The Houston Zoo's herd of elephants, a huge draw through the years, could become a thing of the past if a herpes virus continues to kill young animals.

None of the 14 calves conceived by the zoo's elephants during the last 25 years is alive today.

Six were stillborn or died during pregnancy.

Eight elephants who were born at the zoo later died, six from the nearly always fatal elephant herpesvirus, prompting a scientific research team to characterize the zoo as a "hot spot" for the virus. The virus's most recent victim was Mac, a 2-year-old calf and one of the zoo's most popular attractions, who died last week.

Among the issues raised by the spate of deaths at the zoo and others elsewhere is whether captivity plays a role in the virus's transmission and onset of illness. Some animal welfare groups have called on zoos to halt breeding programs until a cure is found. And some zoos, including Houston's, could find themselves with no Asian elephants to replace aging ones as they die.

"Every year, we have fewer female elephants of reproductive age," said Lauren Howard, an associate veterinarian at the zoo. "That is a huge concern."

The virus is proving lethal to many young Asian elephants, the only species kept at the zoo. Researchers discovered the virus in 1995, and while their understanding of it has progressed, they remain unclear about how it is transmitted and when elephants are exposed to it. A cure or vaccine is even further off.

The disease is plaguing Asian elephants at many zoos worldwide. In North America, at least 17 of 41 zoos that are members of the Association of Zoos and Aquariums have been home to an elephant that died from the herpesvirus or contracted it, according to a report last year by In Defense of Animals, a nonprofit organization that works to protect animals and their natural habitats.

Once the virus is detected, very few elephants have survived. It causes blood vessels to leak and can cause their hearts to stop.

Death can come quickly, from the first day the virus's effects become visible to five days later. In Mac's case, his handlers saw signs of the virus — lethargy and a swollen head — the morning of Nov. 9. He died that evening.

Infection with the virus has caused 20 percent of the deaths of Asian elephants in North American facilities during the last 25 years, according to the International Elephant Foundation, a nonprofit that promotes conservation of elephants. The virus is the No. 1 killer of young elephants in the U.S., according to the National Zoo's Web site.

Targeting one breed

Zoos and other North American facilities hold a small number of the animals — 150 African elephants, 141 Asians, according to the Association of Zoos and Aquariums. African elephants, a different species, not merely a different breed, have been nearly unaffected by the virus.

Zoos don't have the option of importing elephants from the wild because Asian elephants are an endangered species.

Instead, zoos rely on breeding to replace aging elephants. The virus has hampered many breeding programs since it afflicts mostly young elephants.

Until recently, some researchers believed the virus was limited to zoo and other facilities that keep animals, because no cases were confirmed in wild elephants. Animal welfare groups such as In Defense of Animals and Born Free USA have questioned whether the stress of captivity or some other aspect of captive life triggers onset of the virus or causes it to spread.

Researchers from a Berlin wildlife center and other institutions in 2006 reported that Seima, an Asian elephant born in the wild in Cambodia, died from the virus.

However, Seima didn't live in the wild for long. Shortly after her birth, she fell into the hands of traders. Authorities later took her away from the traders. She was then bottle-reared and lived in an elephant sanctuary.

The life expectancy for Asian elephants in the wild and at North American zoos is about 45 years, according to the Association of Zoos and Aquariums.

Virus in the wild

Laura Richman, who discovered the virus in 1995 and heads the Indo Elephant Endotheliotropic Herpesvirus Lab at the National Zoo in Washington, D.C., said that in yet-to-be-published papers, researchers will report the virus has killed other Asian elephants that lived in the wild.

Richman said she and other researchers now are investigating whether most or many adult Asian elephants carry the virus, which usually lies latent.

Adult elephants survive occasional flare-ups. But newborns and calves, such as 2-year-old Mac, sometimes do not, because their immune systems haven't completely developed, Richman hypothesizes.

If her theory is correct, Mac may have caught the virus from one of the four other elephants in the maternally-run zoo herd, including his mom, Shanti and Methai, a surrogate mom.

Methai gave birth to six calves. All died, including ones that were sent to zoos in Berlin, Chicago and Canada. Five of the deaths were caused by the virus.

Of Shanti's two calves, Mac died from the virus. Bella, born in 2004, died after Shanti rejected her.

In Defense of Animals said other virus hot spots include Dickerson Park Zoo in Springfield, Mo., and Rosamond Gifford Zoo in Syracuse, N.Y.

Some stillborn elephants in Europe tested positive for the virus, leading researchers to believe that their mothers passed it on to them, according to a 2005 article on the National Zoo's Web site.

Houston Zoo director Rick Barongi said the six failed elephant pregnancies at the zoo over the last several decades occurred before 1995, when the virus was discovered. The zoo, he said, doesn't know whether the virus caused the failed pregnancies.

Eric Kleiman, research director at In Defense of Animals, said zoos should halt breeding programs until they know how the virus is transmitted and a vaccine or other treatment regimen is developed.

Barongi said, "We know we've got a serious problem here. We know we're at risk unless we find a cure."

But there is no reason to believe that Mac would have been better off if he had not spent two years living with his mom and learning tricks taught to him by keepers and receiving their love, said Barongi.

Halting the breeding program, he said, won't find a cure for the virus.

"What does that get you?" he asked.