Disease Surveillance Flashcards
Recognise the goals for, and purpose of, wildlife disease surveillance schemes
Conservation: • They can impact population numbers • They may lead to extinction risk • A key factor of translocations • There are emerging threats • Illegal trade / persecution
Ecosystem health:
• Climate change is exacerbating problems
• Chemical contamination
Wild animal welfare:
• Anthropogenic causes?
• Large numbers affected
Human health
• Wildlife can be a source of human infection
• E.g. West Nile Virus carried by mosquitoes
Protecting domestic animals
• Wildlife can be a source of domestic animal infection
• E.g. foot and mouth from buffalo to cattle
• Effects trade
Critically evaluate the methods of detecting diseased wild animals
- Routine submission to laboratories
- Examinations of dead animals
- Detection dependent on volunteers – issues
- Bias (convenience sampling is not random) - motivation, human-animal distribution, small population size, size of species, behaviour, disease progression, vast majority not found
Critically evaluate the information that can be gathered from wildlife disease surveillance
Detect new entities, monitor endemic disease spatially and temporally, confirming presence or absence, early detection of pathogens, find results of an intervention
HOWEVER: Cannot infer disease impact, difficult to determine prevalence or incidence
BUT: Improving technology
Critically appraise a plan for a new, or revised, wildlife disease surveillance scheme
AN IDEAL SCHEME:
• investigate all Classes of animals
• clinical and post-mortem investigations
• species standard protocols for analyses
• database
• team of trained professionals
• extensive service laboratories
• mobile reactive investigation team
• links with research on ecology of disease
CONCLUSIONS
• Non-random, convenience, biased samples require careful interpretation
• Surveillance provides information on new disease entities and temporal and spatial changes in existing diseases
Assess the potential benefits and limitations of citizen science reporting networks for wildlife disease surveillance and give examples of how these methods can be optimised.
Benefits to public:
• Science education
• Engagement with nature
• Optimise habitat management for disease prevention
Benefits to science:
• Ability to undertake large-scale surveillance
Opportunistic schemes:
Ad hoc reports of morbidity and mortality & Ad hoc post-mortem examinations
• Variable temporal and geographical observer effort
• Relative over-reporting of charismatic species
• Detection bias – body size, colour/cryptic, habitat type
• Bias towards incidents of “unknown” cause
• Reporting after the event – reduced chance of obtaining meaningful information/ samples
• Unknown impact of disease on wildlife populations
• Limited conclusions
Minimise through expert verification of data
Describe the meaning of the term ‘emerging infectious disease’ (EID), being classified as disease caused by a novel pathogen, or a known pathogen in a new host, location or at increasing occurrence.
Emerging infectious diseases (EIDs) - ‘diseases caused by novel pathogens or known pathogens affecting new host populations, with increased incidence or extended geographic range’
They represent a threat to geographically restricted and critically endangered species as well as common and widely distributes species
Recognise that EIDs have the potential to cause rapid declines of common wildlife species over a wide geographical area within a short time frame and be able to illustrate this understanding with specific wild bird examples.
Garden bird disease
• Parasitic – Trichomonas gallinae (canker)
• Viral – Avian poxvirus
• Bacterial – Salmonella Typhimurium
Explain how the population impact of an EID might be quantified in principle, for example through integrated analysis of long-term population monitoring datasets and disease surveillance.
> Opportunistic surveillance:
• Ad hoc reporting
• Maximum potential observer coverage
• Ability to detect novel incidents
> Systematic surveillance: • BTO Garden BirdWatch • Weekly monitoring year-round • Species distribution & abundance • Observation of sick/ dead birds
> PM examinations: • Scanning (or general) surveillance rather than targeted • Infectious & non-infectious disease • Standardised examination protocols • Case definitions • Incident definitions • Passive reporting method
Define diagnostic test characteristics (sensitivity and specificity)
Sensitivity:
• Ability of a test to correctly detect diseased individuals
• Proportion of diseased individuals that test positive
• “If an animal has the disease, what is the probability that it will test positive?”
Specificity:
• Ability of a test to correctly detect non-diseased individuals
• Proportion of non-diseased individuals that test negative
• “If an animal does not have the disease, what is the probability that it will test negative?”
Explain the effect of imperfect tests on prevalence estimations
sensitivity and specificity need to be incorporated into the calculation
Overestimation and underestimation
Explain the effect of prevalence on test accuracy
Changes in prevalence influence the extent of overestimation due to imperfect reference standard classification
Diagnostic accuracy is affected by the disease prevalence. With the same sensitivity and specificity, the diagnostic accuracy of a particular test increases as the disease prevalence decreases.
Explain how different diagnostic tests can be combined to improve accuracy
Can:
Interpret in parallel: animal considered to have the disease if any of the tests are positive
- Disease is less likely to be missed but false positives are more likely
- Se increases
- Sp decreases
- PPV decreases
- NPV increases
Interpret in series: animal considered to have disease if all tests are positive
- Disease is more likely to be missed but false positives are less likely - Se decreases - Sp increases - PPV increases - NPV decreases
Amphibians can suffer from the same range of types of infectious diseases as other taxa:
Diseases caused by viral, bacterial, fungal, protozoan, metazoan pathogens.
Key pathogens of amphibians include:
Batrachochytrium spp. chytrid fungi, ranaviruses, Pseudocapillaroides xenopi, Ribeiroia ondatrae, SPI
Ranaviral disease can cause:
Explosive outbreaks of fatal disease characterised by internal haemorrhaging of adult or larval amphibians or can cause longer-term lower incidence outbreaks characterised by skin ulceration and limb necrosis.
only asscoiated with pop declines in 2 cases (uk and spain)
climate change likely to exacrebate outbreaks
Batrachochytrium dendrobatidis, Batrachochytrium salamandrivorans and ranaviruses are the only pathogens known to:
Cause long-term amphibian population declines; B. dendrobatidis is by far the most important infectious cause of amphibian declines globally
How to diagnose chytridiomycosis and B. dendrobatidis infection (and being able to differentiate between the two).
A diagnosis of chytridiomycosis rather than solely Bd presence includes consideration of whether clinical signs are present. Laboratory examination of carcasses is recommended where skin swabs from the ventral abdomen, thighs, and digits, alongside skin samples of the ventral abdomen are collected to carry out a duplex real-time PCR test (qPCR) that can detect and differentiate both Bd ad Bsal.
Dissecting toe tips and multiple small rectangles from the ventral abdomen of each carcass and staining allows for histological examination. Identification of sporangia within the tissues under a microscope can diagnose chytridiomycosis rather than simply identifying a Bd infection as a qPCR resembles. Histology also shows inactive infections if empty sporangia are observed, which can aid understanding of disease transmission in terms of spatio-temporal patterns. Classifying the severity of chytridiomycosis via histology allows one to determine whether it was the cause of mortality rather than toxins or other environmental pressures.
true diagnosis of chytridiomycosis disease requires histopathologic examination of tissues
The ecology of B. dendrobatidis and the drivers of B. dendrobatidis emergence
non-hyphal parasitic chytrid fungus
Two hypotheses:
1. Endemic disease. Emerged due to global changes (UV-B, climate, pollution) increasing virulence or decreasing host immunity
- Panzootic. Emerging due to anthropogenic introduction
Bd infection confirmed in:
• Pet trade (e.g. dendrobatid frogs)
• Food trade e.g. bullfrog farms in Uruguay & Brazil (> 1 million p.a. enter USA)
• Lab animal trade (Xenopus spp.)
• Zoo animal trade
• Introduced species e.g. bullfrogs, alpine newts
• Food trade (> 1 million bullfrogs p.a. USA)
• US official trade > 5 million live amphibians imported p.a. (majority wild caught)
• > 2500 tons of frog legs exported annually from China
Bsal is spread through the amphibian trade
Thermal Preference for bd and bsal
- B. dendrobatidis - 17-25 oC
* B. salamandrivorans - 10-20 oC
Host Range for bd and bsal
- B. dendrobatidis - possibly all amphibia
* B. salamandrivorans - newts and salamanders (& some anurans)
Distribution for bd and bsal
- B. dendrobatidis global - wherever there are amphibians
- B. salamandrivorans - SE Asia, Belgium, The Netherlands, Germany & Spain (so far)
- B. dendrobatidis - transmission only via motile zoospores
- B. salamandrivorans - motile zoospores and encysted spores
