Observers have been denied access to horses mustered from Northern Nevada's Calico Complex.
"Advocates continue to request daily site observation and the Bureau of Land Management refuses to co-operate, choosing instead to work in secrecy," the foundation said.
"The bureau denies daily access to the facility by experienced humane observers and chooses to work behind closed doors on private land."
The bureau was maintaining its "closed-door, business-as-usual protocol".
The foundation said the bureau had cancelled last week's only public entrance period of an "insufficient" two hours.
"Advocates understand the Easter cancellation but question why the bureau has denied repeated requests for an alternative public observation period, effectively hiding the public's horses for a 14-day period despite enormous global concern and interest in their well-being."
The foundation also expressed its concern over cases of pigeon fever among the horses.
Veterinarian Richard Sanford, in a report on the outbreak, said the bacteria causing pigeon fever is Corynebacterium pseudotuberculosis, which is found in the soil.
It is most likely transmitted by biting flies and has a long incubation period of weeks or months.
"The disease has nothing to do with pigeons. The name comes from the large chest abscesses that some horses can get, which look like the large breast of a pigeon."
Infections are also also known as dryland distemper or pigeon breast.
Sanford said around 2 per cent of the 1922 horses received at the facility showed clinical signs of healed chest abscesses from recent pigeon fever infection and .25 per cent to .50 per cent showed more recent symptoms or currently were infected with pigeon fever.
"As of March 31, 2010, at the Indian Lakes Road facility, pigeon fever is still noted at the .25 per cent to .50 per cent rate, mostly found in the juvenile horses.
"The incidence of pigeon fever at the Indian Lakes Facility is at the same percentages that exist on the Calico Complex. The chest swellings range from golf ball size to grapefruit size.
"Fly season occurs at the end of summer. Therefore, it is expected that incidents of pigeon fever will decrease over time.
"Disease may or may not reoccur during the 2010 fly season based on environmental factors, such as temperatures, precipitation levels, soil conditions, fly conditions, etc."
Sanford noted that California had severe pigeon fever conditions during the 2009 fly season. "It is speculated those conditions apply to Nevada as well.
"Horses housed at the Indian Lakes Road facility that have active pigeon fever are being monitored. No treatments have been administered to date.
"Abscesses have all resolved without treatment. No deaths or complications have been associated with infection.
"Based on 25 years of past experience with wild horses and burros, pigeon fever can exist in many of our wild herds, depending on current year environmental conditions."
The BLM does not count or report foals who are born and die in the pens, and at least one newborn has died to date, the Cloud Foundation says. "With babies being born almost daily, advocates want them to be identified and publicly reported. More than 75 foals have now been born in the pens with their captured mothers."