A little over two years ago, we commented on a general rise in the frequency of Actinobacillus suis (A. suis) isolation from our swine tissue submission cases, and wondered if it was related to the increased porcine circovirus type 2 (PCV2) activity (National Hog Farmer North American Preview, June 6, 2008). A. suis is a gram-negative bacteria that is present in essentially all swine herds. It can cause respiratory disease and lameness in older pigs and, infrequently, diarrhea in nursing piglets.

The main concern with Actinobacillus suis, however, is sudden deaths in finishing pigs and adult breeding animals due to bacterial septicemia. Losses tend to accumulate over time rather than as an acute episode, so this bacteria doesn’t garner the same degree of attention as other agents that result in large-scale disease outbreaks. The losses of older growing and breeding pigs can add up when A. suis activity increases.

The chart shown in Figure 1 illustrates the point that, at least for submissions to the Minnesota Veterinary Diagnostic Laboratory, the frequency of isolating A. suis from respiratory cases has not dropped off following the widespread use of porcine circovirus type 2 (PCV2) vaccine, but has held steady and may even be increasing slightly. Because we had associated the increased rate of A. suis isolation with PCV2 activity, it is somewhat surprising that A. suis recovery hasn’t dropped along with the incidence of circovirus disease. We will continue to monitor this over time.

Tables 1 and 2 include antimicrobial susceptibility testing information over several years to show some additional trend information. Table 1 summarizes the percentage of A. suis isolates from 2008 and 2009 that were susceptible to the individually listed antibiotics. The last two columns show the MIC50 and MIC90 values. The MIC50 is the minimum inhibitory concentration of antibiotic that inhibits growth of 50% of the isolates that were tested for the year, and the MIC90 is the concentration of antibiotic that inhibits growth of 90% of the isolates. In other words, these are not bacterial colonies from an individual case, but rather are the composite of many cases tested over the course of an entire year.

The minimum inhibitory concentration is the result we report out from antimicrobial susceptibility testing. The lower the value the better, because it indicates that a lower concentration of antibiotic will inhibit the growth of a particular bacterial isolate.

Table 2 shows data over an eight year-period for just three antibiotics that were selected to illustrate some different points. The first part of this table illustrates the susceptibility testing for A. suis against ampicillin. I included this antibiotic because there is a general trend for increased susceptibility over the eight-year time span. The second section of the table refers to susceptibility to ceftiofur, which has remained relatively stable. The third part of the table refers to the impact of chlortetracycline, which has been more variable, with increases and decreases in susceptibility over time. These tables are designed to indicate that susceptibility to antibiotics can be dynamic, but variable in direction (increased or decreased) and rate of change.

Click to view graphs.

Jerry Torrison, DVM
Minnesota Veterinary Diagnostic Laboratory