Hundreds of cows face destruction after being left stranded on board a ship for two months over fears they have bovine bluetongue virus.
The 895 animals began their "hellish" journey on the Karim Allah after setting off from the Spanish port of Cartagena in mid-December.
The shipment was also rejected by several other countries including Libya as it drifted through the Mediterranean before returning to Cartagena on Thursday.
Spanish government veterinarians began inspecting the cows on Friday. If they test positive for the insect-borne virus, which causes lameness and haemorrhaging, they will be immediately destroyed and the shipment would have to be written off as the carcasses cannot be sold.
If the cattle are cleared, they can be resold for live export.
The agriculture ministry said it would take appropriate decisions after analysing information from the inspection. Two vans and a truck carrying an electric generator parked up on the quayside next to the vessel on Friday morning, according to witnesses.
Miquel Masramon, a lawyer representing the shipowner, Talia Shipping Line, said the owners had spent more than 1 million euros (£866,000) to ensure the animals' welfare during the months-long journey.
He claimed that only 15 calves had died, adding: "That's a success because they have been on board for two months without being able to disembark anywhere."
However campaigners fear that as many as 100 had died, while animal rights activists questioned the treatment of the cattle amid reports that the vessel had not received any further stocks of animal feed.
Silvia Barquero, director of the Igualdad Animal NGO, said the cows likely have severe health problems after their "hellish" crossing.
"What has happened to the waste produced by all these animals for two months?" she asked. "We are sure they are in unacceptable sanitary conditions."
Ms Barquero said animals that die during such voyages are usually thrown overboard, in contravention of international regulations, while the alternative of storing carcasses aboard for months would be even worse.
It remains unclear who owns the shipment. According to Mr Masramon, the original exporter says it is not responsible for the cattle as they have already been sold.
Another 1,776 cows are also stranded on board a second ship, the ElBeik, which set sail from Tarragona in Spain in December and was also blocked by authorities in Turkey and Libya. It is currently moored off the Turkish Cypriot port of Famagusta.
Additional reporting by Reuters