By Gregg Hanzlicek, DVM, PhD, Director of Production Animal Field Investigations and Outreach, Kansas State Veterinary Diagnostic Laboratory .
Understanding the etiology of the disease and diagnostic processes can facilitate management and minimize anaplasmosis losses.
The first part of this series, appearing in the March/April issue of Bovine Veterinarian, outlined the prevalence and transmission methods for anaplasmosis in cattle. This installment focuses on temporal effects, types of infection, clinical signs, diagnostic practices treatments and prevention of anaplasmosis.
In Part 1, we noted that ticks serve as the primary biological vector for Anaplasma marginale, while flies can mechanically transfer the disease as can fomites, particularly needles.
Transplacental transfer is one of the major questions we get from both veterinarians and producers. Anaplasma definitely can be passed through the placenta. In three recent studies, the transfer rate was anywhere from 11% to 17%. The animals used in these studies did not have acute infections. They were carrier animals, so their level of bacteremia would be relatively low, and yet the infection rate was between 11% and 17% of the calves in utero. Those calves were born already positive for Anaplasma. Transplacental transmission is probably important in the epidemiology of this disease.
We hear questions about wildlife reservoirs concerning BVD, blue tongue and other diseases. Until about 2008, mule deer were considered to be one of the major wildlife reservoirs for anaplasmosis. Connor, in 2008, conducted a study that indicated they aren’t as big a reservoir as we thought. We have outbreaks in bison in Kansas occasionally, so they can carry it. White tail deer are difficult to infect even experimentally. They aren’t considered to play a major, if any, role, in the transmission of Anaplasma. Black tail deer on the West Coast probably do carry the pathogen, but overall, research has shown that cattle, rather than wildlife, serve as the primary reservoir for this disease.
Producers need to understand that all ages of cattle can become infected. It is true that younger animals are a little refractory to infection, but all ages can become infected with this disease. That’s a good take home message for your producers.
The incubation average is 28 days, so we say a month. It can range anywhere from seven to 60 days. It depends upon the age of the animal, health of the animal, probably the organism strain and of course, the infective dose. The higher dose of infection, the shorter time period before seeing clinical signs.
Table 2 shows the variation in blood concentration during the weeks post-infection. This shows the actual concentration of organisms/ml, not percent of red blood cells (RBC) infected. We see a very quick increase after infection up to 1 billion Anaplasma organisms/ml of blood. Then the immune system, utilizing IgG and interleukins and interferons, kicks in and reduces the blood concentration to very low levels around 100 organisms/ml. The organisms, however are able to change their outer surface proteins and produce what are called mutant variants. They are able to hide from the immune system, allowing the population increases again, not nearly as high as in acute infections, but up to 1 million organisms/ml. The immune system then again reduces the concentration to a low level.
Once an animal is infected, they will be persistently infected and show these fluctuations for life. When we first learned this, we started looking at paired samples, asking veterinarians to collect samples for ELISA and PCR tests from the same cows, which we assumed were carrier cows. When we ran the tests and compared them, we found some cases where the ELISA result was positive and the PCR negative. When we do PCR tests, we use micro-liters of blood, and we suspect that in carrier-state animals, the concentration is so low that we occasionally miss the organisms in the sample. Antibody levels though, stay very constant across these up and down motions of concentration.
These fluctuations generally occur in a cycle of about five weeks, but some studies show the cycle can vary and can be as short as 10 to 14 days. It probably depends on the pathogen strain, the animal and nutritional factors. In any case, these carrier animals go through these phases for the rest of their lives.
Persistent infections occur because, while Anaplasmosis marginale is a gram negative organism, it does not produce endotoxins meaning the immune system doesn’t have an endotoxin trigger. The organisms manipulate host neutrophils and are masters at changing that outer surface protein, hiding from the immune system and suppressing immune response.
Inverse age immunity
When we talk about inverse age immunity, we mean both immunity from infection in young animals and immunity from clinical signs. It is a dual purpose term.
In an older but useful study (Jones et al, 1968), researchers injected 5cc of blood from a carrier cow into weaned calves, aged cows and relatively young steers and heifers, and collected measurements of the percentage of RBC infected. In the five older cows, they found 40% of the RBC infected and two of the cows died. Older animals are more susceptible to the clinical signs.
In the yearling heifers and steers, they found a similar pattern but only 20 to 25% of RBC were infected.
The calves became infected, but infected RBC infected never rose much beyond 10%. So while calves can become infected, we but we normally don’t see clinical signs and they do not experience the anemia found in older animals.
In a 2001 study (Goff et al), researchers inoculated four-month-old calves and three-year-old cows with Anaplasma and measured interleukin 12 and interferon. In this study, calves showed an immediate and measureable interferon and interleukin response, while in older cows there was a delay of five to seven days before they could even measure the two compounds, which are very important for controlling this disease. One theory on why cows experience more clinical signs suggests the delayed response allows the bacteria population to reach levels that cause anemia.
Maternal antibodies probably also help protect calves from anaplasmosis. Research also has shown that splenic NK cells are more effective for clearing pathogens in calves than in mature cows. Also, the bone marrow in younger animals produces RBC faster than in old cows or bulls. These mechanisms could help explain we see inverse immunity to anaplasmosis.
In cattle over two years of age, the number-one clinical sign for producers is when they find one or more adult animals dead in the pasture. In our area the peak time is late summer and early fall, but we see it all year.
Anaplasmosis can cause abortions, and we have seen multiple cases in spring-calving, and this year, in fall calving cows that aborted from this disease. Not all abortions occur in clinically ill cows that are severely anemic, or the calf becomes hypoxic and dies. In some cases, we know these animals are not acute, but are actually carriers. Some of our experienced pathologists who have dealt with this disease for a long time, tell me these calves become septic, with a high concentration of bacteria leading to organ shut-down. I suggest including anaplasmosis on the list if you have an abortion or an abortion storm in your herds.
Another key take-home point is that all ages of animals can become infected. We just consider that once they are infected, just like BLD, they are basically infected for the rest of their lives. They won’t clear the infection.
One positive aspect of this disease is that lifelong carriers will not likely experience clinical anaplasmosis in subsequent years. Whether you treat it or not, if the animal survives the anaplasmosis and the acute infection, they will be carriers, but typically resistant to clinical signs later in life. We’ve always said though, that the key words are “not likely.” We do see some animals, near calving and under stress, experience a down-regulation of the immune system. These are from endemic herds and not acutely infected. We think they are carriers but they suddenly break out with an acute infection if the conditions are right.
We get many calls about vaccines for anaplasmosis. There is an experimental killed vaccine available in some states. The vaccine does not prevent infection, but reports from the fields suggest it reduces clinical signs and reduces cow and bull death losses. The evidence is mostly anecdotal, and cross-protection for various strains remains unclear. There are at least 100 herds in eastern Kansas using the vaccine, and I do not know of any of those producers who have stopped using it once they started. They seem to believe that it does help with clinical signs.
Managing positive herds
One of the goals for Anaplasma-positive herds in endemic areas is to goal is to become endemically stable. Endemically stable means we have a positive herd but we rarely see clinical signs. This occurs if we can infect a high enough percentage of the herd each year, and over time only the young calves will be able to be infected for the first time. I already said that young calves rarely show clinical signs. That is the theory behind endemically stable.
In an Australian study, researchers watched herds over time and found that at very low infection rates there were few clinical signs, which makes sense. They also showed though, that if the infection rate was very high, with at least 75% of the calves infected every year, those herds rarely showed clinical signs. In this study, herds with moderate infection rates experienced the highest incidence of clinical signs. However, as a vector-borne disease, fluctuations in tick and fly populations probably influence the herd’s response to the disease. We often hear from veterinarians and producers saying a herd has been endemic for 15 years, but cattle suddenly begin showing clinical signs.
The Gold Standard diagnostic for determining anaplasma infection is to take blood out of that animal and inject it into a calf that has its spleen removed, but that test is limited to research applications. Veterinarians generally use blood smears, which have sensitivity of 20 to 30%.prior to clinical signs. I’ve not been able to find estimates of diagnostic sensitivity during clinical signs, but it is reported as good. A persistent infection has a low sensitivity for this particular test.
ELISA is a serum antibody test. Some studies by Hans Coetzee and others who inoculated calves showed the sensitivity was 50% before day 10, but went to 99% on day 13 after inoculation. One study examined a calf infested with six to 10 infected ticks. The researchers measured antibodies using ELISA, and found the antibody was positive at the 30% cut-off nine days after infection. In our lab we usually say 30 days is a good time to let these animals sero-convert before we use an ELISA.
In carriers, sensitivity and specificity are very high. ELISA is a really good test to use to find carriers in herds if they have had time to sero-convert.
PCR is an antigen test looking at ribosomal RNA. This will trigger for A. marginale and A. phagocytophilium, which is a white blood cell anaplasma organism. It doesn’t cause disease in cattle in the United States, but does in Europe. We don’t know what the prevalence of A. phagocytophilium is in cattle, but we know that dogs, cats, tortoises, and other species carry it. It is the causative organism of human granulocytic anaplasmosis, and news reports often suggest human cases are related to cattle. However, anaplasmosis in humans more likely originates in dogs, cats, and other species where it is much more prevalent.
For PCR testing, we use whole blood and fresh spleen samples. We don’t really know what the diagnostic sensitivity is on carrier animals. It is just considered to be very good for all PCRs. Also, PCR extraction methods differ between labs. Our question is, if we get down to very low infection levels, are we missing some with the PCR where the ELISA would identify them as positive?