More diagnostics advised for better IB management in broilers
By Lloyd Keck, DVM, ACPV
Senior technical services veterinarian, Zoetis Inc.
Over the past 5 years or so, infectious bronchitis (IB) has been the leading cause of the respiratory disease I see in broilers. My experience has led me to conclude that more diagnostics could go a long way toward helping the industry do a better job managing IB.
As it is now, IB testing is too often initiated after losses are mounting. It’s no wonder considering all there is to do when one poultry operation is raising millions of birds on hundreds of farms. It becomes easy for IB testing to slip through the cracks. Another reason diagnostics for IB isn’t optimal is because there is a limited number of reliable labs to test for the disease.
One case that’s a good example of how diagnostics can help solve an IB problem involved an integrated broiler operation in the Southeast. I was asked to come in to help figure out what was going on. Birds had mild respiratory signs at 3 to 4 weeks of age. There were brief production drops among breeders and some problems with the shell quality of their eggs. The bigger problem, however, was a doubling in broiler condemnations due to airsacculitis.
This scenario pointed to IB, so I proceeded to test for IB virus as well as for other causes of respiratory diseases such as Newcastle disease and infectious laryngotracheitis.
For IB, I always obtain both indirect and direct testing. Indirect testing is based on serology with ELISA or hemagglutination inhibition. These tests will tell you if IB virus is present by detection of IB antibodies, but they won’t tell you the specific IB virus strains that are circulating. Direct testing pinpoints the strains, and for this I submit bird samples for virus isolation and reverse transcriptase-polymerase chain reaction (RT-PCR).
Timing is crucial
Sample collection is vitally important and it’s definitely an art as well as a science. I think cases of IB in poultry are often missed or misdiagnosed due to improper sampling time or due to improper sampling technique and handling. That’s why I usually do the sampling myself.
This isn’t like testing one sick dog or horse. In a flock of birds, multiple samples must be taken to get a good handle on what’s going on. For serology, at least 25 samples should be obtained. It takes at least two weeks and sometimes longer for IB virus antibodies to show up, so I draw blood from birds just before processing.
For RT-PCR, I usually take about 15 swabs from acutely sick birds, but it’s okay to group five samples as one; that means three RT-PCR tests will be run instead of 15, and that helps reduce testing costs. I usually sample from the trachea, although samples from other respiratory tissues can be used. If I’m sampling breeders, I might take oviduct swabs too.
Although it takes weeks for IB antibodies to show up on serology, IB virus will show up on respiratory tissues soon after it’s inhaled by the bird and can be detected in the first several days after infection. The field virus, however, will not be detectable after about 10 days. Since birds don’t even show clinical signs until they’ve been infected for 2 to 3 days, it’s important to sample birds that appear to be on the front end of the infection.
Interpret in context
Interpretation of RT-PCR results must be done in context. If all you get back from RT-PCR is Massachusetts IB, you’ve probably recovered a vaccine virus because this strain of IB doesn’t typically cause a lot of problems anymore. If birds have been vaccinated with other strains of IB virus, you have to consider the source of the sample. For example, respiratory vaccine viruses given at hatch may persist for up to 2 or 3 weeks in most respiratory tissues, but in the cecal tonsils, they may be present for several weeks. This has to be factored in when analyzing the results.
In the case of the Southeast producer, we isolated several variant IB viruses from a few different serotypes. We initiated the use of a three-way combination of IB vaccines that included serotypes the producer hadn’t used before, and it worked well. Morbidity and the problem with condemnations subsided.
Better management of IB could be achieved by initiating a routine surveillance program that would lead to early diagnosis, before losses from the disease really start to mount. Proactive surveys done in the fall can provide timely information should a vaccination program need to be adjusted heading into the peak challenge of winter. A subset of flocks can be tested on the farm at the end of the production cycle or at the processing plant. I like to take samples from three flocks the service staff says have respiratory disease and three that don’t. Sometimes the flocks without signs of respiratory disease turn up positive for IB, but usually the service staff is correct about which flocks have respiratory disease and which don’t.
For cases when IB is suspected but pinpointing the exact IB virus proves difficult, the answer may be the use of sentinel birds. The beauty of this approach is that you’re able to control when the birds are exposed, which in turn helps ensure you can obtain samples at the correct time to find the causative IB virus.
Sentinel birds may or may not be specific-pathogen-free, but they should be birds raised at least in semi-isolation and they must be naïve for IB unless Arkansas vaccine is suspected of masking another variant IB virus.
The birds must be in a wire pen with feed and water. The wire pen enables the birds to have exposure to other birds in the house but also enables you to find them when needed for sampling.
I usually place about 20 sentinel birds when they are about 2 or 3 weeks of age. However, if birds on a farm have been showing clinical respiratory signs at, say, 5 weeks of age, I’d place the sentinel birds at about 4 to 4.5 weeks of age. On days 3, 6 and 9 after placement, I take tissue samples from the lung and cecal tonsils; the samples must be kept separate from each other. Testing at the lab usually involves egg inoculation followed by PCR or RT-PCR.
Using sentinel birds is time-consuming and a bit complicated, so it’s probably not a cost-effective approach for routine use. However, it can be very helpful when you’re having a problem with IB and need a more accurate diagnosis. Considering that IB remains the primary respiratory disease we see and that it can result in substantial economic losses, employing the use of sentinel birds may be well worth the effort.