The two-year-old male red fox, named Basil, was discovered on a car transporter ship travelling from Southampton to New York and was taken in by the Bronx Zoo on February 19.
He was found among the cargo by customs officers after the vessel arrived at the Port of New York and New Jersey.
A spokeswoman for the Wildlife Conservation Society, which operates the Bronx Zoo, said: “Over the course of approximately one month, Basil received three different types of deworming medications as part of a carefully managed treatment plan.
“Following treatment, veterinarians repeated diagnostic testing multiple times to confirm the infection has now been successfully cleared.
“Throughout his treatment, Basil showed significant improvement.”
Basil the fox (Image: WCS Bronx Zoo/PA Wire)
She said Basil had been diagnosed with French heartworm, a rare parasite in the US that is more commonly found in wild foxes and domestic dogs in Europe.
He weighed 11.2lb (5.1kg) when he arrived but now weighs 13.7lb (6.2kg).
The zoo spokeswoman said: “He gained weight, developed a fuller and shinier coat, and demonstrated strong overall health during follow-up examinations.
“Veterinary and animal care staff noted a dramatic improvement in his body condition and appearance during his most recent health check.”
Craig Piper, interim director of the Bronx Zoo, praised Basil’s progress.
Mr Piper said: “Basil has done exceptionally well since arriving at the Bronx Zoo.
“Treating wildlife like this requires extensive co-ordination and expertise from our veterinary and animal care teams.
“We are very pleased with his recovery and continued progress since his long journey across the Atlantic.”
A spokesman for ABP Southampton said: “The Port of Southampton handles everything from cars to containers to cruises, but even we were surprised to find a fox had booked itself a transatlantic crossing.
“Clearly it fancied swapping the Solent for the Staten Island Ferry, though next time we’d recommend it considers the Queen Mary 2, which offers the Southampton to New York route with considerably more comfort.”
The Bronx Zoo is still determining the most suitable long-term home for Basil.