Two dogs died suddenly during separate flights weeks apart onboard U.S.-originating flights in March 2007, according to a new U.S. Department of Transportation report released last week.

The most recent incident occurred March 28 when a traveler flying aboard a United Airlines flight from San Diego to Sao Paulo, Brazil, checked two Lhasa Apso dogs for the journey. Upon arrival, one of the dogs was deceased.

According to the airline, Brazilian authorities refused to release the dog through customs and demanded that it be returned to the United States. Due to the delay, upon arrival back in the U.S. a veterinarian determined an autopsy wouldn’t be viable, and without an autopsy, a cause of death couldn’t be determined.

However, upon examination, the dogs’ kennel was found to be intact and the other dog belonging to traveler was healthy, according to the airline.

The other incident occurred March 9 aboard a Continental Airlines flight from Newark, N.J., to San Antonio, Texas. Upon arrival of the flight, a 10-year-old, 71-lb. Siberian Husky was found dead in his crate.

A necropsy that was performed in San Antonio showed the dog had renal disease and urinary cystitis among other medical problems. The dog’s official cause of death was listed as acute cardiovascular collapse caused by a pre-existing heart condition. His death was ruled not transit-related.

Federal law requires U.S. airlines to report to the Department of Transportation all incidents involving the loss, injury or death of animals during transportation. So far in 2007, there have been six pet-related incidents, according to the DOT: three in March, two in February and one in January.

There were a total of 26 animal fatalities – mostly dogs – before, during or after air transport in 2006. There were also 11 animals injured and 12 animals lost at United States airports last year, according to DOT statistics.