The Mukdahan Provincial Public Health Office on Tuesday confirmed one new case of anthrax, bringing the total number in the latest outbreak to four, with one fatality.
A source said three patients have been hospitalised -- two at Mukdahan Hospital and one at Don Tan Hospital.
Out of 636 at-risk people, 538 have completed a seven-day period of observation. The remaining 98 will wrap this up on Wednesday.
These people were exposed via skin contact or eating contaminated meat. The disease incubation period is seven days.
Authorities have urged the public to avoid raw beef and to only eat meat that is thoroughly cooked at a heat of at least 120C or boiled for 30 minutes to kill any potential anthrax bacteria.
Dr Weerawat Manosuthi, spokesman for the Department of Disease Control, said authorities are monitoring the outbreak in Mukdahan.
All risks remain confined to an area in Don Tan district, currently classified as an outbreak area, with health officials focusing on early detection of the disease among suspected patients and at-risk groups, he said.
He referred to the first three cases, saying three men fell ill after helping to butcher a cow that died in suspicious circumstances.
One has since passed away, and the other two are being treated.
Others involved in the butchering of the same cow are still under observation.
Dr Weerawat urged residents to avoid butchering unsafe meat and not to consume raw meat, especially during local festivals.
Anthrax is commonly found in cattle, buffalo, goats and sheep, but in this case, beef is believed to be the source.
The provincial livestock office has quarantined 124 animals for 30 days and administered antibiotics and vaccines to over 1,200 animals within a 5km radius.
There is no vaccine for humans at this time, he said, but infections can be treated effectively with intravenous antibiotics.
Infection through inhalation is rare but can be severe, Dr Weerawat said.
Skin infections are more common. They are usually less serious but can become fatal if they are left untreated, he added.