You thought your heifers and cows were pregnant until you notice a couple cycling in late summer, then you discover even more females open at preg-checking. Something is amiss, and your veterinarian suspects leptospirosis.

The sad part is that you might have vaccinated for leptospirosis and were confident at vaccinating time that the disease wouldn't hit your herd. Now you shake your head and wonder "What happened?" and "Will some of my spring calf crop be weak or, worse yet, stillborn?"

Research now shows that the "right bugs" may not be in your vaccine.

Leptospirosis is a common industry challenge caused by a single species, L. interrogans, which is subdivided into 200 serovars which have been placed into 26 serogroups. Of the six strains known to infect cattle, the two serovars or strains commonly associated with abortions are L. hardjo and L. pomona.

Research conducted by L. Garry Adams, D.V.M., Ph.D., Texas A&M University's College of Veterinary Medicine, shows that, while L. interrogans serovar pomona is more common in low rainfall areas, L. interrogans serovar hardjo antibodies have a high prevalence through all rainfall areas.

Yes, it appears that hardjo - including hardjo bovis - can lurk everywhere. "Cattle can be the maintenance host for either of these serovars (hardjo or pomona), depending upon the geographic location, but hardjo is the one most associated with cattle," Dr. P.N. Orndorff shared with a veterinary medicine class at North Carolina State University.

The good news for producers is that most vaccines contain antigens that help prevent L. pomona and L. hardjo. The bad news is that there are two hardjo serotypes, with the U.S. strain of L. hardjo bovis missing from most vaccines.

Adding to a producer's frustration is the fact that immunity for leptospirosis tends to be serovar- specific, with little cross-protection.

"For most infectious diseases, the single largest risk factor for acquiring the infection is the subclinically infected animals that are carrying the infectious agent," explained John M. Gay, D.V.M., Ph.D., during a past Washington State University Beef Information Days. "Many infectious agents are host-adapted, meaning that they establish long-term carrier status in a few animals.

"Bovine leptospirosis caused by L. borgpetersenii serovar hardjo type hardjo bovis is one example of this."

Dr. Gay adds that the chronic carrier rate in beef cattle ranges from four percent to 14 percent, with prevalence in dairy cattle hovering at 15 percent.

With hardjo, the host-adapted bacterium lives almost permanently in the kidney of carrier animals and is spread via the urine to susceptible females.

"Cattle are the maintenance hosts for L. interrogans serovar hardjo and are considered to be the only reservoir," Dr. Adams states.

Dr. Adams explains that the source of infection is usually an infected animal which contaminates pasture, drinking water and feed by infected urine, aborted fetuses and infected uterine discharges. He points out that a viable infected fetus can carry the infection for as long as seven weeks after birth. The semen of an infected bull may carry leptospirae, transmitting the disease to heifers and females during breeding.

While infecting micro-organisms, or leptospires, can enter a susceptible host animal through a cut or abrasion, transmission is more likely to come from urine splashing in an eye or through sexual contact. Once transmitted, the micro-organisms multiply in the blood, and, within a few days, there are large numbers in the animal's liver, spleen and kidneys.

Although mortality rate is five percent or less, Dr. Adams says morbidity rate may approach 100 percent of in-contact animals. He adds that abortions can be as much as 30 percent, and that milk production is another area highly affected when an animal has lepto.

Veterinarians agree that prevention of lepto abortion and weak calves centers on the use of bivalent or polyvalent vaccines (vaccines that contain more than one of the known serovars). And now it appears that it's wise to check that the vaccine includes antigens for the U.S. strain known as hardjo bovis. ©