Prosecutors have told jurors that a Texas woman accused of cutting off her infant son's genitals two years ago was a drug abuser who showed no remorse or concern for her child as he was on the verge of death.

Katherine Nadal's defence lawyers, however, said an expert will back up the 28-year-old woman's claim it could have been the family dog, a three kilogram dachshund, that mutilated her son in their suburban Houston apartment in March 2007.

Nadal is on trial on a charge of injury to a child, a first-degree felony. She faces up to life in prison if convicted of attacking her then-five-week-old son, Holden Gothia. Her trial is expected to last at least a week.

During opening statements, prosecutor Denise Oncken told the jury that the night before Holden was attacked, Nadal indicated she was going shopping with a friend but actually "went to a dope house" with her son and got high.

Camden Gothia, Holden's father, testified that when Nadal came back, she was "not sober". Nadal, who authorities say has a history of drug abuse and has prior drug arrests, later tested positive for cocaine, methadone and another drug.

Nadal was still high the day Holden was mutilated, when she claimed the family dog, Shorty, attacked the infant in her bedroom as they slept, Oncken said.

Camden Gothia, 38, who was not married to Nadal but lived with her and his son, was at work when the attack happened.

Doctors who examined Holden will testify the genitals were cut in a perfect square by a sharp instrument, Oncken told jurors. An animal control officer who examined the dog found no blood in its mouth, on its fur or on its paws, she said.

Camden Gothia said the dog was not aggressive.

Nadal told authorities there was a torn, bloody diaper that showed the dog had attacked her son, but no such diaper was ever found, Oncken said.

Edward and Rita Vega, neighbours whom Nadal called for help, testified they didn't find any blood on the dog or anywhere else away from the bed where the infant was found. They also said that Nadal didn't seem upset.

"She showed no compassion for the baby's injury," Edward Vega testified.

Skip Cornelius, one of Nadal's lawyers, suggested his client's lack of emotion was due to shock and disorientation.

Holden, who lost half the blood in his four kilogram body, survived the attack, but the severed body parts were never found.

Allen Isbell, another lawyer for Nadal, told jurors investigators did not find DNA or blood evidence on any instrument or sharp object they tested or in the sink or the garbage disposal.

"There is another reasonable explanation for the injuries to this child," Isbell said. "An expert on dog behaviour ... will testify that in her opinion it is possible a dog could have made those injuries."

Holden, now two, lives with Camden Gothia's sister and her husband, who were given custody after Nadal and Gothia relinquished their parental rights. His father sees him regularly.

He sure won't be Holden anything when he gets older.