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Babesia species are protozoan parasites of domestic and wild animals. They belong to the subclass Piroplasmia 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, horses, 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 now becoming a significant tick-borne disease of man in other temperate climates as well.
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 transovarial transmission.
The ticks which are known to carry the parasite of Babesia bovis are Boophilus microplus, B. decoloratus and B. annulatus.
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 Babesia canis which infects dogs, but it is generally the same in each of the one-tick hosts and the definitive host.
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. Transovarial transmission of Babesia bovis also takes place so that newly hatched onto adult stages can then take place.
Illustration 11-1. Generalized life cycle of the parasite Babesia, which causes the disease babesiosis in man. (SOURCE: CDC/DPDx)
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 erythrocytes.
Babesia microti do not undergo transovarial transmission, but once a larva has become infected from a mammalian host they are able to pass on the infection transstadially to the nymph.
Image 11-1. Diagnosis of Babesia species depends on the observation of the intraerythrocytic organisms in blood smears. Pear shaped microorganisms (2-5µm) and tetrads are the diagnostic shape of the parasite. (SOURCE: PHIL 3899 – CDC/Dr. George Healy)
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, anemia and hemoglobulinuria. Nausea, vomiting and diarrhea 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 six weeks. The illness usually begins gradually, with anorexia and fatigue, plus fever (without periodicity), sweating, rigors and generalized myalgia. Physical examination may reveal only fever, but may also show mild splenomegaly and occasionally mild hepatomegaly.
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 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 Schüffner'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.
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 1.5µm. In very high parasitaemias, extracellular merozoites are found singly or as a syncytial structure. Peak parasitaemia varies between less than 1% to approximately 10%.
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, 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.
The development of the enteroepithelial (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.
Humans can acquire infection by:
· Accidental ingestion of the oocyst shed in the cats feces
· Ingestion of the tachyzoite in infected milk or transplacentally
· Ingestion of the tissue cyst in undercooked or raw meat.
· Transplant of an infected organ in a seronegative recipient
Illustration 11-2. Life cycle of Toxoplasma gondii, causes toxoplasmosis in man. (SOURCE: PHIL 3421 – CDC/Alexander J. da Silva, PhD/Melanie Moser)
Illustration 11-3. Diagrammatic illustration of a Toxoplasma gondii trophozoite in a macrophage of a vertebrate. (SOURCE: Unknown)
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.
Image 11-2. Toxoplasma gondii tissue cyst containing 8–20 parasites (Giemsa stain) (SOURCE: CDC)
This occurs approximately in 1 per 1000 pregnancies. It can cause severe damage to and even death of the fetus. Proliferation of tachyzoites leads to intracellular calcification, choroidoretinitis, hydrocephaly, psychomotor disturbances and convulsions. A small, proportion of babies who are asymptomatic at birth develop retinochoroiditis 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.
Infections with T. gondii are often mild with flu-like symptoms thus they often go unnoticed. However lymphadenopathy is the most easily recognized 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.
Toxoplasmosis has been shown to occur as an opportunistic pathogen in immuno-compromised patients and can cause severe complications. Toxoplasmosis in immuno-compromised patients almost always arises from a reactivation of latent infections. Conditions which can predispose to toxoplasmosis are malignancies, organ transplants, leukemias 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.
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 benchmark 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 centers.
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 amniotic fluid should be used as they do not contain the
latent stages. PCR shows great promise but as yet is still labor
intensive and expensive for routine use in the laboratory.
Trypanosomes are hemoflagellates 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 trypanosomes 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.
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 causative agents of African trypanosomiasis. It is a zoonotic species in that it multiplies in the blood of a range of many mammals including man.
Trypanosoma brucei rhodesiense causes acute 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 inoculation of the metacyclic stage.
Trypanosoma cruzi - The metacyclic trypanosomes occupy a posterior position in the gut of the insect vector and are passed out in the feces - infection is therefore contaminative. This is the causative agent of 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 insect’s feces. Hosts are infected by the contaminative route.
Transmission from one vertebrate to another is carried out by blood-sucking invertebrates, usually an insect. The vector for African Trypanosomiasis is the Tsetse fly, Glossina spp. which cause the diseases Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense.
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 three months.
Illustration 11-4. Generalized life cycle of the Trypanosoma spp. which causes African Trypanosomiasis. (SOURCE: PHIL 3418 – CDC/Alexander J. da Silva, PhD/Melanie Moser)
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.
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 specialized 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 characterized by the occurrence of three types of blood forms (polymorphic), these are:
1) Slender forms: long and thin, about 29µm long, free flagellum.
2)Stumpy forms: thick and short, average length 18µm, typically no free flagellum, but a short one may be present.
3)Intermediate forms: about 23µm long with a moderately thick body and a free flagellum of medium length.
Illustration 11-5. Diagrammatic illustration of a typical trypanosome. (SOURCE: Unknown)
Image 11-3. Trypanosoma brucei gambiense and T. b. 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. (SOURCE: CDC)
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 (Winterbottom's sign). As the disease progresses, edema 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 cerebrospinal 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:
· Examination of blood for the parasites
· Examination of aspirates from enlarged lymph glands for the parasites
· Examination of the CSF for the parasite
· Detection of trypanosomal antibodies in the serum
1. Examination of Blood
a) Thick and Thin Blood Films
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 10ml of citrated blood is centrifuged at 2000rpm 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 5000rpm for 10 minutes.
vi. The supernatant fluid is removed with a pipette and discarded.
vii. The deposit is examined microscopically for trypanosomes.
Heparinized 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 five 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.
3. 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.
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 harbor the parasites, for example opossums, dogs, rates, pigs and cats.
It is transmitted to man by brightly colored bugs belonging to the Reduviidae family, subfamily Triatominae. All stages of these bugs are known to become infected.
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 host’s body.
Image 11-4. Insect vector of Trypanosoma cruzi, belongs to colorful insect Triatominae, also known as the kissing bug. (SOURCE: PHIL 2538 - CDC/World Health Organization)