Based on an examination of serum samples from 115 dairy farm workers collected during July and August, the U.S. Centers for Disease Control and Prevention (CDC) has called for more extensive serologic surveillance to ascertain the incidence rate among individuals exposed to herds with and without bovine influenza H5N1 strain B3.13.

Among the 115 samples, eight yielded antibodies to the specific virus, representing seven percent of those surveyed.  Of those that were obviously exposed and developed circulating antibodies, four reported clinical symptoms including upper respiratory involvement and conjunctivitis with four cases asymptomatic.  This suggests that a proportion of those infected may not develop symptoms after exposure to the current virus circulating in U.S. dairy herds.

Dr. Nirav Shah, Principal Deputy Director of the CDC, stated, “These findings mean that we need to cast a wider net in terms of who is offered a test so that we can identify, treat and isolate those individuals.  Serologic testing at this time is essential to understand the epidemiology of zoonotic bovine influenza. 

To treat and isolate individuals would require rapid testing applying PCR since detection of antibodies is only possible weeks after exposure.  Of greater importance in both antigen and antibody testing is to determine that there is no contagion through assay of nasal swabs and subsequent serum samples collected from household and other contacts with individuals known to have been infected.

It is also of importance to ascertain why only seven percent of farm workers in contact with herds shedding H5N1 acquired infection.  The circumstances favoring zoonotic transmission including the specific tasks or location of workers, the use of PPE or individual susceptibility should be evaluated.

It is anticipated that CDC and specialists in the molecular biology of influenza viruses are conducting sequence analysis to detect any mutations that may favor zoonotic infection or even human-to-human transmission.