For much of the past year, U.S. cattle producers have heard a lot about diseases their animals do not have. With foreign diseases such as bovine spongiform encephalopathy (BSE) and foot-and-mouth disease (FMD) posing significant threats to our industry, the attention is justified should they ever appear here.
Meanwhile, though, several other diseases, already present in the United States, threaten to become more widespread or more difficult to control. Some, such as ana-plasmosis, Johne's disease and neosporosis, have been present in U.S. herds for some time, but apparently are becoming more common or more widespread geographically. Some others, such as a strain of Salmonella known as S. typhimurium DT104, appear to be developing resistance to available antibiotics, creating new concerns for animal and human health.
Timing also could play a role in the spread of these and other diseases. The U.S. beef industry currently is entering a low point in the cattle population cycle. Many producers will begin expanding their herds over the next few years in response to higher prices. Expanding often means purchasing cattle from outside, which without adequate biosecurity can introduce new diseases into your herd.
Johne's disease comes from bacteria called Mycobacterium paratuberculosis. The disease resists detection and control because of its slow development. Cattle typically become infected as calves, but do not show any clinical signs until they are several years old.
Eventually, the bacteria attacks the intestine, causing diarrhea, poor digestion, and excessive weight loss. But an animal can begin shedding the bacteria, infecting a pasture and other cattle in the herd, well before it shows any signs of the disease. Once the disease finds its way into a herd, it is difficult to eliminate. There is no known cure, and culling animals that show signs of disease typically just scratches the surface with more cases turning up later.
Johne's disease is not highly prevalent in U.S. beef herds, but has the potential to become economically important. The U.S. Department of Agriculture's National Animal Health Monitoring System (NAHMS), in it's Beef '97 study, found only 40 positive blood samples from 10,372 cows in 380 herds from 21 states. Those 40 positive animals came from 30, or 7.9 percent, of the tested herds.
Veterinarian David Smith, an epidemiology specialist at the University of Nebraska, says producers usually bring Johne's disease into their herds by purchasing infected animals. Some, he notes, rely on testing purchased animals, but the test is not reliable at the early disease stages. Dr. Smith says some states have begun to establish herd-testing programs, which will prove to be a more valuable tool in the future. Producers knowing the herd health status of their suppliers can make informed decisions regarding purchase of young animals.
Dr. Smith also points out that Johne's disease could have human-health and food-safety implications. Scientists have isolated the M. paratuberculosis bacteria from a few humans with Crohn's disease, along with numerous other bacteria and viruses. Results are inconclusive, but Dr. Smith says that even the chance or the perception that the disease could affect human health puts it high on the priority list.
Control measures for Johne's disease should include reducing exposure and infection of replacements, identifying and removing the most highly infected cattle, and preventing introduction of infection by screening sources of replacements.
Prevalence data from around the country is scarce, but Texas A&M University Veterinarian Kerry Barling has conducted research that suggests neosporosis could cost Texas producers as much as $37 million during 2001. Cow-calf producers would incur much of those losses through abortions, stillborn calves and culling of infected cows that fail to produce live calves.
Dr. Barling notes that abortion is the primary sign of the disease, occurring anytime during pregnancy but especially during the mid-trimester.
The life cycle of Neospora caninum, the pathogen that causes neosporosis, involves dogs and possibly wild canines.
A vertical pattern of infection typically involves a cow becoming infected from dogs, then passing the disease to the calf in utero. Sometimes the calf is born healthy, but carrying the Neospora pathogen. Female calves infected with the disease and added to a breeding herd are likely to abort a calf at some point.
Producers need to work with their veterinarians and state diagnostic laboratories to determine the cause of any aborted calves.
The best control measures are to protect feed and water from exposure to canines, combined with rigorous cul-ling of cows that abort their calves. There is no effective treatment for the disease.
A conditionally licensed vaccine, recently developed by Intervet Inc., has potential to reduce the prevalence of this disease in cattle.
Anaplasmosis is caused by Anaplasma marginale, a rickettsial pathogen that invades the red blood cells. Traditionally, the disease was primarily a regional problem affecting cattle in the southern states. Recent research, however, indicates that anaplasmosis is widespread and is one of the most economically important cattle diseases in the United States.
Biting flies, mosquitoes and especially ticks spread the disease within and between cattle herds, but ranchers also can spread anaplasmosis if they fail to properly sterilize needles, dehorning tools or other instruments.
Primary disease signs are anemia, jaundice, and fever, and outbreaks can occur anytime. The most typical times for outbreaks are during the summer when biting insects are present, or four to six weeks after mechanical transmission from infected instruments. The disease can remain at sub-clinical levels though, only to be triggered by stress later such as during processing or shipping. Healthy carrier animals can serve as reservoirs from which insects and ticks spread the disease.
Control of insects and ticks is an important part of a prevention program. Another is timely treatment with tetracycline antibiotics for clinically ill or carrier animals.
Bovine leukosis virus (BLV)
Veterinarian Chris Chase, at South Dakota State University, says BLV is one of the lesser-known diseases with the potential to cause economic damage in U.S. herds. BLV is a retrovirus infection that causes malignant lymphoma in about 5 percent of infected cattle.
Malignant lymphoma causes weight loss, decreased milk production, enlarged lymph nodes, loss of appetite, rearlimb weakness or paralysis, fever, protruding eyeballs, gastrointestinal obstruction, heart failure, and eventually death.
In its 1996 dairy study, NAHMS found that 89 percent of all U.S. dairy operations and 43.5 percent of all U.S. dairy cattle were seropositive for BLV. NAHMS collected a smaller sample of BLV data in its Beef '97 study and found 38 percent of sampled beef operations and 10.3 percent of tested beef cows were seropositive for BLV.
There is no cure for BLV, and Dr. Chase says culling, monitoring and prevention of transmission are the key control measures. Those strategies carry over to other diseases, he says. As producers bring new animals into their herds, they must be vigilant. Even in the case of diseases for which vaccines are available, such as bovine viral diarrhea (BVD), producers need to use biosecurity measures, testing and monitoring, along with vaccines, for a complete program of prevention.
For more information on these and other important cattle diseases, refer to the following Web sites
- Anaplasmosis information from Texas A&M University: http://animalscienceextension.tamu.edu/publications/13267117anaplasmosis.pdf
- The U.S. Dept. of Agriculture's Center for Epidemiology and Animal Health: www.aphis.usda.gov/vs/ceah/
- Johne's Information Center, University of Wisconsin, School of Veterinary Medicine: www.johnes.org/