Cats and dogs can carry a wide range of parasitic infections which can represent a serious health risk. A new maps service from ESCCAP is designed to show the proportion of pets tested that are positive for a given infection, using available assays (Figure 1). These data are very useful to veterinary professionals, pet owners and the wider public, as they help us to better understand the occurrence and distribution of animals in a given practice area which may be infected with, or had contact with, a particular parasite.

Figure 1. Sample map showing hookworm infections in dogs.

Currently, the ESCCAP maps are based solely on data collected by German laboratories. However, it is hoped that further data will be collated to provide a representation of parasite-associated infections across Europe.

There are twelve maps available to view for the following infections:

Dogs:

• Heartworm
• Giardia spp.
• Whipworm
• Roundworm
• Hookworm
• Ehrlichia
• Anaplasma
• Borrelia
• Leishmaniosis.

Cats:

• Giardia spp.
• Roundworm
• Hookworm.

Factors to consider when interpreting results

As with any data, results can be influenced by a number of factors, including the number of pets tested, the history of the pets before testing, the reason the pets were tested and the nature of the assays used. Understanding each of these factors is critical to accurately interpreting the frequency of positive tests for these important infections.

Sample size

Sample size is an important consideration, and caution is needed when interpreting any percentages generated by testing a small number of pets, particularly when a pathogen’s presence is entirely unexpected. While these unexpected results could be the result of a recent spread or recognition of a disease agent in a given area, they can also occur from a number of other factors, including the background or source of the pets tested (for example, the importation of animals from endemic regions) or the nature of the tests used, rather than true presence of infection. In addition, the data displayed showcases only the current year’s data. Depending on the time of year, this map view may only represent a small number of testing months.

History

A pet’s history before testing may influence results. For example, a pet that has been recently rescued or adopted may not have been protected from infection previously, and is thus more likely to test positive or harbour parasites. In addition, pets that have recently moved into an area may arrive with infections acquired elsewhere. For instance, dogs being imported from a high prevalence area for heartworm and tested after arrival, may result in a number of ‘new’ heartworm cases appearing in a location where historically, infection was relatively rare. Similarly, dogs that have antibodies for the agent of ehrlichiosis may have been imported from southern or south-eastern European countries, where ehrlichiosis is endemic, to an area where this is currently not considered to be the case.

Historical information is not available for the dogs tested. However, unexpected positive results, particularly when clustered around areas with frequent population turnover, such as metropolitan areas, large cities or military bases, should be interpreted with caution as they may represent infections in translocated pets rather than locally-acquired infections.

Reason for testing

The motivation for testing should also be considered when interpreting the data. The diagnostic assays, from which the results are used to create the maps, may be initiated by veterinarians for the diagnosis of suspected infection or for routine screening. Veterinarians are judicious in recommending diagnostic assays for a given patient and respectful of limited client resources. Accordingly, pets that are more likely to harbour infections, including those exhibiting clinical signs of disease or those with previous infections acquired by lifestyle or geographical location, are more likely to receive a test from a veterinarian. When pets that are more likely to be infected are disproportionately tested, the number of positive results increases, as does the calculated percentage of positive test results. This increase in positive results is more likely to be seen in areas where the tests are used more commonly for diagnostic verification of a suspected infection, than for routine screening. Currently, the reasons for testing are not available, and may over- or underestimate local prevalence.

Differences in test sensitivity and specificity

Although assays used in testing pets often appear to be similar, individual tests vary in their sensitivity and specificity, which can influence overall results. Tests with poor sensitivity may underestimate the frequency (and prevalence) of infection in a given population, while tests with poor specificity can overestimate the presence of infection if false-positive results are recorded. In addition, the predictive value of a given assay, or the actual precision of its use, is dependent not only on individual test performance but also the prevalence of infection in the population being tested. For a given test, if sensitivity and specificity are held constant, predictive value tracks prevalence; higher prevalence results in a higher positive predictive value, meaning any given positive result is, on average, more likely to be a true positive, so long as infection is fairly common. When infections are rare, the positive predictive value decreases and the negative predictive value increases.

‘Although assays used in testing pets often appear to be similar, individual tests vary in their sensitivity and specificity, which can influence overall results.’

In effect, the tests perform differently in different geographical areas and in different populations of animals, and therefore the results should be interpreted accordingly. For example, in Germany, Giardia spp. infection is relatively common in young dogs. In this population, a positive test result is likely to be a true-positive (higher positive predictive value). However, for a dog older than 2 years, infection is relatively rare, thus a negative test result is likely a true-negative (higher negative predictive value), while a positive test result is more likely to be a false-positive (lower positive predictive value).

Types of assays used

The assays used detect two different types of biomarkers in the animal’s blood: antigens or antibodies. When a test is positive, and detects antigens, the animal is considered to have an active infection, whereas a positive result for a test that detects antibodies only indicates that the animal has been exposed to the pathogen at some point in the past, so other clinical findings are used to help determine whether the animal has an active infection. Understanding the difference between the two types of tests is of paramount importance when interpreting the maps. The canine heartworm test is antigen-based, so the map is indicative of actively-infected animals. The feline heartworm tests may be antigen or antibody-based, so must be interpreted accordingly. The tick-borne pathogen tests are antibody-based, so the maps are indicative of either an active or a resolved infection with the pathogen.

In epidemiological studies, there is a study population and a target population. The study population includes people or animals that are directly observed during the course of the study. It is also a representation of the target population, or general population. The study population is, therefore, a sample of the target population.

The test results used to produce the maps are from animals presented to a veterinary clinic for a variety of reasons. The vast majority of these animals are owned, while a smaller subset of the animals may be in the custody of a rescue organisation or shelter. As such, the maps only represent the population of owned dogs presenting to a veterinary clinic. Owned dogs not presented to a veterinary clinic, or not tested, stray and feral dogs, plus those in the custody of rescues or shelters that are not presented to a veterinary clinic, are therefore not represented in the maps. It is probable that less preventative care is provided to these animals, although no large-scale studies have been published on this subject.

Along with its industry partners, the ESCCAP’s aim is to protect the health of pets, enhance the safety of the public, and preserve the human-animal bond. Since its conception, the organisation has drawn upon the expertise of its members to produce guidelines, fact sheets and now maps showing the occurrence and distribution of parasite-associated infections. In combination, these resources are designed to keep veterinary professionals informed on the growing risk parasitic infections pose to the health of both pets and humans across Europe.

Conclusions

While the maps are in the early stages of development, it is hoped in time that the data feed will increase and expand to deliver comprehensive reports available for use in veterinary and public health sectors. Mapping parasite distributions is a vital part of parasite control advice and surveillance. Maps based on test results such as these are only possible through parasite testing in first opinion practices across Europe, and hopefully these new maps will encourage further testing and surveillance, while raising the profile of these key canine and feline parasites.