Mass vaccination for the control of PRRSV

The potential value an MLV appears to bring includes reducing shedding of resident virus to reduce risk of infection in gilt entries. 


Article by:

David Smidt

Farms we manage have wrestled with PRRSV control like nearly every production system in the U.S. Part of our strategy for control of PRRSV is the strategic use of Ingelvac PRRS MLV vaccine. The potential value an MLV appears to bring includes reducing shedding of resident virus to reduce risk of infection in gilt entries. While we would like to think use of MLV would magically control all PRRSV challenges, it is apparent that MLV is simply a “tool” to assist in control of PRRS in breeding herds.

PRRS control objectives

Our production team has wrestled with our PRRS con- trol objectives. Do we eliminate the virus? CAN we eliminate the virus given the density of pork produc- tion in NW Iowa? If PRRSV is eliminated, can we maintain a negative status? After serious debate, our primary objective is herd stabilization. If we prove we can accomplish this, then elimination can be pursued.

Expectations of MLV vaccine in our PRRS control program

1. Sow Herd Stability
a. Reduce reproductive losses
b. Produce PRRSV negative weaned pigs

2. Assist in elimination of PRRSV shed from field positive breeding gilts

If one is to state one of the expectations of MLV is to assist in sow herd stability, how does one know the herd is stabile?

We have defined stability as the “demonstrated ability to move PRRS negative pigs from the nursery to the finisher (8-10 weeks of age)”.

1. In our experience suckling pigs PCR negative 4 weeks after second whole herd mass-vaccination (2× mass vaccination 30 days apart).

2. Pigs exiting the nursery (42-49 days in nursery) PRRSV negative; all nurseries routinely bled to determine Elisa negative. All Elisa positive animals are pooled for PCR and sequencing plus RFLP cut test.

3. Finishing pigs 14-16 weeks of age and prior to sale are also monitored at 6-week intervals. 


Use of MLV in stabilizing the sow herd

Why use MLV?

Pro Pork began using the vaccine shortly after introduc- tion of the “tool” in the mid 1990’s. Initial use of the vaccine was limited to use in open animals only and evolved to use quarterly in all animals to its present use as a seasonal tool.

Rationale for using MLV

1. Belief the product was safe
2. Homogeneous immunity is critical
3. Use of MLV reduces shedding of resident field virus
4. Intuitively, we felt it important to reduce field 
virus, not propagate field virus
5. Use of live virus laden serum was not vogue until later in the evolution of control measures 6. Current concern for liability

Current use of Ingelvac PRRS MLV

1. After extensive gilt Elisa monitoring, at gilt selec- tion and 30 days later (30 days before sow herd entry)

2. Seasonal use within the sow herd to anticipate seasonal risks

a. Mid-September b. Mid-October c. April

Past experience with PRRSV outbreaks are associated with seasonal herd stressors and an increased probability of virus survival in cooler temperatures.

Management principles to aid in PRRSV control

Stabilizing the sow herd through use of MLV vaccine does not happen in a vacuum. Additional management “tools” must also be used. It is clear to all that the most important principles in PRRSV control include:

1. Avoid offering negative naïve animals to a popula- tion potentially shedding virus, therefore vaccinate all animals to reduce risk of shedding

2. During times of high risk of virus shed, avoid introduction of animals 

3. Make all attempts to eliminate introduction of new virus strains through “rock solid biosecurity” 

Managing gilt introductions 
1. Received as PRRSV negative isowean pigs, confirmed negative with nursery entry and nursery exit Elisa

2. Monitored Elisa negative at 8 weeks and upon selection at 25 weeks of age

3. In conventional flows, if positive to field virus, dump animals (not happened)

4. Vaccinate with MLV when determined negative

5. Historically moved to centralized breeding for 
movement to smaller farms (now all to on-farm isolation)

6. Moved to on-farm isolation at larger sow farms

7. Introduction is delayed and gilts diverted to off-site 
breeding if there is evidence of sow herd instability to avoid flare up 

Use of MLV in event of a clinical break

As stated, we believe MLV is effective in reduction of shedding resident virus. However, demonstration of protection against newly introduced strains is limited. In event of an outbreak, cases have been managed in the following manner.

1. Herd is closed and is vaccinated twice 30 days apart
2. Gilts are taken off-site to be bred
3. 60 days following second vaccination, piglets are 
PCR negative
4. Gilts are allowed entry into the herd 100 days post second vaccination

Farm results Flow A

Flow A is a commingle system of Farm 1 and Farm 2

Nursery has been monitored negative (all Elisa positive pigs have been PCR negative) See Figure 1.

Farm 1

1. < 0.25 miles from Farm 2
2. 1300 sows
3. Historically received gilts from Gilt Breeder; now 
breed on-site
4. Stabile since 1998 See Figure 2.

Farm 2

1. 800 sows
2. Receives gilts from Gilt Breeder 

3. June 2004 is first break since 1998 4. RFLP Cut 1-8-4 (BI 88873). See Figure 3.

Flow B

Farms 3, 4, 5, 6

1. 2400 total sows
2. Receives gilts from Gilt Breeder
3. 2 farms have had 1 break each over 5 years.  
a. RFLP Cut 1-?-? (BI 66395) 5/04.  b. RFLP Cut 1-8-4 (BI 56289) 2/03
4. Positive pigs diverted from commingled flow

Flow/farm C

1. 3300 sows

2. Receives gilts directly into isolation from Gilt 

3. 3 clinical breaks 6 years

4. Mass vaccination upon first note of break is 
believed to reduce length of clinical episode, however no way to do controlled study

5. Concern is that each break has resulted in isolation of new strain of PRRSV leading one to question biosecurity compliance

Flow D

1. 900 niche product sows
2. Gilts

a. Reared within the finishing system

b. Gilts are sporadically field virus positive

3. 1 clinical break in 3 years

a. RFLP 1-8-3 (BI 56614) 1/03. See Figure 5

4. Biggest problem is production of only 250-275 pigs/week disallowing sites being taken down
5. PRRSV had become endemic in finishing

6. Use of “2× vaccination and 60 day closure” has allowed introduction of naïve animals without acquiring infection
7. 2 sites negative Elisa; 2 additional sites doing 2× again including a sentinel at 45 days post second vx. 

Turn Event Mortality
1 Pre-clinical PRRS 3.3%
2 Clinical PRRS 17.5%
3 Subsequent continuous fill 9.4%
4 Post 2x vaccination 4.7%


1. Ingelvac PRRS MLV has served as an effective tool in “homogenizing” the immunity in the majority of farms.

2. Use of MLV appears to reduce risk of shedding resident field virus to MLV vaccinated, field virus negative gilts.

3. Seasonal 2× vaccination followed by 60 day closure in large sow farms lessens the risk of introduction of naïve animals to those farms.

4. Vaccinated farms receiving MLV vaccinated gilts from the gilt breeder have experienced least frequent PRRSV breaks (cause and effect?)

5. Factors, not limited to herd size, have likely contributed to limiting the effectiveness of MLV in the largest farms.

6. MLV has demonstrated limited success in protect- ing animals from infection from newly introduced field virus (sterilizing immunity) thus heightening the need to practice strict biosecurity.

7. Diligent monitoring of nurseries with both Elisa and PCR technology may allow anticipation of dealing with sow farm PRRSV activity. 

8. 2× vaccination of field virus positive gilts in finishers followed by a 60-day closure allows newly introduced naïve gilts to remain negative within those finishers through reduced risk of infection. 


1. Dee SA, Philips R. Using vaccination and unidirectional pig flow to control PRRSV transmission. Swine Health and Production. 1998; 6(1):21–25.

2. Gillespie TG. Methods of control and elimination of porcine reproduc- tive and respiratory syndrome virus using modified live vaccine in a two- site production system. Swine Health and Production. 2003; 11(6):291– 294.

3. Sanford SE. Production of PRRS Virus-free pigs from vaccinated stable PRRSV-Seropostive sow herds. International Pigletter. March 2002.