23/dec/2008 - (0 comments)
Scott A. Dee

Insects
Insects (mosquitoes (Aedes vexans) and houseflies (Musca domestica)) are commonly observed in swine facilities during the summer months and have been shown to mechanically transmit PRRSV from infected to naïve pigs under experimental conditions. The site of the virus in the insect is the intestinal tract (figure 1). Insects are not biological vectors of PRRSV; therefore, the duration of retention of PRRSV within the intestinal tract of insects is dependent upon virus load post-ingestion and environmental temperature. Transport of PRRSV by insects throughout an agricultural area has been reported for up to 2.4 km following contact with an infected pig population. Finally, control of on-farm insect populations has been demonstrated using a combination of screening of the air inlets of swine facilities along with the use of targeted insecticides and habitat management.

Gastrointestinal tract of a housefly

Figure 1: The site of PRRSV retention in the housefly is the gastrointestinal tract. This slide demonstrates an enlarged image of a housefly that was dissected after feeding on the blood of a PRRSV-infected pig. Live PRRSV was recovered from the blood-filled intestine.

Avian and non-porcine mammalian species
Previous studies have investigated the role of various mammals (rodents, raccoons, dogs, cats, opossums, skunks) and birds (house sparrows and starlings) in the transmission of PRRSV. Results from these investigations have indicated that none of these species were capable of serving as mechanical or biological vectors. However, migratory waterfowl have been proposed as vectors of PRRSV spread between farms, due to their migratory nature and their tendency to nest on or near to swine farm lagoons. Since PRRSV can survive in water for up to 11 days and in swine lagoon effluent for up to 7 days, this appeared to be a plausible hypothesis; however, contrasting results regarding the ability of Mallard ducks to replicate and shed PRRSV to pigs via the fecal-oral route have been reported. However, it has not been possible to reproduce these results under experimental conditions (figure 2).

Pigs with ducks

Figure 2: Despite contact with actively infectious pigs, it was not possible to demonstrate transmission of PRRSV to Mallard ducks.

Aerosols
In recent years, advances have been made in the understanding of the aerobiology of PRRSV. Early data collected during outbreaks in England proposed that the virus can be spread through aerosols up to 3 km, and recent data from a large scale epidemiological study also suggested aerosols as a potential route of indirect transmission throughout swine producing regions. Aerosols have often been blamed for “local spread,” of PRRSV, a term used to describe transmission of the virus throughout a region via undetermined routes. However, results from early experiments evaluating aerosol transmission of PRRSV have been inconsistent, with experimental and field trials reporting different findings. Studies conducted under laboratory conditions have shown that aerosol transmission may occur over short distances; one trial demonstrated that experimentally infected pigs were able to transmit virus to close and indirect contact groups separated by 46 cm and 102 cm in separate trials. Several other studies showed that experimentally infected pigs were able to infect sentinel pigs via aerosols over distances of 1m. Recently, it has also been demonstrated that viable virus could be transported up to 150 m using a negative pressure straight tube model, resulting in the infection of naïve sentinel pigs.

In addition, field trials attempting to transmit PRRSV through aerosols to naïve sentinel pigs were not successful, despite the use of large populations of experimentally infected pigs and commercial conditions. However, these studies all used the same variant of PRRSV, an isolate of low virulence referred to as MN-30100 that had been recovered from a persistently infected sow within an endemically infected farm. This observation led to the question of whether aerosol shedding and transmission of PRRSV may be isolate-dependent. This hypothesis was supported from previously published data involving the use of a mildly virulent reference isolate (VR-2332) and a highly virulent isolate (MN-1b). Results indicated that differences existed in seroconversion rates, recovery of virus from infected animals and transmission of PRRSV to naïve pigs. Recently it was determined that PRRSV isolate pathogenicity significantly influenced virus concentration in aerosols, the frequency of shedding, and transmissibility of PRRSV in aerosols. In these experiments, 2 PRRSV isolates were evaluated: MN-184 (a highly virulent isolate) and MN-30100, an isolate of low virulence. Results indicated significant differences in the frequency of shedding and transmission in aerosols from pigs experimentally infected with MN-184 when compared to aerosols recovered from pigs infected with MN-30100. However, differences in the concentration of PRRSV in aerosols from animals infected with the 2 isolates were low and differences were not significant (table 1). These findings have elevated the importance of the potential for aerosol transmission in North America and provided an explanation for why this route of transmission was so difficult to reproduce experimentally. They also suggest that population size and infection level; along with isolate pathogenicity may be important risk factors for aerosol transmission of PRRSV in the field.

	1	3	5	7	9	11	13	15	17	19	21
PRRSV MN-301000
2 meses	0	0,04	0,4	0,02	0	0	0	0,06	0	0,02	0
6 meses	0	0,1	0,3	0,05	0,1	0	0	0	0	0	0
PRRSV MN-30100 plus M hyo
2 meses	0	0	0	0,025	0,025	0,025	0	0	0	0	0
6 meses	0	0	0	0	0	0,05	0	0	0	0	0
PRRSV MN-184
2 meses	0	0	0,083	0	0,16	0,016	0,033	5,8	0,016	0,15	0,46
6 meses	0	0,2	0,04	0	0	0	1,04	0,25	0,075	0,075	0
PRRSV MN-184 plus M hyo
2 meses	0,35	0	9,62	3,34	265,64	NA	0,4	0,08	152,2	NA	3,33
6 meses	0	1,14	0,58	0,025	0,05	NA	0	0	0,025	0	0

Recent studies conducted using a model of a swine production region involving large population sizes experimentally inoculated with MN-184 have successfully demonstrated airborne spread of PRRSV out to 120 meters. In addition, data from these studies indicate that certain weather conditions, such as wind direction, velocity, relative humidity and barometric pressure are associated with the presence of virus in aerosols and may play a role in its spread between farms. Further studies are under way to re-evaluate these findings as well as determine the distance that the virus can spread via the aerosol route.

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