Why is tertiary syphilis not contagious?  some say tertiary syphilis
results from a hypersensitivity reaction, so is the theory that the
actual treponemal load is low?

"Transmission requires exposure to open lesions with organisms
present, features seen with the primary chancre and with some elements
of secondary syphilis, including mucocutaneous lesions. The incubation
period varies from 10 to 90 days (average about three weeks), being
shorter with larger inocula "

"The pathogenesis of late syphilis is not well understood and may
differ for the different manifestations of the infection. Cellular
immunity is important for control of experimental infection and
undoubtedly is involved in the lesions of late syphilis although the
mechanisms remain unknown."

From: Uptodate.com

Tertiary syphilis is not usually contageous because it tends to
present as a secondary symptom. By that I mean, the symptoms produced
by the tertiary stage (necrosis, blindness, rash and neuropathy) are
only there because of an indirect contact between the body and the
bacterium. The bacteria lays dormant producing toxins which cause the
symptoms, and most of the symptoms are not exposed (no organism-filled
sores etc.) as much as say primary syphilis.

In primary syphilis, the bacterium is DIRECTLY causing symptoms by its
multiplication and toxicity, culminating and growing in the broth
provided for by exposed and open sores called "chancres". Touching
these sores (filled with copious amounts of organisms) results in
transmission.

To answer your second question, the bacteria count can be
significantly lower in tertiary syphilis depending on where you
sample. A blood culture would yield high counts whereas a swab of a
rash would yield lower counts.

Baz :)