Dr. John Andresen’s red-headed patient gives him a wary glance as the doctor approaches with a long needle. The doctor pats him gently before quickly slipping the needle into his neck. After a few seconds, the blood has been drawn and Dr. Andresen pulls the vial out, rubbing the spot where the puncture was made.

His patient didn’t wince once. Dr. Andresen pets the chestnut-colored gelding, who has been nicknamed Vegas, and moves on to the next horse in the stable at Highwind Farm in Mattituck.

For decades, Dr. Andresen — who began his 50th year of practice in 2016 — has served at Mattituck Laurel Veterinary Hospital as one of the only North Fork veterinarians who deal specifically with large and unique farm animals, such as goats, alpaca and horses.

Along the way, he’s developed a loyal base of clients who say they know they can count on him to cure their creatures’ ills.

“He really knows his stuff,” said Kate Nichols of the Little Red Barn farm in Jamesport. Dr. Andresen has been taking care of Ms. Nichols’ animals since she was 12 years old.

“I never thought about using anyone else,” she said.

Dr. Andresen, who will turn 75 next month, was raised in upstate New York and was urged as a teen to pursue engineering, despite his desire to one day become a veterinarian at a zoo. He worked briefly at IBM just before the computing boom, but decided to leave that field when he realized it didn’t interest him.

Instead, Dr. Andresen studied to become a veterinarian. He graduated from Cornell University in 1966 and immediately started working as a vet. He and Dr. Charles Timpone opened the animal hospital in Laurel in 1994.

Since then, more and more people on the East End have horses or livestock that need medical care, Dr. Andresen said.

“There’s been a real interest in livestock recently,” he said. “I think most Americans have a love affair with horses.”

Of course, it’s not just horses that he cares for. He’ll treat all manner of livestock, including cows and goats.

He’s even worked with exotic animals at the Long Island Game Farm, a taste of what he dreamed of as a teenager. Dr. Andresen — who also coaches the Riverhead Blue Waves pole vaulters when he’s not working — recalled once removing a growth from the head of a sedated tiger.

“You don’t realize how big a tiger is until you’re next to it,” he said.

A horse at Highwind Farm pokes his head outside his stall for a treat. (Credit: Paul Squire)

On Tuesday morning, Dr. Andresen — dressed in blue jeans and a striped polo shirt — prepared to make his rounds. Some animals come into the hospital’s office for care. A goat named Honey arrived that morning to have a cast removed from her front leg. But many of Dr. Andresen’s patients require house calls.

As he calls his clients to set up appointments, a sleepy dog — Dr. Timpone’s pet Ava — lies under Dr. Andresen’s desk. Sitting next to him is Katelyn Cameron, his intern and a veterinary student from the University of Rhode Island.

Every Tuesday, Ms. Cameron drives about an hour from her home in Mount Sinai to assist Dr. Andresen with his appointments. She also wants to be a large-animal veterinarian.

“He was the only one I could find who does this kind of thing,” she said.

Their first stop is at Highwind Farm, where Dr. Andresen took blood from a handful of horses to test for equine infectious anemia. Employees there greeted him with a nickname: “Doctor A.”

Later, he swung by Ms. Nichols’ farm to check out at her newest addition, Freckles the horse, who had a cut on his mouth, which Dr. Andresen quickly diagnosed as a type of skin cancer. He then fed dewormer medication to the farm’s goats and when Ms. Nichols warned him that the farm’s pig, Star, might not cooperate, Dr. Andresen barely responded.

He took the syringe and dangled it in front of Star’s mouth until the pig bit down, giving the doctor a chance to squirt the medicine inside.

Ms. Nichols was, as always, impressed by what her favorite vet can accomplish.

“He’s not allowed to retire,” she said with a laugh.

Top Photo Caption: Dr. John Andresen calmly draws blood from Vegas, a horse at Highwind Farm in Mattituck. Dr. Andresen is one of the few vets on the North Fork who care for large animals like horses and goats. (Credit: Paul Squire)