January and February, 1997
In response to Dr. Tim Trayer's letter concerning porcine reproductive and respiratory syndrome (PRRS), published in the September/October issue of Swine Health and Production, I'd like to raise the following points:
It is not surprising to have PRRS problems in multiple-source herds. Multi-site production will not prevent PRRS virus (PRRSV) spread. We have published this in Swine Health and Production, Vol. 1, no. 5, 1993. Therefore, practitioners and producers should not assume that the disease is under control after they have implemented these methods.
PRRS control does not have to be based on the "application of multiple bandages." We have practiced and presented a systematic approach to PRRS control, consisting of diagnosis, gilt management, breeding herd stabilization, and nursery depopulation. This approach has been effective around the world; however, it often takes all four components of this program, applied in an organized, consistent manner, to achieve the desired level of success.
The NPPC recently provided $250,000 to eight different scientific groups in order to address unanswered questions on PRRS. The information from the studies will not be privatized. The NPPC should be congratulated for realizing the need to allocate funds to PRRS-based studies.
As a member of the AASP PRRS Subcommittee, I will say that our group is indeed open to "challenging thinking." In fact, the members of the group have been responsible for a large percentage of the published research data on PRRS. If specific questions exist, the Subcommittee should be contacted and informed of the current concerns. The Subcommittee played a major role in the NPPC program previously described.
In conclusion, although there are still numerous unanswered questions, the information available on PRRS appears to be sufficient to induce successful control of the disease in many cases. I suggest the author review the four-step approach to PRRS control and be sure that all the bases are covered.
Scott Dee, DVM, PhD, Diplomate; ACVM
The following interchange recently occurred on SWINE-L, an AASP-sponsored electronic discussion list on the Internet:
How long will Mycoplasma hyopneumoniae-infected pigs shed the organism?
Pigs are infected with M. hyopneumoniae before weaning unless weaned when less than 14 days old. In batch rearing systems, these pigs usually seroconvert but may not exhibit clinical signs of pneumonia unless they are exposed to excessive stress or other respiratory diseases. Clinical disease generally results from exposure of nursery pigs to older coughing pigs in continuous-flow systems. From the work of Baekbo, et al., (IPVS 1994, pp 135) it appears that infected pigs shed M. hyopneumoniae until they are at least 10 months old. However, in the United States, we have shown that sows (minimum age 1 year) transfer M. hyopneumoniae to their pigs in the farrowing house (Vet Med 1991;86:543-550). Whether adult swine shed this organism beyond this age is unknown to me.
How does M. hyopneumoniae vaccine alter the duration of shedding of infected animals?
I'm not sure that vaccination alters shedding of M. hyopneumoniae unless used in sows to boost passive antibody titer in pigs. Passive antibodies induced by vaccination of sows have been shown to prevent infection in pigs subsequent to challenge. (Kobisch, et al., 1994). Vaccination does reduce lesions induced by M.hyopneumoniae challenge, thus it probably reduces shedding.
Is there an eradication program for M. hyopneumoniae without depopulation? If so, what is the protocol?
One method is discussed by Baekbo, et al., in the previously mentioned paper. Another method is to use SEW technology described in many papers by me and others here in the United States. If you can use early weaning (<14 days) and batch rear pigs, large groups of pigs can be reared free of clinical signs of M. hyopneumoniae and, in good management situations, free of the disease.
-Kirk Clark, DVM, PhD
(Originally posted to SWINE-L)
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