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Predisposing factors

Infected pigs and contaminated equipment play key roles in PRRS transmission between herds and spread of the virus within farms. Movement of infected animals (that might not be showing clinical signs) within a farm, and mixing piglets from different sows will help spread the virus around the farm and increase the chance of a new strain of virus emerging.

Buying-in replacement gilts and sows and/or infected semen that come from positive herds represent a risk. However, the risk can be reduced by knowing the disease status of the farm where the pigs come from, and by keeping the newly brought in animals separate from the rest of the herd for the quarantine period recommended by your veterinarian.

Aerosol transmission plays an important role in PRRS transmission from one farm to another and within the farm. Climatological factors such as wind direction may favour this transmission route. Pigs do not shed large quantities of PRRS virus in aerosol; but, population size and pig density seem to be particularly important factors in transmission via aerosol.

The closer a non-infected herd is to an infected herd the greater the risk of cross-infection. The probability increases with the density of PRRS positive neighbouring herds. In other words, the more positive herds near-by the more likely it is for a herd to become infected. The risk decreases as the distance to the nearest positive herd increases. Poor biosecurity practices will increase the risk of PRRS virus entering the farm and spreading through it.

Herd size and pig density are important predisposing factors. The probability of PRRS virus persisting in a farm increases with the herd size. This is mainly because big herds are more likely to have, at the same time, pigs with the disease (i.e. infected pigs), pigs that are eliminating the virus (i.e. infectious pigs) and pigs that have not been infected (i.e. susceptible pigs); and also, because different age groups are present, so 'new' susceptible pigs are either coming in or being born. So the virus is maintained at the farm, and there are bigger chances for the virus to mutate leading to sporadic outbreaks. In the long term the presence of susceptible animals is vital to maintain infection in the herd.

What is persistence and fade out cycle? Why is it important for control measures?

As we know virus recombination plays an essential role in PRRS virus biology. Virus mutations can occur even in the same herd. However, a spontaneous extinction of PRRS virus is also possible, and this phenomenon is called fade out. Cycles of persistence of the virus and fade out are observed in some farms. Why does this happen? As you remember the pig immune system will develop antibodies against pathogens that have invaded the body. If there are no pigs coming onto the farm, it will reach a point where no susceptible pigs will be around because all pigs have antibodies against the strain that is circulating in the farm, and the virus cannot reproduce. So, if there is no introduction of new pigs (i.e. new susceptible pigs) and the virus does not mutate, the virus will fade out (as all pigs in the farm will develop antibodies against the strain that is circulating). This in fact has been the basis for some control and eradication programmes as we will see later.

To summarize, list the main predisposing factors for PRRS

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