Your cattle could be infected with the reproductive disease neosporosis, and you may not know it. But just because you can't see outward signs of the illness, doesn't mean neosporosis isn't affecting your herd's performance.

Texas A&M's Kerry Barling calls neosporosis a "mystery disease" because the symptoms are subtle. But, he says, if animals in your herd have Neospora infections, chances are they're causing significant production losses in your calf crop, both pre- and post-weaning.

In the feedlot, Barling has seen decreased weight gains, lighter carcass weights and lower feed efficiency due to neosporosis infections. In the cow herd, a cow infected with neosporosis is six times more likely to abort her fetus than an uninfected cow.

Those losses can't be ignored, and neosporosis - long thought to primarily affect dairy herds - is now being noticed by beef producers.

"Neosporosis has been easier to identify in dairy herds because they're raised in confinement," says J.P. Dubey, a researcher with the Agricultural Research Service in Beltsville, MD. In California dairy herds, as many as 19% of all aborted fetuses are diagnosed with this infection, he says.

But Dubey says neosporosis is prevalent among beef herds, and beef producers need to take steps to prevent it.

How Do Cattle Acquire Neospora? Caused by the protozoal parasite Neospora caninum, cows can be infected with neosporosis two ways.

The most common form of transmission is "vertical," when an infected cow transmits the parasite to her fetus. An infected cow is six times more likely to abort the calf, usually mid-gestation. But, some cows infected with neosporosis will carry the fetus full term.

These calves have an increased likelihood of being stillborn or born with neurological problems, but it's also possible that the newborn calf will appear healthy. These apparently healthy calves, however, have as high as an 80% chance of being infected through vertical transmission from the cow, according to Barling.

If Neospora carrier calves are retained as replacement females, they will pass the infection to offspring, thus transmitting the disease to the next generation. Infected animals that go on to the feedlot will likely have decreased performance, Barling points out.

A second source of transmission to a cow herd can be from a definitive host - an animal that sheds the Neospora oocysts in its feces. Cattle become infected by ingesting oocysts in contaminated water or feed. Just last year, dogs were identified as the definitive host of Neospora, Dubey says.

With that finding, researchers now believe the rate of horizontal transmission from the definitive host may be higher than once thought, according to Kynan Sturgess, a senior technical services veterinarian with Bayer Animal Health in Canyon, TX.

Numbers Unknown Because neosporosis can be transmitted so easily and unknowingly, it's unclear how prevalent neosporosis is nationwide. Among 1,000 calves in the Texas Ranch to Rail program last year, 13% tested positive to carrying the Neospora infection, Barling says. "That surprised us," he says.

Further research revealed that among the consignors to the Ranch to Rail program, 40% of the consignments were negative for Neospora infections, but 60% of consignments had at least one animal that tested positive for Neospora. Barling suspects Texas' findings are similar among beef herds nationwide.

The National Animal Health Monitoring System (NAHMS) plans to release numbers on the prevalence of neosporosis this fall, says NAHMS' Dave Dargatz. The information will be based on blood samples collected nationwide from beef herds in 1993.

"There is a lot of interest in Neospora, and it is something that needs to be dealt with," Dargatz says.

Reduce Your Herd's Risk Because the definitive host for Neospora was unidentified until last year, little is known about neosporosis and methods to combat it. Bayer Animal Health has a conditional license for a Neospora caninum vaccine and is conducting field trials to determine field efficacy.

Sturgess says Bayer is working on experimental challenge models in order to obtain a conventional license approval from USDA.

Until the effectiveness of a vaccine has been proven, Texas A&M's Barling recommends management practices to help prevent the introduction of neosporosis to a herd.

To determine if a herd has a high rate of vertical transmission, he suggests taking blood samples from replacement heifers first. "If the heifers have a high to moderate infection rate, it could indicate a high rate among the cow herd, and the cows should be tested," he says. "If a low rate of infection is found, it may not be economical to test the cows."

He recommends culling replacement heifers and cows that test positive to eliminate them as a source for vertical transmission. In herds with high infection rates, it may be practical to cull 10% of infected cows per year.

But, Barling cautions, "Culling won't work if you've got a continual outside source of exposure bringing Neospora into the herd."

To reduce the risk of outside sources, Barling recommends:

* Restricting access of dogs and wild animals to feed and water supplies. Feeds stored on the ground should be covered or fenced.

* Promptly removing aborted fetuses and placentas to prevent animals from ingesting these potential sources of infection.

* Controlling stray dogs, fox and coyotes to keep canine numbers around a cow herd to a minimum.