Early spring cow losses in southwest South Dakota

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From early March through April 2009, numerous ranches in southwest South Dakota—mostly Jackson and Shannon Counties and adjacent areas— reported higher than normal numbers of cows in late gestation on pastures dying. Typically these cows became thin, developed rough hair coats, became weak, and were unable to rise after lying down. For the most part, these cows were alert and aware of their surroundings. Some would remain down for one or two days before dying.

Q: How many cows were affected in these herds?
A: Although true numbers are hard to come by, it’s estimated from veterinarian’s reports that nearly 200 cows were lost from 16 herds recorded. The numbers affected per herd range from 1 to 55. According to information compiled, the median number of cows affected from these herds was six.

Q: Much was made about ticks as the cause of these losses. What was learned about the role of ticks?
A: One of the interesting aspects of these cases was that some—but not all—of the affected herds reported that the affected cows were covered with ticks. Early on, the onset of these cases of down, weak cows coincided with an unusually early tick hatch in those areas. Other herds reported finding only a few ticks on their affected cattle.

Q: What kind of tick was found on these animals in South Dakota?
A: Two species have been identified in these affected herds: Dermacentor andersoni (Rocky Mountain Wood Tick) and Dermacentor albipictus (Winter Tick). The normal ranges of these ticks are generally in the Rocky Mountain states, so finding these ticks here may represent a bit of a stretch outside their normal range.

Q: What problems could ticks cause for cattle?
A: First, ticks are the source of blood loss for cattle due to their feeding activities. This could cause some borderline anemia, and would affect the animal’s ability to gain weight or produce milk. In addition, irritation from heavy infestations of ticks will likewise interfere with weight gains or production. This could be a very

important issue in cows in late gestation that are already partitioning quite a bit of their energy and protein into the developing calf inside them.

Second, ticks can carry and spread certain viral, bacterial, and protozoal diseases. Dermacentor andersoni has been implicated in transmission of Rocky Mountain spotted fever, tularemia, Colorado tick fever, and Q fever in the USA, and is one of the causes of tick paralysis. Dermacentor albipictus may play a role in the transmission of anaplasmosis and Rocky Mountain Spotted Fever. However, Rocky Mountain Spotted Fever (caused by Rickettsia rickettsii), and Colorado Tick Fever (caused by a tick-borne virus) have not been documented in cattle.

Although tularemia is endemic to this area of South Dakota, cattle are generally believed to be resistant to clinical effects. The causative organism, Francisella tularensis, was not isolated from any of the cattle samples submitted.

One of the modes of transmission of Q Fever may include ticks and wild animals; however, its characteristic signs of late term abortions were not observed in these cases.

Tick paralysis, caused by a toxic substance in the tick’s saliva, has been reported in cattle; however, it is usually observed in calves and younger cattle, rather than older cows.

"Tick fever” or “Texas tick fever” is caused by a protozoal organism carried by Boophilus spp. ticks, which have been eradicated from the United States, except for a permanent quarantine zone in south Texas. Another syndrome also called “tick fever” or “tick-borne fever” is carried by a Ixodes ricinus, found only in Europe.

Q: What about the possibility of anaplasmosis?
A: Anaplasmosis is due to an infection caused by Anaplasma marginale, a bacteria that infects red blood cells of cattle. The bacteria may be spread from animal to animal by mechanical or biological means (biting flies or ticks). Infected red blood cells are removed by the animal’s immune system, and anemia results.

Clinical signs depend on the degree of anemia, and may include pale gums, weakness, dehydration, and lack of appetite. Dyspnea and belligerence may be noted as the condition progresses.

Q: How many of the cattle tested were positive for anaplasmosis?
A: None. Anaplasmosis can be diagnosed directly by examination of blood smears or by PCR techniques. Indirect evidence through measurement of A. marginale antibodies may be obtained with competitive ELISA serology. Neither blood smear examinations nor serology tests were positive for anaplasmosis in any samples obtained from cattle in these herds during this period. In general, it takes 3 to 8 weeks from the introduction of the organism into the animal to the time when enough red blood cells are affected and removed by the body for clinical anemia to result. It is unlikely that appropriate vectors had been around long enough by the time clinical illness was apparent in these cattle.

Q: In some (but not all) of these areas, there were large numbers of deer dieoffs  Can deer get anaplasmosis, too?
A: Experimentally, deer can become infected with anaplasmosis, as diagnosed with blood smears and by showing a long -lasting antibody response. But even experimentally, deer did not show anemia or clinical signs, let alone mortality. It is generally believed that the low level of bacterial infection in deer blood cells is below the threshold that would result in efficient transmission to cattle from biting flies.

The SDSU ADRDL examined blood samples and tissues from deer that died in the area. None had blood smears positive for A. marginale. Two of eight deer had positive serology, indicating exposure at some point.

Q: What caused the death losses in the deer in these areas?
A: Deer in these areas were subject to the same environmental, nutrition, and tick infestation factors that are present in the cattle populations. There was no evidence that cattle losses were caused by a reservoir of a disease in the deer, nor was there evidence that deer losses were caused by a reservoir of disease in cattle.

Q: What other lab results have come from the cows that were examined?
A: Most of the cows tested showed serologic evidence of exposure to bluetongue virus and epizootic hemorrhagic disease (EHD). However, the agents were not been found in blood or tissues. Cattle are generally resistant to clinical signs, let alone death losses, from infections with these viruses. EHD has been known to cause large die-offs of wild deer, but not usually infect cattle. In 2007 in Ohio an outbreak of EHD in cattle occurred, resulting in mild oral lesions and lameness in affected animals.

None of those signs or lesions were identified in these cattle, nor was EHD confirmed as a cause of mortality in the deer. Additionally, blood samples from one deer revealed very high titers to Ehrlichia canis, a bacteria related to the causative agents of anaplasmosis and Rocky Mountain Spotted Fever. The serology test may have detected exposure to another Ehrlichia spp. There are no reports of clinical disease due to Ehrlichia in deer. Several cattle blood samples submitted to Colorado State University were positive on a rickettsial PCR panel, but subsequent typing could only be performed for Ehrlichia canis, for which all samples were negative.

Q: In summary, what most likely caused the mortality in these cattle?
A: Likely, it was a combination of factors. Older cows in late gestation were more likely to be affected, and in many cases, tick infestation coincided with clinical signs. Whether an individual infectious agent such as a rickettsia   contributed, was not proven. It is reasonable to believe that a combination of nutritional factors, weather stress, blood loss due to tick infestation, the demands of the fetus or newborn on the mother’s nutritional stores, and pregnancy-related immunosuppression all played varying roles in causing cows to become weak and eventually die. Although it is not unusual for beef herds to contain a few cows that are unable to maintain body condition for various reasons, late gestation is a critical time to pay attention to cows’ nutritional demands.

The demands from the developing calf, and in many cases from adverse environmental conditions, may draw down a cow’s nutritional reserves quickly. As stated above, it may be difficult to change this for a certain percentage of cows.

Q: What are other recommendations for next year’s late gestation cows?
A: Application of insecticides for ticks and other external parasites early next spring in these areas should be considered. Producers should use veterinary input in choosing a product. Most of these products will not result in long-term residual protection and multiple applications may be necessary. In some affected herds, treatment with antibiotics (especially tetracyclines) showed positive results. Producers should work with their veterinarians in making treatment decisions. Realize that cows may show some of these same signs when affected by conditions such as milk fever or grass tetany.

This information was excerpted from the August 2009 Animal Health Matters newsletter from the South Dakota State University Veterinary Science Department and the Animal Disease Research and Diagnostic Laboratory. See the newsletter here.

 

 


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