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Consequently, the parasite ' s geographical expansion is believed to occur through a combination of sporadic longdistance introductions alongside slower, continuous spread within local fox populations. Previous evidence suggests this pattern is already underway, as demonstrated by the sudden emergence of angiostrongylosis cases in Scotland in 2009 [ 4 ], as well as the progressive establishment of infection in regions that previously exhibited a low prevalence, such as Gloucestershire. This mode of spread results in an uneven, dynamic and ever-changing distribution, characterised by a mosaic of areas with high and low parasite prevalence that can change over time. For this reason, it is important that veterinary professionals have a good understanding of what is happening in their own practice and local area, while also recognising that the local situation has the potential to change gradually over time, or quite suddenly in the event of a long-distance introduction of disease.
Understanding the risk of parasitic diseases can guide effective diagnoses and preventative regimens. For A. vasorum, risk factors can be split into fixed factors and more changeable factors. Examples are shown in Table 1. The risk of transmission of A. vasorum to dogs is highest in late summer and autumn, when the warm and wet weather increases slug and snail activity. Because of the lag between infection and signs of disease becoming apparent, this means that the peak risk periods for A. vasorum disease are generally considered to be winter and spring [ 5 ]. However, some risk is present year round, since in southern( or warmer) areas, large slugs can overwinter in protected environments such as compost bins, while some smaller species continue to breed during winter, and snails can live for several years. Slug seasonality is shown in Figure 2.
Table 1. Risk factors for infection with Angiostrongylus vasorum.
Fixed risk factors
Dog age, with younger dogs being at greater risk
Breed, possibly. Observed breed-associated risks are inconsistent and may be influenced by behaviour, and a distinct genetic risk has not been reliably proven
Home address
Changeable risk factors Climate and weather
Slug and snail populations, and exposure to them. For example, risk may be higher at dusk / dawn, on damp / wet days or when dogs are off lead
Wildlife host populations
Dog behaviours: scavenging, coprophagia and grass chewing are likely to be positive risk factors for angiostrongylosis, as are outdoor feeding, puddle drinking, and leaving food bowls and toys outdoors
Although it is advisable to understand the risk factors for A. vasorum disease, there are currently no robust models to predict individual risk. Therefore, in much of the UK, mitigating individual risk completely is not considered possible while also maintaining a good quality of life and outdoor freedom for pet dogs.
Figure 2. Slug seasonality cycle. Courtesy of Professor Eric Morgan.
Volume 41( 2) • April 2026
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