Leptospirosis is an infectious disease that dogs can contract from water or soil contaminated by rat urine. Lepto causes liver or kidney disease or inexplicable bleeding. It is a tough disease to diagnose and to treat.
There are many different strains of Lepto, with names such as “Canicola” and “Bratislava”. Vaccines are available that protect dogs from either two (L2) or four (L4) different strains of this infectious disease.
In the UK, studies at the Bristol vet school suggest that “Bratislava” may be the most common Lepto strain affecting dogs in Britain.  The L2 vaccine doesn’t protect against “Bratislava”.  L4 vaccine became available and in 2013 we switched from L2 to L4.
The following year the Daily Telegraph published a story saying 120 dogs had died after L4 vaccination and that the World Small Animal Veterinary Association (WSAVA) advised vets not to use the vaccine.  Both statements were false. The WSAVA replied immediately that this was ‘blatantly untrue” and the Veterinary Medicines Directorate (VMD) responded that adverse reaction reports (at seven for every 10,000 injections) were (and still are) statistically as rare as they are for L2.
While 120 dogs may have died after vaccination they did not die because of vaccination.  However this “false news” gained a life of its own on the web so we think it’s worthwhile to give you the most recent update on adverse reactions from the UK’s Veterinary Medicines Directorate and from the European Medicines Agency.
First, some definitions.
A common adverse reaction is one that occurs once in every 10 to 100 individuals.
An uncommon one occurs once in every 100 to 1000 individuals.
A rare adverse reaction occurs once in every 1000 to 10,000 individuals.
The least frequent of all, the very rare adverse reaction occurs once in every 10,000 or more individuals.
Here is the most recent pharmacovigilence report on L4 vaccine.”
“In rare cases mild and temporary increase in body temperature (= 1°C) may occur for a few days after vaccination, with some puppies showing less activity or a reduced appetite. A small, temporary swelling may occur at the injection site, which will either disappear or reduce in size within two weeks after vaccination. An occasional temporary, acute (short-term) hypersensitivity (allergic) reaction may occur.
In very rare cases, there have been reports of immune-mediated reactions, including immune-mediated haemolytic anaemia, immune-mediated thrombocytopenia and immune-mediated polyarthritis.” 
“Very rare” means a one hundredth of one percent risk of that adverse reaction.

If you ever read inserts that come with medicines you know that every single medicine we take ourselves or give to our pets triggers a range of adverse reactions. The ones to worry most about are the common or uncommon ones. ‘Rare’ and ‘Very Rare’ are exactly what they say they are. At the London Vet Clinic we feel that for dogs exposed to rat urine the benefits of protection from Lepto far outweigh the extremely low incidence of rare  adverse reactions.

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