Mapping deer diarrhea - a survey study

Background
Within the framework of wildlife disease surveillance, the Swedish Veterinary Institute (SVA) regularly receives reports of roe deer (Capreolus capreolus) with diarrhoea. Autopsies often reveal emaciation and signs of chronic inflammation in the gastrointestinal tract, but in many cases no specific cause can be determined. Knowledge of the underlying causes of diarrhea and intestinal inflammation in deer is limited, and there are few studies that have systematically investigated the extent and etiology of the problem.

A research project at SVA in 2023 and 2024 investigated whether the single-celled parasite cryptosporidium (Cryptosporidium spp.) could be a cause of deer diarrhea. The results indicate that cryptosporidium may be involved in parts of the problem, but that diarrhoea in deer is not solely explained by the presence of the parasite.

Similar problems with deer with diarrhoea are reported from several European countries, including Denmark and Norway. There is indications from Norway that the incidence of diarrhea in deer has increased in recent years. In a Swiss study that included 1,571 deer over a 50-year period, gastroenteritis with diarrhea was the second most common disease-related cause of death (Pewsner et al. 2027). The study identified several different pathogens and parasites, including Clostridium spp., Erysipelothrix rhusiopathiae, paratuberculosis (Mycobacterium avium subsp. paratuberculosis) and various endoparasites, but each individual pathogen was only present in a few individuals.

In Sweden, an autopsy study of 985 deer showed that 15% of the animals died of gastrointestinal-related disease, and in a large proportion of these cases the etiology was unknown (Aguirre et. al. 1999). The same study showed that parasites in the gastrointestinal tract were present in approximately 11% of the deer. In Denmark, the presence of bovine viral diarrhea virus (BVDV) in cervids has been investigated without demonstrating infection in deer (Nielsen et. al.
2000).
Previous investigations in Sweden have also indicated that parasites may play a role in some outbreaks. In a major outbreak of diarrhoea among deer in Dalsland, abundant occurrence of gastrointestinal parasites and Giardia spp. was demonstrated, probably linked to high infection pressure in a relatively dense population in a limited area. There are indications that reports of diarrhea are more common during spring and autumn, but this has not yet been quantified in a systematic way. Since the cause is most likely multifactorial, knowledge is also needed about when and in which types of populations the problems arise. Against this background, there is a need to better describe the extent of the problem at the population level, with the possibility of adding other factors, such as weather and population data.

Aim and project plan

The aim of the project is to map the occurrence, geographical distribution and seasonal variation of diarrhea in deer in Sweden through questionnaire-based data collection. The project also aims to identify overall patterns and potential risk factors that can contribute to an improved understanding of underlying causes and form the basis for future in-depth studies.