Canine Leptospirosis:
Current Issues on Infection and Vaccination
Leptospirosis, a contagious disease affecting both animals and humans and spread by infection with a bacterial pathogen called Leptospira, may result in chronic liver and kidney disease and fatality in the dog. Over the past 30 years, preventative vaccination against two of the most common Leptospires, L. canicola and L. icterohaemorrh agiae, have nearly eradicated clinical disease associated with these strains among the inoculated population. Though not without potential side effects associated with allergic reactions to inoculant in a small number of dogs, the risks of not vaccinating for Leptospirosis once far outweighed risks of vaccine-reaction. In recent years, however, new outbreaks of Leptospirosis have been reported in the population of vaccinated dogs. Clinical evidence now suggests that these new cases are associated with the once, less-common Leptospires for which current vaccines do not protect against. In light of these findings, the process of vaccinating dogs with the current Leptospirosis vaccines is being seriously questioned.
Infectious Leptospirosis
The Leptospira Organism. Leptospires are known as "aquatic spirochetes": they thrive in water and appear long and helical with a characteristic hook on one or both ends. These organisms are divided into two species, Leptospira biflexa and Leptospira interogans, the latter of which is pathogenic in animals and humans. L. interogans is divided into strains, or serovars, based upon the types of antigens (cell-surface markers against which the infected host will make antibodies) on their surface. These cell surface antigens provide little cross-immunity against one serovar and the next; that is, a dog that has developed immunity to one strain by either previous infection or vaccination will not be able to immunologicall y fend-off an infection of a different, subsequent strain.
Serovar prevalence. As recent as the 1980s, L. icterohaemorrh agiae and L. canicola were identified as the most prevalent serovars causing Leptospirosis in the canine. By the 1990s, however, an increased incidence of L. grippotyphosa and L. pomona was observed in conjunction with a resurgence of Leptospirosis disease suggesting a changing trend in the epidemiology of this disease. It is speculated that these changes in serovar prevalence are related to two primary factors that may strongly influence the epizootiology of Leptospira serovars. These factors are: 1) preventative vaccination has all but eradicated clinical disease in the domestic dog and 2) there has been an increased migration of wildlife, for which serovar infections with L. grippotyphosa and L. pomona are most prevalent, into suburban areas.
Modes of Disease Transmission. Leptospira thrive in spring and autumn when wet soil conditions and moderate temperatures support their otherwise poor environmental survivability. Infection by contact with infected urine or ingestion of urine-contaminated water is the most common means of transmission of the disease. Less common modes of infection include transmittance of the organisms during breeding, gestation, or through the membranes of the eyes, abrasions or bite wounds, or ingestion of the flesh from infected animals such as rats, raccoons, skunks or opossums. A serovar infects the dog as a maintenance host, using the dog to carry out most, if not all of the organism's life cycle. Under these conditions, the kidneys of the infected dog become the "breeding" grounds for the serovar, some of which will be shed in the urine where they may gain access to other dogs and continue the infectious cycle.
Prevention. Commercial vaccines are available and protect against clinical disease associated with the L. icterohaemorrh agiae and L. canicola serovars. Inoculation does not, however, prevent infection and development of a carrier state whereby the dog will be clinically asymptomatic for disease yet provide a source of contagion through the shedding of serovars in its urine. Additionally, vaccinating against these specific serovars does not afford protection against other serovars.
Annual Revaccination and Leptospirosis
Current concerns in canine immunology have addressed issues related to overuse of vaccines in dogs and cats. General consensus among specialists in the field is that yearly vaccination against viral infections associated with canine distemper virus, canine parvovirus and canine adenovirus are generally unnecessary since active immunity induced by these vaccines provide at least several years of protection. This consensus, however, does not apply and should not be generalized to bacterin vaccines, which immunize against diseases associated with bacterial organisms. In fact, clinical evidence suggests that bacterin-derived vaccines including those which protect against Bordetella bronchiseptica (kennel cough), Leptospira (Leptospirosis), and Borrelia burgdoferi (Lyme disease) probably don't even provide protective immunity for 12 months suggesting that more frequent vaccination for these diseases are required. It is perhaps the common use of combination (all-in-one) vaccines containing bacterins, which immunize against bacterial infections such as Leptospirosis and/or kennel cough in addition to common viral infections, that gave rise to the practice of frequent vaccine administration . Indeed the incorrect generalization of long-term immunity, associated with vaccination against viral immunogens, to bacterin-based vaccines may lead to a decrease in annual vaccination for bacterial-based diseases and subsequently give rise to a resurgence of outbreaks of bacterial disease in the coming years. In light of this, annual re-boostering against bacterial diseases should continue despite discontinuatio n of yearly vaccination against viral diseases.
The Current Leptospirosis Vaccine
Recent serological studies on wildlife and domestic dogs suggests that L. grippotyphosa and L. pomona have replaced L. icterohaemorrh agiae and L. canicola as the prevalent serovars responsible for Leptospirosis in the United States today. As such, current commercial vaccines, which protect against the formerly prevalent serovars, would not be effective at providing immunity against Leptospirosis caused by L. grippotyphosa and L. pomona. For this reason, there has been some conjecture that current commercial vaccines should be considered obsolete for protecting against Leptospirosis. Leptospirosis vaccines, as mentioned above, protect against clinical disease but do not prevent subclinical infection to a "carrier" state.
When all the facts are considered, these findings do not necessarily suggest that L. icterohaemorrh agiae and L. canicola no longer pose a threat to dogs. Rather, this information should be taken into consideration when determining potential risk of infection in dogs that may be candidates for side effects associated with vaccine-reaction. Leptospirosis-containing vaccines are associated with a higher risk for side effects, particularly, anaphylactic reactions (see Canine Anaphylaxis). Taken together, benefits of vaccinating dogs, who live in areas where icterohaemorrh agiae and canicola incidence is low and who may have a higher predisposition for vaccine side effects with current Leptospirosis inoculants (see Vaccines, Infectious Diseases and the Canine Immune System), may not outweigh risks of vaccine reaction.
New Leptospirosis Vaccine Immunizes Against L. grippotyphosa and L. pomona
Fort Dodge now offers the Duramune Leptospirosis vaccine that immunizes against L. grippotyphosa and L. pomona serovars as well as L. icterohaemorrh agiae and L. canicola . This vaccine has been formulated through the new subunit technology that uses only the antigen component of the organism (that will produce an immune response) instead of the entire organism. As such, subunit vaccines greatly reduce vaccine side-effects that occur with higher incidence with bacterin-based vaccines while providing durable protection from the disease.