Leptospirosis is an important bacterial disease of dogs. Infection occurs through contact with infected urine. It additionally presents a public health concern due to its zoonotic nature. Leptospira can affect multiple organ systems, but the classical presentation is renal or hepatic dysfunction. However, other manifestations have since been recognised. Leptospirosis is perceived to, generally, be an acute illness but there are reports of chronic forms of disease. Disease can range from mild to severe, multi-organ failure or death.
One of the key issues with leptospirosis is the vast range of tests available for diagnosis. Timing and choice of test provides a continued challenge to clinicians.
For the past 50 years, a bivalent vaccine has been utilised for protection in canines. However, several years ago tri- and tetravalent vaccines were released to the veterinary market in response to emerging concerns to changing patterns of leptospirosis.

Which animals can be infected with leptospirosis?
Leptospirosis affects most mammalian species. Animals can be classified as maintenance hosts, eg. rodents, or incidental hosts, eg. dogs. Host-adapted leptospiral infections in maintenance hosts are typically asymptomatic with urinary shedding. This is in contrast with infection in incidental hosts usually resulting in clinical disease. An animal that is a maintenance host for one leptospira serovar can be an incidental host for many other serovars (Bharti et al, 2003; Ellis, 2010).
Rodents have a highly important role in maintenance and shedding of leptospires. There is variation between rodent species in their degree of renal carriage. This variation in renal carriage and distribution of rodent species around Europe is an important contributory factor to distribution of serogroups (André-Fontaine, 2006).
The significance of leptospirosis in cats, both as an asymptomatic reservoir and as a causative agent of kidney disease, is still being explored (Lapointe et al, 2013; Schuller et al, 2015).

What is the geographical distribution of leptospirosis in the UK and Ireland and which serovars are most important?
At present, there have been no large-scale surveillance studies undertaken in the UK or Ireland. Leptospirosis has not been a notifiable UK disease since 2010, so there is no mandatory reporting of cases in people (Forbes et al, 2012). Our knowledge of circulating serogroups and their prevalence is largely based on smaller studies. The serogroups most frequently identified in European studies are listed in Table 1 below.
Leptospirosis is an endemic disease to Ireland and the UK (Ellis, 2010). It is likely that the serovars seen in Ireland will be similar to the UK because there are no export/import regulations. There have been several studies examining Irish dogs specifically. Rojas et al, (2010) found 7% of a subset of healthy-dog urine samples at the University College Dublin Veterinary Hospital contained pathogenic leptospires. Another Irish study identified 6% of dogs not suspected of leptospirosis having significantly elevated antibody titres to the following non-vaccinal serovars: Ballum, Bratislava, Mozdok, Altodouro and Hardjo (Schuller et al, 2015). A study on Irish beef suckler herds found sero-prevalence to Hardjo to be over 80% (O’Grady et al, 2012).

What is a serogroup or serovar?
One aspect of understanding leptospirosis that is complex is the ever-changing nomenclature of the bacteria, the key terms are defined in Table 2 below. The evolving classifications and grouping provides challenges for clinicians.
Serovar and sero-grouping are important from an epidemiological and diagnostics context as this system has been used since early research into leptospires in the 1900s. There is no correlation between serovar/serogroup and genomospecies organisation (Levett, 2001). As applies to all pathogenic bacteria, viruses and parasites, the immune response in infected humans and animals as well as protection against re-infection are mainly serotype-specific. Therefore, Leptospira genomospecies are of minor importance to the epidemiology of leptospirosis.

How might leptospirosis present in a patient?
Infection occurs through contact with urine-contaminated water onto mucus membranes or broken skin. Once leptospires enter the blood stream, various organs can be affected. Renal and hepatic involvement is most common but additionally lungs, eyes and the reproductive tract can be affected (Delaude et al, 2017; Ellis, 2010; Schuller et al, 2015). Although leptospirosis is more commonly reported as an acute disease, it is important to highlight that it can be involved in chronic renal or hepatic disease (McCallum et al, 2018; Timoney et al, 1974). Table 3 summarises major clinical signs reported.

There are also rarer reports of reproductive issues; such as abortion and infertility, and skin conditions, such as calcification (Munday et al, 2005). The role of leptospirosis in reproductive disease in livestock is well recognised but it has not yet been fully explored in dogs. Leptospires have been isolated from bitches with reproductive issues (André-Fontaine, 2006; Ellis, 2010; Graham & Taylor, 2012; Mori et al, 2017; Rossetti et al, 2005).
In recent years, studies have documented cases with severe respiratory tract involvement. This presentation is called leptospiral pulmonary haemorrhage syndrome (LPHS [Klopfleisch et al, 2010; Kohn et al, 2010; Major et al, 2014]). In one Swiss study, 68% of dogs diagnosed with leptospirosis had respiratory signs as part of their clinical presentation (Major et al, 2014). LPHS is recognised in human leptospirosis and has mortality rates associated with over 50% (Dolhnikoff et al, 2007). Although the pathogenesis is not well understood, it is thought that LPHS is associated with an immune-mediated response rather than a high bacterial load. Other bleeding disorders such as epistaxis, haematemesis and petechiae are seen with leptospirosis. This may be due to leptospires triggering vasculitis but the exact mechanisms have not been elucidated (Wagenaar et al, 2007).

What are common laboratory-test abnormalities in leptospirosis?
Laboratory tests will often reveal a range of non-specific abnormalities which might raise suspicion of leptospirosis and prompt confirmatory diagnosis. The most common abnormalities are listed in Table 4.

When to test for leptospirosis?
Due to the wide range of clinical presentations, it is important to keep leptospirosis as a differential for the following presentations:

• Other causes of acute kidney injury (AKI)
• Toxic, eg. non-steroidal drugs, ethylene glycol, aminoglycosides
• Infectious, eg. generalised sepsis, pyelonephritis, babesiosis
• Other, eg. urethral obstruction
• Other causes of acute hepatitis
• Toxic, eg. NSAIDs, xylitol zinc, onion
• Infectious, eg. infectious canine hepatitis (CAV-1), bacterial infections
• Other causes of coagulation disorders
• Toxic, eg. rodenticide
• Immune-mediated, eg. immune mediated haemolytic anaemia or thrombocytopaenia, disseminated intravascular coagulopathy (DIC)

Which diagnostic tests can be performed?
In order to confirm the diagnosis of leptospirosis, specific tests must be performed on blood or urine samples. There are numerous tests available and knowing when to use them is crucial to diagnosis. Tests are divided broadly into two categories:

• Serological tests that measure antibody response to leptospirosis infection (serological testing).
• Molecular tests that detect leptospiral DNA (molecular testing).

Table 5 lists which types of tests fall into these categories.

What are the limitations with the currently available tests?
The MAT test is the current gold standard for leptospirosis diagnosis. Interpretation of MAT results can be challenging. The result given is an antibody titre (the highest dilution of sample where >50% of the sample has agglutinated). Examples of MAT test agglutination results can be seen in Figure 1.
There is variability of the cut off for a positive sample but veterinary medicine generally follows the same guidelines as Leptospirosis Burden Epidemiology Research Group (LBERG): ≥1:400 for a single sample and at least a four-fold increase between paired samples (Schuller et al, 2015). Paired sampling is highly recommended as it is impossible to differentiate between post-vaccine antibodies or active or previous infections from a single elevated antibody titre (Ellis, 2010; Schuller et al, 2015). Although vaccine response antibodies generally do not reach as high titres or remain elevated for as prolonged periods as post-infection antibodies, this is not consistently the case (Martin et al, 2014; Miller et al, 2011). High uptake of vaccination in UK dogs (between 65-95%, depending on the study) and the inability of the MAT test to distinguish between these antibodies is a key limitation of this test (Ball et al, 2014; Sánchez-Vizcaíno et al, 2018). A major drawback of serological testing is sampling too early in the disease course (before IgM antibody levels have risen) leading to a false negative diagnosis.
Although polymerase chain reaction (PCR) results are not challenging to interpret, there is the risk of false negative results depending on timing of antimicrobial therapy and, if it is a urine sample, intermittent shedding of leptospires. In case of sampling outside a hospital, storage and transport conditions and time until sample processing have an impact on PCR results due to possible disintegration of leptospiral DNA. If possible, samples for PCR should be taken prior to initiation of antibiotic therapy. Additionally, PCR results do not provide any information on infecting serogroup (Musso & La Scola, 2013).
Unfortunately, the majority of tests require external laboratory analysis, so it can take seven to 14 days to receive results. Due to this lag, modified ELISA patient-side tests, such as SNAP Lepto (IDEXX Laboratories) and Immunocomb (Biogal-Galed laboratories) can be useful for increasing confidence in a diagnosis of leptospirosis. However, both the European College of Veterinary Internal Medicine (ECVIM) consensus statement and test manufacturers recommend these tests be performed in conjunction with other serological and/or molecular tests. The ECVIM Consensus statement also recommends PCR testing be done in conjunction with MAT testing (Schuller et al, 2015).

When should each test be used?

How should leptospirosis be treated?
Treatment of leptospirosis requires antimicrobial and supportive therapy. The most common antimicrobials used are penicillin derivatives and doxycycline. The dosages are listed in Table 7. Penicillin and penicillin related antimicrobials are useful for treating the leptospiraemia but will not prevent leptospires colonising renal tubules. The use of doxycycline is required to eliminate persistent renal carriage. However, doxycycline is indicated only once the patient is eating and not vomiting, so more critically ill patients should begin therapy on intravenous penicillin.
In cases with severe acute kidney injury (AKI grade 4 or creatinine >440umol/L), the antimicrobial dosing interval should be increased (Schuller et al, 2015). Supportive therapy required will vary depending on which organ systems are affected. Patients are frequently managed with intravenous fluid therapy, antiemetics and analgesia.
Dogs with coagulation abnormalities may require plasma transfusions. Dogs with severe AKI (>grade 4) may be candidates for renal replacement therapy (RRT) at certain referral clinics (Schuller et al, 2015; Sykes et al, 2011). Dogs presenting with LPHS are treated with oxygen therapy (Sykes et al, 2011). Additionally, as a result of infection, animals may have developed chronic renal or hepatic insufficiency and need lifelong management of this. One study of dogs treated for AKI (of various aetiologies) found that 50% of dogs had permanent renal damage after discharge (Kis et al, 2012).
Monitoring of biochemistry, particularly renal values and electrolytes, should be done every 24 hours while hospitalised. Additionally, careful monitoring of fluid ins and outs should be undertaken (Schuller et al, 2015; Sykes et al, 2011).
It is recommended to treat other dogs in the household with prophylactic antimicrobial therapy. Additionally, owners should be advised to disinfect thoroughly and avoid contact with contaminated urine while their dog is undergoing treatment (Sykes et al, 2011).

What factors might be associated with poorer outcomes?
Sykes et al (2011) reported survival rates of 80%. Major et al (2014) found that the presentation of hepatic dysfunction was most negatively associated with survival. This has been recognised in human studies where the presence of jaundice was associated with higher mortality than other presentations (Taylor et al, 2015).
Additionally, it is reported that some serogroups are associated with more severe disease than others. For example, André-Fontaine (2006) reported Australis and Sejroe serogroups being associated with milder or more chronic forms of leptospirosis than the Autumnalis and Grippotyphosa serogroups. Goldstein et al (2006) found dogs infected with the Pomona serogroup to have higher mortality rates. Although this relationship has not been examined formally in more robust veterinary studies, similar relationships between serogroup and disease severity have been acknowledged in human literature also. In a large human meta-analysis, patients infected with serovar Icterohaemorrhagiae had highest mortality when compared to all other serovars (Taylor et al, 2015).

How is leptospirosis prevented?
Vaccination is a cornerstone of preventing leptospirosis in dogs and also minimising the risk of urinary shedding of leptospires to humans (Klaasen et al, 2014). Leptospirosis vaccines contain inactivated whole-cell leptospires (Klaasen & Adler, 2015). Vaccination does not generate cross protection between serogroups. It is only protective against the serovars included in the formulation and those closely related to them. The duration of immunity from vaccine administration is approximately 12 months (Klaasen et al., 2014). Hence, these vaccines must be administered annually. Bivalent vaccines have been widely used since the 1960s and provide protection against the Canicola and Icterrohaemorrhagiae serogroups.
In 2013, a broader version of the vaccine was released throughout Europe providing coverage against a further two serogroups, Grippotyphosa and Australis. These serogroups were included due to increased seroprevalence in continental Europe studies (Ellis, 2010; Klaasen & Adler, 2015; Klaasen et al, 2013; Renaud et al, 2013; Schuller et al, 2015). In the US, a tetravalent vaccine was also released but with the inclusion of Pomona rather than Australis serogroups. This reflects higher incidence of infections with the Pomona serogroup in the US than in Europe (Sykes et al, 2011).
Prior to the introduction of a bivalent vaccine in the 1960s, L. interrogans Canicola and icterrohaemorrhagiae were the most important serogroups associated with disease in dogs. Since then, leptospirosis has remained an important disease of dogs but the importance of the Canicola serogroup has diminished whilst identification of disease due to other serogroups has increased. Dogs are the only maintenance host of Canicola therefore high levels of vaccinated dogs have created a degree of herd immunity to this serogroup (André-Fontaine, 2006).
There has been a lot of dog owner concern with respect to tetravalent vaccines and adverse reactions towards them. The Veterinary Medicines Directorate (UK) investigated this and found the incidence of adverse events with bivalent vaccines to be 0.015% (two in 10,000) and tetravalent vaccines 0.069% (seven in 10,000). According to this, leptospirosis vaccines fit into the category of ‘rare’ adverse effects (<10 in 10,000) (Veterinary Medicine Directorate, 2017). This incidence rate is similar to what has been reported for other core vaccines such as DHP (distemper, adenovirus and parvovirus).

Future perspectives
Leptospirosis remains an important pathogen to consider in a wide range of clinical presentations in dogs. Future work must focus on robust epidemiological studies of UK and Irish canines and improved diagnostics. Improved characterisation of clinical presentation in these canines will also improve clinician recognition of the disease. The importance of other species' (particularly livestock) role in spread of leptospirosis must also be explored.