1. Transmission within herds

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Swine

I. DIRECT TRANSMISSION:

a. HORIZONTAL TRANSMISSION:

Routes of shedding:

Infected pigs can shed virus from multiple routes and for a long time; mainly by nasal secretions, saliva and semen; less frequently in milk and colostrum, and rarely in urine and faeces. The amount of virus shed, together with the duration of the shedding, vary significantly among strains.

Routes of exposure:

Swine can be infected by several routes of exposure, including oral, intranasal, percutaneous, and sexual (intrauterine and vaginal). The probability of infection is highly dependent on the route and the dose of virus.

Chronic/persistent infection (carrier animals):

The virus can be detected in blood for long periods of time. The length of viraemia is strongly influenced by the strain and the animal’s age; it may extend to weeks in piglets (up to 90 days), whereas in adult pigs it may last for only a few days. After viraemia, PRRS virus can persist for several weeks, particularly in tonsils and other lymphoid tissues. Although most pigs clear PRRS virus within 90 or 120 days, some may remain persistently infected for several months; virus may be isolated from tonsils up to 150 days post-infection. Persistent infection occurs at all ages, but maximum values have been observed in in utero infected foetuses. It has been suggested that the persistence of PRRS virus involves continuous viral replication; it is not a true steady-state persistent infection.

Obviously, prolonged viraemia and persistent infection increase the possibility of transmission. As with other members of the genus Arterivirus, pigs recovered from viraemia and/or acute phase can shed PRRS virus at low levels or intermittently. Therefore, a negative result for viraemia or serum antibodies in a previously infected pig does not rule out that the animal could be shedding the virus.

Importance of semen in PRRS virus transmission:

Semen can play a crucial role in PRRS virus transmission. This is due to the special features of the virus shedding by this route and the extensive use of artificial insemination.

Though viraemia usually is very short in boars, from few a days to a maximum of two-to-three weeks, they can shed the virus in semen for several weeks after infection. In addition, amount of virus necessary to infect a susceptible sow by sexual exposure tends to be much lower that the amount of virus usually present in semen from an infected boar. Nevertheless, transmission by semen appears more likely to occur at the early stages of infection due to the high titres of virus in the semen. It is also important to note that a high variability in virus shedding among boars is observed, and that boars are intermittent shedders. Consequently, a negative isolated PCR result in serum or semen does not rule out the possibility of shedding by this route; more than one sample from a given boar should be analysed at different time periods.

Parental exposure:

Pigs are extremely susceptible to infection by parenteral exposure; very few PRRS virus particles are sufficient to infect pigs by this route. Therefore, any event, practice or contaminated material that could result in breaks in the skin barrier can potentially facilitate the transmission of the virus, such as: teeth clipping, ear notching, tail docking, inoculations with medications (needles)… In relation to the latter point, it has been demonstrated that infection can occur following the use of the same needle used to vaccinate infected pigs. In addition, because the virus is present in oral fluids, normal pig social behaviour and aggressive interactions (bites, scrapes, abrasions…) can also result in parenteral infection.

b. VERTICAL TRANSMISSION:

It is well-known that abortions during the first and second third of gestation due to PRRS virus infection are rare. However, the virus can efficiently cross the placental barrier in the last period of pregnancy and it can replicate in the endometrial and placental tissues. As a result, foetal death can occur; alternatively, piglets that survive pregnancy, which are usually weak, succumb to other infections. Otherwise, they will survive and remain infected. As we have stated before, piglets infected during the foetal phase could remain as positive for a long time and infect other piglets as late as three months post-farrowing.

II. INDIRECT TRANSMISION:

Several routes of indirect transmission by fomites and contaminated residues, as well as airborne transmission, have been demonstrated (please see the section “Transmission between herds (introduction into the farm)”. Within the herd, we should pay particular attention to hands/gloves, coveralls, boots, and mainly to needles, since as we have seen, pigs are most susceptible to infection via parenteral exposure.