Proceedings of the 10th International Colloquium on Paratuberculosis

Epidemiology Perspectives Lecture
Epidemiology, transmission and economics <strong>of</strong> Johne’s disease: state <strong>of</strong> knowledge,
knowledge gaps, and possible future directions
Ian Gardner
University <strong>of</strong> California-Davis, USA
Evidence-based medicine is predicated on <strong>the</strong> principle that risks and benefits <strong>of</strong> interventions can be explicitly
quantified through scientifically-valid studies. These studies can <strong>the</strong>n inform decisions about <strong>the</strong> economic
value <strong>of</strong> interventions whe<strong>the</strong>r for a single herd or flock, or for national and regional programs. Current
knowledge indicates that <strong>the</strong>re are multiple sources <strong>of</strong> MAP infection and consequently, <strong>the</strong> recommendation
lists for producers to control Johne’s disease are long and <strong>of</strong>ten unprioritized. There are few published
randomized controlled trials (RCT) or cohort studies assessing <strong>the</strong> effects, let alone <strong>the</strong> cost-benefits,
<strong>of</strong> interventions. Evidence from <strong>the</strong> U.S. Johne’s Disease Demonstration Herd Project and experiences
elsewhere indicate that effective control is possible, but eradication is unlikely in intensive production systems
such as large dairy herds. Data on <strong>the</strong> benefit-cost ratio for individual or multiple interventions or entire control
program are sparse. This is a critical need for this information for dairy herds because Johne’s disease is
typically not a high priority disease from a financial perspective. This may change with increasing pressure
from public health authorities to mitigate risks from dairy and meat products. Any expectation that interventions
that are applied on farm or along <strong>the</strong> production chain can ever be linked to a reduction in Crohn’s disease
incidence is very unrealistic.
Investigation <strong>of</strong> <strong>the</strong> epidemiology, transmission and economics <strong>of</strong> Johne’s disease <strong>of</strong>fers numerous challenges,
which can be broadly categorized as biological, financial and methodological. I would like to focus on
<strong>the</strong> latter and <strong>of</strong>fer some perspectives on <strong>the</strong> future. First, <strong>the</strong>re has been inadequate use <strong>of</strong> longitudinal
designs, especially RCT, to evaluate interventions and as a consequence, <strong>the</strong>re are no systematic reviews
<strong>of</strong> <strong>the</strong> effects and economics <strong>of</strong> interventions. Cost and feasibility are <strong>the</strong> key limitations. Second, <strong>the</strong>
lack <strong>of</strong> uniformly-applied quality standards for publication <strong>of</strong> risk factor studies is symptomatic <strong>of</strong> <strong>the</strong> reality
that we lag behind our human health colleagues (e.g. CONSORT and STROBE in human health). Casecontrol
and cross-sectional studies have outlived <strong>the</strong>ir usefulness for bovine and ovine paratuberculosis.
Third, statistical methods exist to incorporate prior knowledge into risk factor analyses and adjust for nondifferential
misclassification <strong>of</strong> <strong>the</strong> outcome (e.g. ELISA, culture or PCR-based test results) but <strong>the</strong>se methods
are infrequently used. The challenge in <strong>the</strong> next 5 years is to design, finance and implement well-planned
longitudinal studies to answer critical research questions through networks and programs such as JDIP, JDRC
and ParaTBTools.
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