STAT: CDC Explains Delay to MO H5N1 Human Serology Results
Results now expected in mid-October -also validation work reported on blocking ELISA influenza serology test for cattle
Helen Branswell of STAT last night reported the latest information regarding delayed serology results from convalescent blood samples collected from the confirmed H5N1 patient from Missouri, a symptomatic household contact, and several symptomatic health care workers. Here is a link to the article (subscription may be required):
CDC testing of Missouri's bird flu case hits obstacles, delaying answers (statnews.com)
In summary, changes to the viral genome of this isolate require revised serology assays reflecting changes in H5N1 viral surface antigen presented. Because CDC scientists lack a virus isolate from the MO cases, they must reverse engineer a virus to reflect the surface antigen changes for use in the serological assays that reflect the changed surface proteins.
But the CDC has had to develop a new test to look for those antibodies because key genetic changes to the main protein on the exterior of the virus found in the Missouri case meant the agency’s existing tests might not have been reliable, Demetre Daskalakis, director of the CDC’s National Center on Immunization and Respiratory Diseases, said in an interview. He suggested it will be mid-October before the work can be completed.
“The antibodies that would grow in the person exposed to that virus would then be different then the antibodies that would grow in a person who had a virus without those mutations,” Daskalakis said.
Developing the new test has been challenging because the sample from the patient contained so little viral material that the CDC was not able to grow whole viruses from it. Instead its scientists have had to reverse engineer H5N1 viruses that contain the changes to use them as the basis for the new serology test, he said.
While the delay is unfortunate, this is a case where accuracy trumps speed. It’s speculative, but the serum from the patient may well have provided less than clear positive results, indicating that more sensitive test(s) would be required to assess the serological status of patient contacts. Serology is not straight-forward and easy to over-interpret or under-interpret; read the full article for a good discussion of some of the steps utilized to clarify interpretation.
Speaking of human serology, on a CDC ZOHU call yesterday, Dr. Eric Deeble of USDA stated that a second set of serology results are in the queue from farm workers in Michigan, and human serology studies are underway on Colorado poultry responders and dairy workers. It will be interesting to see those results, as well as see more discussions regarding use of revised viral antigens in the Hemagglutination Inhibition (HI) serology testing of these subjects.
I would imagine that the CDC work is also of interest to animal health lab specialists, evaluating HI testing in cattle and other species. For all the limitations of serological testing, it’s the best retrospective tool we have for looking at populations for exposure and possibly infection specifically by H5N1 2.3.4.4b B3.13. As the MO case work indicates, even having the correct viral clade antigen for testing may not be sufficiently sensitive and specific for definitive answers.
Blocking ELISA serology tests have been used for several years for mass screening for Influenza A antibodies in poultry, pigs, and other species. However, this test does not differentiate between H subtypes because it measures antibodies to influenza viral nucleoprotein in serum, which are formed to all influenza A subtypes.
Cattle are not widely infected by other influenza A viruses; therefore, blocking ELISA-positive results in cattle serum and milk can serve as a great measure of H5N1 infection in cattle serum samples. Commercial Influenza A H5 blocking ELISA test kits have been available for several years for use in poultry and other species. A complete validation study of a commercial kit for use in cattle serological and milk testing was published this week:
Anti-influenza antibodies to monitor H5N1-IAV 2.3.4.4b outbreaks in dairy cattle were evaluated with a commercial IAV enzyme-linked immunosorbent assay (ELISA) (Cat# 99-0000900; Idexx Laboratories Inc., Westbrook, ME, USA) designed to detect anti-NP antibodies in multiple species.
Please read the entire study carefully. This technology applied to milk testing has applications in assessing herd recovery, especially when paired with PCR testing for active virus.
However, I’d urge that researchers and clinicians utilize ELISA testing more fully for epidemiological intra-herd and inter-herd viral spread assessments. On the CDC ZOHU call yesterday, Dr. Deeble stated that understanding spread of H5N1 within and between herds remains “challenging”, with a third round of studies now nearing completion at ARS NADC. I hope that the results of 2nd and 3rd rounds of studies will soon be disclosed. However, we could also learn much about viral spread through larger scale serological sampling. Now that USDA seems a bit more flexible in granting that transmission is not fully explained by mechanical (fomite) spread of milk within and between farms, it’s time to utilize resources like ELISA serology for infected farm sub-population-based analyses.
The test could also be used for retrospective testing of asymptomatic contact herds. Perhaps serological testing as part of the Federal Order needs to be reviewed, since positive results only indicate exposure, not current infection. Producers and veterinarians should not be unwilling to test due to fears of quarantines.
Perhaps the most critical issue the dairy industry refuses to face right now is risk of infection and transmission between non-lactating dairy cattle. If these animals are widely susceptible and infectious, then movements between herds without testing are high-risk activities for further spread of virus. A series of serological surveys in affected and contact herds and their sub-populations could bring scientific clarity. This issue can no longer be ignored.
John