Experimental infection of weanling pigs with A-swine influenza virus. 2. The shedding of virus by infected animals.

As part of a study described in a previous paper observations were made to determine whether and for how long experimentally infected young pigs would transmit their infection to new groups of weanlings maintained in contact with them. When groups of 4 or 5 susceptible weanlings 2-3 months old were placed in contact for a month with infected pigs 42 days or 3, 6, 9 or 12 months after experimental infection, no antibody rises were observed in the contact pigs. However, a strain of virus identical with the infecting strain was isolated from lung suspensions from 2 of the 5 contact pigs exposed to pigs infected 3 months previously. Possible sources of technical error such as laboratory contamination could be almost certainly excluded. It is considered that a shedder state of virus had occurred some time during the fourth month following experimental infection. There was suggestive serological evidence that the shed virus acted as a booster dose to previously infected pigs.

Experimental infection of weanling pigs with A-swine influenza virus. 3. Immunity in piglets farrowed by antibody-bearing dams experimentally infected a year earlier.

Pigs experimentally infected as weanlings with swine influenza virus, as described in previous papers, were bred from when mature. Attempts to isolate virus at parturition from the placenta and from different organs of some of the piglets immediately after birth gave negative results. Antibody levels were determined in the sows and remaining piglets at different times after birth, and the clinical course, immunity and antibody formation were studied in some of the piglets challenged with swine influenza virus 10 days after birth. The levels were found to be generally higher in the sows than those in their offspring. Specific antibodies were detected in piglets that had presumably not ingested colostrum, but the possibility of unobserved ingestion of colostrum could not be excluded. Colostrum-fed piglets had specific antibody in their sera; an increase in antibody titre occurred by the 10th day after birth, remained until the 20th day and decreased steadily to the 30th day. Colostrum-fed piglets receiving antibodies from immune dams were resistant to a challenge of virulent swine influenza virus, and did not respond with an antibody rise during a 30-day observation period after challenge.