Most bovine veterinarians consider anaplasmosis to be a disease of little consequence in the Southeast. But anaplasmosis, caused byA. marginale and commonly transmitted by ticks and contaminated equipment is proving to rob profits and crop up in places where it hasn’t been seen before, such as the Northeast.
Hans Coetzee, BVSc, PhD, Dipl. ACVCP, Kansas State University, says the introduction of anaplasmosis into regions where the disease has not previously been reported is likely associated with the movement of infected
animals. Furthermore, local changes in climate may support vector populations in areas where these were previously less prevalent.
“Many states have relaxed requirements for anaplasmosis testing prior to importation which may have contributed to the spread of the disease,” Coetzee explains. “This has increased the risk of introducing anaplasmosis into naïve herds resulting in endemic instability and clinical disease outbreaks. Movement of infected cattle misclassified as seronegative by use of the complement fixation test into areas in which anaplasmosis is endemic may also have resulted in the introduction of newA. marginale strains.”
Jessica Laurin, DVM, Animal Center of Marion County, Marion, Kan., has dealt with anaplasmosis in her practice area on the edge of the Kansas Flint Hills and has seen an increase in the past eight years. “We had a few mild winters in a row and increased deer numbers, and I think both contributed to the increased prevalence,” Laurin says. “When we test, we will find 40–60% positive in a herd showing active infection.”
And what’s the economic damage? It is estimated that the introduction of anaplasmosis into a previously naïve herd can result in a 3.6% reduction in calf crop, a 30% increase in cull rate and a 30% mortality rate in clinically infected adult cattle, Coetzee states. Economic loss to the United States livestock industry as a result of anaplasmosis is estimated to be >$300 million per annum. The cost of a single case of anaplasmosis is estimated to be around $400/animal. The Canadian Food Inspection Agency (CFIA) has concluded that anaplasmosis is currently not found in Canada. However, direct production losses associated with the establishment of anaplasmosis in Canada have been estimated to be $12–36 million in 2003 with a CFIA cost to prevent the spread of anaplasmosis of $3 million.
Coetzee notes that studies of A. marginale strains in the United States have revealed that many more genotypes of A. marginale exist in nature than have been described. This genetic diversity most likely is attributable to the phenomenon of infection exclusion toA. marginale that results in one genotype/animal. “Once a new genotype is introduced into a particular region, it will be maintained in ticks and cattle by independent transmission events and the disease will likely become endemic,” he says. Persistently infected cattle and ticks could both serve as reservoirs of the introduced genotype. “This is important because these diverse A. marginale genotypes may not provide cross-protection, thus greatly increasing the challenge to use vaccination as a method of anaplasmosis control. Therefore, correct classification of disease status is essential to restrict movement of infected cattle to prevent diversification of A. marginale isolates.”
Clinical signs of anaplasmosis can include weakness, fever, depression, constipation, decreased milk production, jaundice, abortion and death in some cases. Laurin says the “hallmark” anaplasmosis-infected cow is 2–3 years old, and it will be seen in older cows in a herd that has just become exposed that season. Affected cows in her area typically start to exhibit illness from August to November. “They will come off grass with an active anemia, and when the temperatures start to fluctuate with our first one to two dramatic temperature drops, clinical signs will become noticeable,” Laurin says. When those clinical signs start appearing in the in fall in unthrifty cows, Laurin say her rule outs also include bovine leukosis , parasitism, hardware disease, and Johne’s disease.
Overt signs will be due to severe anemia. Cows will become anxious and mean or protective and can start to charge or act high-headed, and even drop dead from stress. “It is not atypical as a veterinarian to come onto the farm, step into a pen with the cow in question, and watch the cow drop in front of your eyes,” Laurin notes.
Coetzee explains that death is due to erythrophagocytosis or destruction of the red blood cells in the spleen of the host. The anaplasmosis infection within the red blood cell causes the host to recognize these cells as “foreign” and therefore to mount an immune response against them. These cells are removed from circulation by the spleen causing a severe anemia. Destruction of the red blood cells is the primary cause of death however many of the complications of the disease such as rumen stasis can make treatment difficult.
Mild to moderate anemia from anaplasmosis allows the cows to also show other signs such as late summer pneumonia. They will present with light to pale mucous membranes, elevated heart rates and increased lung sounds, Laurin adds. They will also be off feed, and may be constipated.
It’s more difficult to identify and diagnose calves affected with anaplasmosis, Laurin says. Cattle are growing, have an active thymus, so they produce enough red blood cells to get by. “They are typically pulled as a respiratory case that does not respond to medication, and they start to dwindle in weight or condition. A hindsight diagnosis becomes easier especially when there is no history of treatment with an oxytetracycline injectable.” They are also more susceptible to other infections, and Laurin believes anaplasmosis may be a contributing factor. “I have found positive calves in chronic pens in yards where injectable tetracyclines were not a part of a treatment protocol.”
Abortion has been noted as another clinical sign of anaplasmosis, but Laurin has not been involved with any herds where that was found to be the case. However, she says, “It could be that producers aren’t finding the abortions or our time of year for exhibiting illness may not correspond with the timing of abortions.”
Persistence and carrier animals
Any class of cattle can become infected with anaplasmosis. It is noteworthy that calves in utero can become infected with anaplasmosis if born to infected cows, Coetzee says. This occurs in approximately 16% of cases. These animals are born clinically healthy but remain persistently infected with anaplasmosis and are therefore potential reservoirs of infection. Anaplasmosis infection in calves less than 1-year old is generally self-limiting and rarely fatal.
Once cattle become infected with anaplasmosis these remain persistently infected carriers for life even when treated with therapeutic doses of oxytetracycline during acute clinical infection. Carrier cattle appear clinically healthy but are capable of transmitting infection to ticks and other animals through equipment contaminated
with infected blood. “Persistent infections are maintained because anaplasmosis organisms change their antigenic structure every five to seven weeks,” Coetzee explains.
In order to control these emerging new antigenic variants, a competent host immune response is needed. If cattle suffer from immune suppression associated with poor nutrition, calving, lactation stress or concurrent disease, this may reduce their ability to control this new antigenic variant, he adds. “Therefore, these stressed carrier animals may be more likely to develop acute anaplasmosis. Because the tetracycline antimicrobials we use to treat clinical anaplasmosis appear to prevent the replication of anaplasmosis organisms and not sterilize infections at the physiological concentrations achievable in the animal, a competent immune response is likely needed to work in concert with the host immune system to adequately control these acute infections.”
This may explain why some classes of cattle, such as early lactation dairy cattle, do not always respond favorably to treatment with tetracyclines. Furthermore this may explain why cattle with acute anaplasmosis infections become persistently infected even if treated with oxytetracycline.
Coetzee says reports indicate that cattle on a higher plane of nutrition develop more severe anaplasmosis than animals maintained on a lower energy plane. One study reported 122 cases of clinical anaplasmosis in 2,036 feedlot calves aged between 5 and 10 months. The study found that calves diagnosed and treated for anaplasmosis during the late finishing period (day 35 to slaughter) had a reduction in average daily gain of 96 g/day. “Clinical anaplasmosis therefore has significant economic implications in feedlot operations,” he says.
Coetzee’s group recently conducted a study to investigate the seroprevalence of Anaplasma marginale in Iowa feed-lots and its association with morbidity, mortality and treatment costs. Blood samples were taken from 659 calves from 31 consigners at processing and classified as seropositive toA. marginale using a competitive enzyme-linked immunosorbent assay (cELISA) test with a 30% cutoff. Health and production parameters were modeled by A. marginale serostatus with mixed model regression analysis. The apparent prevalence of seropositive cattle was 15–17% (100/659).
There was no significant association between A. marginale serostatus and production parameters. However, seropositive status had a weak association with undifferentiated fever, Coetzee says. Although prevalence of anaplasmosis in Iowa feedlots is higher than reported in Montana-sourced calves arriving in Canadian feedlots, this was not associated with increased production costs.
Veterinarians should exercise caution before making a definitive diagnosis of acute anaplasmosis solely on the basis of a positive result for the cELISA and clinical signs such as fever, anemia, and icterus, cautions Coetzee.
Differential diagnoses that should be ruled out based on these clinical signs include acute anthrax, leptospirosis, bacillary hemoglobinuria, oak poisoning, poisoning caused by ingestion of Brassica species, multicentric lymphosarcoma and foreign animal diseases such as babesiosis, theileriosis, and trypanosomiasis. “In these circumstances, it would be advisable to detect intraerythrocytic inclusions of A. marginale by examination of blood films to assist in differentiating between anaplasmosis and these other diseases,” he says.
Microscopic examination of stained blood films is commonly used to detectA. marginale organisms in erythrocytes of infected animals. However, this diagnostic technique may be unreliable when cattle have minimal
infections or in advanced cases of the disease when animals are severely anemic. “In a study conducted at Kansas State University we observed that the cELISA accurately identified all infected cattle before the number of A. marginale–infected erythrocytes exceeded percent parasitized erythrocytes (PPE) of 1%,” Coetzee says. This suggests that the cELISA may be more sensitive than examination of stained blood films for identifying early clinical cases.
Coetzee’s research group found in a study that compared the sensitivity of the complement fixation (CF) and a new cELISA tests that the overall sensitivity of the cELISA and CF tests was 94.8% and 26.5%, respectively. “These results indicate that the cELISA has superior sensitivity for the serological detection of A. marginale,” Coetzee says. “It is however significant that both tests demonstrated a high percentage of false negatives during the prepatent period. For the purpose of identifying anaplasmosis carrier cattle, this new commercially available cELISA test is reported to have a sensitivity of 96% and specificity of 95%. Practitioners should interpret results in the 30–40% range with caution as these animals should ideally be re-tested to confirm their serological status.”
Laurin uses the cELISA test on serum to indicate infection and active carriers. “We have gone into herds and given injectable tetracycline with multiple runs through the chute to get 10 days active blood level of the antibiotic,” she says. “For some herds, we then come back and test to see which cows have cleared vs. which are carriers.”
The cELISA test at Kansas State University Veterinary Diagnostic Labor-atory is run on serum. Blood should be collected in serum (red top) tubes, and serum removed by centrifugation to avoid hemolysis. Serum can be frozen and shipped on ice to the diagnostic laboratory. Alternatively serum tubes can be shipped overnight in cold packs. At Kansas State, the anaplasmosis cELISA test costs $9.00/sample for the first 10 samples and $6.00/sample for each subsequent sample. Check with your diagnostic laboratory for information on anaplasmosis testing.
Molecular biological tests may be the future of definitive anaplasmosis identification and control strategies in very early stages of infection and Coetzee says that currently polymerase chain reaction (PCR) is being researched at Kansas State. An RNA-based PCR test may be useful to identify cattle in the early stages of infection or to determine if attempts to eliminate persistent anaplasmosis infections with antimicrobials have been successful.
Anaplasmosis prevention and treatment
There are currently no vaccines that are universally efficacious in preventing anaplasmosis in cattle. The best way to prevent the introduction of anaplasmosis into a naïve herd is to maintain a closed herd and to test cattle prior to purchase to ensure that these are seronegative. Observing biosecurity practices such as using clean needles and disinfecting equipment between animals is absolutely necessary to prevent the spread of disease. Strategic fly and tick control during the peak vector season may also help prevent the spread of disease.
In endemic areas, maintaining a carrier state may be advantageous because persistently infected cattle are generally resistant to new infections. However, even in endemically stable herds some losses should be expected especially when the animals are under stress. “We do not advocate intentionally infecting animals with anaplas-mosis as this may increase these losses and may not provide cross-protection against newly introduced strains of the disease,” Coetzee says. “This would also limit a producer’s ability to market cattle in non-endemic areas and may spread other diseases such as bovine leukosis .”
Burning of pastures has had a positive effect in Laurin’s area, how-ever, neighboring pastures that don’t burn can then have high tick counts and more chance of becoming infected, she says. Some research also suggests that wildlife are attracted to the new growth accelerated by burning and therefore burned areas rebound fast and often have higher tick populations.
Laurin says there has been a tradition of keeping chlortetracycline (CTC) medicated mineral out in summer to help prevent incidence, but in the past few years it has seemed to help less. Year-round feeding of tetracycline antimicrobials is advocated in some areas to control active anaplasmosis infections. Coetzee adds that although this strategy may prevent clinical cases of the disease occurring, research has shown that persistent anaplasmosis infections can be sterilized if 2 mg/lb bodyweight/day of CTC is fed for over 50 days.
“We have shown that cattle that are chemosterilized with chlortetracycline are susceptible to reinfection with anaplasmosis. Therefore it is possible that year round CTC feeding may inadvertently clear infection from some carrier animals making these susceptible to new infections and creating endemic instability,” Coetzee says. Researchers from Washington State University recently found evidence of genetic material that may convey multi-drug resistance in the A. marginale genome. However, to date, resistance has not been documented with anaplasmosis.
CTC and oxytetracycline (OTC) are the only compounds approved for use against acute anaplasmosis in the United States. Chemosterilization has been reported in cattle fed CTC at dosages ranging from 0.5 mg/lb for 120 days to 5 mg/lb for 30 to 60 days. CTC is only approved for control of active infection of anaplasmosis caused by Anaplasma marginale susceptible to CTC when delivered in a free-choice feed at a dose of 0.5 to 2.0 mg CTC/lb body weight/day.
Results of a Kansas State University study demonstrate that CTC administered at 2 mg CTC/ lb bodyweight/ day for at least 50 days may be used to eliminate persistent A. marginale infections. “These research findings have important implications for cattle producers,” Coetzee says. First, this regimen may offer a way to clear infection from carrier cattle in non-endemic areas where the disease has been recently introduced however this may not occur with all isolates and is dependent on cattle consuming sufficient amounts of medicated feed.”
Second, this study demonstrated that the commercially available cELISA test offers a practical and cost effective means of determining the success of disease clearance because chemosterilized cattle will test cELISA negative in three to four months. Third, it appears that chemosterilization is not dependent on the concentration of tetracycline that animals are exposed to but rather the duration of therapy. This suggests that an adequate host immune response is required to work in concert with the antimicrobial to eliminate the infection.
Lastly, clearance of persistent anaplasmosis infections at the approved dose may not be desirable in endemic areas because these cattle are completely susceptible to reinfection with the same anaplasmosis isolate. “It is not known to what extent this may contribute to outbreaks of anaplasmosis in endemic areas but feeding tetracyclines at the lower end of the approved dose range or pulse-feeding may be considered as alternatives to year-round exposure to CTC,” Coetzee says.
Ticks and other types of transmission
Anaplasmosis in cattle is a rickettsial disease caused by Anaplasma marginale and commonly transmitted by ticks. Kathy Kocan, PhD, says in the U.S., anaplasmosis is transmitted by Dermacentor ticks (D. andersoni in western United States, D. variabilis and D. albipictus in other areas of the United States). D. andersoni and D. variabili are 3-host ticks, in which larvae, nymphs and adult stages feed on separate hosts (larvae and nymphs feed preferentially on small mammals and adults feed on large mammals), whileD. albipictus is a one-host tick in which all tick stages feed on the same host.
D. variabilis and D. andersoni transmit A. marginale from stage to stage (interstadial transmission) or by transfer of male ticks (intrastadial transmission within the tick stage) which can acquire infection from an infected cow and then readily transmit infection to susceptible cattle during subsequent feedings. Male ticks are intermittent feeders that transfer readily transfer among individual cattle and therefore have the potential to transmit A. marginale to many cattle. Therefore, male ticks may be the main tick stage transmitting A. marginale throughout a herd without requiring molting from one tick stage to the next. However, transovarial transmission of A. marginale one tick generation to the next via eggs does not occur and without a source of infection the tick population will not be infected in subsequent generations.
Wildlife reservoirs of A. marginale may impact the epidemiology of anaplasmosis. While deer (white-tailed and mule) do not appear to serve as reservoirs of A. marginale but they may drive tick populations in certain areas. Bison from both Canada and the U.S. were found to be a reservoir for A. marginale which was infective for cattle and ticks.
Procedures such as vaccination, ear tagging, hormone implanting, dehorning and castration performed soon after cattle arrive at feedlots are common practices. Blood-contaminated needles and surgical instruments are recognized methods of transmission of anaplasmosis. Hans Coetzee, BVSc, PhD, Dipl. ACVCP, says a review of several reports of anaplasmosis outbreaks attributed to iatrogenic infections described a case involving 1,500 animals where anaplasmosis was associated with exposure to contaminated dehorning and horn tipping equipment.
“In another report, an outbreak of 105 cases of anaplasmosis was attributed to contaminated vaccination equipment,” Coetzee adds. “Recently Brandon Reinbold DVM, PhD at Kansas State University found that 6/10 naïve calves became infected when exposed to a contaminated injection needle used on an infected animal.” Disease transmission was not observed when calves were exposed to a needle-free injection system. The apparent prevalence of anaplasmosis in calves arriving at feedlots described previously emphasizes the importance of using clean equipment to minimize disease transmission at processing.
Coetzee adds that it is noteworthy that some isolates of anaplasmosis are not tick-transmissible and are primarily spread on the mouth parts of biting flies and insects.