Malaria

Diagnosis

• Blood films
• Must be made within 3-4 hours
• Thick
• Detection
• Unfixed
• Giemsa or modified field (rapid) stain
• X100 objective
• If negative despite strong clinical suspicion, examine larger area or repeat testing
• Thin
• Identification
• Giemsa or leishman stain at pH 7.2
• A rapid modified field stain will allow identification of P falciparum, but not the other species
• If thin film negative
• May be able to speciate on thick film
• Treat as falciparum
• Antigen testing to confirm falciparum

Quantification

• P. falciparum
• Quantify percentage of parasitized cells
• Examine 1000 red cells
• Double infection, quantification applies only to falciparum
• Exchange transfusion may be indicated if parasitaemia >10%
• Should be repeated daily until no parastites (other than gametocytes)

Confirmation

• Examined by two trained observers
• All positive cases should be sent to a reference centre

• Notifiable disease

• Supplementary tests
• Parasight F
• Tests for soluble falciparum antigen in the blood
• Sensitivity is equivalent to thick films
• Useful to confirm falciparum esp. if mixed infection
• Histidine rich protein2
• May remain positive for 14 days even after treatment
• False postivies with rheumatoid factor
• Parasite LDH
• Only produced by viable parasite, so becomes negative after 2-3 days
• Quantitative buffy coat blood parasite detection
• Fluorescence microscopy
• Exposure of blood to acradine orange
• Can be used as an initial screen
• Disadvantages
• Expensive
• HJ bodies and reticulocytes also fluoresce with acridine orange
• Speciation not possible
• Ab tests
• Not useful in acute attack, because stay after positive after successful treatment
• Can be useful in excluding malaria in chronic PUO

Life cycle

• Injected sporozoites enter hepatocytes and divide
• Rupture of hepatocyte releases merozoites, attach to red cell membrane
• Digestion of Hb produces haemozoin = pigment
• Asexual replication in red cell results in schizonts
• Some form gametocytes, some rupture and reinfect other cells
• Ovale and vivax have a dormant hypnozoite stage in the liver
• Maximal immunity takes about 10 years and is lost over 1-5 years
• In pregnancy, parasites are sequestered to placenta and can cause placental insufficiency
• Falcip = SSA
• Vivax = S Asia, India etc

Anaemia

• Parasitized red cells are removed by spleen
• Severe malarial anaemia
• Also removal of non-parasatized cells
• Reticulocyte suppression
• Hyper reactive malarial splenomegaly
• Hypersplenism
• Blackwater fever
• Intravascular haemolysis
• Assoc with quinine and G6PD deficiency
• Dyserythropoiesis in the marrow for weeks after acute infection

• P vivax needs duffy antigen on red cells as a receptor
• Often lacking in certain black races and thus protective

Treatment

• Falciparum:
• Chlorquine if sensitive (mostly central/ S America)
• Quinine if likely resistance
• Alternatives include malarone, artesunate, mefloquine
• Vivax/ ovale:
• Primaquine for the hypnozoite stage
• Chloroquine or quinine depending on likely resistance pattern
• Malariae
• Chloroquine

P. Falciparum – at different stages in thick and thin films – see the WHO website:  http://malaria.who.int/docs/hbsm_diagnosis.htm

Babesiosis

• Not a tropical disease
• Primarily a disease of animals, but rarely infects humans
• Transmitted by tick bite
• B. divergens – Europe
• B. microti – N. America
• Infect red blood cells and are confused with falciparum
• Presenting features
• Fever, prostration, hepatosplenomegaly, jaundice, haemolytic anaemia (usually DAT negative, despite being complement mediated), haemoglobinuria
• May be fatal in splenectomised patients
• ‘Maltese cross’ forms on blood film
• Parasitaemia does not correlate with severity
• Treatment
• Clindamycin and quinine
• Exchange transfusion

Filariasis

Lymphatic filariasis

• Wucheria bancrofti
• Sheathed, nucei NOT to end of tail
• Asia, Africa, America and pacific islands

• Brugia malayi
• Sheathed, more tightly coiled with a sub-terminal and terminal nuclei, often widely separated
• China and far east 

• Worms 4-10cm long and live in lymphatics
• Microfilaria are produced by the female
• Released into the blood after 3-8 months
• Exhibit daily periodicity to coincide with mosquito activity
• Presenting features
• Lymphangitis (spreads distally), fever, redness and pain over lymphatic vessels
• Chronic
• Hydrocele, lymphoedema and elephantiasis, chyluria

Onchocerciasis

• Second commonest cause of infectious blindness in the world
• Caused by oncocerca volvulus
• Transmitted by black fly – simulium damnosum
• Causes dermatitis and keratitis which leads to blindness if untreated
• Simbiotic relationship with Wolbachia species (bacteria)
• Treated with ivermectin or doxycycline (which kill the wolbachia)

Other filariae

Loa loa

• Sheathed, nuclei to the end of the tail
• Central Africa
• Migrates through subcutaneous tissue, including the conjuctiva

Mansonella perstans

• Unsheathed, nuclei to the end of the tail
• Non-pathogenic, but frequently co-exist with W. bancrofti

Mansonella ozardi

• Also probably non-pathogenic
• West indies and S America

• Eosinophilia
• May develop tropical pulmonary eosinophilia (>10)
• Immunological hyper responsiveness to microfilaria in the lungs
• Microfilaria on lung biopsies
• Visualise worms by USS in lymphatics
• Rapid response to diethylcarbamazine
• Diagnosis
• Blood film
• Best to take sample at midnight or midday
• Wet preps
• Thick and thin films
• Antigen detection
• ELISA or ICT for bancrofti
• Antibody not helpful as most in endemic areas have Abs to crude filarial antigens

African trypanosomiasis

• Trypanosoma brucei gambiense in west and central Africa
• Trypanosoma brucie rhodesiense in East Africa
• Transmitted by tsetse fly
• Local multiplication of tryps at site of bite, followed by entry into blood stream
• Clinical features
• Lymphadenopathy esp. posterior cervical
• Mild splenomegaly
• Anaemia, haemorrhages and petechiae
• Protracted febrile illness
• Parasites invade CNS and death inevitable if untreated
• Anaemia
• Extravascular haemolysis
• Failure to incorporate iron into red cell precursors
• Mott morular cells in marrow
• BM is hypercellular with areas of gelatinous degeneration
• Diagnosis
• Wet preps of fluid aspirated from LNs
• QBC
• Serological agglutination test
• CSF examination to stage disease
• Treatment
• Pentamidine and suramin
• Poor CNS penetration
• Melarsoprol for late stage disease
• Mortality 4-12%
• Eflornithine

American trypanosomiasis

• Trypanosome cruzi
• Transmitted by triatome bugs
• Americas
• Clinical features
• Chagoma at site of bite
• Fever, hepatosplenomegaly, lymphadenopathy
• Multiply intracellularly in muscle tissue
• Heart, colon and oesophagus
• Chronic disease is associated with heart disease in 30%
• Arrythmias and cardiomegaly
• Asymptomatic infection is common
• Screened for in blood donations
• Diagnosis
• Serological
• Treatment
• Benznidazole

Leishmaniasis

• Transmitted by sand fly
• Visceral form
• L. donovani and L. infantum
• Increasing with HIV infection
• Clinical features
• Diarrhoea, arthralgia, weight loss, bleeding gums
• Muscle wasting, hepatosplenomegaly, fever, anaemia
• Splenomegaly results in hypersplenism
• Diagnosis
• Intracellular amastigotes in macrophages
• Splenic > BM aspirate
• PCR
• AB tests but may be negative in imunocompromised
• Treatment
• ambisome