079 - Bacillus para-pertussis: A Species Resembling Both Bacillus pertussis and Bacillus bronchisepticus but Identical with Neither

This is a topic I've heard mentioned before as related to the effectiveness (or lack thereof) of the pertussis vaccine. And perhaps more distantly related to other vaccines. The idea is that there are other, rare, related pathogens or mutants of common pathogens that may take the place of pathogens made rare by vaccination, essentially refilling the niche. 

Or, in a more conspiracy-prone vein, that after a vaccine is widespread, cases of the vaccine-preventable disease are recorded instead as caused by a different strain, species, whatever; essentially changing the definition of disease so that artificially it looks like the rate of the disease declines drastically post-vaccine.

Personally I haven't found much evidence to bear out the latter idea yet. Definitions do change, a notable example being the broadening definition of autism artificially increasing the number of cases, but to say it's as simple to change a case rate as to reassign a set of symptoms to a different cause is to ignore the fairly modern ability to identify pathogens using very precise molecular techniques and such.

The former is an interesting idea, but I don't think it has borne out well in history. Though it may sometimes appear that there is a parallel increase in a parallel disease as one declines due to vaccination, often that's just because the vaccine-preventable was so common before, it masked instances of the parallel one, and now that's not so common, the parallel one becomes more apparent.

Anyway, the specific case today is infection with the bacterium Bordetella parapertussis as a rare alternative cause of whooping cough. Normally the disease is caused by Bordetella pertussis, which seems generally more severe than its cousin. Grace Eldering and Pearl Kendrick worked on pertussis a lot in the early 20th century, and in this study they identified B. parapertussis as similar but distinct from its cousins.¹

These researchers had collected almost 1500 isolates from whooping cough patients over 5 years. But 10 of them seemed unusual: on agar, colonies grew larger than expected over time, and could grow without blood in the medium. Another species, B. bronchisepticus, also caused disease, but was also different in some ways from this new isolate (chiefly, motility).

Of the cases from which these isolates came, half were less than moderately severe, but almost all of them whooped, so it seemed like regular whooping cough. Apparently at least one had a co-infection with this isolate and regular B. pertussis.

Then Eldering and Kendrick did a bunch of biochemical bacteriological tests on one of the isolates, called 309, and compared it to B. pertussis and B. bronchisepticus. This was neat because I remember learning about most of these in basic microbiology lab, and here I see them applied, many decades ago. Overall, 309 was similar to B. pertussis in some ways and similar to B. bronchisepticus in others, but to neither in all.

Finally they tested the immunological characteristics of the three strains. They found when antibodies were generated to any one of them, they cross-reacted somewhat with each of the others, but not completely. So they seem to share some antigens.

So apparently B. parapertussis is a separate agent, that occasionally (around 0.7% of cases) causes whooping cough in humans. One important question related to vaccines is the one raised at the beginning (if B. parapertussis could come fill a niche left if B. pertussis is eliminated by vaccines); more on that later, but J.J. Miller Jr. mentioned this in a study about a decade later:

"Some apparent failures of immunization in vaccinated children are due to infections with the Bacillus parapertussis. These infections will seldom be diagnosed correctly, as the cough is clinically indistinguishable from that of pertussis: in other words they are cases of whooping cough but not of pertussis."²

Another question, kind of on the other side, is whether a vaccine against B. pertussis could give cross-protection against B. parapertussis (or vice versa). Since they do share antigens, it's not unlikely. Today's study doesn't say much about this, but I think Eldering and Kendrick did address this question in later studies.

A final note on taxonomy: it seems like the taxonomy of Bordetella was pretty confused at least until the 1950s, so B. pertussis was referred to as Bacillus pertussis, Hemophilus pertussis, and probably others until people finally settled on Bordetella. So if you see those other names, you know.

References:

1.

Eldering, G. & Kendrick, P. Bacillus para-pertussis: A Species Resembling Both Bacillus pertussis and Bacillus bronchisepticus but Identical with Neither. J. Bacteriol. 35, 561–572 (1938).

2.

Miller, Jr., J. J. Immunization procedures in pediatrics. JAMA 134, 1064–1069 (1947).