Chronic Giardia infections in dogs: Longitudinal analysis of cyst excretion and fecal consistency in young and adult dogs.

Giardia infections are common in dogs, yet uncertainties remain regarding their clinical relevance and whether effective immunity develops over time. This study investigated Giardia infection dynamics in young and adult dogs through a longitudinal follow-up, assessing cyst excretion, potential indicators of immune development and associations with fecal consistency. Puppies were monitored monthly for five months after arriving in new households, while adult group-housed dogs across five study groups were followed biweekly over two months, with cyst excretion quantified by immunofluorescence microscopy. A substantial proportion of puppies exhibited chronic infection, consistently shedding high cyst levels, suggesting incomplete or delayed protective immunity. Infection status upon arrival strongly predicted chronicity, with puppies from professional breeders at higher risk compared with those from non-professional breeders. Similarly, chronic and high-intensity infections were prevalent in group-housed adult dogs, indicating an apparent absence of effective immune development. Although fenbendazole treatment was administered in some study groups, it only temporarily reduced cyst excretion; infection prevalence and cyst burdens remained persistently elevated. In puppies, Giardia-positive individuals showed a non-significant trend toward looser stools compared with negatives. In adult dogs, infected individuals had significantly higher odds of looser stools (OR: 2.32, p = 0.007). Furthermore, cyst excretion levels were positively correlated with fecal consistency scores in adult dogs. These findings emphasize the chronic nature of Giardia across age groups and suggest early infection status and environmental contamination significantly influence infection persistence. This study provides the first evidence linking Giardia cyst load to fecal consistency in adult dogs, highlighting potential clinical relevance of giardiosis.