Omen and Maynard, the two dogs that died late this week in the Iditarod, had fluid in their lungs, race marshal Mark Nordman reported Saturday.
Necropsies showed that both dogs had pulmonary edema, possibly because cardiac abnormalities prevented their hearts from moving fluid out of their lungs. Further tests are pending, Nordman reported.
Omen, a member of Rick Larson's team, died on the trail Friday. Maynard, a member of Warren Palfrey's team, died Thursday night.
Their deaths bring to five the number of animals that have died in this year's race, the most since five died in the 1997 race.
Also on Saturday, two other mushers scratched.
Aaron Peck of Grande Prairie, Alberta, Canada pulled out of the race at the Elim checkpoint. Peck, 29, arrived in Elim on Friday afternoon with six dogs, but decided at about 9 a.m. Saturday he shouldn't take his team on, race officials said.
And David Sawatzky of Healy scratched late Saturday afternoon only a few miles from the finish in Nome. Sawatzky, 56, had about 15 dogs and was walking ahead of the team for several miles after leaving Safety before deciding to call it quits at about 5 p.m., officials said.
In light of the latest dog deaths, People for the Ethical Treatment of Animals escalated its annual letter-writing campaign to persuade Iditarod sponsors to withdraw their support. More than 2,700 people have participated, according to spokeswoman Desiree Acholla.
"Racing dogs to their death is indefensible, yet the death toll rises year after year and the race continues with business as usual," she said.
Major sponsors of the Iditarod include Exxon Mobil Corp. and Wells Fargo. Both companies say they support the race.
At least one animal welfare group -- the Humane Society of the United States -- no longer actively campaigns against the Iditarod, although officials aren't endorsing it either.
They acknowledge, however, the event's significance in commemorating the work of dog teams that were sent on the trail in 1925 to bring diphtheria serum to Nome to combat a deadly outbreak.
"I would like to see the Iditarod celebrate the history and culture of the event and not be just a timed event, but they're trying to make it as safe as they can for both the animals and humans," said Dave Pauli, the humane society's Western region director.