Blood parasite investigation (Guidelines)

Audience

• Highland HSCP

A number of parasites can be detected by blood film microscopy including malaria, microfilaria, trypanosomes and babesia. Timing of sample and clinical details is critical for accurate interpretation.

Malaria

There are five recognised species of malaria parasites affecting humans: Plasmodium falciparum, P vivax, P ovale, P malariae and P knowlesi. Patients can present with fever, myalgia, headache, nausea, diarrhoea, vomiting, lethargy, anorexia and rigors. Rarely symptoms of malaria can manifest many years after exposure.

In patients returning from a malarial endemic region presenting with symptoms concerning for malaria the following initial investigations are suggested

• Full blood count
• Thick and thin blood films (the laboratory will automatically calculate level of parasites in cases of P falciparum and P knowlesi)
• Malaria rapid diagnostic test (this will be done automatically by the laboratory)
• Renal and liver function
• CRP

It is vital that details of anti-malarial medication and travel history are given with each request. When investigation for malarial parasites is requested the sample should be transported to the laboratory as soon as possible. A negative malarial screen does not exclude a diagnosis of malaria. If there is strong clinical suspicion of malaria repeat the test in 12, 24 and 48 hours. If concerns please discuss with an infectious diseases specialist.

Microfilaria

Filariasis in humans can give rise to a variety of clinical features; these include eosinophilia, chyluria, elephantiasis, lymphadenopathy, Calabar swelling, urticaria, subcutaneous nodules and blindness.

In patients returning from an endemic region presenting with symptoms concerning for microfilaria the following initial investigations are suggested

• Full blood count
• Thick and thin blood films
• Filaria serology

The timing of specimen sampling is critical in the detection of microfilaria. Times for collection should be selected in accordance with the patient’s clinical symptoms and travel history. An absolute minimum of a sample taken during the day (around 1pm) and one at night (around midnight) should be examined. It is vital that details of travel history are given with each request. If concerns please discuss with an infectious diseases specialist as normal blood film microscopy does not exclude this diagnosis.

Trypanosomes

Patients infected with trypanosomes can present with fever, headaches, joint pains, itching and lymphadenopathy. The symptoms can vary depending on the type of trypanosome.

In patients returning from an endemic region presenting with symptoms concerning for trypanosome infection the following initial investigations are suggested

• Full blood count
• Thick and thin blood films
• Trypanosome serology

If concerns please discuss with an infectious diseases specialist as normal blood film microscopy does not exclude this diagnosis.

Babesiosis

Babesiosis occurs following tick bites; particularly in immunocompromised patients and patients who have hyposplenism. Babesia infection is characterised by the presence of haemolytic anaemia and nonspecific flu-like symptoms (e.g. fever, chills, myalgia, weakness, fatigue). Some patients have splenomegaly, hepatomegaly, or jaundice. Risk factors for severe babesiosis include asplenia, advanced age and other causes of impaired immune function (e.g. HIV, malignancy, corticosteroid therapy, chemotherapy).

In patients returning from an endemic region presenting with symptoms concerning for babesiosis the following initial investigations are suggested:

• Full blood count
• Thick and thin blood films

It is vital that details of travel history are given with each request. If concerns please discuss with an infectious diseases specialist as normal blood film microscopy does not exclude this diagnosis.