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

Breed Occurrence
There is no specific breed susceptibility, though working dogs and cats on a farm are most likely to be exposed to contaminated rodent urine.

Human infection is most often contracted from :

• Farm livestock
• Rodents
• Dogs
• Cats ?

Human workers most at risk include :

• Abattoir workers
• Agricultural workers
• Miners
• Sewer workers
• Veterinarians

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 40^o C or 104^o 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

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.

Prognosis

The prognosis is guarded.

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.