Ticks are obligate bloodsucking arachnid parasites (subclass Acari) that cannot survive without blood. Larvae, nymphs and adults, both males and females need to suck blood. Of all the parasites of dogs and cats, ticks can be the most harmful due to the fact that they can transmit numerous diseases.

There are approximately 900 species, of which about 700 species are hard ticks (ixodid) and 200 species are soft ticks (argasid). Of these, there are approximately 200 ixodid and 40 argasid species present in Africa. Many of the ticks and tick-borne diseases occur usually in specific geographical areas.

Dogs are usually more affected than cats because cats are much more likely to get rid of the ticks because of their intensive self-grooming behaviour.

Before taking their blood meals ticks are rather small – the larvae are smaller than 1 mm, nymphs are only 1-2 mm long, unfed adult ticks are usually 0.3 to 1 cm long, depending on the species. But engorged (those that have had a blood meal) adult females of certain species can be up 3 cm long (50 to 100 times larger than if they are not engorged) when they are ready to lay eggs.

Unlike insects, the tick’s body is not segmented (divided) and do not even have a proper head. Instead they have a series of mouthparts grouped into the so-called capitulum (also called a false head). These mouthparts are made up of:

  • a pair of palps
  • a pair of chelicerae to cut and open the skin of the host
  • The hypostoma that is like a needle that penetrates the skin to suck blood. The hypostoma has backward pointing barbs that help keep the tick attached to the host's skin.

Ticks only have an abdomen that contains all the major organs (digestive tract, reproductive organs, nervous system, etc.). The body is covered with a hard skin that is called the exoskeleton which protects the body organs. The exoskeleton of the abdomen is tough and resistant to pressure, but not rigid and can expand considerably to allow for expansion while taking a blood meal.

Tick larvae only have three pairs of legs, whereas, nymphs and adults have four pairs of legs. The legs are covered with small spines that end with a claw. The spines and claws help the tick crawl and remain attached to the host or to climb in the vegetation while waiting for a host to come past.

All ticks undergo a so-called metamorphosis; the developmental process into adulthood not only requires increasing in size, but also changing shape. Due to exoskeleton, size increase is only possible through moulting. Each stage between moults is called an instar, the first instar being larva, then nymph and lastly adult. Hard ticks are mostly three-host tick and require three blood meals to complete development from each life stage to the next.

Ticks go through four major developmental stages: 

  1. Egg
  2. Larva
  3. Nymph
  4. Adult

The duration of the complete life cycle varies between the tick species and is strongly dependent on the climatic and ecological conditions. It can vary between two to three weeks to more than two years. A typical feature of many tick species is their extreme ability to survive without feeding, and species can wait more than a year without feeding (the record is 20 years). The digestive process of the blood meal takes place almost entirely in the body cells and undigested blood can act as a food reserve to be consumed of a period of months, or even years while the ticks wait for the right conditions. If they become desiccated, they can absorb water from the atmosphere.

The saliva of some ticks have a glue-like substance that helps the hypostoma to stay firmly attached to the skin of the host, this normally dissolves after the tick has finished feeding. Unlike fleas, tick saliva contains anti-coagulants which prevent the blood from clotting during the blood meal. The bite of a hard tick causes little or no immediate pain to the host, but due to the fact that the bite lasts for days, the injected saliva will cause immune reactions that lead to inflammation of the skin, the intense itching causes scratching, rubbing and biting of the affected areas. Bites of soft ticks are generally more painful.

Mating occurs on the host. A female may begin feeding without the presence of a male but will not become fully engorged unless she mates with a male. The adult female feeds on a host for about one week after mating and then detaches from the host, drops off and finds a safe area to lay eggs typically cracks or crevices in or near a home, garage or dog kennel, the eggs are not laid on the host. Egg laying can happen as soon as four days to several weeks after detaching from the host, and egg laying can last a few weeks, during this period the female can lay 2000 to 20 000 eggs (dependent on species). The female's body empties itself almost completely, once all the eggs have been laid, the female will die.

Dependent on climate, the larvae can take a few days or weeks to hatch out of the eggs, hatching is faster if the weather is hot and humid. Once hatched, they will instinctively climb to the tip of grasses, leaves or bushes in the vegetation, where they will wait for a suitable host to pass by. This behaviour is called questing and they can perceive the environment with sensory organs (the Haller's organ) in the legs. A suitable host is recognised host by its body heat and exhaled carbon dioxide (CO2).  Once a suitable host passes by and touches the grass or the leaves, the questing ticks will hang on to the host.

Questing behaviour means that most hard ticks do not jump onto their prey, nor drop down from the trees. Hard ticks are not transmitted from one animal to another and not from pets to humans. A hard tick of all stages that has found a suitable host will soon attach for feeding and have its blood meal for a few days, and is unlikely to pass to another animal before or after the blood meal.

A heavy tick burden can lead to blood loss causing serious anaemia. A single female hard tick of the larger species (e.g. Rhipicephalus) can take something between 1 and 5 ml blood during a meal that lasts for 1 to two weeks. 100 such ticks would suck up to 0.5 liters, 1000 ticks up to 5 liters of blood!

Ticks often build clusters, typically in the ears, between the toes and on the back. When there are many ticks are biting in a small area of skin open wounds can develop that can be easily get infected.

The tick's saliva contains a complex mix of several substances that strongly affect the host's immune system, causing toxicosis (poisoning) of the host, toxicosis can lead to paralysis of the host and can be fatal.

Ticks are vectors and can transmit numerous viral, bacterial, fungal, rickettsial and protozoan diseases to livestock, pets and humans. Typical tick-borne diseases in Southern Africa are ehrlichiosis, babesiosis, haemobartonella etc. Within a tick population, not all ticks are infested with pathogens, sometimes only 10% of the ticks are actually infected and the level of infestation of the infected ticks is also a variable in that some ticks may carry thousands of pathogens, others only a few ones. The virulence of the pathogens can also be different, whereby a very few pathogens of a very virulent strain can be as harmful as many pathogens of a rather benign strain.

 

 There are about 900 tick species worldwide, but the three worth mentioning for the South Africa region are:

  •  The brown dog tick (Rhipicephalus sanguineus) (hard tick).

The adult stage of the brown dog tick is red-brown in coloration, it lacks distinctive markings or body patterns and has an elongated body shape and hexagonally shaped mouth parts. Males are similar in appearance to females but slightly smaller in size.

All developmental stages feed on the same host species, and each life stage requires a new host to feed on. The brown dog tick prefers to feed on dogs, although it will occasionally feed on other hosts (e.g. humans, cats, etc.).

The brown dog tick has been reported to transmit numerous pathogens of medical significance:

  • Ehrlichia
  • Hepatozoonosis in cats
  • Babesiosis (Biliary)
  • Yellow dog tick (Haemaphysalis leachi) (hard tick)


The yellow dog tick has been reported to transmit numerous pathogens of medical significance:

  • Babesiosis (Biliary)
  • Tick bite fever (humans)
  • Ehrlichia
  •   Bontlegged tick (Hyalomma truncatum) (hard tick)


The common name of this tick is derived from the fact that at the end of each segment of the brown legs, there is a circled by ivory coloured bands. These ticks are mostly not present in the southern parts of KwaZulu-Natal.

Although this tick is most known for its transmission of a toxin causing sweating sickness in cattle and especially calves, they are able to transmit numerous other pathogens of medical significance:

  • Tick Toxicosis
  • Tissue necrosis
  • Tick Bite fever (humans)

Common diseases and medical conditions caused by ticks

Babesiosis (Biliary) is caused by protozoa that parasitises the red blood cells in the host, and is transmitted by the Brown dog tick as well as the Yellow dog tick.  This zoonotic disease is common in Southern Africa and can also be transferred to humans if an infected tick bites (dependant on the strain of Babesia).

The protozoa are carried in the saliva of an infected tick, and the tick must have a blood meal for at least 48 hours before the disease is transmitted.

Ticks become infected with the disease when they have blood meal from a dog that is infected. When the tick becomes a carrier, it will carry the disease to its offspring.

 The incubation period for the disease is 10-28 days, so the tick that transmitted the disease may be gone long before the dog develops the infection.

 Once the parasite enters the body, it invades the red blood cells (the cells that carry oxygen to various parts of the body) in the bloodstream of the dog. A single parasite can multiply to as many as 64 parasites, with each one infecting a new blood cell. The disease gains momentum as the parasite multiplies and the process becomes faster that the production of red blood cells, causing low oxygen carrying capacity which leads to multiple organ damage or failure.

 Once the immune system recognises the parasites in the red blood cells, it destroys the parasites, along with the red blood cells. Infected cells are identified by the liver and spleen, which is then removed from the bloodstream, and destroyed in these organs.

 Sometimes large numbers of red blood cells rupture in the bloodstream, leading to a large amount of red pigment (haemoglobin) in the blood, which is excreted in the urine, causing a red discolouration of the urine.

Yellow faeces can also be seen due to the dead red blood cells that are transformed by the liver into bile and excreted into the stool.

 Symptoms:

  •  Lethargy
  • Loss of appetite
  • Fever (over 39°)
  • Anaemia
  • Anorexia
  • Fever
  • Yellow mucus membranes and gums
  • Dark red urine
  • Severe lethargy
  • Vomiting
  • Dehydration
  • Laboured breathing/panting
  • Nervous Symptoms
  • Seizures
  • Death

Ehrlichiosis is a rickettsia bacterial infection, although there are many different strains of this bacteria, dogs are most commonly infected with Ehrlichia canis. These bacteria infect and live within the white blood cells of the hosts. Different types of Ehrlichia live in different types of white blood cells. Ehrlichia canis lives in what are called monocytes (a different type of white blood cell).

Ehrlichia canis is reported primarily in dogs, but has also been documented in felines as well as humans. Normally Rhipicephalus sanguineus (brown dog tick) acts as the primary vector transferring the pathogen between hosts during blood meals. Dogs act as reservoir hosts for this pathogen. The ticks become carriers of the pathogen when they take a blood meal from a dog carrying the pathogen.

Pathogens of the disease are stored in the mid-gut and salivary glands of the infected tick and transferred via the saliva to hosts during blood meals. If ticks are infected during the larval stage, they will retain the pathogen through the next two life stages and can transmit to hosts during blood meals in both the nymph and adult stages.

There are three phases of illness with Ehrlichiosis:

1.   The Acute Phase is normally a mild phase that occurs 1 to 3 weeks after the host is bitten by the infected tick. The organism replicates during this time period and attaching to white blood cell membranes. The dog will be listless, off food, may have enlarged lymph nodes, and may be accompanied by fever. This phase rarely kills dogs and most will clear the organism if they are treated in this phase but if no treatment is received, it will progress to the next phase.

2.    During the subclinical phase the dog will appear normal, and can stay this way for months, or even years. The organism is hidden away in the spleen, the only hint that Ehrlichia is hiding is through a blood test.

3.    The chronic phase will cause the dog to get sick again, and up to 60% of dogs infected with Ehrlichia canis will have abnormal bleeding due to reduced platelets numbers. Deep inflammation in the eyes may occur as a result of the long-term immune stimulation.

Symptoms include:

  • Fever
  • Lethargy
  • Anorexia
  • Vomiting
  • Diarrhoea
  • Abnormal bleeding such as nosebleeds, bleeding under skin that looks like little spots or patches of bruising
  • Lack of blood clotting
  • Inflammation of the eye
  • Loss of stamina
  • Stiffness and reluctance to walk (due to the disease causing arthritis and muscle pain)
  • Weight loss
  • Enlarged Lymph Nodes
  • Enlarged spleen
  • Swelling of the limbs or scrotum
  • Discharge from nose and eyes
  • Coughing
  • Difficulty breathing
  • Neurological symptoms such as incoordination, depression, paralysis, etc.
  • Seizures
  • Symptoms of organ involvement may also occur in the chronic form, especially kidney disease

 

Haemobartonella or Mycoplasma haemofelis is a parasitic bacteria, and is usually the causative agent of Feline Infectious Anemia (FIA).

This leaves M. haemofelis inextricably dependent upon its host for the provision of amino acids, cholesterol, vitamins and fatty acids.  Ticks are thought to be the primary source of infection, although it is also known to be transmitted from queen to kitten and following blood transfusion

Symptoms include:

  • Anemia
  • Lethargy
  • Fever
  • Anorexia

Recent evidence suggests that M. haemofelis may be transmissible to humans.

Hepatozoonosis is and organism that is transmitted by the brown dog tick.  The organism is picked up by the tick while feeding off an infected host, when the tick falls off and gets onto a new host and that host eats the tick, the uninfected dog or cat will then pick up the disease.  The tick will not infect the host animal when biting, only if the infected tick is eaten.

Infection is often subclinical. However, symptoms of clinical infection include:

  • Fever
  • Lack of appetite
  • Wight Loss
  • Bloody diarrhea
  • Hyperaesthesia (heightened sensitivity of the skin and musculature) over the back and sides
  • Muscle wasting
  • Proliferation of the outer layer of bones, causing pain
  • Kidney failure

Hepatozoonosis may affect the bones, liver, spleen, muscles, small blood vessels in the heart muscle, as well as the intestinal tract.

Tick Paralysis is the only tick-borne disease that is not caused by an infectious organism. The illness is caused by a neurotoxin that is produced in the tick's salivary gland, which causes the loss of voluntary movement and which is caused by a disease of the nerves that connect the spinal cord and muscles. With lower motor neuron paralysis the muscles stay in an apparent state of relaxation.

The toxin causes symptoms within 6-9 days after the tick has attached to the host.  Symptoms may include:

  • Regurgitation
  • Vomiting
  • Unsteadiness
  • High blood pressure
  • Fast heart rate and rhythm
  • Weakness, especially in the hind limbs
  • Partial loss of muscle movements
  • Complete loss of muscle movement (paralysis) (commonly seen in advanced stage)
  • Poor to complete loss of reflexes
  • Low muscle tone
  • Difficulty in eating
  • Disorder of voice
  • Asphyxia due to respiratory muscle paralysis in severely affected animals
  • Excessive drooling
  • Enlarged oesophagus
  • Excessive dilatation of pupil in the eye

Tick related diseases in humans:

Tick Bite Fever is caused by Rickettsia conorii and is transmitted by hard ticks. It is usually a mild disease, especially in children and young adults, but in middle-aged, immune compromised and elderly patients, it may be severe form. During these severe attacks the central nervous system may be involved and marked disorders of liver and kidney function sometimes lead to kidney failure and the need for treatment and dialysis in an intensive care unit.  The incubation for this disease is 5-7 days.

Symptoms include:

  • Black Mark at the site of the bite, similar to spider bite approx. 2-5mm diameter. May be ulcer-like with a black spot in the middle
  • High fever
  • Severe headache
  • Delirium
  • Stupor
  • Rash, consisting of small red marks on the skin, slightly raised which normally starts on the arms and legs, and then later spreading
  • Occasionally coma
  • Rarely development of gangrene of the fingers and toes

 

 

Written by Cherece du Plessis


©Cherece du Plessis

 

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