Infectiousness of pigs infected by the Porcine Reproductive and Respiratory Syndrome virus (PRRSV) is time-dependent

Abstract

The time-dependent transmission rate of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) and the correlation between infectiousness, virological parameters and antibody responses of the infected pigs were studied in experimental conditions. Seven successive transmission trials involving a total of 77 specific pathogen-free piglets were carried out from 7 to 63 days post-inoculation (dpi). A semi-quantitative real time RT-PCR was developed to assess the evolution of the viral genome load in blood and nasal swabs from inoculated and contact pigs, with time. Virus genome in blood was detectable in inoculated pigs from 7 to 77 dpi, whereas viral genome shedding was detectable from nasal swabs from 2 to 48 dpi. The infectiousness of inoculated pigs, assessed from the frequency of occurrence of infected pigs in susceptible groups in each contact trial, increased from 7 to 14 dpi and then decreased slowly until 42 dpi (3, 7, 2, 1 and 0 pigs infected at 7, 14, 21, 28 and 42 dpi, respectively). These data were used to model the time-dependent infectiousness by a lognormal-like function with a latency period of 1 day and led to an estimated basic reproduction ratio, R0 of 2.6 [1.8, 3.3]. The evolution of infectiousness was mainly correlated with the time-course of viral genome load in the blood whereas the decrease of infectiousness was strongly related to the increase in total antibodies.

Porcine reproductive and respiratory syndrome virus (PRRSv) modified-live vaccine reduces virus transmission in experimental conditions

“…In SPF inoculated but non vaccinated pigs, clinical and virological data were close to other results obtained previously with another genotype 1.1 strain with hyperthermia observed as soon as 2 DPI persisting 5-6 days and also with a similar evolution of the viral genome load [8]. The observed duration of viraemia, slightly shorter with the strain used in the present experiment might be related to the sensitivity of the challenge strain-specific qRT-PCR slightly lower because of the definition of primers not matching with the vaccine strain.…”

“…In the non-vaccinated pigs, the estimated durations of the latency and infectiousness were similar to previous estimates in an experiment involving another genotype 1.1 strain [8]. However, in this study, the basic reproduction ratio (R0) estimate (2.6) was lower than the one estimated in the present study (5.4).…”

“…The latency period being relatively short in regards to the infectious period [8], we modeled the latency assuming an exponential distribution, for which the rate parameter ( ) was estimated. The transmission rate and parameter governing the distribution of the latency were estimated by Bayesian inference using Monte Carlo Markov Chain (MCMC) with uninformative uniform prior distributions.…”

“…We showed the efficacy of a modified-live vaccine based on a genotype 1.1 strain on reduction of transmission of a heterologous genotype 1.1 strain, which is also representative of the strain variability encountered in a densely pig production area such as Brittany, France. Piglets have been monitored individually every 3 days and on a long term, which is pivotal for this virus because of long lasting viraemia and the possibility of transmission to susceptible contact pigs at least until 28 days post-infection [8].…”

“…the ability to significantly reduce virus transmission and spread within the population is not well known. To date, PRRSv propagation within pig populations has been essentially studied in non-vaccinated pigs in field [7] or experimental [8] The impact of vaccination has been evaluated in one study but eventually the evaluation of the vaccine effect was complicated by the abnormally low transmission rate of the challenge strain even in the non vaccinated group [9]. Another study has compared PRRSv transmission in vaccinated and non vaccinated pigs with a basic reproduction ratio (R0) estimate in the non vaccinated group comparable to other published estimates.…”