Evolution ensures survival for this multi-species virus.
Swine Influenza Virus (SIV) continues to evolve. During the last decade, many strains commingled with genes from avian, human or swine species to create new swine flu strains.
SIV infects both breeding and growing animals. A sudden onset of respiratory conditions with fever and depression of the animals is an early sign of the disease. High fever in pregnant animals can cause abortions.
Flu virus can be difficult to control on farms. Relying on vaccines can be of limited value if the vaccine strains don't closely match the herd strain. Fortunately, diagnostic tests give us very specific strain identification and comparison results.
Diagnosing SIV infections is best done through testing that demonstrates the presence of the virus itself or one of the viral antigen proteins. Nasal swabs or fresh lung tissue provide the best specimen sources for these tests. Blood testing can yield confusing results because of the cross reactivity of the strains.
A 1,200-sow farm produced weaner pigs for three-site production. The sow farm was negative for porcine reproductive and respiratory syndrome (PRRS) and produced over 25 pigs/sow/year. The nursery manager told me he had difficulty starting the pigs on feed after weaning. In the last three weeks, weaning weights had dropped by 1½-2 lb. Some pigs entered the nursery with a mild cough.
Upon visiting the nursery, I noticed that about 20% of the previous week's weaned pigs were depressed and inactive. They had rough hair coats and a gaunt appearance. Rectal temperatures ranged from 104-106 °F. About 10% of the pigs had a mild cough. Tissues were collected from four pigs and sent to the diagnostic lab.
Meanwhile, I scheduled a visit to the sow farm to follow up the weaner pig problem. The owner was confused by the recent news of the weaner pigs because breeding and farrowing appeared normal.
However, the breeding manager commented she had difficulty getting the weaned sows to express estrus and also noticed more tearing around the eyes. I blood tested several sows and collected nasal swabs.
The lab results for the sow unit and nursery indicated both were negative for PRRS virus but were positive for SIV. The SIV for both sites was sequenced at the lab and found to be the same H3N2 strain.
The clinical signs at the sow unit were minimal, yet causing subtle problems. A commercial vaccine provided blanket immunization in the sow herd and also raised colostral antibody levels for suckling pigs. Nursery pigs were treated symptomatically with water medication and injections.
An 1,800-sow, breed-to-wean unit owner was alarmed because sow death loss had increased to 20% (annualized) and abortions had more than tripled. The sow herd was PRRS-negative, and had never seen serious health issues.
When I walked into the gestation barn, only 30% of the animals jumped up. This was odd, because sows were an hour overdue for feeding. They were obviously depressed and off feed. Rectal temperatures of several off-feed sows were 105-107 °F.
I bled the sows for PRRS virus and collected eight nasal swabs from tear-eyed sows. Upon leaving the barn, I asked the unit manager if he had brought in any new animals recently. He replied half the staff had been sick for two weeks, so there was no time to move any gilts in from isolation. Flu had been present in many of the local schools. Some of the farm employees had been coming into work when they were still sick.
The lab returned a positive SIV test. Five days after my visit, most of the sows were back on feed. Employees were told to stay home if they had a cold or a fever.
A 2,500-head finishing barn full of 250-lb. pigs was one week from marketing. The owner said that he had received a panicked phone call from the caretaker at the barn where nine pigs were found dead that morning.
When I stopped late that afternoon, eight more pigs were dead. Most wouldn't get up, even after I stepped into the pens. Rectal temperatures on 10 pigs averaged 105 °F. Rapid onset of acute respiratory signs with widespread coughing made me suspect an influenza break.
Since the pigs were very close to marketing, I elected to place the pigs on liquid aspirin in the water to let the virus run its course. In five days, 95% of pig activity was back to normal again.
Lung tissue analysis confirmed SIV on all samples sent.
Swine flu continues to cause substantial losses. Discuss SIV with your veterinarian and the role it may play in your production system.