs for this disease and increase the activity of this Bd in the environment, leading to the infection of adult specimens (27, 73). Therefore, we recommend that relevant personnel pay attention to the detection Bd of tadpoles and juvenile amphibians, and control the fungal loads in environments where tadpoles live to reduce the prevalence.

Water sources can be one of the environmental reservoirs of Bd, because it can survive in water alone for several weeks (8, 71, 74). In a subgroup analysis of sample sources, the Bd prevalence of amphibians inhabiting streams was shown to be higher than in other places (Table 1). However, species living on roads had the lowest prevalence. Therefore, especially cooler and shaded lentic streams might be more conducive to Bd transmission among amphibians (75, 76). Thus, we analyzed the subgroups of different hosts, and frogs had the highest prevalence (P < 0.05; Table 1). There was a great difference in the degree of Bd infection between anurans (frogs and toads) and caudates (salamanders and newts) (72). According to statistical data, 36% of caudates are terrestrial, while only 1.7% of anurans are terrestrial (77). Therefore, species living longer in water were more susceptible to infection (78). In the light of the IUCN classification of amphibians, the prevalence of Bd in vulnerable species was the highest at 43.08% (Table 1). Many species were listed as threatened by the IUCN, because they were infected with Bd resulting in population declines (79). Therefore, we propose strengthening the protective measures for those habitats that are suitable for Bd growth, such as manipulating the habitat environment for temperature or humidity and for endangered species, captive breeding may be established to ensure populations.

In our research, there were 20 medium-quality articles and 91 high-quality articles (Table 1). In the medium quality articles, we found that some of them did not have random sampling, definite sampling time, sampling location or clear detection methods (Supplementary Table 4). Since these medium-quality articles did not provide detailed data, the potential risk factors of Bd infection cannot be accurately measured, leading to biased results. Thus, we recommend that researchers should find as many factors as possible influencing fungal infection, so as to more effectively prevent these diseases and lay scientific groundwork for future research work.

This study has many advantages, such as its rich sample size, comprehensive coverage, clear method of analyses and wide range of risk factor assessments, which provides many evidence-based substantive suggestions for combating Bd spread. However, this study has also some shortcomings. First, although corresponding search formula is applied to collect articles, all relevant studies could not be fully found. Second, there was no information about the susceptibility of a species to Bd infection, it could be a potentially important risk factor for the implementation of controls. Third, only 13 out of 111 studies included in the investigation reported explicitly using random sampling, but others could not be ruled out the possibility of random sampling, which could result in a slight publication bias in the funnel plots and Egger's test, even through no publication bias was detected in this study. Therefore, the future study design still need to be comprehensive, such as clearly disclosing their sampling details. However, we are convinced that our report can reflect the situation of chytridiomycosis spread worldwide.

Chytridiomycosis has a wide distribution range and affects various amphibian species at all ages. In the last decade, the prevalence of Bd has shown a decreasing trend, and temperature and elevation have significantly affected it. In addition, the use of real-time PCR methods could help researchers obtain a more accurate estimate of Bd prevalence in amphibians. Meanwhile, we thus should focus on highly sensitive species and those listed as threatened by the IUCN. Therefore, we suggest that governments should formulate a reasonable management based on different geographical conditions, habitat types and species status. This study helps further elucidate potential risk factors for Bd infection and also provides a theoretical basis for related personnel to prevent and control chytridiomycosis transmission.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

ZL performed the data extraction and drafted the manuscript. ZL and QW analyzed the data. KS and JF developed the paper concept, designed, supervised, and revised the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by the National Natural Science Foundation of China (grant numbers 31961123001, 32171525, and 31770403).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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