Tired, swollen and blistered feet are among the biggest challenges for the thousands of Central American migrants making their way through southern Mexico in the hope of starting new lives in the United States.

Three weeks of pounding the highways every day takes a toll. Whenever possible, the migrants discard damaged footwear, replacing them with donated shoes found at stops along the way or with spare pairs they carry in backpacks.

The most gruelling days demand treks of more than 100 miles.

Migrants eager to find strength in numbers must maintain that pace to remain with the group. And they do so in cheap shoes and sweat-drenched socks, which they eagerly peel off at the end of each day.

The migrant caravanMigrants bathe in the rain water outside a temporary shelter set up in Veracruz. Mexico (AP)

Blisters are a nasty foe for migrant feet, young and old alike. Red Cross personnel at waystations bandage swollen feet or apply antisceptic to broken blisters. Children wince as their wounds are treated. Flies gather on open sores, and the risk of infection is high.

“These are extreme conditions,” says Ignacio Escotto, a Mexican vascular surgeon.

Unrelenting contact with hot pavements will cause the feet to swell, he said, while dehydration and malnutrition also wreak havoc on soft tissue.

The Mexican government said around 3,000 migrants have applied for refuge in Mexico in recent weeks and about 500 have asked for assistance to return to their countries of origin.

Migrant caravanShoes have been donated for the migrants as they continue their long journey on foot (AP)

Many of the about 4,000 migrants still with the caravan have now covered more than 800 miles since setting out from Honduras on October 13, hitching rides on flatbed trucks whenever possible. They face another more than 800-mile trek to the nearest US border crossing.

Calls have grown in recent days for buses to transport scores of people from the caravan to the Mexican capital, where the fatigued travellers hope to find respite and medical treatment. Those buses have not arrived.

The group now finds itself in the Gulf state of Veracruz, traversing what some call the “route of death” because of the large number of migrants who have disappeared in the state in recent years.

It is unclear what part of the US border they will aim for eventually, but their latest overnight stay in Veracruz could be one of their last before they head to Mexico City, a potential launching spot for a broader array of destinations.

Inspired by their progress and an outpouring of support from townspeople along the way, several smaller caravans of migrants have formed in Central America in recent weeks in an attempt to improve their odds of making it to the US.