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Researchers monitor amphibian populations for chytrid fungus by pairing repeated population surveys with pathogen sampling. They may capture amphibians, collect individual skin swabs using contamination-control procedures, and test extracted DNA for Batrachochytrium dendrobatidis (Bd), B. salamandrivorans (Bsal), or both using real-time PCR. To assess effects on populations, they interpret infection results alongside abundance, survival, life stage, habitat, and environmental conditions. A positive test shows pathogen detection in a sample; it does not, by itself, show that a population is declining or establish why.
What researchers are trying to measure
Population monitoring and pathogen surveillance are related but distinct tasks. A population survey may track whether a species occupies a site, how many animals are present, survival, or change over time. Pathogen surveillance asks whether infection is detected and how its prevalence or pathogen load varies among animals, places, or sampling periods.
Researchers can link these kinds of observations to investigate possible population effects. For example, USGS describes research relating Bd prevalence and pathogen load to environmental stressors and green treefrog survival (USGS research on chytrid fungus). Infection prevalence is the proportion of tested animals that test positive in a defined sample; it is not a measure of mortality, abundance, or population decline.
How a monitoring program works
1. Define the question and survey design
Researchers first decide whether they need to estimate occurrence, abundance, survival, infection prevalence, pathogen load, or a combination. They then choose sites, sampling periods, host species, and life stages to match that question. Repeated observations are important when the goal is to compare sites or track change over time.
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2. Select sites and timing
Site selection may be broad and designed for comparison, or targeted toward a particular threat. For emerging Bsal surveillance, USGS describes county-level risk assessments that consider fungal ecology, trade-related information, salamander diversity, mitigation, and prior surveillance (USGS Bsal risk assessment).
A 2016 USGS workshop report identified early detection opportunities at key import locations, among high-risk wild populations, and through archived samples (USGS workshop report on Bsal surveillance). Sampling date matters too: pathogen prevalence can vary across seasons and among life stages, so results need to be interpreted in light of when and which animals were sampled.
3. Collect population and pathogen observations
Field teams may capture amphibians by hand, traps, or nets, then sample each animal individually. In a USGS national survey, amphibians were swabbed using methods intended to prevent contamination between samples; extracted DNA was tested with real-time TaqMan PCR for both Bd and Bsal (USGS amphibian chytrid fungus surveillance).
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The appropriate sample depends on the study. USGS work also includes non-lethal tissue samples, soil and water samples, and environmental measurements. These are different forms of evidence and should not be treated as interchangeable: the sampling method and assay need to fit the pathogen, host, and research question.
4. Test samples and report what was detected
For a molecular test, a laboratory extracts DNA from the collected material and uses an assay such as real-time PCR to look for targeted pathogen DNA. Reports should identify the pathogen tested, host species and life stages, sampling location and date, sample type, assay, and relevant details of the survey design. A result described as “not detected” means that the target was not detected in the tested material under those conditions; it does not establish that the pathogen is absent from every animal or location.
5. Interpret infection alongside population evidence
Researchers compare infection measures with population observations and relevant ecological information, such as survival, life stage, habitat, and environmental conditions. A positive pathogen result documents detection in a sample, but demonstrating a population effect requires additional evidence about demographic outcomes and how they change over time.
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Why Bd and Bsal must be distinguished
Bd and Bsal are separate amphibian chytrid fungi. Bd is associated with amphibian chytridiomycosis; Bsal is an emerging chytrid threat focused particularly on salamanders. A study may test for one or both, so “chytrid detected” is not sufficiently specific when reporting results. The pathogen name and sampled host should be stated together.
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Why sampling context changes the result
Prevalence is specific to the animals and dates included in a survey. In a study of boreal toads in southeast Alaska and northwest British Columbia, Bd prevalence peaked in June. Adult prevalence was 37.5%, compared with 11.2% among juveniles and metamorphs; these are study-specific findings, not general rates for amphibians (Hossack et al., 2020).
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That example shows why a prevalence figure without its host, life stage, place, and sampling period can mislead. A survey weighted toward adults, juveniles, or a particular season may report a different prevalence from one with another sampling composition, even within the same broader region.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What national Bsal surveillance results do—and do not—show
As reported by the USGS National Wildlife Health Center in January 2026, its surveillance program had tested more than 15,000 samples from 39 U.S. states and over 55 species since 2016, with no Bsal detected in that program (USGS Bsal risk assessment). This describes the program’s reported samples, geography, and period. It does not establish global absence or guarantee that Bsal will not be detected in later samples.
Choosing a field-survey reference
For general amphibian population survey methods, USGS’s record for Measuring and Monitoring Biological Diversity: Standard Methods for Amphibians describes ten standard sampling procedures, including implementation protocols, equipment and personnel needs, and data-analysis suggestions (USGS standard methods for amphibians). It is a broad field-methods reference, not a diagnostic manual for Bd or Bsal testing.
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