Safety of Porcine Reproductive and Respiratory Syndrome Modified Live Virus (MLV) vaccine strains in a young pig infection model

Abstract

The objective of this study was to compare the safety of all modified live virus vaccines commercially available in Europe against Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) under the same experimental conditions. For this purpose, one hundred and twenty three-week-old piglets, divided into five groups, were used. On day 0 of the experiment, nine pigs per group were removed and the remaining fifteen were vaccinated with the commercial vaccines Ingelvac PRRS MLV, Amervac PRRS, Pyrsvac-183 and Porcilis PRRS by the IM route or were mock vaccinated and used as controls. On day 3, the nine unvaccinated pigs were re-introduced into their respective groups and served as sentinel pigs. Clinical signs were recorded daily and lung lesions were determined on days 7, 14 and 21, when 5 vaccinated pigs per group were euthanized. Blood samples and swabs were taken every three days and different organs were collected at necropsy to determine the presence of PRRSV. None of the vaccines studied caused detectable clinical signs in vaccinated pigs although lung lesions were found. Altogether, these results indicate that all vaccines can be considered clinically safe. However, some differences were found in virological parameters. Thus, neither Pyrsvac-183 nor Porcilis PRRS could be detected in porcine alveolar macrophage (PAM) cultures or in lung sections used to determine PRRSV by immunohistochemistry, indicating that these viruses might have lost their ability to replicate in PAM. This inability to replicate in PAM might be related to the lower transmission rate and the delay in the onset of viremia observed in these groups.

Figure 1

Average Daily Weight Gain (ADWG) recorded for vaccinated pigs of all experimental groups. All vaccinated pigs were weighted every three days. Individual weights of days 0, 6, 15 and 21 were used to estimate the ADWG of each group in periods 0 to 6; 7 to 15 and 16 to 21.

Figure 2

Mean viral titers in positive serum samples of vaccinated pigs at each sampling day. Titers are expressed as tissue culture infectious doses 50 (TCID₅₀) per mL of serum. Bars represent the standard deviation.

Figure 3

Mean viral titers in positive tissue samples of vaccinated pigs at each necropsy day. Titers are expressed as tissue culture infectious doses 50 (TCID₅₀) per gram of tissue. Bars represent the standard deviation. LSLN: left superficial lymph node; RSLN: right superficial lymph node; LSILN: left superficial inguinal lymph node; RSILN: right superficial inguinal lymph node; Est.LN: esternal lymph node; Med.LN: mediastinic lymph node; Mes.LN: mesenteric lymph node

Figure 4

Proportion of viremic sentinel pigs from each experimental group in each sampling day. Blood samples were taken from sentinel pigs every three days. The percentages of sentinel pigs found viremic in each experimental group in each sampling day are represented in the figure.

Figure 5

Proportion of PRRSV seropositive sentinel pigs from each experimental group in each sampling day. Seroconversion of sentinel pigs was assessed by ELISA test. The percentage of sentinel pigs positive by ELISA test in each experimental group in each sampling days are represented in the figure.

Figure 6

Percentage of PRRSV positive swabs obtained from infected sentinel pigs during the experiment. Oral, rectal and nasal swabs were obtained from sentinel pigs every three days. Swabs from sentinel pigs considered to be infected (based on the determination of viremia in the same days or in previous days) were used for virus isolation. The percentage of positive swabs obtained from infected pigs in each experimental group each sampling day is represented in the figure.