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.
icterohaemorrhagiae, 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.
The following article provides a
detailed examination of infectious Leptospirosis in the canine and the recent
clinical findings and misconceptions surrounding the controversy of using
current vaccines to immunize dogs.
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 immunologically fend-off an infection of a different,
subsequent strain. Despite this, however, these antigens may be cross-reactive
in serological testing; that is, diagnostic testing to differentiate one
serovar infection from another may lead to false-positive results because some
antigens from one strain may have similarities to antigens from another
strain.
Serovar prevalence. As recent as
the 1980s, L. icterohaemorrhagiae 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.
Symptoms of disease. During the
first 4-12 days following infection with Leptospira, the dog may experience
sudden symptoms of fever (103-105oF), depression, vomiting, loss of appetite,
conjunctivitis, and generalized pain. Within 2 days of the onset of these
primary symptoms, body temperature may drop suddenly and there may be a
noticeable increase in thirst. A definite change in the color of the dog's
urine and/or jaundice (icterus) is often noticed and may be the only
indication of disease. Color intensity of the urine may vary from lemon to
deep orange. Additionally, frequent urination and subsequent dehydration
(uremia) are consistent with invasion of the kidney tubule cells by the
Leptospira organism and usually present within a few days of the primary
symptoms. In advanced cases of infection, profound depression, difficulty
breathing, muscular tremors, bloody vomitus and feces are often observed as
the infection progresses to include the liver, gastrointestinal system and
other organs. Course and severity of the disease is often dependent upon the
serovar responsible for the infection. Serovars associated with liver
infection and symptoms of urine discoloration and/or jaundice (icterus),
elevation of liver enzymes, and gastrointestinal symptoms include L.
icterohaemorrhagiae and grippotyphosa. The serovar grippotyphosa is also
associated with symptoms of renal failure as is the serovar pomona.
Diagnosis. Given the nonspecific
symptoms often associated with Leptospira infection, definitive diagnosis must
be based on the combination of symptoms and results from laboratory and
serologic tests. Despite this, however, Leptospirosis should be among the
primary suspected causes of illness in dogs presenting with sudden-onset
kidney dysfunction. Laboratory testing of blood chemistry and urine provide
evidence of abnormalities of components of the blood, elevation in liver
enzymes, electrolyte imbalances, and active urinary sediments all consistent
with vascular, liver, and kidney disease associated with Leptospira infection.
The most commonly used serologic test includes the microscopic agglutination
test (MAT), which titrates reactivity of antibodies in the patient's serum
with live leptospires. Limitations to MAT include false-negative results early
in the course of the disease, reduced positive response in vaccinated dogs
that may be harboring chronic infection, and cross-reactivity excluding the
ability to distinguish between serovars. Other serological tests including the
enzyme-linked immunosorbent assay (ELISA) and microcapsular agglutination test
(MCAT) are more specific, reducing false-positives associated with vaccinal
responses and providing earlier detection by monitoring immunoglobulins
specific for immune response to infection (IgM), respectively.
Treatment. Antibiotic therapy in
the early course of Leptospirosis infection is efficient in shortening
duration of the disease, reducing the time period for risks of contagion, and
decreasing the severity of liver and kidney damage. In advanced cases,
supportive therapy to compensate for abnormal blood, kidney and liver function
may be required. Therapy to restore urine production, kidney filtration and
blood flow are essential to reversing kidney failure. In cases of severe liver
disease, a decrease in clotting factors in the blood may lead to bleeding
disorders requiring treatment by transfusion. Since Leptospirosis poses a risk
of contagion to other animals and to humans, special precautions must be taken
to prevent transmission of Leptospira from the dog to other animals and human
companions or caretakers. All blood, urine, and tissues from a dog suspected
or determined to have Leptospirosis must be handled as biologically hazardous
waste. Infected dogs should be quarantined and areas of contamination should
be washed and disinfected with an iodine-based solution. It is important to
note that even after treatment and control of the active disease state, dogs
continue to shed serovar in their urine and therefore, may pose an infectious
risk to other animals and to humans up to 3 months following infection.
Prognosis. Fatalities as a
direct result of Leptospirosis do not usually exceed 10% and usually occur
5-10 days after initial onset of the disease. Death arising from secondary
complications associated with progressive kidney and liver damage are common
but may not occur for long periods following the initial disease.
Prevention. Commercial vaccines
are available and protect against clinical disease associated with the L.
icterohaemorrhagiae 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.
Current Issues Relating to
Leptospirosis and Vaccination
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 discontinuation 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. icterohaemorrhagiae 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. There are several factors that should be
considered prior to drawing this conclusion. First, Leptospirosis vaccines, as
mentioned above, protect against clinical disease but do not prevent
subclinical infection to a "carrier" state. That is, a dog that is annually
vaccinated may harbor infectious organisms of L. icterohaemorrhagiae or L.
canicola which will pose a risk of contagion to dogs that are not vaccinated
or in which vaccination for these serovars has been discontinued. Without
serological testing, low clinical incidence of these formerly prevalent
serovars may be a result of the currently large population of vaccinated dogs.
If this is the case, discontinuing administration of the current Leptospirosis
vaccine may result in a resurgence of clinical disease. Furthermore, it is
important to note that samples from these studies are not necessarily
representative of all regions of the US.
When all the facts are
considered, these findings do not necessarily suggest that L.
icterohaemorrhagiae 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. Taken
together, benefits of vaccinating dogs, who live in areas where
icterohaemorrhagiae and canicola incidence is low and who may have a higher
predisposition for vaccine side effects with current Leptospirosis inoculants
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. icterohaemorrhagiae 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.
