Leptospirosis is mainly a water-borne disease and rodents are major reservoirs. Man is an accidental host, becoming infected through occupational or recreational exposure. Leptospirosis has a worldwide distribution, but human cases are more frequently reported from the tropics, such as Hawaii for the United States, where the average annual
incidence is 1.08 per 100,000 population, while it is 0.05 per 100,000 population for the United States as a whole (Sasaki et al., 1993).
The etiological agents of leptospirosis belong to the more than 200 pathogenic serovars within the 23 serogroups of Leptospira interrogans (Andre-Fontaine et al., 1994). Many of these serovars are capable of causing disease in humans and dogs, but until recently the main serovars involved in zoonotic transmission between canids and humans were L. canicola and L. icterohaemorrhagiae (Farr, 1995).
More recently, canine outbreaks caused by
L. pomona and L. grippothyphosa have been reported in Europe (Andre-Fontaine et al.,1994) and in the United States, and to a lesser extent by L. australis, L. automnalis, or some other serovars. Dogs are the natural carrier host for L. canicola, but can also be infected with various other serovars and will shed these organisms in their urine for up to several weeks. Humans can become infected through licking from, or when petting, an infected dog. Recently, many cases of leptospirosis have been reported in dogs in the United States caused by L. grippothyphosa, especially in the north-eastern states. The reservoirs are more likely to be racoons, opossums, and skunks. The specific role of dogs as source of human infection is not well quantified, but is reported to be high. Human infection with L. icterohaemorrhagiae is associated with exposure to infected dogs and is the most commonly diagnosed leptospiral infection in humans (Heath and Johnson, 1994). As most dogs are vaccinated against L. canicola and L. icterohaemorrhagiae in developed countries, suspicion of leptospirosis is often ruled out in a differential diagnosis. However, dogs may be infected by other serovars, and be potential carriers and shedders of all serovars. In
California, 10% of 61 leptospirosis cases in
humans over a 20-year period resulted from pet contact (Meites et al., 2004). The course of infection caused by exposure to a leptospiral agent is largely dependent on host adaptation of the serovar. Infection of an animal with its species’ host-adapted serovar usually results in a mild disease state and high likelihood of development of a chronic carrier and shedder state. Such individuals represent reservoir or maintenance hosts. Infection with a non-host-adapted serovar typically causes severe acute disease characterized by hepatitis, haemolytic crisis, and organ failure (Heath and Johnson, 1994). In dogs, leptospirosis can range from an acute septicaemia with haemolytic anaemia, hepatorenal failure, uncontrollable vomiting and bloody diarrhoea, to a subacute form with fever and jaundice, or to
milder forms with chronic nephritis. In humans, after an incubation period of 7–12 days, most persons will have a subclinical infection or anicteric febrile disease, often misdiagnosed as influenza. In its initial febrile phase, which usually lasts for 4–7 days, fever,
headache, myalgia, conjunctivitis, nausea, and vomiting are commonly seen (Heath and Johnson, 1994). In severe cases, by the end of the 1st week, jaundice and renal failure may begin. By the 3rd week, severe icterus with high levels of bilirubin is observed, usually associated with severe glomerulonephritis or interstitial nephritis. The mortality rate may reach 10–20%. Leptospira spp. may be isolated from the patient’s blood or cerebrospinal fluid during the 10 days of infection, or the urine after 21 days, and identified by dark-phase microscopy or culture. Recent molecular techniques, such as PCR or immunoblotting may reduce the time for diagnosis, as culture may require several weeks, but seems to be less sensitive than serological diagnosis (Levett, 1999). Laboratory diagnosis is still mainly based on serology, especially micro-agglutination. Agglutinins will appear between the 6th and 12th days of illness. A specific diagnosis is usually based on the demonstration of a fourfold rise in antibody titre. Several serological tests have been developed, including ELISA or IFA. Leptospira spp. are very sensitive to penicillin G and doxycycline, which are the most effective antibiotics in dogs and humans, especially when administered in the early phase of the disease. Prevention is based on rodent control and exposure reduction, as well as dog vaccination.