Challenges to PRRS virus management
Perhaps more than any other production disease, PRRS demonstrates the need for a step-by-step, systematic approach to control. The unique nature of the PRRS virus has forced producers to adjust some traditional assumptions about managing disease out of a herd or pig flow:
PRRS virus infection results in inconsistent exposure throughout the adult population. That inconsistency creates subpopulations that differ in infection/immune status.
Those subpopulations can co-exist in a herd for extended periods. Plus, animals can change from one subpopulation to another.
Field strains of PRRS virus appear to spread more readily or chronically circulate when carrier pigs shed virus to subpopulations of naïve pigs in the same population. Research shows that regularly introducing those naïve pigs into infected populations helps maintain the infection there.
The existence of those susceptible pockets is fundamental for maintaining chronic field virus circulation and persistent PRRS viral transmission from carrier animals. PRRS virus can also persist in infected sows and pigs, which allows it to cycle back to naïve contacts. Studies demonstrate PRRS can be transmitted from infected pigs to susceptible contacts for up to 86 and 99 days post infection, and it can persist beyond 100 days. Current data suggest that dealing with PRRS via herd closure would require a closure of at least 120 days.
Systematic PRRS control begins by using diagnostic investigations to determine where, and at what stage of production, the virus is actively circulating in a production system. Once the pattern of viral circulation is understood, intervention strategies can be tailored based on goals of the farm and risk factors that affect the potential success.
Dee S.A.; et. al. An update on the transmission and elimination of the PRRS virus. Proc. AASP, Feb 2001:191-192.
Issues
Will your control plans succeed?
Test your control plans against the hallmarks of a workable PRRS-management plan, according to Boehringer Ingelheim Vetmedica Technical Services Veterinarian Reid Philips:
- Is it a plan that can be implemented and executed on the farm?
- Is it easily enough executed to encourage everybody involved to comply with all steps?
- Will the results be consistent and sustainable?
- Does it return more money than it's projected to cost?
- Are all stakeholders in the plan in agreement about the optimal intervention methods?
- Will it enable you to measure and monitor progress, objectively evaluate possible new intervention methods as you move through plan execution, and allow for adjustments to the plan as needed?