African swine fever interference with foot-and-mouth disease infection and seroconversion in pigs

Initial oral infection of pigs with either highly virulent (L-60) or moderately virulent (DR-2) African swine fever virus (ASFV), followed in 3 days with exposure to foot-and-mout h disease virus (FMDV) (tongue inoculation and contact), failed to cause FMDV infection or seroconversio n in 18 of 22 L-60-infected pigs and 13 of 34 DR-2-infected pigs. Of the 13 DR-2-infected pigs remaining free of foot-and-mout h disease (FMD), 2 pigs survived to 24 days without antibody to FMDV, despite constant contact with clinically infected pigs with FMD. Three other DR-2-infected pigs never developed FMD lesions but did develop low levels of antibody to FMDV by day 17. A group of larger pigs (in which DR-2 is less virulent) infected with DR.2 and then FMDV had a rapid but suppressed immune response to FMDV. Contact pigs introduced 3 days postin- oculation and inoculated with FMDV only all became infected with ASFV by contact and died. This remarkably long lasting l-way interference with FMD infection during acute and subacute African swine fever was not anticipated. Infection with ASFV may have blocked the initial target cells (possibly dendritic cells) necessary for establishment of FMDV infection. African swine fever (ASF) was first described in 1921 inoculated with RPV, and pigs vaccinated for 122 days as an acute fatal viral disease of pigs causing necrosis were inoculated with FMDV or AHCV. All pigs were and hemorrhage in the spleen and lymph nodes and challenged 3-8 days later with the virulent L-60 strain other organs. 37 In domestic pigs, virulent ASF was le- of ASFV. About half of each group had died by 2 weeks thal and self-limiting. Only after ASF became estab- after inoculation, and these pigs had pneumonic lesions lished in Portugal and following a vaccination cam- attributed to ASF. High fevers were reported between paign in 1963 with an attenuated live vaccine 30 did 2 and 6 days postinoculation (DPI) with both rinder- subacute and chronic ASF occur frequently. With the pest and foot-and-mouth disease (FMD). No lesions modification of the disease in the Iberian peninsula, or deaths were reported due to these viruses, but it the finding of field isolates of low and moderate vir- probably can be assumed that FMDV caused typical ulence stimulated studies to determine the effects of vesicular disease or it would have been mentioned. ASF on the immune system. Recovered pigs were found Another study (Gamett W, Ferris DH, unpublished) to have high levels of circulating antibody concurrently was undertaken following laboratory reports from Spain with high levels of ASF virus (ASFV), 7,9 which sug- that HCV and ASFV were present simultaneously in gested that the immune system was not impaired in field samples. Pigs were inoculated with virulent HCV pigs infected with less virulent isolates. To test this and moderately virulent (DR-2) ASFV simultaneous- hypothesis, pigs were vaccinated with an attenuated ly, and infection with both was detected for up to 2 ASFV and 2 months later were challenged with foot- weeks by direct immunofluorescence of tissues and and-mouth disease virus (FMDV). 6 A normal antibody antibody was detected to both by serologic tests. response was reported 21 days later. None of these studies addressed the dicotomy of an Following a report from Portugal that pigs vacci- apparent lack of a humoral immune response to vir- nated with the attenuated ASF vaccine were suffering ulent ASFV, although there is an essentially normal from postvaccinal "stress" and chronic pneumonia, 30 immune response in pigs infected with moderately vir- the possible role of stress was investigated in vacci- ulent ASFV, albeit not neutralizing. 19 An ASF patho- nated pigs. 8 The "stressors" applied were virulent rin- genesis study 35 showed that early viral replication in derpest virus (RPV), FMDV, and attenuated hog chol- the tonsil and mandibular lymph node occurred in era virus (AHCV). Pigs vaccinated for 66 days were reticuloendothelial cells of the paracortex. The authors suggested that these cells might be antigen-prese nting cells, and their infection may modulate the immune From the Foreign Animal Disease Diagnostic Laboratory, Na-