Objective—To characterize the genetic diversity of
Haemophilus parasuis field isolates with regard to
serovar, herd of origin, and site of isolation.
Sample population—Isolates of H parasuis obtained
from pigs in 15 North American herds and multi-farm
Procedure—98 H parasuis isolates were genotyped
with the enterobacterial repetitive intergeneic consensus
based-polymerase chain reaction (ERIC-PCR) technique
and serotyped via agar gel precipitation test.
Genomic fingerprints were analyzed and dendrograms
were constructed to identify strains from the same
serovar group, herd of origin, or isolation site and to
evaluate the genetic variability within these categories.
Results—Serovar 4 (39%) and nontypeable (NT) isolates
(27%) were most prevalent. Thirty-four distinct
strains were identified among the 98 isolates, using a
90% similarity cutoff. Strains from serovar 4 and NT
isolates had high genetic diversity (12 and 18 strains,
respectively). One to 3 major clusters of prevalent
strains could be identified in most of the evaluated
herds. Haemophilus parasuis strains isolated from the
upper respiratory tract were either serovar 3 or NT
isolates. Potentially virulent strains (isolated from systemic
sites) were either serovars 1, 2, 4, 5, 12, 13, or
14, or NT isolates.
Conclusions and Clinical Relevance—Although H
parasuis had high genetic diversity overall, only a few
strains caused disease in these herds. The ERIC-PCR
technique was more discriminative than serotyping,
and a broad genetic variety was observed within particular
serovar groups. (Am J Vet Res 2003;
Objective—To determine effects of vaccination protocols with modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine on persistence and transmission of virus in pigs infected with a homologous isolate and determine clinical and virologic responses following heterologous viral challenge.
Animals—Four hundred forty 6- to 8-week-old PRRSV-naïve pigs.
Procedures—Pigs were allocated into 5 groups. Groups A to D were inoculated with wild-type PRRSV VR2332. Group A (positive control pigs) received PRRSV only. Groups B, C, and D received modified-live PRRSV vaccine (1, 2, or 3 doses). Group E served as a negative control group. To evaluate viral transmission, sentinel pigs were introduced into each group at intervals from 37 to 67, 67 to 97, and 97 to 127 days postinoculation (DPI). To evaluate persistence, pigs were euthanized at 37, 67, 97, or 127 DPI. To assess clinical and virologic response after challenge, selected pigs from each group were inoculated at 98 DPI with a heterologous isolate (PRRSV MN-184).
Results—Mass vaccination significantly reduced the number of persistently infected pigs at 127 DPI. Vaccination did not eliminate wild-type PRRSV; administration of 2 or 3 doses of modified-live virus vaccine reduced viral shedding after 97 DPI. Previous exposure to wild-type and vaccine virus reduced clinical signs and enhanced growth following heterologous challenge but did not prevent infection.
Conclusions and Clinical Relevance—Findings suggest that therapeutic vaccination may help to reduce economic losses of PRRSV caused by infection; further studies to define the role of modified-live virus vaccines in control-eradication programs are needed.
Objective—To evaluate retention of porcine reproductive
and respiratory syndrome virus (PRRSV) in
houseflies for various time frames and temperatures.
Sample Population—Fifteen 2-week-old pigs,
two 10-week-old pigs, and laboratory-cultivated
Procedure—In an initial experiment, houseflies were
exposed to PRRSV; housed at 15°, 20°, 25°, and 30°C;
and tested at various time points. In a second experiment
to determine dynamics of virus retention,
houseflies were exposed to PRRSV and housed
under controlled field conditions for 48 hours.
Changes in the percentage of PRRSV-positive flies
and virus load per fly were assessed over time, and
detection of infective virus at 48 hours after exposure
was measured. Finally, in a third experiment, virus
loads were measured in houseflies allowed to feed on
blood, oropharyngeal washings, and nasal washings
obtained from experimentally infected pigs.
Results—In experiment 1, PRRSV retention in houseflies
was proportional to temperature. In the second
experiment, the percentage of PRRSV-positive houseflies
and virus load per fly decreased over time; however,
infective PRRSV was found in houseflies 48 hours
after exposure. In experiment 3, PRRSV was detected in
houseflies allowed to feed on all 3 porcine body fluids.
Conclusions and Clinical Relevance—For the conditions
of this study, houseflies did not support PRRSV
replication. Therefore, retention of PRRSV in houseflies
appears to be a function of initial virus load after ingestion
and environmental temperature. These factors
may impact the risk of insect-borne spread of PRRSV
among farms. (Am J Vet Res 2005;66:1517–1525)
Objective—To determine whether flies can acquire
porcine reproductive and respiratory syndrome virus
(PRRSV) and disperse the virus throughout a designated
Animals—60 four-month-old pigs.
Procedure—On day 0, 28 of 60 pigs were inoculated
with PRRSV MN 30-100 (index variant). On the same
day, 100,000 pupae of ochre-eyed houseflies and
100,000 pupae of red-eyed (wild-type) houseflies
were placed in the swine facility for a release-recapture
study. Flies were recaptured at 2 locations within
the swine facility, 6 locations immediately outside
the facility, and 30 locations 0.4, 0.8, 1.3, 1.7, 1.9, and
2.3 km from the facility. Traps were emptied on days
2, 7, 8, 10, and 14. Samples derived from flies were
tested by use of a polymerase chain reaction assay,
virus DNA was sequenced, and viruses were tested
for infectivity by means of a swine bioassay.
Results—PRRSV RNA homologous to the index
PRRSV was detected in trapped flies collected inside
and immediately outside the facility and from 9 of 48
samples collected at 0.4 km, 8 of 24 samples collected
at 0.8 km, 5 of 24 samples collected at 1.3 km, and
3 of 84 samples collected at > 1.7 km from the facility.
Two samples collected at 0.8 km contained genetically
diverse variants of PRRSV. Swine bioassays
revealed the virus in flies was infectious.
Conclusions and Clinical Relevance—Flies
appeared to become contaminated with PRRSV from
infected pigs and transported the virus ≥ 1.7 km. Flyborn
transmission may explain how PRRSV is seasonally
transported between farms. (Am J Vet Res 2004;65:1284–1292)