The introduction of a highly virulent PRRSV strain in pig farms is associated with a change in the pattern of influenza A virus infection in nurseries

Abstract

The present study aimed to determine the dynamics of influenza A virus (IAV) infection in two endemically infected farms (F1 and F2), where a longitudinal follow-up of piglets was performed from birth to 8-12 weeks of age. During the study, a highly virulent isolate of porcine reproductive and respiratory syndrome virus (PRRSV) was introduced on both farms. This allowed us to examine the impact of such introduction on the patterns of infection, disease, and the antibody response of pigs to IAV infection. The introduction of the new PRRSV strain coincided with a change in the dynamics of IAV infection on both farms. In F1, the cumulative incidence of IAV increased from 20% before the outbreak to 67.5%, together with the existence of animals that tested positive for IAV (RT‒qPCR) in nasal swabs for two or more consecutive samples. In F2, the cumulative incidence of IAV increased from 50% before the PRRSV outbreak to 70%, and the proportion of prolonged IAV shedders increased sharply. Additionally, some animals were infected with the same IAV twice during the observation period. In contrast to previous reports, our study revealed that prolonged shedding was not related to the titres of maternally derived antibodies at the time of infection but was significantly (p < 0.05) related to PRRSV infection status. Notably, both before and after the PRRSV outbreak, a high proportion of IAV-infected piglets did not seroconvert, which was significantly (p < 0.05) related to the hemagglutination inhibition titres against IAV when infected.

Keywords: Influenza A virus; coinfection; maternally derived antibodies; porcine reproductive and respiratory syndrome virus.

Figure 1

Dynamics of swIAV and PRRSV infection in the studied farms. Weekly mortality rates for each sampling period and farm. Left: Farm 1, Right: Farm 2; upper: longitudinal study 1 (L1), lower: longitudinal study 2 (L2). Red bars indicate the number of animals that tested positive for swIAV in nasal swabs for the first time; yellow bars indicate the number of animals that tested positive for swIAV for two or more consecutive samplings; dashed bars indicate the number of animals that tested positive for swIAV after being negative; and grey bars indicate the animals that tested negative for swIAV. The dotted red lines show the proportion of PRRSV-viremic animals at each timepoint. The black dotted line shows the mortality rate between two consecutive periods.

Figure 2

Results of the serological analyses for swIAV performed at Farm 1 and longitudinal studies 1 and 2. L1 = longitudinal study 1; L2 = longitudinal study 2. From left to right, the results obtained by ELISA, HI, and VNT are shown. The box and whisker plots indicate the minimum, maximum, median, and 25% and 75% quartiles. Each dot represents an individual result. For the ELISA and HI plots, the grey area delimits the threshold for positive results (S/N < 0.6 for ELISA) or a predicted correlate of protection (1:40 in HI). The number over the box indicates the proportion of examined individuals over the threshold, with the number in red corresponding to the proportion of seropositive animals at the moment when swIAV incidence was the highest for that batch of pigs. For VNT, no threshold is indicated, as no standard threshold has been determined for pigs. The average S/N values or titres were compared between consecutive weeks and are indicated in the graphs. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; n.s. = nonsignificant difference.

Figure 3

Results of the serological analyses for swIAV performed at Farm 2 and longitudinal studies 1 and 2. L1 = longitudinal study 1; L2 = longitudinal study 2. From left to right: ELISA, HI and VNT. The box and whisker plots indicate the minimum, maximum, median, and 25% and 75% quartiles. Each dot represents an individual result. For the ELISA and HI plots, the grey area delimits the threshold for positive results (S/N < 0.6 for ELISA) or a predicted correlate of protection (1:40 in HI). The number over the box indicates the proportion of examined individuals over the threshold, with the number in red corresponding to the proportion of seropositive animals at the moment when swIAV incidence was the highest for that batch of pigs. For VNT, no threshold is indicated, as no standard threshold has been determined for pigs. The average S/N values or titres were compared between consecutive weeks and are indicated in the graphs. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; n.s. = nonsignificant difference.

Figure 4

Seroconversion and HI titre when animals were infected by swIAV for the first time. The graph shows the number of animals for which an increase in the HI titre ≥ X2 was observed after infection with respect to the HI titre determined when infected by swIAV for the first time. The animals were examined 2–4 weeks after infection, depending on the periodicity of sampling.

Figure 5

Characterization of prolonged shedders and pigs reinfected with swIAV. A The graph shows the distribution of HI titres at the moment when the first swIAV infection was detected in single shedders, prolonged shedders, and reinfected pigs. B Variation in C_t values between the first and consecutive detections of swIAV in nasal swabs from prolonged shedders. Upper: C_t values for cases in which the second detection resulted in a decrease in the viral load higher than 3.3 C_t units; middle: C_t values for animals that showed similar viral loads in consecutive samplings; and lower: C_t values for cases where viral loads increased between consecutive samples. C Distribution of HI titres when swIAV was detected for the first time in the animals shown in B.