Intervention and Investigation of Outbreaks of Health Care-Associated Infections
An outbreak is defined as an increased occurrence of infection or disease above the expected baseline (endemic) level. Outbreaks are an ever-present risk in all health care facilities, veterinary or otherwise, and outbreaks will occur despite rigorous infection control protocols.
As stated regarding surveillance, if HAIs are not being identified in health care settings, then methods are not sufficient to detect their occurrence. It follows that you cannot manage what you have not detected. The general principles of outbreak management are to contain the outbreak and reduce the overall impact on individual animals and the facility. It is critical to note that despite the stigma often attached to outbreaks, an outbreak of disease does not necessarily indicate poor management or malpractice. Making no attempt to identify HAIs, neglecting signs of a developing outbreak, or responding inappropriately to the spread of HAIs could all be construed as poor management and even malpractice. Thus there are many reasons why plans for a logical, aggressive, organized, and cooperative response should be developed before outbreaks have ever been identified. Consultation with internal and external experts in epidemiology, infectious diseases, microbiology, and infection control can be useful. It is unlikely that the person involved in coordinating the outbreak response will be an expert in all areas. Consulting others does not indicate a lack of ability on the part of the response team; rather, it indicates a logical approach to typically complex problems. It makes sense to consult with those who, through interactions at multiple facilities, have gained experience that usually cannot be obtained while concentrating on primary responsibilities as a clinician at a single institution. Ideally, these individuals are consulted early in the outbreak, and it may be prudent to develop contingency plans and contact consultants in anticipation of potential problems. The use of external consultants can be particularly useful because they can examine a situation and facility without the same degree of inherent bias that develops through experience and habit of regular daily practice in a facility.One of the most important factors in outbreak intervention is early recognition of “clusters” of cases that may be related. Ideally, laboratory testing, clinical disease reporting, or syndromic surveillance will be used to aid in systematic efforts for early identification of suspicious clusters of cases. First-generation transmission—that is, transmission from the index case (directly or indirectly) to other animals—can be difficult to detect, particularly in situations where the index case is infectious prior to developing clinical disease. The most important control aspect related to the first generation of cases is prompt identification so that an outbreak can be recognized early enough to implement measures to reduce the risk of further transmission to a second generation of cases. Often outbreaks are not identified or addressed until multiple generations of transmission have occurred, thereby increasing the overall morbidity and mortality and complicating the response.
General principles of outbreak response include identification of the infectious agent, identification of infectious animals, identification of animals that have likely been exposed, determination of the mode of transmission, and prevention of further transmission by means such as isolation, implementation of barrier precautions, active surveillance, environmental surveillance, increased environmental disinfection, restriction of animal and personnel movement, and under extreme conditions minimizing new exposures through restricting admission of new patients.
While useful, initial intervention measures do not require definitive identification of the etiologic agent. In the absence of a confirmed etiology (i.e., recognizing a cluster of patients with diarrhea of unknown cause), outbreak intervention can still be performed using general infection control practices combined with knowledge of appropriate response regarding the most likely causes.
General principles such as isolation of infected or affected animals and their direct contacts, use of barrier precautions, close attention to personal hygiene, restriction of animal and human movement, regular examination of animals and careful attention to cleaning and disinfection are important measures that are useful for most types of infectious disease outbreaks. Indeed, these measures alone will often constitute the full response even after an etiologic agent is identified.Identification of animals likely to be shedding the agent responsible for the outbreak is critical. Definitive confirmation is not required, and a syndromic response is often better. For example, amid a Salmonella outbreak, it is wise to consider all animals with diarrhea, depression, or fever as likely to be infected and shedding bacteria even if culture results have been negative for some. The same is true for horses that develop fevers when others suspected of having herpesvirus myeloen- cephalitis or influenza are hospitalized in the same facility. This will result in some misclassification, but erring on the side of being overly sensitive in identifying infectious animals is more appropriate when trying to halt the spread of disease. The main problem with this type of response is imposing excessive strain on available resources (e.g., isolation stalls, personnel time) because of excessive misclassification.
Identification of potentially exposed animals is also critical. Depending on the pathophysiology of disease, these animals may or may not require segregation or isolation. For some pathogens, clinical disease precedes development of the infectious state, so exposed animals can be monitored and isolated if required. For example, horses exposed to S. equi consistently develop fever prior to shedding the organism;78 as such, regular monitoring of body temperatures of exposed animals and prompt isolation of animals developing fever may be sufficient to prevent propagation of an outbreak.
For many pathogens, screening of all potentially exposed animals can be useful. In particular, the use of rapid screening techniques that can provide a turnaround time of a few hours (e.g., antigen detection test for equine influenza, S. equi PCR) can be very useful. However, testing strategies should also include some use of culture so that whole organisms will be available to aid epidemiologic investigations. Sampling should be based on the pathophysiology of infection and disease (i.e., sampling when shedding is most likely to occur). Widespread screening of hospitalized animals will often be required to determine the extent of the outbreak and facilitate removal of infectious animals from the population. Therefore if imposed as a response to suspected spread of HAIs, it is essential that screening not be an elective procedure. As such, it is prudent for facilities to have previously considered how costs for diagnostic testing will be distributed in the event of suspected HAIs. Surveillance for shedding of specific contagious agents at the time of admission can facilitate prevention of outbreaks as well as intervention responses. In situations in which recently exposed animals do not necessarily develop consistent overt clinical signs or rapid diagnostic testing is not available, it may be necessary to segregate all potentially exposed animals, either in an isolation unit or in the main hospital.Determination of the likely mode(s) of transmission is also required for initiation of an effective response. This is usually relatively clear based on the identity of the known or suspected pathogen. Regardless, all agents of concern for HAIs can generally be spread through contact and common vehicle (fomite) transmission, and thus controlling all methods of contact exposure to patients will be critical in response to suspected outbreaks. Although droplet transmission might be most important for respiratory agents, practicalities of cleaning hospital environments for large animals usually necessitates use of large volumes of water, which can also create hazards related to droplet transmission for other agents such those with oral-fecal transmission.
As such, the potential for droplet transmission should also be tightly controlled in response to suspected outbreaks. Generally, only highly contagious viral respiratory agents such as influenza virus pose a true hazard for airborne transmission. However, in situations in which routes of transmission are not clear, it is prudent to assume the worst and manage the outbreak as if the pathogen is highly transmissible by multiple routes until proven otherwise.Prevention of further transmission is obviously one of the most important goals of outbreak response. A variety of factors need to be considered to achieve this. In addition to identification of infectious animals, measures need to be established to prevent direct and indirect contact of these animals with other animals and the general hospital environment. Isolation and the use of barrier precautions can achieve this. The response to implemented infection control measures should be monitored closely to determine their adequacy, with an understanding that there is an inherent lag time from implementation of measures to control of the outbreak that is proportional to the incubation period for infectious diseases.
At times, partial or complete closure of the facility may be useful or necessary to protect patients that would be admitted in the future. Restricting new admissions decreases the number of newly susceptible individuals that might promote propagation of an outbreak. Further, if new infections appear to be occurring despite implementation of best control efforts, it is important to consider whether it is prudent or ethical to continue admitting new patients. It is also critical to consider the tremendous burden that can be necessitated by the need to deal with infection control measures and the possible effects on patient care. During outbreaks, stall space, particularly isolation space, may become limited, and decreasing the number of admissions facilitates depopulation of the facility for proper cleaning and disinfection.
Clinical personnel are often overworked during outbreaks because of the number of ill animals, and excessive workload can contribute to breaches in disease prevention protocols. Although there may be reluctance among some people to restrict hospital admissions because of potential negative impacts related to public relations and finances, early, short-term closure of a facility to stem a small but developing outbreak is much better than a subsequent longer closure in response to a larger outbreak with more affected animals. A key component of facility closure is proper communication to staff and the public, emphasizing the proactive approach that is being taken to protect patients and clients and to prevent a major problem from developing. Regardless of whether it is deemed necessary to restrict admission of new patients, as described previously, whenever outbreaks of HAIs are suspected, it is critical to consider whether standard informed consent procedures suitably convey the risks associated with hospitalization to clients.If environmental contamination is suspected of being widespread, complete depopulation of a ward or even the entire hospital may be necessary to allow for thorough cleaning and disinfection. The availability of multiple wards can facilitate this process, as individual problem wards can be closed and disinfected while allowing the hospital to remain functioning. Assuming that sensitive culture or other antigen detection methods are available, postdisinfection environmental screening may be indicated for certain pathogens (e.g., S. enterica) prior to reopening.
Although vaccination is often perceived as a cornerstone of infection control, the indications and limitations of vaccination must be considered. No vaccine confers protection to all vaccinated animals, and vaccination should never be used as a replacement for good infection control practices. A more appropriate perspective is that vaccination is principally useful when methods used to prevent exposures to infectious diseases have failed. Requiring vaccination prior to hospital admission is an area that has received limited consideration in large animal practices. Although vaccination requirements are difficult to enforce in animals admitted for emergency procedures, it may be reasonable for facilities to consider encouraging or requiring vaccination against relevant pathogens prior to admission for elective procedures. For example, requiring vaccination for influenza in horses or bovine viral diarrhea (BVD) in pregnant cattle prior to admission could decrease the risk of infection and transmission in hospitals. Problems that might be encountered with such an approach include the lack of relevant efficacy data for many vaccines, potential concerns regarding adverse effects with some vaccines, difficulty ensuring compliance, and competitive issues if all competing practices do not have the same standards.
Vaccination may be a useful intervention measure during some outbreaks, but the time lag between vaccination and protective immunity limits the potential role of most vaccines. Further, the efficacy of vaccines typically have not been evaluated in the face of outbreaks. It is important to remember that three possible outcomes are associated with any intervention, including vaccination: it may help, it may have no effect, or it may be harmful. For vaccination during an outbreak to be helpful in control efforts, the vaccine must be given to susceptible animals prior to exposure, must initiate a protective response before natural exposure, and must not be harmful when administered to animals that are incubating disease. These criteria are often difficult or impossible to meet during an outbreak. The greatest potential use for vaccination during an outbreak is likely intranasal influenza vaccination, since this type of vaccine can provide a rapid and effective immune response and is not contraindicated in exposed animals.