Swine Resp Disease Epidemiology and Economics

The flow of swine production goes from sow farm -> nursery -> finisher. Where do we see the most respiratory disease and why?

Nursery and finisher

Mixing and maternal antibody loss

Infectious dose (ID50)

The dose producing infection in 50% of exposed hosts

Infectious vs contagious

Infectious - capable of transmission at all

Contagious - high combined probability of the whole transmission cascade succeeding

Prevalence risk (formula)

Number of positive individuals divided by total number in population

Incidence risk (formula)

Number of newly positive individuals divided by number at risk at start

Incidence rate (formula)

Number of newly positive individuals divided by accumulated time at risk

Relationship between prevalence and incidence in stable populations

Prevalence ≈ incidence rate × average duration of disease

As incidence increases, what happens to transmission rate?

It increases

If case numbers are increasing, are we on the early or late side of transmission?

Early

Sporadic disease pattern

Random, clustered disease episodes occurring at variable, unpredictable intervals

Endemic disease pattern

Stable disease prevalence with a predictable incidence rate over time

Can still be high prevalence

Epidemic disease pattern

Incidence rate rising significantly above the expected baseline level, often from a change in virulence, ecology, or a novel agent

Vector borne diseases

Absolutely requires an animate vector to move between hosts

Examples of vector borne diseases

Japanese encephalitis virus

Vesicular stomatitis virus

How do we control/eradicate vector borne diseases?

Control the vector - disease is eradicable in principle, but practically difficult

Short-cycle pathogens

Pathogens causing rapid shedding and immunity, have a short infectious period, and requiring large susceptible host populations

Tend to become epidemic and then die out

Short-cycle swine pathogen examples

Influenza A virus

Porcine Respiratory Coronavirus

TGEV

PEDV

Control strategy for short-cycle swine pathogens

Establish closed or All-In-All-Out populations to remove susceptible host supply

Eradication is achievable

Long-cycle pathogens (Definition)

Pathogens establishing persistent infection where hosts remain contagious long-term

Often vertical transmission

Tend to become endemic

Long-cycle swine pathogen examples

PRRSV

M. hyopneumoniae

APP

Aujeszky's disease virus

Lawsonia intracellularis

Leptospira

Brachyspira

Erysipelothrix

Control strategy for long-cycle swine pathogens

Herd depopulation or whole-herd test-and-removal with DIVA diagnostics

Resistant pathogens

Pathogens forming environmentally stable stages (spore, ova) independent of host population density

Resistant swine pathogen examples

Bacillus anthracis

Clostridium perfringens

Ascaris suum ova

Control implication for resistant pathogens

Site eradication is nearly unachievable once environmental contamination occurs - environmental persistence for years

Commensal pathogens

Normally harmless flora causing disease only in the presence of cofactors or stressors

Commensal swine pathogen examples

PCV2

Haemophilus parasuis

Streptococcus suis

Staphylococcus hyicus

Pathogenic E. coli

Control strategy for commensal swine pathogens

Manage stress cofactors and vaccinate

Elimination from population is unfeasible

HARDEST TO CONTROL

Necessary Cause

A factor that must be present for the disease to occur, but may not act alone

Sufficient Cause

A combination of factors that inevitably produces disease in exposed individuals

For short cycle or vector borne pathogens, what control strategy is often the more economical choice in the long run?

Elimination