Abortions occurring in an anaplasmosis "endemically stable" cow-calf herd
By Drs. Gregg Hanzlicek and Cynthia Bell
Background: Bovine anaplasmosis is an infectious, non-contagious red blood cell disease caused by the gram negative bacteria Anaplasma marginale. Any age of animal can become infected. Infection occurs through the transfer of red blood cells from a positive to a negative animal. The major sources of bacteria transfer are through some tick species (Dermacentor sp.) and tabanids (horse and deer flies). Iatrogenic transfer also occurs through the use of multi-use needles and tattoo devices. Fetuses can also become infected through their positive dam.
History: A herd of 86 cross-bred adult beef cows experienced 6 abortions over a five day period. The gestational age for all the aborted fetuses was estimated at to be 240-270 days. The herd owner was aware that his herd had been anaplasmosis positive for over 20 years, and that his operation was in an endemic area.
Clinical signs: One cow was found dead and 4 others were experiencing common clinical anaplasmosis symptoms (dyspnea, ataxia, aggressiveness) at the time the abortions were occurring. Gross appearance of the aborted fetuses was unremarkable.
Diagnostics: A necropsy was not performed on the deceased cow. Blood samples from 3 of the clinical cows were submitted for anaplasmosis PCR. All three were positive with Ct values ranging from 14 to 23. Entire sets of fresh tissues were submitted from three fetuses (lung, thymus, heart, liver, spleen, kidney, lymph node, and placenta). Abomasal contents and thoracic fluid were also submitted. In addition to the fresh tissues, a complete set of fixed tissues including those listed above and the brain were submitted. Water and feed samples were collected and frozen in case toxicology diagnostics were found to be necessary.
Gross examination of the fetal tissues did not reveal anything remarkable. Histopathology revealed meconium and squamous epithelial cells in the lungs. Although not definitive, abundant meconium in the lungs may indicate fetal stress. Testing for known major abortion pathogens was completed, and all results were negative. All fetal splenic tissue samples were Anaplasma PCR positive (Ct values: 21-24).
Outcome: The entire adult herd was treated, per label, with an injectable chlortetracycline product, and no further abortions or clinical cows were reported.
Take home messages: It was difficult to convince the owner that the clinical signs and abortions were due to anaplasmosis because the herd had been positive for many years, and it had been many years since any clinical signs were observed.
One of the recommended management programs for positive herds in endemic areas is to let the herd become "endemically stable". This term describes a positive herd that rarely experiences clinical anaplasmosis. Theoretically, becoming endemically stable makes sense but research suggests that it is not simple. Recent work conducted in Australia indicated that the greatest opportunity for clinical issues are in herds where the infection rate has been or is moderate.1 This means that for endemic stability to be effective, the infection rate (force of infection) must be either very low or very high. A very low rate equates to few animals becoming infected, thus few animals will be able to become clinical (Figure 1). At very high rates, over time, only young calves will be able to be infected (the older animals will already be positive before they reach adulthood) and because young calves rarely become clinical, clinical signs should be rare (Figure 2). When the infection rate is moderate, there are plenty of positive animals to serve as a reservoir AND significant numbers of negative adults that are susceptible to clinically disease (Figure 3).
The problem with depending solely on this philosophy to manage herds is that the infection rate must remain either consistently low or consistently high from year to year. For example, those animals who are young during periods of low to moderate infection rate become negative adults then, if the force of infection increases greatly, they may become initially infected as a group, resulting in a clinical epidemic.
Anaplasmosis is primarily spread through insect vectors, so it seems logical that, during some years, the force of infection will vary with changes in the vector populations. Many factors impact vector populations and, unfortunately, few of them are within our control. The weather is the most obvious example, but other factors include changes in local land use that might provide more or less breeding ground for vectors.,.
Even for herds with a long history with anaplasmosis, but rarely if ever experiencing clinical signs, there are situations when clinical signs may occur. Endemic stability is a logical management philosophy but probably should not be the only management tool practiced in some herds.
1Is endemic stability of tick-borne disease in cattle a useful concept? Trends in Parasitology, Vol. 28, Issue 3, 2012, pages 85-89. Nicholas N. Jonsson, Russell E. Bock, et al.
|Figure 3||Key to figures|
1. Large Animal Internal Medicine. Bradford Smith, Fourth Edition 2005. Pages 1276-1278
2. Veterinary Medicine. Otto Radostits, et al. Ninth Edition, 2000. Page 760