Contact tracing the best intervention for controlling swine disease spread

Man with pigs

By Rob Hannam, Farm Health Guardian

Quickly controlling disease spread is a key priority in the event of a suspected outbreak, especially for foreign animal diseases (FADs) that may lead to quarantining of farms and movement restrictions. When implementing any quarantine strategy, other important considerations include animal welfare, minimizing impact on the uninfected premises and on business continuity. The best intervention is one that will control the disease, get the industry moving in the shortest length of time and will be very targeted so that fewer properties need to be quarantined. Avoiding quarantines on farms that may not be needed is a real benefit in keeping the industry operating.

A recent paper by Machado et al utilized mathematical modeling to show the effectiveness of different control strategies on African swine fever (ASF). The study uses real animal movement data to simulate the propagation of two ASF strains among more than 13,000 pig farms. The researchers showed that contact tracing was the best intervention compared to any other control strategies in the model. Contact tracing resulted in 95% disease control, which was higher than control from radius-based quarantine zones, in addition, 15-day contract tracing also resulted in far fewer properties needing to be quarantined.

The Machado study considered eight scenarios, including:

  • 15 km radius quarantine zone around the infected property
  • 15 km radius zone plus a system-wide quarantine of all farms within the same pig producing company/production network
  • 15-day contact trace

Of all the scenarios tested, contact tracing always resulted in the best control from both a disease control and an economic standpoint as shown in the table below:

Control strategy # infected farms quarantined # healthy farms quarantined % control of transmission
15 km quarantine around index farm 14 63 85%
15 km quarantine + system-wide quarantine of all farms in the same company* 16 2100 89%
15 days of contact tracing 18 12 95%
30 days of contact tracing 18 28 99%

* The system-wide quarantine includes all farms within the same pig producing company/production network (not just those within the 15 km quarantine zone).

Combining contact tracing with a 15 km quarantine zone plus system-wide quarantine, did not show any significant increase in control of transmission compared with contact tracing on its own, but it did result in 2100 healthy farms being quarantined unnecessarily. The contact trace strategy targeting infected farms had the lowest number of quarantined farms compared with all the other strategies modelled. The model predicted that a 15-day contact trace would quarantine less than 1% of uninfected farms, while 15 km ring-based quarantine would majorly increase the direct cost of the disease through the unnecessary quarantining of uninfected farms. Furthermore, “ring quarantine zones regardless of size (i.e., 5 km, 15 km) were outperformed by backward animal movement tracking,” according to Dr. Machado.

“Ring quarantine zones regardless of size (i.e., 5km, 15km) were outperformed by backward animal movement tracking.”

– Dr. Machado

Since contact tracing has the most effective control and the least negative impact on uninfected farms, this study confirms it should be the chosen strategy for foreign animal disease control. Contact tracing is more targeted than ring quarantine zones alone and as a result, will have the least negative economic and animal welfare impact.

This study gives us important information by measuring the effectiveness as well as the unintended impacts of disease control strategies. Let’s ensure these learnings are given due consideration now, so that they are put into action for the next disease intervention.

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