What are the differences between type 1 and type 2 PRRSV vaccines currently available with regards to immunogenecity in sow herds?


Answered by: Tomasz Stadejek 

Published on: April 28, 2016

We should remember that the protection means different things for virologists and practitioners and producers. The best is when immunization protects completely against the infection. This is rarely achieved in the field conditions.What we are aiming at, using immunization protocols, is to protect against the clinical symptoms, and restricting virus shedding and transmission. In other words we are aiming at clinical protection and limiting the virus presence in the farm environment.


The cross protection is best if induced by the same strain that challenges the pigs. Some scientific reports show that it can be lifelong, not only in terms of clinical symptoms but also can protect against the infection. The further away antigenically is the challenging virus from the immunizing virus , the lower is the cross protection. The problem is that we don’t know how to measure antigenic relationship between strains that would allow us to predict the level of cross protection (also vaccine induced).
Today there are modified live virus vaccines produced from multiple genetic lineages of either Type 1 or Type 2. In theory better protection is provided by the vaccine of the same genotype as the wild type virus. Genetic classification of the vaccine strain within the genotype is less important. However, there are reports showing good efficacy of “American” vaccines against “European” strains, and vice versa. We have to remember that the vaccination efficacy is a result of the vaccine antigenicity , vaccination technique, the properties of the wild type strain and its dose (the presence in the environment). All these factors influence the outcome of the vaccination protocol. “Weak” vaccine could be good enough if properly administered in animals staying in clean pens and are fed well. “Strong” vaccine will not work if administered as split dose, in animals living in poor conditions, coinfected with other pathogens that may compromise their immune system, and in a population of mixed immune status regarding PRRSV. Large farms can be coinfected with multiple strains of PRRSV . It is good to know it, and to monitor the changes in virus populations during the control programs.
In summary, the differences in immunogenicity between the vaccines do exist but in the field conditions the most important is to be consistent in executing the vaccination program with a given vaccine, to improve management practices and to monitor the effects of the program in terms of changes in virus circulation (laboratory diagnosis) as well as in terms of production parameters. 

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Have you noticed a significant increase in the number of cases of kyphosis in piglets following an acute clinical outbreak of PRRS?

  

It is not something that is easily observed, but the link between PRRS and kyphosis has been described in several papers. In fact, a recent study (published a couple of years ago) by José Pallarés, from the University of Murcia, evaluates a clinical case to try and discover what causes the increase in the presentation of this condition.Besides genetic causes, the relationship with infectious diseases such as PRRS and PCV2 is investigated. Lesions have been found in animals with kyphosis that can also occur in animals infected “in utero” by PRRSV and PCV2, such as arteritis, peri-arteritis, lymphocytic myocarditis and lymphocytic myositis.

These lesions are also associated with kyphosis in piglets affected by other conditions such as alopecia areata.

To this day, the direct link between infection and kyphosis has not been confirmed, but it has been suggested based on field observations and, as mentioned before, the type of lesions found. We must not forget that kyphosis has also been described in cases of phosphorus deficiency, for example, when there is an interaction between high doses of zinc oxide and phytases.

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Once PRRSV enters your farm, how long does it take to spread to the whole farm? What are the first animals that will show clinical signs?

  
Answer from Dr. Tomasz Stadejek. Warsaw University of Life Sciences. Poland.

The question is very difficult to answer as the outcome of PRRSV infection in a naïve farm is depending on many factors that differ a lot between farms. Some of the factors are:
Type of farm: farrow to finish, farrow to wean or multisite.

Construction of barn compartments.

Pig flow: continuous flow or all in-all out.

Level of internal biosecurity, including personnel flow, disinfection etc.

PRRSV strain characteristics.

In a single site, farrow to finish farm, with continuous flow and low level of internal biosecurity the spread of virus in a population can be rapid. On the other hand, in a multisite farm with high internal and external biosecurity the virus spread can be limited to a single site or barn. The lower level of general farm management, the easier and faster the PRRSV spread.
It has to be remembered that PRRSV strains differ in a level of viremia and shedding by infected pigs. “Milder” viruses will be easier handled by infected animals, the level of shedding can be lower, so the spread of such strain could be slower even in conditions of farrow to finish farm. Recently in Belgium more aggressive PRRSV type 1 strains were described that efficiently replicate in nasal mucosa that helps shedding in aerosols, and in effect, in virus spread.
Clinical symptoms of PRRSV infection can be from very mild to severe, depending on the strain characteristic and existing co-infections in a population, as well as environmental conditions. The earliest symptoms post infection are not specific and most often transient: depression, lethargy and anorexia. Some animals can be feverish and exhibit dyspnea and cough. The better health status of pigs and better environmental conditions, the milder the course of the disease can be. However, there are strains of PRRSV type 1 and particularly of type 2 that can cause high fever and severe pneumonia, even in specific pathogen free pigs. Usually younger pigs are more affected than older pigs, in which the acute phase of infection can be asymptomatic. This phase will start in a part of a farm population where the virus first enters, end eventually the symptoms will roll over the rest of the population, depending on the aforementioned conditions. As the clinical respiratory symptoms are not specific to PRRSV, they can be confused with other disease conditions. Laboratory detection (PCR) of PRRSV in blood, oral fluid or lungs or lymphoid tissue of symptomatic pigs is necessary for early detection of virus infection in a previously naïve farm.
The most prominent clinical symptoms of PRRSV infection is reproductive failure: late term abortion, premature farrowings, stillborn and weakborn piglets and high preweaning mortality. Often this is when PRRSV is first suspected in a farm. It has to be remembered that these symptoms can be caused by PRRSV only if sows or gilts were infected and become viremic in third trimester of gestation. Only then the virus can infect placenta, cause lesions and cross fetal barrier, inducing reproductive failure. Even then not every litter will be affected in the same way. Some litters can be born apparently healthy but infected, and shedding PRRSV to littermates and between litters. Sows that became infected in first half of gestation, control viremia before the virus can cross placenta, so the litters are protected from in utero infection. Such piglets are protected also by colostral antibodies until several weeks of age. Most of cases of reproductive failure have non-infectious etiology so laboratory diagnosis is necessary to confirm the role of PRRSV. Detection of antibodies to PRRSV in sows that, let say, aborted, and virus by PCR in piglets in affected litters confirms PRRS case in a reproductive herd.
As reproductive herd shows delayed effect of PRRSV circulation in a farm, one has to assume that if the reproductive symptoms are evident and confirmed to be linked to PRRSV, the virus must have circulated in a population already for several weeks, unnoticed.
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Effects of PRRS in pigs on a confirmed PRRS positive farm

  
Clinical and productive effects of PRRS in pigs on a positive farm will basically depend on the immunization status of the herd and the virulence of the PRRS virus strain infecting the farm. The immunization status of the herd would depend on the previous PRRS virus circulation on the farm and the vaccination strategies applied.
Once the PRRS virus is introduced into a farm, the circulation of PRRS in pigs of the herd will depend on different factors (Figure 1) such as the production system (i.e: 1-site vs. 2-sites, AIAO vs. continuous flow…), internal biosecurity, replacement gilt management, vaccination and immunization strategies or the presence of subpopulations of negative sows. Generally, it will tend to circulate indefinitely if no control measures are applied and periodical clinical outbreaks can be expected. However, some rare cases of spontaneous elimination of the virus have been described, especially on very small farms:

  
For PRRS infected farms, we can generally distinguish between 3 different situations according to the infecting PRRS virus strain and the previous PRRS status of the farm:

Re-emergence or re-circulation of the same PRRS strain already present on the farm: In this situation, the presence of negative subpopulations can play a crucial role in the clinical presentation and virus circulation. Negative subpopulations can be susceptible to the infection with the PRRS virus present on the farm, experiencing clinical problems and shedding the virus. Since a positive herd population presents homologous immunity against the circulating PRRS, clinical presentations tend to be a temporary mild impairment of productive and reproductive parameters.

Lateral infection introducing a new PRRS virus strain into a positive farm: Depending on the virulence and the immunization level of the herd and the presence of negative subpopulations as well, clinical PRRS problems can range from no effect to a severe outbreak. 

New introduction of the PRRS virus into a negative farm: PRRS infection introduced into a naïve farm will most probably cause serious clinical outbreaks, especially if the PRRS virus strain is highly virulent. Typical reproductive disturbances such as late term abortions and increased numbers of stillborn and weak-born piglets at farrowing. At the same time, increased preweaning mortality can? be observed. Moreover, after this reproductive clinical episode, the PRRS virus will be spread rapidly to weaners and growers.

Brief glossary and background regarding airborne PRRS virus

Airborne Infectious diseases represent a major challenge to human and animal health, mainly due to the difficulty of controlling the air quality within a facility that hosts susceptible individuals.
This challenge is even greater when the cause is an agent like PRRS virus , capable of traveling long distances carried by aerosols. Here are some of the most common terms used when referring to airborne PRRS virus.


Airborne Diseases (AD): Diseases caused by pathogens and transmitted through the air.
Airborne PRRS virus: A systemic swine virus that can be shed in many body secretions and is exhaled in air1.
Airborne Transmission (AT): AT occurs from infected to naïve individuals, when pathogens travel on dust particles or on small respiratory droplets that are stirred up into the expelled air due to coughing, sneezing or exhalation.
Airborne transmission of PRRS virus: American researchers documented for the first time2 the airborne transmission of PRRS virus in 1997. A more recent study suggests that on large sow farms with good biosecurity in swine-dense regions, approximately four-fifths of PRRSV outbreaks may be attributable to aerosol transmission3.
Aerobiology: It is the study of the processes involved in the movement of microorganisms in the atmosphere from one geographical location to another, including the aerosolized transmission of disease. It employs contemporary techniques including computational fluid dynamics to study airborne particle flow, polymerase chain reaction (PCR) methodologies to identify infectious agents and quantify airborne particle concentrations in various settings, and epidemiology to track the spread of disease5.
PRRS aerobiology: Since the late 90s to date, it has been issuing increasingly strong evidence about the role of aerosol transmission of PRRS virus. Recently, the particle concentration, size distribution, and infectivity of PRRS virus emitted by infected pigs, were found associated with a wide range of particle sizes that can deposit throughout the respiratory tract and later be swallowed. PRRS virus viability was particle size dependent with virus isolated only from particles larger than 2.1 µm, supporting the relevance of the aerosol route in the transmission PRRS4.

Air filtration: In human medicine, air filtration is part of the so called -respiratory protection against biologic agents present in bio-aerosols-. These programs include individual and collective protection, mainly in health centers and other places with some risk of airborne diseases.
PRRS virus and air filtration: After several publications on PRRS virus aerosol transmission, not always conclusive or based on partially controlled studies, in 2009 a relevant study using a production region model was published7. It involved a PRRS virus infected population of pigs (source), and two naïve populations of pigs (recipients), housed in separate buildings located 120 meters away. The two recipient barns employed identical biosecurity protocols apart from the installation of bio-aerosol filtration in one barn. Non-filtered populations became infected in 8 of 26 replicates over a one year period, while none of the 26 recipient replicates in filtered barns were infected. It was demonstrated that strict bio-security measures alone were not sufficient at preventing introduction of PRRS virus into the non-filtered populations. For the first time, the effect of bio-aerosol filtration was being studied under semi-controlled conditions5, 6. In this video the principles of air filtration are explained: