This is from the 1997 REDBOOK, considered the "bible" for infections disease
by US pediatricians:
Pertussis can be a severe disease in infants; duration can be 6-10 weeks of
cough.  According to surveillance data from 1980-89 in the USA, pneumonia,
seizures, and encephalopathy occurred in 21.7%,3.05, and 0.9%, respectively,
of infants with pertussis.  The case-fatality rate was 1.3% in children
younger than 1 month and 0.3% in those 2-11 months of age.
Treatment:
Infants younger than 6mo . . . often require hospitalization for supportive
care to manage coughing paroxysms, apnea, cynaosis, feeding difficulties, and
other complications.
Antimicrobials given during the catarrhal stage may ameliorate the disease.
After paroxysms are established, however, antimicrobials ususally have no
discernible effect on the course of illness and are recommended primarily to
limit the spread of the organism to others.  The drug of choice is
erythromycin (40-50 mg/kg/d po divided into four doses; max 2g/d); some
experts prefer the estolate preparation.  The recommended duration of therapy
to prevent bacteriologic relapse is 14 days.  Because of in vitro
susceptibilities, clarithromycin and other macrolides also are likely to be
effective and thus are alternative for patients who cannot tolerate
erythromycin.
Trimethoprim-sulfamethoxasole (8-40mg/kg/d po in two divided doses) is another
possible alternative, but its efficacy is unproven.

Septra use in the mother should be of NO CONCERN with breastfeeding.  However,
in my opinion, the HCP would be wise to use erythromycin to treat this family,
rather than Septra, since its efficacy in unproven.

I agree with Dr. Newman--there is no need for mom to pump & dump and she
should continue to breastfeed.

Linda L. Shaw MD FAAP