Other strategies proposed for PRRS control
When faced with a sudden rise in reproductive failure in sows plus respiratory disease in finishing pigs, this could be a sign that there is a new strain(s) causing an outbreak of PRRS. In this situation, it is recommended that herd closure is implemented immediately. What do we mean by herd closure and how does it work?
As we saw in the previous unit (PRRS-causes and predisposing factors), when a pig gets infected with PRRS virus, persistent infection with the virus rarely lasts more than 200 days. The pigs that are exposed the virus develop antibodies and this gives them subsequent protection, provided they overcame the first bout of the disease. The concept of "herd closure" was developed with this evidence in mind. Basically, all pigs in the farm are exposed to PRRS virus (either by vaccination or inoculation with virulent virus). Then the herd should be closed to all animal entry for 200 days. This means that no further pigs (including gilt replacements) are introduced for a period of 4 to 8 months. During this period the herd is 'stabilised', which means that all sows and gilts on the farm will become protected against the current strain and after 200 days no pig will be carrying or eliminating the virus anymore.
After this period extra care needs to be taken when replacement animals are being introduced. They must come from a negative herd to avoid the introduction of a new strain, and strict biosecurity has to be in place.
At this stage it may be worth refreshing your memory about the dangers of having multiple PRRS strains circulating on a farm? (see What is virus recombination? and What is persistence and fade out cycle? in the previous unit).
If the herd closure strategy is implemented during an outbreak, it can be helpful to get the virus sequenced (i.e. send samples to the lab to determine which strain is circulating). This has been used in the USA to try and locate the source of the virus.
The "herd closure" concept has been used successfully in regional and national elimination programmes. For example, it was used in the Chilean programme, in combination with other control measures, to eradicate PRRS.
Chile eradication programme
PRRS was first diagnosed in 2001 when a study carried out by the Ministry of Agriculture found that 30% of the herds sampled were positive. The virus was found to be similar to the American genotype and luckily just one strain was circulating. After the result of this study, an eradication programme was implemented to get rid of the disease using the following approach.
Firstly, farmers and vets had to report farms suspected of PRRS to the Ministry of Agriculture. In other words the disease was made notifiable.
An agreement was made between the public and the private sectors in which an eradication programme would be regulated by the Ministry, and PRRS vaccination would not be available for the country's herds.
Secondly, the strategy began at multi-site enterprises. Site 1 (breeders) would adopt a herd closure strategy for 6 months and sites 2 and 3 (weaners and finishers) would undergo depopulation (all in - all out).
In the case of mono-site units, a de-population / re-population strategy was carried out 4 years after the programme started. The mono-site producers joined together and rented an empty farm where they could establish a new herd of PRRS-negative sows for repopulating their units once those premises were cleaned and disinfected.
Thirdly, other measures within the programme included surveillance at livestock auction markets; improving biosecurity at the positive farms (including movement restrictions, washing and disinfection protocols at boundary fences); farm staff training on herd biosecurity and hygiene. Also, records were made of the geographical location of all farms and import regulations were reviewed and improved.
What were the key factors that made PRRS eradication in Chile possible?
Taking the Chilean experience, do you think it would be possible for a similar programme to work in your country? Why?
Test and remove
'Test and removal' is an additional procedure that can be used when controlling PRRS. It consists of testing the breeding herd in order to detect sows and boars that are infected but do not show clinical signs. Positive animals are culled. This method has been successful in eliminating PRRS virus from endemic farms, but has the disadvantage that the test is expensive. Additionally, animals that were exposed to the virus but no longer have the virus might test positive, so unnecessary elimination of some animals might happen.
Filtering air entering pig houses has been proposed as a mean of reducing the risk of introducing a new strain of PRRS virus through air transmission. Results for preliminary trials seem to be promising. However, other biosecurity measures are still needed in conjunction with this approach. It is important to point out that the studies carried out so far have been done on farms that were PRRS virus free (by previously carrying out herd closure or de-pop /re-pop protocols) and so the same results might have happened just by applying strict biosecurity measures.
Where house flies and mosquitoes are an issue, the use of insect screens (nets) can reduce the mechanical spread of PRRS by insects. However, this should be accompanied by removing standing water and maintaining pens as clean as possible.