Description
Leptospirosis is an infectious disease which can affect several primary hosts
including dogs, rodents (rats, mice, voles), pigs, cattle, horses, the skunk,
raccoon and the opossum. In these species the infection may be subclinical,
however when it is transmitted to other incidental hosts (species such as humans
and cats) the infection is much more severe, results in clinical signs and these
hosts shed the organisms for much shorter periods than the primary hosts.
Leptospirosis is also known as Weil's Disease or Fort Bragg Fever
Cause
Leptospires are thin, flexible, motile, filamentous bacteria called
spirochetes. At least 8 serovars are thought to be important in causing disease
in cats and dogs, though only two of these - L. interrogans canicola and L
interrogans icterohaemorrhagiae are included in current Leptospira vaccines. In
the USA there is serological evidence that exposure to the serovars
grippotyphosa, pomona and bratislava are increasing in frequency.
Generally, serovars canicola and grippotyphosa cause renal disease in dogs,
whereas the serovars icterohaemorrhagiae and pomona produce liver disease.
Transmission to humans usually occurs by one of the following routes :
- Inhalation
- Ingestion
- Skin contact and inoculation - especially contact with urine
Signs
Cats :
Although leptospiral antibodies are found in cats, disease due to Leptospirosis
is very rare. Rodents are the natural prey of cats and it is reasonable to
assume that they may have inherited some natural resistance to Leptospirosis
which is so common as a subclinical infection amongst rats and mice.
Dogs : Most Leptospiral infections are subclinical or chronic, but acute
and peracute forms are seen.. Puppies are more severely affected than adults,
and large outdoor breeds of dog are more commonly affected.
The sudden presence of large numbers of Leptospires in the blood (Leptospiraemia)
- a peracute infection - can cause rapid death with few signs.
Acute infections cause a very high body temperature (up to 40o C
or 104o F ) shivering and painful muscles, followed by vomiting,
dehydration and shock. Blood clotting defects and injury to blood vessels
results in haemorrhage in vomit, faeces, in the skin and nose bleeds.
Hypothermia and depression precede death. Various compensatory signs relating to
shock may be seen - including increased respiratory rate, increased heart rate
and poor capillary refilling rate. Inflammation of the uvea of the eye (anterior
uveitis) causes photophobia.
Less acute cases develop a variety of signs including hyperaesthesia,
injected mucous membranes with petechial and ecchymotic haemorrhages.
Respiratory signs ( most often associated with icterohaemorrhagiae infection)
include a cough, dyspnoea , tonsillitis, conjunctivitis and rhinitis. The
kidneys are badly injured which can lead to chronic renal failure. Jaundice is
frequently seen and, if the liver is severely affected, bile output might cease
causing very pale faeces. Liver failure can result.
Sometimes intussusception can occur in association with inflammation of the
intestinal tract and abortion or infertility can result if transplacental
infection takes place.
Humans
Signs include :
- Fever
- Hepatitis
- Meningitis
- Renal Failure
- Uveitis
Vaccination can reduce the severity of the clinical course in infected
animals but it does not prevent the carrier state - which presents a risk for
humans.
New vaccines are being developed which will cover a larger number of
serovars, and which will produce high, protective antibody titres after only 2
weeks.
Diagnosis
Live Leptospires can be identified by their writhing movements on dark-field
examination of wet-mount preparations. Specific fluorescent antibody staining
techniques have also been developed. Urine contains the highest concentration of
organisms , but they can also be identified in blood and other fluids.
Leptospires are difficult to culture - especially from blood and CSF as they
are only present in significant numbers for the first week of infection. Urine
samples collected directly from the bladder (cystocentesis) are preferred. Blood
samples need to be collected into heparin anticoagulant - not citrate.
Microscopic agglutination (MA) test is the standard serological test for
Leptospires and it must be performed in a commercial laboratory. One problem
with the test is that dogs may have positive antibody titres following natural
subclinical infection, or following vaccination. Demonstration of a rising
antibody titre is the only way to confirm that an active infection is present.
ELISA tests have been used to measure IgG and IgM antibody concentrations.
The IgM increases within a week of infection and peaks within 14 days; whereas
the IgG develops in 2-3 weeks and peaks after about 30 days. Dogs that die
within a week have high IgM, but ormal MA and IgG titres.
Most clinical cases of leptospirosis have azotaemia with increased blood urea
(82-100%) and creatinine (82-100%) concentrations. The majority have
hyperphosphataemia (47-83%) and a large number have increased alkaline
phosphatase (33-65%) and alanine aminotransferase (22-35%) concentrations.
Treatment
In peracute and acute forms of the disease treat the dehydration and shock
first with fluids, and plasma or blood transfusions if there is evidence of
blood loss.
Antibiotics should be administered as soon as possible - penicillin G or it's
derivatives (e.g. ampicillin) are usually recommended as the drugs of choice for
treating Leptospiraemia, followed by tetracyclines or one of the other
recommended antibiotics to eliminate the carrier state. Other drugs that have
been recommended include : amoxicillin, erythromycin and doxycycline.
An antiserum is available for administration to infected dogs.
Postmortem
Many findings can result from leptospirosis including enlargement of various
organs (tonsils, lymph nodes, kidneys, liver) with surface haemorrhages (petechiae
or eccymoses) and pale discolouration of affected organs - notably kidney, liver
and lungs.
Leptospires can be cultured from kidney tissue, but special silver staining
techniques are needed to identify them in tissue sections.