Introduction to MHASpread

Welcome! This vignette introduces the core concepts and philosophy of MHASpread without requiring technical expertise.

What is MHASpread?

MHASpread is a computer model that simulates how foot-and-mouth disease (FMD) spreads across livestock farms and evaluates disease control strategies.

Why Should You Care?

If you work in:

Veterinary epidemiology: Understand disease dynamics and control effectiveness
Livestock policy: Evaluate trade-offs between strategies (vaccination vs. culling)
Emergency response: Plan preparedness for potential FMD entry
International trade: Assess disease risk and surveillance requirements

Then MHASpread can help you make evidence-based decisions.

The Disease & The Challenge

What is FMD?

Foot-and-mouth disease is a:

Highly contagious viral infection of cattle, swine, sheep
Major trade barrier: Infected countries face export restrictions
Economic threat: Single outbreak can cost millions in lost production
Global problem: Endemic in Africa & Asia; sporadic in Americas

Why is FMD Hard to Control?

Multi-species spread: Cattle ↔ Swine ↔ Sheep transmission varies
Fast dynamics: Animals become infectious within days
Hidden spread: Subclinical infection means farms don’t “look sick”
Network complexity: Animal trade creates long-distance jumps
Limited resources: Depopulation, vaccination capacity finite

How MHASpread Helps

Core Questions MHASpread Answers

“How big will an outbreak become?” → Model estimates farm infections under different scenarios
“How fast will it spread?” → Spatial & network transmission rates quantified
“Which control strategy works best?” → Compare depopulation vs. vaccination vs. combinations
“Is our response capacity enough?” → Test if depopulation/vaccination speed sufficient
“What does control cost?” → Economic models integrate epidemiological outcomes

What MHASpread Doesn’t Do

Predict when an outbreak will occur (stochastic timing not modeled)
Represent multi-species competition or co-infection
Model long-term herd dynamics (focuses on acute phase)
Incorporate human behavior/psychology beyond protocol compliance

Key Concepts (Non-Technical)

The Compartmental Model

MHASpread tracks where every animal is in its disease journey:

Susceptible → Exposed → Infectious → Recovered
                ↓
            Vaccinated (stays protected)
                ↓
            Depopulated (removed from farm)

At each farm, every day: Some susceptible animals become infected; infected animals either recover or die.

Why Multiple Host Species?

The Problem: Swine and cattle behave differently:

Swine: Shed virus 10–100× more than cattle, confines tightly → rapid transmission
Cattle: Lower shedding, often pasture-based → slower spread
Sheep: Lowest transmitters → least amplification

A mixed cattle-swine farm is a “danger zone”—swine amplify infection rapidly.

Spatial Transmission (“Invisible Spread”)

Disease spreads from one farm to neighboring farms through:

Environmental sources: Wind-blown virus, contaminated equipment
Fence-line contact: Animals near boundaries
Animal movements: Trade shipments (most important long-distance pathway)

Key insight: Nearby farms at risk even if no direct contact.

Control Zones

Authorities draw three circles around detected infected farms:

              ← 15 km (Surveillance Zone)
         ← 7 km (Buffer Zone)
    ← 3 km (Infected Zone)
       [INFECTED FARM]

Infected zone: Cull all animals on infected farms
Buffer zone: Vaccinate susceptible cattle
Surveillance zone: Actively monitor for new cases

The Outbreak Scenario

Timeline: A Typical MHASpread Simulation

Day 0: Single farm infected with FMD (index case; unknown to authorities)

Days 1–20 (“Silent Spread”):

Infection spreads within farm, then to neighbors via kernel
May reach 10–50 farms before detection
Meanwhile, animal trade continues unimpeded

Day ~21 (“Detection”):

Normal surveillance detects first clinical case
Authorities establish control zones

Days 22–30 (“Emergency Response”):

Depopulation begins (limited capacity)
Vaccination mobilized (15–20 day delay to organize)
Movement standstill enforced
Contact tracing identifies linked farms

Days 30–60 (“Active Control”):

Steady depopulation of infected farms
Vaccination progresses in susceptible zones
New infections decrease if control effective

Day 60+ (“Resolution”):

No new infections
Remaining infectious animals recovered/removed
Trade restrictions lifted
Herd rebuild begins

Decision Points: Evaluating Strategies

Question: Should We Vaccinate or Depopulate?

Classic trade-off:

Strategy	Pros	Cons
Depopulation	Fast; permanent; no disease risk	Animal welfare; expensive; market disruption
Vaccination	Preserves herds; faster recovery	Leaves vaccinated animals (vaccination needed periodically); higher disease risk
Combination	Speed + herd preservation	Most expensive

MHASpread finding (Brazil case): Combination slightly more cost-effective (~€500 vs. €600 per farm protected).

Question: How Capacity Should We Invest In?

Resource allocation:

Low capacity (1 farm/day depopulation): Slow response, outbreaks large
Moderate capacity (3–5 farms/day): Balances cost & effectiveness
High capacity (10+ farms/day): Expensive but marginal improvement

MHASpread analysis: Diminishing returns at high capacity (investment may not justify benefit).

Real-World Application Examples

Example 1: Brazil’s 2000–2001 FMD Outbreak

Context: Rio Grande do Sul state, mixed cattle-swine farming
Outcome: 2,000+ farms affected, $100 million+ in losses

What-if simulation with MHASpread:

With faster depopulation (5 farms/day vs. actual ~1–2): Outbreak would have been 50% smaller
Addition of emergency vaccination: Could have prevented additional 200 farms

Lesson: Surge capacity important; preparedness saves money.

Example 2: Bolivia Risk Assessment (2023)

Context: Mixed cattle-llama system, resource-limited

Scenario tested: “What if 1 infected farm enters Bolivia from Argentina?”

MHASpread result:

Without control: 100+ farms affected in 3 months
With realistic capacity (2 farms/day depopulation): 15–20 farms affected
Cost of control (~€300k) far less than uncontrolled outbreak (~€5–10 million)

Conclusion: Justified investment in surveillance and response infrastructure.

Key Takeaways

FMD spreads fast: ~20 day “silent spread” creates large undetected outbreaks
Early detection critical: Reduces number of infected farms at control start
No perfect control: Depopulation fastest but costly; vaccination preserves herds but riskier
Capacity matters: Doubling depopulation speed can halve outbreak size
Context varies: Brazil ≠ Bolivia ≠ Chile; customized strategies needed

What’s Next?

Explore MHASpread in more depth:

Want to understand the model? → Read Model Structure Vignette
Ready to learn about transmission? → Transmission Processes Vignette
Interested in control strategies? → Control Strategies Vignette
Want to see real examples? → Case Studies Vignette
Ready to run simulations? → Consult example_script.md and Data Requirements

Glossary (Quick Reference)

SEIR: Susceptible-Exposed-Infectious-Recovered (disease stages)
β (beta): Transmission rate (infected-to-susceptible encounters per day)
Latent period: Days between infection and becoming infectious
Infectious period: Days an animal sheds virus
Kernel: Mathematical function describing transmission vs. distance
Metapopulation: Network of farms as interconnected units
Stochastic: Incorporating randomness (outcomes vary between runs)
Depopulation: Culling all animals on infected farms
Vaccination: Emergency immunization of susceptible animals
Standstill: Ban on animal movements
Zone: Geographic region with specified control intensity