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Leptospirosis

Introduction

Leptospirosis is a zoonotic disease caused by the bacterial genus Leptospira. Leptospires are spirochetes, a group of Gram-negative bacteria with long, thin, spiral structures and an internal flagella used for movement. The size of a typical leptospire is around 0.1μm wide and 6-20μm long. This narrow, helical structure enables them to burrow in to tissues, within tissues they may adopt a more spherical or granular appearance.

Etiology

The primary pathogenic strain of Leptospira is Leptospira interrogans, however there are also non-pathogenic strains such as Leptospira biflexa which is an environmental saprophyte (i.e. consumes dead organic matter).There are currently around >16 species of identified Leptospira.

Serovars & Serogroups

Serovars are groups of organisms, categorised depending on the antigens they present on their surface. Therefore if a number of organisms within a Leptospira species share the same antigens on their surface, they will be grouped together into one serovar. Differences in surface antigens occur within the same species for example, within the pathogenic L. interrogans species, around >250 serovars have been identified worldwide.

Serogroups are clusters of serovars, as there may be hundred of serovars within a species, it is useful to group those together which share similar properties. Important serovars of L. interrogans include:

  • Canicola (Primary reservoir host – Dog)
  • Icterohaemorrhagiae – (Rat, mouse)
  • Bratislava – (Rat, pig, horse)
  • Pomona – (Cattle, pig, skunk)
  • Grippotyphosa – (Rodents)
  • Hardjo – (Cattle)

Each serovar is not limited to its primary reservoir host however. They be transmitted to incidental hosts fairly easily. Incidental hosts include humans, dogs and cats as well as other domesticated animals.

Different serovars are also responsible for different clinical conditions which can range from abortion to haemorrhagic disease. An individual serovar may also cause different clinical conditions in different species, for example L.interrogans serovar hardjo causes abortion and still births in cattle, but in humans it can cause an influenza-like illness or liver/kidney diseases.

Epidemiology

Transmission Cycle

The transmission cycle of a typically Leptospira species is as follows:

  • Rodents shed Leptospira in their urine
  • Direct transmission of Leptospira to humans may occur at this stage
  • The urine contaminates the environment (e.g. soil, water) with Leptospira
  • Indirect transmission to humans may occur at this stage
  • Leptospira may be transmitted to other domestic animals via the environment
  • These animals may become infected and can shed Leptospira in their urine which can lead to direct transmission to humans or contaminate the environment as before

Rodents can acquire Leptospira from the urine contaminated environment, thus creating a cycle of transmission

Transmission via direct contact usually occurs by urine which contains the Leptospira organisms. However direct transmission may also occur via veneral or placental transfer as well as bite wounds or the ingestion of infected tissue material. Crowding of animals (such as in kennels or intensive farming) will enhance transmission of Leptospira. Animals which recover from the disease, may still be infected, thus making them carriers of Leptospira which can still be excreted chronically in their urine. This continues the spread of infection

Transmission via indirect contact can also occur. Methods of indirect transmission generally requires exposure to contaminated sources such as; soil, food, bedding or water sources. The bacteria enters susceptible hosts from the contaminated source via damaged skin or exposed mucous membranes such as in the nose, mouth, eyes etc. Leptospira remains viable in the environment (still able to cause infection) for months, this further enhances transmission.

Environmental Factors

The optimal habitat for Leptospira depends on their environment, if aquatic, optimal conditions are stagnant or slow moving waters. If terrestrial, a neutral or slightly alkaline soil pH is preferred. However, organisms may survive transiently in undiluted acidic urine. A typical temperature range of 0-25C is preferred, this often leads to seasonal fluctuations in the incidence of Leptospirosis.

Pathogenesis

Leptospira enters the host by penetrating mucous membranes via vunerable areas such as damaged skin, eyes, nose or the mouth. Their helical shape and flagella aids in tissue penetration. Upon entering the blood system, they begin to multiply rapidly. The presence of bacteria in the blood is called bacteraemia. They are then distributed around the body via the blood stream.

Once distributed around the body, they then further replicate in target organs and tissues (including the kidney, liver, spleen, central nervous system, eyes and genital tract). The incubation period is around 7 days, this factor depends on the species and the strength of the host immune system however.

The initial immune response will usually remove all Leptospira organisms from the blood and tissues but some will persist in the kidney tubules where they can continue to replicate. The Leptospira organisms in the kidney tubules manage to evade the host immune response by avoiding phagocytosis.

The damage done to the host’s organs and tissues is variable and depends on the virulence of the Leptospira serovar and how susceptible the host immune system is. The most serious of diseases occur in the incidental hosts, i.e. not the primary reservoir host.

Overview of Pathogenesis

Diagnosis

At present there are three different methods of leptospirosis diagnosis:

  • Detect leptospire antigens – Leptospire antigens will induce agglutination of antibodies. This can be tested using a microscopic agglutination test (MAT).
  • Isolation of Leptospires – Leptospires are isolated from the urine or infected tissues. However this can be very labour and time intensive as Leptospira species are slow to culture using growth medium, meaning it can take weeks before a positive/negative result is returned. Despite this, this method of diagnosis is probably the most reliable.
  • Polymerase Chain Reaction (PCR) – Molecular methods of diagnosis (such as PCR) are gaining popularity for diagnosing Leptospirosis, however PCR is unable to distinguish between serovars.

Prevention

For dogs there are currently two forms of vaccine available:

  • In the UK, a bivalent vaccine is used which protects against two serovars – canicola and icterohaemorrhagiae.
  • In the USA however, a quadrivalent vaccine is used, this protects against four serovars – grippotyphosa and pomona as well the canicola and icterohaemorrhagiae which the bivalent vaccine covers.

Some preventative measures are also being taken in cattle to protect against Leptospira borgpetersenii serovar hardjo.

The widespread use of these bivalent vaccines may be responsible for the observed decline in classic canine Leptospirosis infections, however this vaccine does not provide cover for other serovars.

In the USA canine leptospirosis has been classified as a re-emerging disease due to the increasing amounts of newly diagnosed cases. This may be due to the prevalence of grippotyphosa, pomona and bratislava in wild reservoir species which are spreading Leptospira through the domesticated animal population. This is good reasoning behind the introduction of the quadrivalent vaccine as it protects against these serovars (not bratislava, however cases of bratislava are low).

In the UK, rural cases of canine leptospirosis are greater than urban cases, possibly hinting at a greater transmission via wildlife. This is a breakdown of serovar cases diagnosed in the UK:

  • 60% – L. icterohaemorrhagiae
  • 20% – L. canicola
  • 6% – L. icterohaemorrhagiae copenhageni
  • 1.3% – L. bratislava

Opioid Analgesics (Morphine) & Equine Colic (Butorphanol)

Introduction

The primary effect of opioids is to temporarily remove pain when used at therapeutic levels; this is done by binding to opioid receptors found primarily in the central nervous system (some receptors are found in the gastrointestinal tract). When larger doses are given, opioids can induce beneficial and non-beneficial pharmacological effects such as sedation, respiratory depression or constipation. The term given to non-synthetic opioids is opiates; opiates are derived from the naturally occurring opium alkaloids found in the resin of the opium poppy.

The main uses of opioids include:

  • Treatment of acute pain (e.g. post-operative)
  • Palliative care to alleviate serve chronic pain (e.g. cancer)
  • Surgical premedication regimes (due to their calming, sedative action – also reduces the amount of post pain relief required)
  • Neuroleptanalgesia (a state of quiescence, altered awareness, and analgesia produced by a combination of an opioid analgesic and a neuroleptic – a tranquilliser). And neuroleptanaesthesia (a form of anaesthesia achieved by the administration of a neuroleptic agent, a narcotic analgesic, and nitrous oxide with oxygen. Induction of anaesthesia is slow, but consciousness returns quickly after the inhalation of nitrous oxide is stopped)
  • Restraint
  • Antitussive (the alleviating or suppressing coughing)

Pharmacology

The body naturally releases endogenous opioid peptides or endorphins which bind to opioid receptors in the body. There are three primary receptor types, each with different functional responses and these are:

  • μ (mu) – Responsible for supraspinal (above the spine) analgesia, respiratory depression, euphoria and physical dependence of opioids (misuse and abuse of opioids)
  • κ (kappa) – Responsible for spinal analgesia, miosis (pupil constriction of the eye) and sedation
  • δ (delta) – Responsible for hallucinations and dysphoria (agitation and anxiety)

Exogenous opioids (synthetic or natural) mimic the body’s own endogenous opioids and are therefore able to bind to the above receptors – resulting in a response specific to the receptor they bound.  Opioids are able to either stimulate or depress the receptors, meaning opioid drugs can be classes as; agonists, antagonists or both – agonists-antagonists. Agonists bind to receptors and induce pharmacological responses whereas antagonists bind to receptors and do not produce a response, this makes them able to counteract the effect of other drugs or endogenous compounds.

Agonists are used for the primary reasons listed earlier, mainly analgesia. Examples of opioid agonists are:

  • Morphine
  • Pethidine
  • Methadone
  • Fentanyl
  • Etorphine.

Antagonists are primarily used to reverse the effects of agonists i.e. analgesia. They do this by binding to μ and κ opioid receptors, which together are responsible for analgesia. Examples of opioid antagonists include:

  • Naloxone (Narcan)

Agonists-Antagonists have both agonistic and antagonistic properties. This means they are able to antagonise the pure agonists (e.g. morphine) at μ and κ opioid receptors but they also have their own milder agonistic effects. The agonist effect is sufficient enough to be used as analgesics. Examples of opioid agonists-antagonists include:

  • Butorphanol
  • Pentazocine (Fortral)
  • Nalorphine
  • Diprenorphine (Revivon)
  • Buprenorphine (Temgesic)

The principal usage of opioids in medication is for analgesia. Analgesia is the loss of pain perception. Opioids effect both the physical and psychological perception of pain, physically blocking or raising the threshold of pain stimulation and removing the association of pain with fear. Associated with analgesia is sedation which is not considered hazardous, respiratory depression (which can also be associated with opioid analgesia) however can be a distressing side effect. A list of unwanted opioid effects includes:

  • Sedation
  • Excitement
  • Respiratory depression
  • Cough suppression
  • Nausea
  • Vomiting
  • Constipation

Opioid Selection

There are many opioids available for use, each with different properties, when selecting an opioid it is important to consider its potency, how quickly it acts (speed of onset) and how long it lasts (duration). The best analgesics are those which have a mild potency, rapid onset and a long duration of effect. When combining an opioid with a neuroleptic for neuroleptanalgesia, the desired properties of the opioid are slightly different; strong potency, rapid onset and brief period of duration.

As opioids can have an effect on the gastrointestinal system, (as opioid receptors are also found in the gastrointestinal tract) if they are to be given orally then they must have low lipid solubility.

Another point to consider is whether to use an agonist or an agonist-antagonist as both are able to produce analgesia. The main consideration is that pure agonists are more reliable and predictable than agonist-antagonists, but the agonist-antagonists produce fewer side effects such as vomiting, sedation and respiratory depression. Also as agonist-antagonists have antagonistic effects, any further use of analgesics may be compromised.

Below is a comparison of the potency of certain opioids relative to morphine, the most potent being Etorphine. Etorphine (or Immobilon) is extremely powerful and typically only used to immobilise large mammals (e.g. elephants). Due to its potency it can prove lethal to man.

Drug Relative Potency
Meperidine 0.1
Morphine 1
Butorphanol 1-2
Hydromorphone 10
Alfentanil 10-25
Fentanyl 75-125
Remifentanil 250
Sufentanil 500-1,000
Etorphine 1,000-3,000

Opioids are often used as part of a pre-medication routine i.e. before surgery as a pre-emptive form of analgesia. This is because once pain has been established (i.e. during surgery) pain relief drugs prove less effective. As a result larger doses would be needed to prevent the pain which increases the onset of associated side effects e.g. respiratory depression.

Examples of Opioids and their Properties:

Morphine

Morphine (agonist) is considered the standard opioid with all other forms of analgesia being compared against it. It is the most potent natural analgesic, more potent derivatives have been artificial synthesised. Morphine produces a mixture of stimulant and depressant actions depending on the size of the dose as well as the species and absence or presence of pain.

Differences between the species can be observed e.g. in the dog, the cortex is depressed and little excitement is produced. In the cat, very small doses are able to induce excitement and in the horse morphine will not produce excitement if no pain is present (effect is less predictable in horses however). Despite this morphine is safe to use in all species as long as the correct dosage is used, the presence of excitement tends to increase with dose.

The duration of morphine is about 4 hours in all species, it is eventually metabolised by the liver. It is normally injected subcutaneously at a dose of around 0.1mg/Kg (in dogs and cats).

Use of morphine can either stimulate the medulla (which is followed by depression of the medulla) or directly depress the medulla.

Morphine has a number of effects on the gastrointestinal tract. Initially it may invoke vomiting and defaecation which is followed by constipation. Constipation is due to local effects on the small/large intestinal opioid receptors. Segmental tone of the intestines increases, along with sphincter tone but the action peristalsis decreases. This increases the time taken for intestinal contents to pass.

Other areas affected by morphine include:

  • The chemoreceptor trigger zone (CTZ) of the medulla is stimulated by morphine – this induced vomiting.
  • The occulomotor centre is stimulated which is responsible for producing miosis.
  • The cough centre is depressed – reducing coughing but making post-operation mucus accumulation a possible problem.
  • The vagal centre is stimulated, which increases gastrointestinal activity and is responsible for the initial defaecation. If a large dose is administered bradycardia may be induced (slowed heart rate <60bpm) by myocardium depression.
  • The respiratory centre is easily depressed by morphine, even with a low dose. This is due to a reduced response to elevated CO2 levels. The mechanisms involved in the regulation of respiratory rhythm are also affected – contributing to the overall depression of the respiratory centre.

Butorphanol

Butorphanol (agonist-antagonist) is a widely used sedative and analgesic in dogs, cats and horses – combined with tranquillisers for sedation. It is around 1-2 times as potent as morphine, but it does have a slightly shorter duration of action at around 2-3 hours (Morphine – 4 hours). It has a much less profound effect on the respiratory system as the dose increases compared to morphine.

One major use of Butorphanol is for intravenous administration in the horse (0.1mg/Kg) to alleviate abdominal pain associated with torsion, impaction, intussusception (intestinal prolapse) and spasmodic colic.

Equine Colic

The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. Some examples are:

  • Spasmodic colic – Increased peristaltic contraction
  • Impactive colic – Caused by irritation to the lining of the bowel or ileum due to diet or ingestion of large amounts of sand/ dirt
  • Obstructive colic – Obstruction of the bowel by large food masses
  • Flatulent colic – Build of intestinal gases causing distension and pain
  • Parasitic colic – Intestinal pain from parasites such as roundworm or tapeworm
  • Idiopathic colic – From another cause which remains unknown

There are also many diagnostic tests for equine colic:

  • Increased heart rate with decreased circulating volume
  • Distinctive behavioural signs
  • Auscultation – Listening to internal body sounds
  • Abdominocentesis – The extraction of fluid from the peritoneum which can be useful in assessing the state of the intestines
  • Nasogastric Intubation – Insertion of a tube from the nose to the stomach which can be used to drain excess liquid from the stomach – for therapeutic reasons and for diagnosis
  • Rectal/Faecal examination

There are also many drugs/treatments available to treat the symptoms:

  • Analgesics
  • Spasmolytics
  • Lubricants/laxatives
  • Antizymotics – Used against disease producing organisms e.g. bacteria
  • Anthelmintics – Used against parasites
  • Fluid therapy

The major analgesics used against colic are α2-agonists (xylazine, romifidine and detomidine), opioids (butorphanol) and NSAIDs (flunixin).

Butorphanol is usually used alongside small doses of xylazine, romifidine and detomidine. This is because it has minimal effects on the cardiovascular system (which is not true for xylazine, romifidine and detomidine). Both butorphanol and the α2-agonists have a duration of around 2-3 hours and they both reduce intestinal motility/activity.