Babesia species

Introduction

Babesia species are protozoan parasites of domestic and wild animals. They belong to the subclass Piroplamsia and are commonly referred to as ‘piroplasms’ due to the pear-like shaped merozoites which live as small intra-erythrocytic parasites. They commonly infect mammals, particularly cattle, sheep, goats, hosres, pigs, dogs and cats and occasionally man.

Species infective to humans are the cattle form Babesia bovis which can often be fatal and Babesia microti which is less pathogenic. Until recently B. microti was confined to the United States but is becoming known as an important tick-borne disease of man in temperate climates.

Human infections have been found in Europe and North America. The infection is known as Babesiosis, it can also be described as ‘redwater fever’ or ‘tick fever’ and has a major impact on the livestock industries in many countries.

What is unusual with this parasite’s life cycle is the development in the vector. They use the one-host ticks, belonging to the genus Boophilus. The parasites are passed to the eggs and hence to the larval stages which can thus become infective after the adult tick dies. This process is known as transovarian transmission.

The ticks which are known to carry the parasite of Babesia bovis are Boophilus microplus, B. decoloratus and B. annulatus.

Life cycle

Human babesiosis is a zoonosis, acquired by tick bite when individuals accidentally interact with the natural life cycle of the parasite.

The life cycle is best known for B. canis which infects dogs, but it is generally the same in each of the one-tick hosts and the definitive host. (Fig. 1)

Babesia bovis

When the tick bites, sporozoites are injected into the blood stream and penetrate the erythrocytes. In contrast to the malaria life cycle, there is no tissue stage for Babesia bovis. Babesia multiplies in the red cell by budding in contrast to schizogony in Plasmodium species. The red cell ruptures and daughter parasites invade new erythrocytes for further asexual multiplication. Some of the sporozoites injected by the tick vector follow a different path of intra-erythrocytic development, growing slowly and "folding" to form accordion-like structures which are destined to undergo further development in the tick vector. Within the intestine of the tick, the accordion-like stage eventually fuses with another, to form a zygote. Further development outside the intestine occurs in a variety of tissues, the salivary glands and ovaries being especially important for transmission. Sporozoites in tick salivary glands are injected into the mammalian host at the next blood meal. Trans-ovarial transmission of Babesia bovis also takes place so that newly hatched onto adult stages can then take place.

Figure 1. Generalised life cycle of the parasite Babesia, which causes the disease babesiosis in man. (Adapted and redrawn from Smyth, J.D, 1994)

Babesia microti

In the small mammal host of Babesia microti, sporozoites from the tick vector first enter lymphocytes and undergo merogony, the daughter parasites of which then enter erythorcytes. (Fig. 2)

Babesia microti do not undergo trans-ovarial transmission, but once a larva has become infected from a mammalian host they are able to pass on the infection trans-stadialy to the nymph.

Figure 2. Diagnosis of Babesia species depends on the observation of the intraerythrocytic organisms in blood smears. Pear shaped microorganisms (2-5 µm) and tetrads are the diagnostic shape of the parasite. (Giemsa stain). (www.cdfound.to.it)

Clinical disease

Babesia bovis - Patients who are particularly at risk are those who have had a splenectomy. The patient may feel vaguely unwell at first but by the time the diagnosis has been made, is usually very ill, with fever, prostration, jaundice, anaemia and haemoglobulinuria. Nausea, vomiting and diarrhoea have also been recorded. Unlike malaria symptoms, the symptoms of babesiosis do not exhibit periodicity.

Babesia microti - Most human infections are subclinical. Where clinical illness develops, the incubation period is 1 to 3 weeks, occasionally up to 6 weeks. The illness usually begins gradually, with anorexia and fatigue, plus fever (without periodicity), sweating, rigors and generalised myalgia. Physical examination may reveal only fever, but may also show mild splenomegaly and occasionally mild hepatomegaly.

Laboratory diagnosis

Definitive diagnosis depends upon finding parasites on blood film examination which can be detected 2 to 4 weeks after a tick bite. Hamster inoculation and serology have also been used for diagnosis.

1. Microscopic Examination

Babesia bovis

Babesia bovis are pear shaped, oval or round and may exist in pyriform pairs. There may be 1 to 8 parasites per red cell. Ring forms can be confused with malaria parasites, especially Plasmodium falciparum. However, in contrast to Plasmodium species, Babesia do not form pigment, do not cause alterations in red cell morphology and finally do not exhibit the Maurer's clefts of Plasmodium falciparum, the Schuffner's dots of Plasmodium vivax, or the James's dots of Plasmodium ovale.

The "Maltese cross form" is unique to Babesia but in its absence it may be very difficult to distinguish young ring forms of Plasmodium falciparum, from Babesia. The absence of pigment cannot be relied upon, as young rings of Plasmodium do not exhibit pigment. Babesia, are smaller than malaria parasites, and in some of the larger rings there is white vacuole containing erythrocyte stroma, instead of the pink vacuole seen in malaria. Babesia parasites do not form schizonts.

Babesia microti

Ring, rod shaped, pyriform, amoeboid, and "Maltese cross" forms are seen. In heavy infections different stages may be noted in the same red cell. Intra-erythrocytic stages measure approximately 2 by 1.5mm. In very high parasitaemias, extracellular merozoites are found singly or as a syncytial structure. Peak parasitaemia varies between less than 1% to approximately 10%.

2. Serodiagnosis

The Indirect Fluorescent Antibody Test (IFAT) is available for both B. bovis and for B. microti and is the most useful serological test for early diagnosis.

3. Animal Inoculation

This is not routinely used for diagnosis but B. microti grows well in hamsters and can serve as a confirmatory test.

Toxoplasma gondii

Introduction

Toxoplasma gondii, the causative organism of toxoplasmosis, was first observed in 1927 in the gondi, a North African rodent. The first human case of toxoplasmosis was also reported that year. The organism is a coccidian protozoa belonging to the sub-phylum Apicomplexa and has a world wide distribution occurring in all warm-blooded animals.

Cats are the definitive hosts and they become infected by ingesting oocysts or cysts in tissues of paratenic hosts, such as mice, or transplacentally. Man becomes infected either by direct ingestion of oocysts from a cat or by eating raw or undercooked meat. Those who handle raw meat are particularly at risk. Infection can be transmitted transplacentally.

Life cycle

The development of the entereoepithelial (sexual) cycle in a cats intestine is brought about by the ingestion of sporulated oocysts of a mouse with cysts. The pre-patent period up to the shedding of the oocysts varies with the stage of T. gondii ingested, for example only 3 – 10 days if the cat has ingested a mouse containing cysts, but about 19 – 20 days or longer after direct infection with oocysts or ingestion of a mouse containing only tachyzoites. Women most at risk of delivering an infected infant are those who acquire the infection just prior to gestation. (Fig. 3, 4 & 5)

Humans can acquire infection by;

1) Accidental ingestion of the oocyst shed in the cats faeces

2) Ingestion of the tachyzoite in infected milk or transplacentally

3) Ingestion of the tissue cyst in undercooked or raw meat.

4) Transplant of an infected organ in a seronegative recipient

Figure 3. Life cycle of Toxoplasma gondii, causes toxoplamosis in man.

Figure 4. Diagrammatic illustration of a Toxoplasma gondii trophozoite in a macrophage of a vertebrate. (Adapted and redrawn from Smyth, J.D, 1994)

Clinical Disease

Serological evidence has shown that approximately one third of the world's population has Toxoplasma antibodies. This suggests that the majority of infections are benign with most people exhibiting few or no symptoms, but fever and swelling may be seen. However Toxoplasma gondii can cause severe illness in congenital infections, acquired infections and in immunocompromised patients. This may lead to ocular toxoplasmosis and ultimately to fatal CNS disorders such as encephalitis.

Figure 5. Toxoplasma gondii tissue cyst containing 8 – 20 parasites (Giemsa stain) (www.cdfound.to.it)

Congenital toxoplasmosis

This occurs approximately in 1 per 1000 pregnancies. It can cause severe damage to and even death of the foetus. Proliferation of tachyzoites leads to intracellular calcification, corioretinitis, hydrocephaly, psychomotor disturbances and convulsions. A small, proportion of babies who are asymptomatic at birth develop retinocoroiditis or mental retardation as children or young adults. When a mother is first exposed to the parasite in later pregnancy the infant is likely to be less severely damaged or asymptomatic.

Acquired infections

Infections with T. gondii are often mild with flu-like symptoms thus they often go unnoticed. However lympadenopathy is the most easily recognised symptom and it can be accompanied by fever, headache and myalgia. Toxoplasmosis may also produce infectious mononucleosis like symptoms. Ocular toxoplasmosis is also a common manifest however it is not yet proven whether this is due to congenital or acquired infections. Other manifestations of Toxoplasma infections are meningoencephalitis, hepatitis, pneumonitis and myocarditis.

Immunocompromised patients

Toxoplasmosis has been shown to occur as an opportunistic pathogen in immunocompromised patients and can cause severe complications. Toxoplasmosis in immunocompromised patients almost always arises from a reactivation of latent infections. Conditions which can predispose to toxoplasmosis are malignancies, organ transplants, leukaemias and patients with acquired immune deficiency syndrome (AIDS). In immunocompromised patients, the central nervous system is primarily involved with diffuse encephalopathy, meningoencephalitis or cerebral mass lesions. Toxoplasma encephalitis has been reported as a life-threatening among patients with AIDS.

Laboratory Diagnosis

1. Serological techniques

The detection of toxoplasma specific antibodies is most commonly used in clinical laboratories. Specific IgG antibodies typically persist for life whereas specific IgM antibodies begin to decline after several months. Most laboratories carry out preliminary tests for IgG antibodies and more definitive tests including IgM and IgA are carried out in reference laboratories. The Sabin-Feldman Dye Test is the gold standard for detecting the presence of specific antibodies. It measures the total amount of specific antibody in a serum which is capable of participating in antibody-mediated killing of tachyzoites by complement. This test involves the use of live tachyzoites which are derived from infected mice or rats. Because of the use of live organisms, this test is not recommended in the use of routine laboratories and is thus only employed in reference centres.

2. Isolation Techniques

Culture of parasites in animals is the best overall method but it can take up to six weeks before the result is available and is thus a disadvantage. Tissue culture is more rapid taking three or four days to obtain a result, but is not as sensitive.

3. Antigen detection

The direct detection of very small amounts of specific nucleic acid has been made possible by the introduction in 1985 of the polymerase chain reaction (PCR). This technique results in the amplification of a specific fragment of DNA from within the parasite genome which is detected by ethidium bromide staining, following gel electrophoresis. PCR is so sensitive it should detect Toxoplasma DNA in one cyst. However this may indicate a latent infection rather than an active infection. However its sensitivity may create problems since it will detect very small amounts of DNA from latent as well as active infections and it does not differentiate between cyst and tachyzoite DNA. Thus samples like blood, CSF, urine and and amniotic fluid should be used as they do not contain the latent stages. PCR shows great promise but as yet is still labour intensive and expensive for routine use in the laboratory.

Trypanosoma species

Introduction

Trypanosomes are haemoflagellates and three species of the genus Trypanosoma are responsible for disease in humans such as sleeping sickness.

Trypanosomes occur in the blood of the majority of vertebrate animals. The life cycle involves intermediate host, which usually is an insect. Many species of tyrpanosomes can live in harmony with their hosts producing no pathogenic effect, but the best known species are those that are pathogenic to their definitive hosts. The disease in caused by the pathogenic types is called, trypanosomiasis.

Salivarian trypanosomes

Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense - The metacyclic trypanosomes are found in the proboscis of the insect vector - infection is therefore inoculative. The above are the aetiological agents of African trypanosomiasis, it is a zoonotic species in the fact that it multiplies in the blood of a range of animals, domestic and wild animals as well a man.

Trypanosoma brucei rhodesiense causes an acute form of sleeping sickness in East Africa, while T. b. gambiense causes chronic sleeping sickness in West Africa.

These are known as salivarian trypanosomes as they complete their development in the salivary system (anterior portion of the vector). Transmission takes place by innoculation of the metacyclic stage.

Stercorarian trypanosomes

Trypanosoma cruzi - The metacyclic trypanosomes occupy a posterior position in the gut of the insect vector and are passed out in the faeces - infection is therefore contaminative. This is the aetiological agent of South American trypanosomiasis.

These trypanosomes are known as stercocarian as they complete their development in the posterior region of the vector, so that the infective forms appear in the insects faeces. Hosts are infected by the contaminative route.

African Trypanosomiasis

Life Cycle

Transmission from one vertebrate to another is carried out by blood-sucking invertebrates, usually an insect. The vector for African Trypanosomes is the Tse tse fly, Glossina, and the species which cause the disease are T. b. gambiense and T. b. rhodesiense. (Fig. 6 &7)

Metacyclic (infective) trypomastigotes are inoculated through the skin when a tsetse fly takes a blood meal. The parasites develop into long slender trypomastigotes which multiply at the site of inoculation where ulceration occurs. The trypanosomes continue to develop and then may invade the lymphatic tissues, the heart, various organs and in later stages, the central nervous system. Trypomastigotes are taken up by the tsetse fly (male and female) during a blood meal. The parasites develop in the midgut of the fly where they multiply. 2-3 weeks later the trypomastigotes move to the salivary glands transforming from epimastigotes into metacyclic (infective) trypomastigotes. The tsetse fly remains infective for life i.e. about 3 months.

Figure 6. Generalised life cycle of the Trypanosoma sp. Which cause African trypnaosomiasis.

The mode of transmission mentioned above, metacyclic transmission, requires to be separated from mechanical transmission, a process in which trypanosomes survive, for a short time, on and about mouth parts of an insect and are inoculated into a new host when the vector bites again, without undergoing any developmental cycle.

Metacyclic transmission requires a lapse of time to allow the trypanosomes to reach an infective stage by a particular developmental sequence in the vector, usually a period of several days.

Morphology

The parasite is an elongated cell with single nucleus which usually lies near the centre of the cell. Each cell bears a single flagellum which appears to arise from a small granule - the kinetoplast. The kinetoplast is a specialised part of the mitochondria and contains DNA. The length and position of the trypanosome’s flagellum is variable. In trypanosomes from the blood of a host the flagellum originates near the posterior end of the cell and passes forward over the cell surface, its sheath is expanded and forms a wavy flange called an undulating membrane.

Development is characterised by the occurrence of three types of blood forms (polymorphic), these are:

1) Slender forms: long and thin, about 29mm long, free flagellum.

2) Stumpy forms: thick and short, average length 18mm, typically no free flagellum, but a short one may be present.

3) Intermediate forms: about 23mm long with a moderately thick body and a

free flagellum of medium length. (Fig. 8 & 9)

Figure 8. Diagrammatic illustration of a typical trypanosome. (Adapted and redrawn from Smyth, J.D, 1994)

Figure 9. Trypanosoma brucei gambiense and rhodesiense: two forms of trypomastigote can be seen in peripheral blood: one is long slender, 30 µm in length, and is capable of multiplying in the host, the other is stumpy, not dividing, 18 µm in length.

Clinical disease

The early stages of African trypanosomiasis may be asymptomatic and there is a low grade parasitiaemia. This period may last for several weeks to several months. The disease may terminate untreated at this stage or go on to invade the lymph glands. Invasion of the lymph glands is usually accompanied by a high irregular fever with shivering, sweating and an increased pulse rate. The lymph glands near the bite often become swollen, in T. b. gambiense the glands at the back of the neck and T. b. rhodesiense usually the glands under the jaw are affected (Winterbottoms sign). As the disease progresses oedema of the eyelids, face and sleeplessness are features along with increasing lethargy and listlessness.

Trypanosomes may invade the central nervous system giving symptoms of meningoencephalitis, confusion, apathy, excessive sleeping and incontinence. At this stage, the cerebro-spinal fluid (CSF) usually contains mononuclear cells and a few trypanosomes may be detected. If untreated, character changes, mental deterioration and coma develops, finally resulting in death. Such signs are more commonly seen with gambiense than in rhodesiense in which patients often die before these symptoms develop fully.

Laboratory diagnosis of African trypanosomiasis

Laboratory diagnosis of African trypanosomiasis is by:

1. Examination of blood for the parasites

2. Examination of aspirates from enlarged lymph glands for the parasites

3. Examination of the CSF for the parasite

4. Detection of trypanosomal antibodies in the serum

1. Examination of blood

a) Thick and thin blood films blood film

Thick and thin blood films are made and stained with Fields stain and examined as for malaria parasites

b) Triple centrifugation technique

This method is carried out as follows:

(i) 5 to 10 ml of citrated blood is centrifuged at 2000 rpm for 5 minutes to pack the red blood cells.

(ii) The plasma and white cell layer are removed by a Pasteur pipette and transferred to a clean centrifuge tube.

(iii) This is centrifuged for a short time in order to deposit any red blood cells carried over.

(iv) The supernatant fluid is removed by pipette to a clean tube.

(v) This is centrifuged at 5000 rpm for 10 minutes.

(vi) The supernatant fluid is removed with a pipette and discarded.

(vii) The deposit is examined microscopically for trypanosomes.

c) Miniature anion-exchange centrifugation technique (ref. Transactions Royal Society of Tropical Medicine and Hygiene. 1979. 73. 312-317)

Heparinised blood is passed through an anion exchange column. As the blood travels down the column the red cells are adsorbed while the less strongly charged trypanosomes are washed through with saline. The eluate is centrifuged and examined microscopically for motile trypanosomes.

d) Buffy coat examination

Trypanosomes are centrifuged in a microhaematoctit tube for 5 minutes. Parasites can be seen microscopically at the junction of the packed red cells and plasma.

2. Examination of lymph gland aspirates

The aspirate can be examined microscopically by making a wet preparation, or if there is not much material, it can be allowed to dry on a slide and then stained with either rapid Field’s stain or with Giemsa and examined microscopically.

4. Examination of CSF

In the late stages of African trypanosomiasis, trypanosomes may be found in the CSF together with IgM - containing morula (Mott) cells, lymphocytes and other mononuclear cells. Once the CSF has been collected it must be examined as soon as possible. The parasites are unable to survive for more than 15-20 minutes in CSF once it has been removed. The parasites become inactive, are rapidly lysed and will not therefore be detected. The CSF should be examined wet and spun down in a sterile tube and a film made from the deposit. The film is then stained with rapid Field’s or Giemsa and examined microscopically.

NB. It is impossible to distinguish between T. b. gambiense from T. b. rhodesiense on a stained film as the two subspecies which infect man are identical.

South American trypanosomiasis

Trypanosoma cruzi occurs throughout South and Central America, especially in Brazil, Argentina and Mexico causing the disease known as Chagas’ disease. It is estimated that over 24 million people are infected with this species. It is a zoonotic parasite with over 150 species of wild animals known to harbour the parasites, for example opposums, dogs, rates, pigs and cats.

It is transmitted to man by brightly coloured bugs belonging to the Reduviidae family, subfamily Triatominae. All stages of these bugs are known to become infected. (Fig. 10)

The bugs live in the crack of the walls and vegetal roofs of the poorly maintained houses, coming out at night to feed on the exposed parts of the hosts body.

Figure 10. Insect vector of Trypanosoma cruzi, belongs to colourful insect Triatominae, also known as the kissing bug.

Life Cycle

Metacyclic trypomastigotes are deposited in faeces on the skin as the triatomine bug (reduviid bug) feeds. The bug usually bites round the edges of the mouth and eyes. The trypomastigotes are either rubbed into the skin by

scratching the irritated area or penetrate the conjunctiva or membranes of the nose and mouth. Trypomastigotes become amastigotes in localised reticulo endothelial cells and multiply. The amastigotes develop into trypomastigotes which are released into the blood when the cell ruptures. No multiplication of the parasite takes place in the blood in its trypomastigote stage. The trypomastigotes reach tissue cells especially heart muscle, nerves, skeletal muscle and smooth muscle of the gastrointestinal system by way of the blood and lymphatic system. The trypomastigotes become amastigotes and multiply
forming pseudocysts. Within the pseudocyst some amastigotes become elongated and develop first into epimastigotes and then trypomastigotes. When the cell ruptures the trypomastigotes are released into the blood and

Figure 11. Generalised life cycle of the South American trypanosomes, Trypanosoma cruzi, which is known to case Chagas’ disease.

Continue to circulate whilst others invade further tissue cells. The life cycle completes when a triatomine bug vector ingests circulating trypomastigotes. In the vector the trypomastigotes transform and develop into epimastigotes, multiply by binary fission in the gut of the bug. After about 10 - 15 days, metacyclic trypomastigotes are formed and can be found in the hindgut of the bug. (Fig. 11)

Morphology

Trypanosoma cruzi has a single form (monomorphic), about 20mm in length, and characteristically curved. The kinetoplast is large, considerably larger than the Trypanosoma bruceii species already discussed. They sometimes appear as a bulge at the posterior end. The flagellum is medium in length. (Fig. 12 & 13)

Figure 12. Trypanosoma cruzi causes a disease known as Chagas’ disease, and is caused by the haemoflagellate Trypanosoma cruzi. T.cruzi in blood sample (Giemsa stain) (www.cdfound.to.it)

Figure 13. Multiplication of Trypanosoma cruzi in man only occurs in the amastigote phase, which grows in a variety of tissue cells especially muscle. In vitro infected fibroblast showing a large number of intracellular amastigotes. (Giemsa stain) (www.cdfound.to.it)

Clinical Disease

Many people infected with T. cruzi remain asymptomatic and free from Chagas’ disease or experience only an acute infection without progressing to the chronic stage. The most severe form of the disease is most commonly seen in children younger than 5 years of age. Multiplication of T. cruzi at the site of infection can produce an inflamed swelling (chagoma) which persists for weeks. Trypomastigotes or amastigotes may be seen in the aspirate of the chagoma. Regional lymph nodes may become infected which frequently involve one side of the face. Unilateral oedema of the upper and lower eyelid may occur along with conjuctivitis. This is known as Romana’s sign.

In the acute stage of infection trypomastigotes can be found in the blood. Symptoms may pass unnoticed, but there may be fever, malaise increased pulse rate and enlargement of lymph glands, liver and possibly spleen. Muscle aches and pains are characteristic at this stage and parasites may be seen in blood films. The acute form is most often seen in young children and occasionally can cause serious damage to the heart and other complications leading to death caused by central nervous system involvement.

Chronic manifestations include signs of cardiac muscle damage with a weak and irregular heartbeat, oedema, heart enlargement leading to heart failure. Dilation of the digestive tract resulting in megaoesophagus and megacolon may also occur. About 10% of persons infected with T. cruzi develop chronic Chagas cardiopathy.

Laboratory Diagnosis of South American trypanosomiasis

Laboratory diagnosis of South American trypanosomiasis is by

1. Examination of blood.

2. Xenodiagnosis

3. Blood culture

4. Serology

1. Examination of blood

a) Thick and thin blood films are made and stained with Fields stain and examined as for malaria parasites. Wet preparations of blood can also be examined for motile trypanosomes.

b) Buffy coat examination - Trypanosomes are centrifuged in a microhaematoctit tube for 5 minutes. Parasites can be seen microscopically at the junction of the packed red cells and plasma. This technique is rapid and sensitive.

Trypanosoma cruzi can often be seen in C, U or S shapes in stained films.

2. Xenodiagnosis

Xenodiagnosis is useful in chronic and sub acute (low parasitaemia) disease. Sterile bugs are fed on patients by attaching a black bag containing the bugs to the arm of the patient and allowing them to feed for 30 minutes. Twenty five to thirty days later the bugs are dissected and the contents of the hindgut and rectum are examined microscopically for the presence of trypanosomes.

3. Blood Culture

Blood culture is as sensitive as xenodiagnosis but it requires sterile conditions.

4. Serology

Serology tests include:

(i) IFAT indirect fluorescence antibody test

(ii) CFT complement fixation test

(iii) IHAT indirect haemaglutination test

(iv) ELISA enzyme linked immunoabsorbent assay

Other lab findings include:

Raised ESR, marked lymphocytosis with atypical mononuclear lymphocytes

NB. In certain areas of S. America where Trypanosoma rangeli (non pathogenic species transmitted by Rhodnius bug) is found with T. cruzi all positive preparations should be checked to confirm T. cruzi.

Leishmania species

Introduction

Leishmaniasis is caused by parasites of the genus Leishmania and is endemic in many parts of Africa, Asia and South America. It is transmitted by Phlebotomus species, sandfly.

Leishmania species are mainly parasites of man and other animals, especially dogs and rodents. They cause diseases collectively known as Leishmaniasis; causing 3 types of disease i.e. visceral leishmaniasis, cutaneous leishmaniasis and muco-cutaneous leishmaniasis. These are all debilitating and disfiguring diseases which occur throughout the Old and New World. The parasites are unusual in that they live entirely within the cells of the reticulo-endothelial cells, they have become perfectly adapted as the proteolytic enzymes which attack other foreign bosies in the blood stream do not destroy them.

Visceral leishmaniasis

Human visceral leishmaniasis (VL), sometimes known as Kala-azar, is caused by Leishmania donovani complex; L. donovani and L. donovani infantum in the Old World and L. donovani chagasi in the New World. The clinical features -azar caused by these species are similar, but they have different epidemiological features. The parent species L. donovani occurs in Asia (Northeastern China, India and Iran) and Africa (primarily Sudan, Kenya and Ethiopia) and can affect people of all ages. The parasite (L. d. infantum) which causes VL in countries bordering the Mediterranean, (Southern Europe as well as North Africa) affects young children as well as infants. It is now being seen in the immunocompromised. In the New World also, VL is mainly a disease of young children, with the causative organism L. d. chagasi being closely related to, but slightly different from, L. donovani. The main geographical foci of VL in Latin America are in northern and northeastern Brazil. Small foci are found in northern Argentina, Columbia and Venezuela. Sporadic cases are found in central American countries, including Mexico.

Cutaneous leishmaniasis

Cutaneous leishmaniasis is caused by L. tropica, L. major and L. aethiopica in the Old World and L. mexicana complex in the New World. Leishmania tropica is widely distributed around the Mediterranean basin, Afghanistan, Kenya, Kenya, Armenia, Azerbaijan, Turkmenistan and Uzbekistan. Leishmania aethiopica is seen in the highlands of Ethiopia and L. major occurs in the Middle East, West Africa, North Africa and Kenya. Leishmania mexicana complex is found in Central America and the Amazon Basin.

Mucocutaneous leishmania

Is caused by the L. braziliensis complex and is found in Brazil, Eastern Peru, Bolivia, Paraguay, Ecuador, Columbia and Venezuela.

Life cycle

All forms of infection starts when a female sandfly (Phlebotomus species) takes a blood meal from an infected host. Small amounts of blood, lymph and macrophages infected with Leishmania amastigotes are ingested. Once ingested the amastigotes transform to promastigotes in the sandfly, the non-infective promastigotes divide and develop into infective metacyclic promastigotes. These are formed in the midgut of the sandfly and migrate to the proboscis. When the sandfly bites the extracellular inoculated promastigotes at the site of the bite is phagocytosed by macrophages. After phagocytosis, transformation to dividing amastigotes occurs within 24 hours. Reproduction at all stages of the lifecycle is believed to occur by binary fission. No sexual stage has been identified. (Fig. 14)

Figure 14. Generalised life cycle of Leishmania sp., which by using a sandfly as a vector can cause Leishmaniasis in man,

Morphology

Leishmania exist as flagellated extracellular promastigotes in the Sandfly vector and as a flagellar obligate intracellular amastigotes within mononuclear phagocytes of their vertebrate hosts. The various species are not distinguishable morphologically from one another. When stained with Romanowsky stains such as Giemsa, amastigotes appear as round or oval bodies ranging from 2 - 3mm in diameter with a well defined nucleus and kinetoplast, a rod shaped specialised mitochondrial structure that contains extranuclear DNA. The flagellated promastigote form is spindle shaped, measuring 10 - 20mm in length, not including the length of the flagellum. As in the amastigote form a nucleus and kinetoplast are clearly visible. (Fig. 15 & 16)

Figure 15. Amastigote of Leishmaniasp. in a macrophage of a vertebrate host. (Adapted and redrawn form Smyth, J.D, 1994)

Figure 16. Leishmania promastigote. This stage of the parasite are seen in the vector, sandfly.

Clinical Disease - Visceral leishmaniasis

The incubation period of VL may vary between 2 weeks and 18 months. The onset of VL is usually insidious with fever, sweating, weakness and weight loss. The most prominent findings are fever, hepatosplenomegaly and anaemia. The sites mainly affected are the liver, spleen and bone marrow. Enlargement of the liver is due to hyperplasia of Kupffer cells which are packed with amastigotes. The bone marrow is infiltrated with parasitised macrophages. Some organs, notably the kidneys, may show pathological changes secondary to deposition of immune complexes. In advanced cases, ascites and oedema can develop. Deaths are usually due to secondary bacterial infections such as pneumonia, tuberculosis or dysentery.

Laboratory Diagnosis of visceral leishmaniasis

1. Microscopy

Parasites may be found in a splenic aspirate, liver biopsy or bone marrow biopsy. These techniques, especially splenic aspirate and liver biopsy, can be hazardous and require previous expertise in the procedure.

a) Air dry smears.

b) Fix in methanol for 1 minute

c) Stain with Giemsa 1 in 10 in buffered distilled water pH 6.8 for 30 minutes (or use the rapid Field’s stain)

d) Wash the slide in buffered water and drain dry

Amastigotes of leishmania should be seen in positive smears. They are approximately 2-4µm in size, oval and are frequently seen within the cytoplasm of the macrophage. The amastigotes possess a nucleus and a rod - shaped kinetoplast within the cytoplasm. In many samples a very small number of parasites are present. Extensive searching of the film is necessary.

2. Culture

The aspirates can be cultured in Novy-Nicolle-MacNeal (NNN) or Schneider's Drosophila medium. In culture the amastigote stage converts to the promastigote stage. However, this is not a rapid technique, as the parasites may take anything from 10 - 21 days to grow.

3. Serodiagnosis

VL produces large amounts of specific IgG which can be used for diagnosis. Currently the most used sero diagnostic tests are Indirect-immuno Fluorescent Antibody Test (IFAT), Enzyme Linked Immunosorbent Assay (ELISA) and Direct Agglutination Test (DAT).

Clinical Disease – Cutaneous Leishmaniasis

Following a bite from an infected sandfly, a small red papule appears at the site of the bite about 2 – 8 weeks later. The papule increases in size centrifugally. The patient then mounts either a hypersensitive response or an anergic response. In a hypersensitive response, the papule eventually ulcerates, becomes depressed and then eventually heals through scarring. The patient is now immune from subsequent bites. In an anergic response, the nodule grows and spreads over large areas of skin. This resembles leprosy.

Laboratory Diagnosis of Cutaneous leishmaniasis

1. Slit skin smear.

The margin of the lesion contains amastigotes whereas the centre contains debris and dead skin material. This the margin of the lesion is aseptically punctured with a hypodermic needle and syringe containing a small amount of saline. The aspirate which is drawn up into the needle is examined microscopically and/or cultured using the method described in visceral leishmaniasis.

2. Polymerase chain reaction

Gene amplification techniques are powerful and sensitive methods and are useful in diagnosis of cutaneous leishmaniasis particularly when organisms cannot be detected microscopically. It is also very useful for the speciation of Leishmania parasites thus the correct treatment can be administered.

Clinical Disease – Mucocutaneous leishmaniasis

Mucocutaneous leishmaniasis or espundia initially develops like cutaneous leishmaniasis but develops into lesions in the mucocutaneous junction of the pharynx resulting in the break down of the palate of the mouth and nose or more rarely the genitalia or anus. This occurs from a few weeks to several years after the cutaneous lesion has healed. These lesions result in disfiguring deformities of the nose and mouth.

Laboratory Diagnosis of Mucocutaneous leishmaniasis

1. Microscopy

Finding the organisms in a histological section of the lesion provides definitive diagnosis of mucocutaneous leishmaniasis. However, the organisms are very rare in this form of the disease and culture can be a more sensitive method (see visceral leishmaniasis).

2. Polymerase chain reaction

The PCR method has the advantage of not only low numbers of parasites in aspirates but also histological sections. This makes this a very sensitive method in diagnosing mucocutaineous leishmaniasis when parasites are difficult to detect.

References

Murray, PR, Drew, WL, Koyayashi, GS & Thomson, JH: Medical Microbiology. Mosby Books Inc., New York (1990)

Peters, W & Gilles, HM: Tropical Medicine & Parasitology. Wolfe Medical Publications Ltd.

Jeffrey & Leach: Atlas of Medical Helminthology and Protozoology. E & S Livingstone Ltd.

Garcia, LS & Bruckner, DA: Diagnostic Medical Parasitology. Elsevior Science Publishing Co. Inc.

Smyth, J.D: Introduction to Animal Parasitology. Cambridge University Press (1994)

Snell, JJS, Farrell, ID & Roberts, C: Quality Control, Principles and Practice in the Microbiology Laboratory. Public Health Laboratory Service. ISBN 0 901 144 312.

Brown, VC. A Longitudinal study of the prevalence of intestinal helminths in baboons (Papio doguera) from Tanzania. (1994) Thesis, Liverpool.