It comes as no surprise that porcine epidemic diarrhea virus (PEDV) has surged to become the most common enteric disease diagnosed in porcine tissues submitted to the Iowa State University Veterinary Diagnostic Laboratory (ISUVDL).  PEDV was present in over one-third of the cases of enteric disease diagnosed in the past 12 months (Figure 1.)

PEDV has spread rapidly in the U.S., particularly in the period from December 2013 through March 2014. Although losses are severe, this disease has illuminated gaps in our national, regional, local, and farm-level biosecurity programs.  Many producers and veterinarians have shored up biosecurity protocols with emphasis on consistent execution on a daily basis – doing it right ALL the time. If biosecurity really matters, might one expect changes in the trends in frequency of diagnosis of other diseases?

Disclaimer: Data from a diagnostic laboratory should not be used to imply prevalence, incidence or relative impact of disease agents on farms. The purpose of this report is to provoke thought rather than offer conclusions. In this article, all data and diagnoses are from cases in which there was detection of the offending pathogen and a compatible microscopic lesion with evaluation of tissues by a veterinary pathologist. These types of cases are sometimes referred to as “tissue cases.” Positive tests from feces, environmental samples, or submission in which pathology was not performed are not included (simply a positive polymerase chain reaction test ((PCR)) on feces was not counted as a case).  

Figure 1 illustrates the percentage of pathologic diagnosis (from “tissue cases”) of various causes of enteric disease, with blue bars representing the average from three years prior to April 30^th 2013, when the first U.S. cases of PEDV were diagnose,  and the red bars representing the year since PEDV emerged in the U.S. The number of enteric “tissue cases” presented to ISUVDL increased by 25% over the past year, but both the percentage and the frequency of detection of some of these diseases has decreased. These observations may portend changes in trends of disease detection and perhaps impact on farms.

Figure 1: Annualized % of causes of diarrhea detected from “tissue” submissions. (ISUVDL: May2010-Apr2013 versus May2013 to Apr2014) Blue bars: Average of 3 years prior to PED; Red bars: year since PED

A brief summary of ISUVDL diagnostic data from cases which had tissues and pathological assessment as part of the diagnostic process (not just samples tested by PCR) is found in Table 1.

Table 1: Changes in frequency of diagnosis of selected disease agents

Of particular interest is the decrease in frequency of transmissible gastroenteritis (TGE) diagnosis. Since records started the 1950s, cases of TGE have increased in winter months and subsided in the summer. Figure 2 illustrates the recent seasonal nature of TGE diagnosis. In recent years, most cases of TGE have occurred at wean-finish sites, with transportation serving as a common source of virus transmission. However, in the last year, TGE cases have been virtually eliminated, with no seasonal increase in 2013-14; indeed, no cases detected from July 2013 to year’s end, and only one case detected in 2014 (Feb). This does not mean there were not positive tests for TGE on submissions that contained only feces or environmental samples. All cases referred to in this article are based on submission of tissues and have compatible microscopic lesions.

One could speculate that increased biosecurity measures implemented for PEDV may have some impact the frequency of diagnosis of diseases. Improved external biosecurity would be expected to prevent introduction of “new” disease introductions to farms or sites while improvement in internal biosecurity (e.g. improved hygiene, disinfection, traffic patterns) would be expected to decrease overall impact from disease agents already endemic on the farm.

It is not possible to know if the frequency of diagnosis of the agents in Table 1 represents a “new introduction,” or new outbreaks on a farm, or if those cases are simply confirmation of disease on farms or flows previously known to be positive. For example, the strongly beta hemolytic Brachyspira (hyodysenteriae or hampsonii) cause swine dysentery, and are likely introduced to many farms in a manner similar to TGE (through transportation or carrier pigs), but this agent will have a longer incubation period, move slower through the group, and can remain endemic for long periods of time. External biosecurity should prevent a new introduction, but one should expect some of the preventative benefit to also come from the internal biosecurity practices. Appropriate hygiene, implementing preventative management practices and controlling traffic patterns are expected to be quite useful in mitigating disease expression with Brachyspira.

Similar general statements regarding disease introduction (external biosecurity practices) or disease expression (internal biosecurity practices) may apply to salmonellosis (disease caused by Salmonella sp) or Lawsonia (aka. proliferative enteritis or ileitis). External biosecurity undoubtedly has a role in preventing new introductions of porcine reproductive and respiratory syndrome (PRRS) virus. Internal biosecurity and management practices can also impact PRRS transmission or disease expression on endemic farms. It is more difficult to assess the role of biosecurity in occurrence of clinical expression of respiratory pathogens such as Mycoplasma hyopneumoniae or swine influenza virus (SIV).

The presence of PEDV on a farm will undoubtedly cause continuing losses. The interaction of the PED virus with other pathogens is not well studied. There are a multitude of reasons why disease trends will change over time. Diagnostic laboratory data is highly skewed and does not represent true incidence, prevalence or disease impact. That said, it remains plausible that the added emphasis on biosecurity for PEDV – what we do OUTSIDE farms and what we do INSIDE farms – may be having benefit on transmission or expression of other disease agents.