Retrospective evaluation of the seasonality of canine tetanus in England (2006–2017): 49 dogs

Objective: To evaluate the seasonality of canine tetanus in England. Methods: Medical records of a single referral hospital in England were reviewed. Dogs diagnosed with localized or generalized tetanus between January 2006 and June 2017 were studied. Results: Forty-nine cases were included. The prevalence of tetanus in England was significantly higher in the winter when compared with the summer ( P = 0.002) and autumn ( P = 0.024), with the highest number of cases recorded in February. Conclusions: The prevalence of canine tetanus in England was significantly higher in winter months, especially in February.


INTRODUCTION
Canine tetanus is an uncommon neuromuscular disease caused by neurotoxins of Clostridium tetani. Several studies in human literature reported seasonal variation in tetanus prevalence. [1][2][3][4][5] Although there are no data regarding seasonality of canine tetanus, anecdotal evidence suggests that the prevalence of tetanus in dogs may be higher during certain periods of the year. Knowledge of seasonal variation in occurrence of canine tetanus may direct further research toward understanding environmental exposure factors and animal characteristics contributing to contraction of the disease. This, in turn, could increase awareness of this rare disease among veterinary professionals and dog owners and the development of disease prevention strategies targeting populations at risk. This study aimed to determine if there is a seasonal variation in the prevalence of canine tetanus. It was hypothesized that a seasonal variation in occurrence of canine tetanus exists in England.
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There was a statistically significant difference in the monthly prevalence in canine tetanus presentation (P = 0.048), with the highest number of cases seen in February (n = 11) and no cases recorded in August. There was also a statistically significant difference in case distribution between seasons of the year (P = 0.003). A significant difference in seasonal case distribution was noted between winter and summer (P = 0.002) and between winter and fall (P = 0.024).
There was no correlation between the prevalence of tetanus and monthly rainfall in England. There was, however, a weak negative correlation between the prevalence of tetanus and the average monthly temperatures in England (r S = -0.30, n = 138, P < 0.001; Figure 3).

DISCUSSION
This retrospective case series aimed to assess whether there was a seasonal prevalence of canine tetanus. The prevalence of canine tetanus in England was found to be significantly higher in winter months when compared with summer and fall, with the highest number of cases recorded in February. The tetanus population evaluated in this study was similar to that described in previous retrospective studies consisting of predominantly medium and large breeds and a younger population of dogs. [6][7][8] The seasonality of tetanus has been reported in people in several studies, although findings are contradictory. In 1 study, the highest prevalence of tetanus among people living in India was in the postmonsoon season (September-December). 1 The authors found that one third of tetanus cases occurred in the transition period between the hot and wet monsoon season and the cool and dry winter season. It was suggested that this was likely related to the occupation of the affected people (farmers and manual laborers), who were more likely to come into contact with the toxin via injuries sustained at work during this period. In Nigeria, higher numbers of adult tetanus have been recorded in the hot and dry season. 2 The authors hypothesized that this was due to reduced activity, increased wearing of shoes, and increased washing away of animal waste that littered the streets in wet weather. A higher prevalence of human tetanus in warmer months has also been noted in Italy, Canada, and Japan and has been associated, by some authors, with greater outdoor activity and increased exposure to soil. [3][4][5] Given these differing findings in the human data and lack of definitive reasoning for the seasonal variations seen, it is unclear why a seasonal prevalence in canine tetanus was noted in England. As there was not a similar variation in hospital caseload over the same months, the variation in prevalence is thought to be a true finding. The correlation between tetanus caseload and lower temperatures suggests that cooler weather conditions may lead to increased canine exposure to C. tetani spores. One possible explanation is that mild winter weather conditions support bacterial growth and survival. A combination of environmental factors, such as temperature and humidity, may stimulate favorable anaerobic conditions or production of nutrients

F I G U R E 1 Monthly prevalence of canine tetanus in England
F I G U R E 2 Seasonal prevalence of canine tetanus in England promoting spore germination. 1 Dogs walked in England during winter months are often exposed to wet and muddy conditions, less pronounced in warmer seasons, which may predispose to increased con- period of 3 days to 3 weeks. Therefore, although the seasonal prevalence of canine tetanus appeared to peak in the winter months, it is likely that the actual onset of the disease process was up to a month earlier.
The prevalence of canine tetanus in England was significantly higher in winter months, especially in February. Further studies could investigate the effect of different climates, other environmental factors, and activity levels of exposed dogs on the prevalence of canine tetanus.