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Infectious respiratory disease: making the right diagnosis

What are the important things to consider when trying to diagnose a respiratory infection in horses?

03 September 2020, at 8:30am

Outbreaks of respiratory infections on yards are an ever-present threat facing the UK equine population. The saying “prevention is better than a cure” could not be truer in order to avoid the widespread consequences of disease outbreaks. But when faced with a snotty horse in the middle of a busy day of calls, what approach should veterinary surgeons be taking to obtain a diagnosis and why does making the correct diagnosis matter?

An effective approach to diagnosis and control of infectious disease is only possible with a good understanding of the epidemiology of the UK’s most common equine respiratory infections, and there are many online resources to keep you up to date with the latest diagnoses in the UK. This article will focus mostly on equine influenza, equine herpes virus-1 and -4, and strangles (Streptococcus equi).

Which sample and testing method to choose?

Obtaining a diagnosis of the causative agent of an infectious respiratory case/outbreak may appear to be a challenge, particularly if clients are reluctant to sample due to financial constraints. However, a mismanaged outbreak due to a slow or incorrect diagnosis will have a much greater economic and welfare cost and this should be clearly communicated with clients if they appear reluctant in the first instance. Diagnostic success can be hampered by clients contacting veterinary surgeons too late on in the infectious process, making negative agent detection test results more likely and yard control measures less successful due to widespread exposure having already occurred. Veterinary surgeons should actively encourage owners to understand how to closely monitor their horses, particularly when the risk of exposure to infections may be higher, such as after attendance at an event, and veterinary surgeons must encourage clients to contact them without hesitation if they have any concerns. The choice of sample and the laboratory technique to use is made under the pressure of many factors in a clinical setting. A clear understanding of optimal sample choice, sample method and diagnostic test is necessary to assist those initial choices and ensure greatest diagnostic gain. Correct interpretation of laboratory results is also paramount.

Agent detection tests

These diagnostic tests identify the infectious agent and therefore are run on samples taken from anatomical sites where the agent is most likely to be present. Factors affecting sampling success include incorrect sample choice, suboptimal sampling technique, incorrect sampling materials used, poor timing of sampling or incorrect diagnostic test choice. Polymerase chain reaction (PCR) is the most frequently used modality for agent detection and this molecular diagnostic test uses limited but specific genetic sequences to identify pathogen DNA or RNA in a sample. PCR is rapid (around two hours, giving rise to the possibility of same day results), relatively simple, relatively cheap and very sensitive (ie detects a small amount of agent and rarely produces false negative results). The potential incubation and latency periods vary between agents, and careful history taking and physical examination will help determine if agent detection testing will be successful in the specific case presented. Population medicine should always be applied to yards, despite implementation challenges due to multiple owners and veterinary practices being involved. If the case presented is not the most optimal to make a diagnosis through agent detection techniques, additional horses may need to be sampled and may provide a higher likelihood of a successful diagnosis.

Serological tests

Diagnostic testing methods measuring an immunological response to infection are used to analyse samples for the presence of infection-specific antibodies. Their presence demonstrates an immune response (at some point) either to infection by, or after vaccination against, the specific pathogen (many tests cannot readily differentiate responses to infection from those induced by vaccination unless marker vaccines are used).

Optimising testing

Careful history taking will help to determine the most likely differential diagnoses. If agent detection is timed when signs may not be specific and because simultaneous multiple infections can occur, it is advisable to submit swabs to be tested by agent detection for all three endemic diseases. When obtaining a history, case- and population-specific questions should be asked (Table 1). Following history and physical examination, optimal sample methods should then be chosen based on findings of these initial investigations, taking into account each disease’s epidemiology.

TABLE (1) Pertinent questions to be asked when taking a history of a suspect respiratory infectious disease case(s)
TABLE (1) Pertinent questions to be asked when taking a history of a suspect respiratory infectious disease case(s)

Submission of diagnostic samples

All sampling materials should be confirmed to be in date prior to use. After taking a swab, it should be placed in an appropriate transport medium for the agent being tested for, to optimise detection of that organism in subsequent laboratory testing, which is usually provided by the receiving laboratory. If no medium is available, a maximum of 7ml of sterile saline in a universal container will suffice. Packaging of biological samples is subject to Health and Safety Executive legislation and the sample must be in a leak-proof primary receptacle, be individually wrapped with absorbent material and placed in a leak-proof secondary packaging material (eg a sealed plastic bag). The outer envelope should be appropriately labelled. A sample should, wherever possible, be received by a testing laboratory within two days of sampling and guaranteed delivery is recommended. Laboratory submission forms should be filled out with as much information as possible to ensure the laboratory is provided with sufficient history to aid result interpretation.

Result interpretation

Diagnostic test results can be challenging to interpret and the potential reasons for each obtained result should be considered, with the case's history being paramount for correct result interpretation. Information from initial investigations and diagnostic tests should be assessed in combination with case factors and factors specific to the entire population in which the case resides. Infectious disease diagnosis is not black and white and veterinary surgeons are encouraged to discuss presentations and results with veterinary epidemiologists to help make a correct diagnosis. Veterinary epidemiologists will also be able to assist in optimal diagnostic choices and control methods.

In reporting serological sample results, positive and negative phrasing is used with caution as a case could have a “negative” serology result but still have the infection, with the most common reason for this being a result of serological sampling in the acute phase of the infection. It is also beneficial to be aware that a titre does not need to be zero for a result to be interpreted as “no evidence of infection”, as each test has its own levels for suspicion of infection. When interpreting results from paired samples, the original acute serum sample should be tested alongside the second convalescent sample, to provide a valid paired test result. Only samples run on the same test run can be reliably compared due to the possibility of inter-assay variation.

When the original sample is re-run, each assay will have an inter-assay variation value that is deemed to be acceptable. Occasionally, results may fall out of this allowable bound-ary and laboratories should have internal quality controls implemented to monitor and help prevent such scenarios; where this occurs, paired samples should be retested.

Preventing disease outbreaks

Veterinary surgeons are actively encouraged to discuss with clients their approaches to new arrivals, precautions taken at equine events, general yard hygiene, equipment sharing and protocols for visiting personnel. Risk mitigation is integral to equip owners with the best defences against infectious disease outbreaks, ensuring the health, welfare and performance of their horses. Simple discussions at time of routine vaccinations provide a great opportunity to suggest the best approaches to biosecurity, with these being yard specific depending on yard type and facilities available. Given the fact that risk cannot be completely eliminated, a fast, correct diagnosis assists in the implementation of optimal control measures, limiting the negative effects of infectious disease outbreaks.

Conclusion

Successful infectious disease diagnosis can be a challenge, but there are a multitude of benefits to obtaining a correct diagnosis. Cases will commonly require both agent detection and serological testing. Obtaining samples for agent detection at the correct time and just prior to instigating treatment enhances the chances of an accurate diagnosis. It is vital to utilise the test in conjunction with the specific case's history (including vaccination status and reason for testing) and clinical signs. If a test result is unexpected, this should be acknowledged in partnership with the testing laboratory, as all diagnostic testing has the potential for incorrect results. Infectious diseases come with many challenges and veterinary surgeons may feel pressurised in their diagnosis and management, but having a clear understanding of infectious disease epidemiology and laboratory testing options and carrying out sampling in a concise and timely manner will assist to reduce the consequences and stigma of infectious diseases.

The latest equine infectious disease diagnoses are available online.

Fleur Whitlock, BVetMed (Hons), MRCVS, worked as an equine vet within the Animal Health Trust (AHT)'s Epidemiology and Disease Surveillance team prior to the AHT’s closure at the end of July 2020. Since then, with ongoing support from the UK equine industry, the team is continuing to work closely with vets and equine industry stakeholders to ensure the health of the equine population. Their work focuses on infectious disease diagnosis, control and prevention.

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Richard Newton, BVSc, MSc, PhD, FRCVS was the Director of Epidemiology and Disease Surveillance at the Animal Health Trust until its closure in July 2020. He continues to work closely with the UK equine industry in conducting global disease surveillance and advising on disease prevention strategies and management of outbreaks

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