U.S. researchers have examined samples gathered from an ailing three-year-old orca swimming off the West Coast and say a parasite may be the reason the young killer whale is so thin.

The distressed southern resident killer whale, known as J-50, has been the focus of a cross-border emergency response from fisheries officials and veterinarians for several weeks after images revealed the whale had lost 20 per cent of its body weight.

The scientists tested fecal samples gathered last weekend from three members of the whale's pod — called J-pod — including J-50.

They found a moderate level of a parasite known as Contracaecum, according to a release from the U.S. National Oceanic and Atmospheric Administration (NOAA).

<a href="https://twitter.com/hashtag/J50?src=hash&ref_src=twsrc%5Etfw">#J50</a> Update Part 1 of 5 Results from health samples are coming in from several top labs around the country. 1st finding is from a fecal sample collected last wknd from J16, J42, & J50. It found moderate levels of Contracaecum, a nematode parasite sometimes found in marine mammals <a href="https://t.co/QiRYsa7kg9">pic.twitter.com/QiRYsa7kg9</a> —@NOAAFish_WCRO

The worm is not usually a problem in healthy animals, the statement from NOAA said, but in thin, weak or compromised animals the parasite can penetrate the stomach lining and cause a bacterial infection in the bloodstream or bore into internal organs.

Officials say they can't be sure the positive sample came from J-50, but their veterinary team treating the orca will use a dewormer with an antibiotic in upcoming treatments.

NOAA says the treatment will help reduce the bacterial and parasitic burden on its system, so it can begin to regain lost weight.

It says both treatment options have been safe and successful for use in other cetaceans.

Earlier this month, scientists attempted to feed the killer whale live chinook salmon off a boat following a novel antibiotic treatment by dart at sea.

Read more from CBC British Columbia