Management still best solution for foothill abortion disease

News
May 7, 2010

If a cow aborts her calf, she loses her one chance to compensate for her yearly maintenance, not to mention the time and money invested in buying bulls, breeding, preg checking and all the necessary elements of her reproductive upkeep. And while a random 1 percent to 2 percent abortion rate in a cow herd may be unavoidable, widespread abortions caused by disease often have devastating consequences for ranchers.

At least on one front, however, it appears that science is making headway against disease-induced abortion in cattle. Central and northeastern California and Great Basin ranchers have long struggled with losses due to foothill abortion, also known as Epizootic Bovine Abortion (EBA), a disease spread by the bite of the Pajaroello (pa-ha-WAY-lo) tick. According to Dr. John Maas, DVM, University of California, Davis (UC Davis) extension veterinarian, EBA has been blamed for up to $6.2 million of annual out-of-pocket losses to California ranchers alone, and estimates say 5percent to 10 percent of the California calf crop (45,000 to 90,000 head) may be lost each year to the disease. But these numbers may soon change. A team of researchers from UC Davis and University of Nevada Reno (UNR) lead by Dr. Jeff Stott are making steady progress towards developing a vaccine which could soon make devastating foothill abortion losses a thing of the past. In the meantime, however, minimizing the effects of EBA can still be achieved through adaptive management practices and a sound understanding of the disease.

EBA is caused by a bacteria which is picked up and spread by the Pajaroello tick and is found throughout California and the Great Basin. "It’s a regional disease," explains Maas. "It occurs in most all of California, a lot of Nevada, part of [Oregon] all the way over from Klamath Falls across to the Jarbridges in Idaho. We’ve seen cases out of all of those areas."

The Pajaroello is a soft tick which prefers to live in dry soils around trees, rocks and brush. These areas are also favored by cattle and deer for bedding down, which makes them ideal locations for ticks to feed. Typically, the ticks are not found in wet areas and irrigated pastures.

Unlike other types of ticks that commonly feed on livestock and may attach and feed for up to two weeks, the adult Pajaroello tick only will feed for 10 to 20 minutes before detaching itself from its host. For this reason, spray products and insecticidal ear tags are not believed to be effective against preventing bites. Due to the very short feeding period, it is also highly uncommon to find these ticks on cattle, and their presence must be determined by other means, such as using a dry ice (CO2) trap.

The ticks are most active in the driest and warmest months, which vary according to location. They have little activity in the winter. That said, Pajaroello ticks frequently survive through the winter and have been observed in captivity to survive for over three years without a blood meal. The ticks are also affected by rainfall and drought; wet, cool weather reduces their activity while hot, dry weather accelerates it.

When an infected tick bites a pregnant cow before she is at least six and a half months along in her pregnancy, the bacteria can infect the fetus, causing its death and abortion. The time frame is important. After six and a half months gestation, it is believed that the fetus will have a strong enough immune system to overcome the infection.

Explains Maas, "The organism can cause abortion if the cattle are a month or so pregnant, up to about six and a half months pregnant ... Six months is not quite there."

Aside from aborting her calf, the cow shows no sign of infection and is not harmed.

Once a cow is exposed to the EBA agent through a tick bite, either while pregnant or open, she will develop a temporary immunity to the infection for one to two years. Although it may seem counterproductive to infect cows with the disease, one of the best tools for managing for foothill abortion is to expose cows to tick-infested pastures either while they are open (as with replacements), or after they are more than six and a half months pregnant, in order to give them immunity. Also, cows that do abort one year will very likely acquire immunity and carry their next year’s calf to term.

Though it is not known exactly how long immunity lasts, it is not thought to be long term. According to Dr. Mike Teglas, assistant professor of animal biotechnology at UNR, recent research has demonstrated that some cattle can maintain immunity for up to three years. However, it is recommended that cows be exposed to the ticks every one to two years to maintain immunity.

Additionally, it is suspected that heifers must reach breeding age before they are able to acquire immunity. Remarks Maas, "We’re still kind of unsure [when immunity starts]. We haven’t done definitive research to pin down that time frame."

Producers who suspect EBA as the cause of abortion in their cattle should contact their vet, says Maas.

"I guess the caution to the producers is: Don’t just assume that it’s foothill because you live in foothill country, and don’t just assume it’s not if you don’t live in foothill country. It’s really easy to get fooled on these abortion diseases."

A visual examination by a vet of an aborted fetus is usually sufficient to verify EBA as the cause, and is an important first step in managing for the disease. Aborted calves with EBA typically have a fluid-filled abdomen which makes them appear "pot-belied." They also have swollen lymph nodes in front of their shoulder blades, and frequently present with tiny red hemorrhages around the eyes, nose and mouth. For an absolute diagnosis, fetal tissue can be sent to a lab.

Dr. Rod Ferry, DVM, a veterinarian out of Lakeview, OR, is similarly cautious. He explains, "If it’s a ranch where I know I’ve got a problem, and we get that typical calf when they bring it in... that’s usually as far as they go. If it’s some place we’ve never really diagnosed foothill but you get that aborted fetus and it looks like foothill, I’ll usually send it in for laboratory confirmation on the first go-round."

After a positive diagnosis, the next step to managing foothill abortion is to identify which fields are inhabited by the Pajaroello tick. Because the ticks are so seldom found on livestock, the recommended method is a CO2 trap. Common practice advises that several unsuccessful attempts to trap ticks are necessary to conclude that a field tests negative for ticks. A single trapped tick is an indicator that a field is positive for ticks.

Once tick-inhabited areas are identified, these fields should only be used by open cows, cows that are more than six and a half months pregnant, or by cows which have recently been exposed and are therefore immune. They may also be safely used by stockers or bulls.

Another option is to use infested areas in cold seasons, or change calving schedules to ensure that cows will not be in the first six and a half months of gestation on infested pastures during warm weather.

Producers managing for foothill abortion can learn to use their infested fields as a tool to give cows immunity by turning them out on tick-infested fields when they are not susceptible to abortion (while open, or more than six and a half months pregnant). Cows already exposed to the disease also need to be re-exposed every year or two to keep their immunity. Maas elaborates: "[Infested fields] are really good places to push cattle that are breeding-age heifers that haven’t been bred yet, that are open cows, that are cows that are seven months-plus pregnant. You can push them into those areas to make sure they get bitten."

However, it is essential that pregnant cows without immunity are kept off infested pastures:

"Don’t bring cattle in from other areas unless you manage them with the realization that they could come down with foothill."

While intelligently managing for the disease can reduce death losses significantly, the collaborative research effort between UC Davis and UNR, funded by the California Cattlemen’s Association (CCA) and the Nevada Cattlemen’s Association (NVA), has resulted in a prototype vaccine currently undergoing testing of dosages. Remarks Teglas, "If [CCA and NCA] didn’t show the kind of support they’ve been showing recently, we never would have gotten this far."

The current vaccine is described by Teglas as a rough draft, a first attempt that will be refined in subsequent versions. One of the drawbacks to this first generation EBA vaccine is that it can only be used safely in open cows, a limitation that will allow it to be used only by producers who process cows before bulls are turned out.

Says Teglas, "The idea in the long term is to make a vaccine ... that will be useful in both pregnant and non-pregnant animals. This one is a "stop gap" measure. We’ve been very well supported by both the California and Nevada Cattlemen’s and we feel like we need to give them something right now that they can use. But this isn’t the end point."

At this stage, the vaccine will essentially replicate a tick bite and, therefore, if given to a pregnant cow, could cause her to abort. Says Teglas, "[The vaccine] is basically going to be a live bug. You wouldn’t want to put it into a pregnant animal."

However, trials have found the vaccine to be very safe for open cows, with no detectible side effects. Explains Teglas, "It doesn’t influence their ability to rebreed or anything like that. It doesn’t affect their fertility in any way ... They never get sick; they never run a fever. It’s hard to tell that they’re infected."

The good news is that science seems to be gaining on one long intractable health issue in cattle. The bad news is that the Pajaroello tick has been moving into new territory, which makes the need for a viable vaccine all the more urgent. Says Teglas, "The tick has spread beyond its original [range]. It’s perfectly reasonable to think that if the tick gets introduced somewhere else, it will take the disease with it." — Andy Rieber, WLJ Field Correspondent

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