Faithful dogs are walking thousands of miles to the US-Mexico border with their migrant owners, only to be abandoned in droves once there.

Migrants trekking to the US either bring their pets or pick animals up along the way without realizing animals are subject to strict rules for entry at the border.

Pets are also not allowed into federal processing centers, causing many to ditch their dogs right there, leaving them to fend for themselves in the harsh conditions at the southern border.

The Post spotted dozens of domestic breeds of dogs, now strays, wandering on both sides of the border at Eagle Pass, Texas, this month.

Many were simply abandoned like the piles of clothes, backpacks, shoes and children’s toys discarded all along the banks of the Rio Grande.

Some were injured. Many looked terrified and flinched even when just being passed a bowl of water.

Most were starving and reduced to scrounging for food wherever possible.

Authorities, including members of the National Guard, feed and care for the animals they encounter on the streets in the area.

But the problem is much bigger than any well-meaning person or group can tackle on their own.

The Centers for Disease Control and Prevention requires dogs to have a valid microchip and rabies vaccination certificate, among other strict criteria, before they enter the country, in order to protect the health of US citizens.

Dogs from countries that are deemed at high risk for rabies — including Colombia, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Belize, Bolivia and Peru, among others — or which have traveled though those countries in the last six months are automatically disqualified from entry to the US, causing a problem for many who journey from South and Central America.

Several animal rescue groups have launched missions to help local law enforcement reunite dogs with their migrant owners — almost all of whom are long gone.

Even if all other criteria were met or dogs and their owners are smuggled over the border illegally, the people often don’t then have the paperwork or money for their pets to fly with them.

Most buses also will not allow pets to ride on the grueling, days-long journeys to other parts of the country.

The number of dogs at the border is also multiplying rapidly as new litters are born to the abandoned ones.

A Customs and Border Protection (CBP) spokesperson told The Post that if an owner does not transfer possession to friends or family, the agency “works with local officials from animal health services, such as a local humane shelter, to see if there can be placement for the animal while its owner remains in custody.”

However, local shelters in Eagle Pass are overrun with the sheer number of homeless dogs along the border.

CBP did not respond to questions regarding how many animals had been turned away by the agency or what their fates were.

The problem also exists inland and has been reported in Colony Ridge, a top destination for newly arrived migrants located some 35 miles north of Houston. Several advocates last month told told The Post the area has more than 1,000 neglected animals in desperate need of rescue.

“The conditions are heartbreaking,” said John Rouke, a veteran who’s engineered previous animal rescues around the southern border.

Independent rescuer Lisa Noble said there are hundreds of starved and emaciated dogs at a time — “maybe more” — living in the area.

Animal rights group People for the Ethical Treatment of Animals (PETA) confirmed to The Post it has received reports that migrants bring animals “as varied as guinea pigs and small dogs” to the US border.

“But we know that the border processing facilities don’t allow it,” PETA spokesperson Catie Cryar said.

“We have also heard of an informal network of people on the Mexican side who are looking after animals who have been denied entry.

“Many animals are a cherished, integral part of the family and, just like children, should not be separated from that family.”

The organization is calling on the Biden administration to allow animals into the US “if they are vaccinated or can be quarantined” so they can be reunited with their families, Cryar told The Post.

During fiscal year 2023, which ended Sept. 30, more than 2.47 million migrant encounters were recorded along the southwestern border, according to US Customs and Border Protection.

A one-month record of 302,034 people were encountered on the Mexican border in December, according to the government agency’s statistics.

On Sunday, a long-awaited, $118 billion bill was introduced in the Senate that combines new and stronger border measures with aid packages for Ukraine, Israel and other allies.

However, some House Republicans have called it a “non-starter.”

Meanwhile, thousands of migrants continue to pour in over the border every day.

And as their wet and unwanted gear continues to pile up along the banks of the river and by broken fences, so too do the dogs that came with them, now abandoned to the wild and all alone.