When a smell so foul wafted across Cape Town this week that locals described it as “unimaginable”, the search for the source of the stench choking the picturesque South African tourist destination led to the city’s port.

Nearly a mile from the dock, Terence van der Walt, a local wine merchant, was stuck in traffic Monday morning when the smell, made worse by the hot summer weather, began to invade his car. With such an enveloping smell, it seemed pointless to roll up the windows.

“It was so lazy,” Mr. van der Walt described his experience on Tuesday. “It would have been green if this was a cartoon.”

After the smell hovered over Cape Town for several hours, a team from the local environmental health agency discovered the source: a Kuwait-registered, 623-foot-long cattle truck with 19,000 cows on board.

The freighter Al Kuwait docked at the busy port of Cape Town on Sunday to replenish feed supplies during its voyage from the Rio Grande port in Brazil to Iraq, according to shipping data. The animals had been on board for more than two weeks.

It was the ship’s first docking in South Africa, said Jacques Peacock, a spokesman for the national Society for the Prevention of Cruelty to Animals. Therefore, before the ship’s arrival, the organization had obtained a court order allowing inspectors from the group to board the ship and inspect its cargo.

On board, they found a buildup of feces and ammonia in the animals’ cramped stalls on several decks. An “unimaginable” smell was created, the group said in a statement on Monday. On Tuesday, after inspectors spent several hours on board, the organization released images of cows whose coats were caked in trash. Veterinarians found several dead and injured cows and euthanized eight animals, the group said.

“This smell is an indication of the terrible conditions in which the animals suffer,” it said.

The group has campaigned against the transport of live animals by sea in South Africa and has lobbied the country’s government to ban the practice in its waters. Such ships often have poor ventilation and unsanitary conditions, the group said, adding that animals risk being trampled or injured when traveling over rough seas, and the ships rarely have a veterinarian on board.

Although the South African government issued new guidelines for exporting animals from the country last year, Peacock said the SPCA now plans to introduce stricter guidelines for ships from other livestock-exporting countries.

The vessel is owned by Kuwait-based Al Mawashi, which specializes in livestock trading and transportation and has offices in Dubai, South Africa and Australia.

The ship was chartered by a customer and Al Mawashi knew nothing about the delivery of 19,000 cows or the conditions in which they were kept, said Sithembele Qomoyi, a spokesman for the group’s South African division. The Al Kuwait had complied with animal welfare regulations, he added. The carrier had recently passed inspection in Australia, where Al Mawashi also does business.

In South Africa, the Society for the Prevention of Cruelty to Animals is in an ongoing court case with the livestock trading company over the export of live livestock.

Mr. Qomoyi said the company considers “any ship that transports live livestock by sea to be a ‘death ship’.”

The freighter remained in port on Tuesday and was expected to leave South African waters by Wednesday.

Officials have ordered the local authority that operates the port to ensure the ship does not pump waste into the port. For his part, Mr van der Walt said he swam in the sea on Tuesday and found the water to be clear.

Although the smell now came from outside the city, it was a worrying reminder to locals who had to contend with another source of foul matter: the city’s dilapidated sanitation infrastructure.

Councilors in the mayor’s office acted quickly to reassure residents that the recent noxious odor was not coming from raw sewage, as was the case just weeks earlier when a water pump collapsed in a northeastern suburb.

Last fall, heavy rains damaged pipes in another suburb and sent wastewater into rivers and wetlands, said Caroline Marx, director of Rethink the Stink, a water activist group in Cape Town. And since then, there have been about a dozen contaminated sewage spills in the area, she said.

Although the city has increased its sanitation budget, Cape Town is struggling to keep up with rapid urbanization, Ms Marx said. Away from the luxury hotels and affluent suburbs, residents of the ever-growing shanty towns without basic services often share a water pump and mobile chemical toilets.

“The city is years behind where it would like to be,” Ms. Marx said.