Laboratory techniques in the investigation of toxoplasmosis.

Introduction Infection by the protozoan Toxoplasma gondii is one of the most common parasitic infections of warm blooded animals including man. The definitive host is the cat in which the sexual life cycle takes place. Human infection may arise in utero or be acquired by ingesting tissue cysts in undercooked meat or by ingestion of oocysts in soil or via contaminated foods. Seroconversion increases with age and varies according to geographical area and eating habit. Reported prevalence rates, as determined from the presence of serum antibodies, are 20-40% in Great Britain, 50-60% in the USA and 80-90% in France.`2 Acute toxoplasma infection in the immunocompetent patient is often asymptomatic but may cause lymphadenopathy or a glandular fever-like illness. Congenital infection may cause foetal death or result in cerebral damage and retinochoroiditis in severely affected cases. Toxoplasmosis is now well recognised as an important opportunistic infection of the immunocompromised. Cancer patients, organ-transplant recipients and patients with the acquired immune deficiency syndrome (AIDS) are all at risk of severe, sometimes fatal, toxoplasmosis. The infection in these patients has a predilection for the central nervous system (CNS).3 Since the recognition of AIDS, toxoplasmic encephalitis has become one of the most common causes of encephalitis in the USA.45 It is the most common cause of intracerebral mass lesions in patients with AIDS and is possibly the most common opportunistic infection of the CNS. CNS toxoplasmosis is due principally to reactivation of endogenous infection acquired in the past and the risk of an AIDS patient with positive toxoplasma serological tests developing CNS infection has been estimated at 30%.6 Published estimates of the incidence of CNS toxoplasmosis in AIDS patients vary enormously and probably reflect prevalence of parasite infection in different populations. Involvement of other organs is rare but pulmonary toxoplasmosis is said to occur in 1% of AIDS patients7 and a diffuse retinochoroiditis8 has been recorded. The early diagnosis of CNS toxoplasmosis in AIDS patients requires a high index of suspicion. The patient may present with focal or generalised neurological abnormalities. Computed tomography (CT) is extremely useful for investigating suspected CNS toxoplasmosis. Lesions, single or multiple, and isodense or hypodense are usually seen in the cerebral hemispheres often with surrounding oedema and mass effect (fig). Contrast studies reveal either ring or nodular enhancement in most cases. Magnetic resonance imaging, if available, may detect lesions not demonstrated by CT. Most often a diagnosis is made in AIDS patients on the basis of compatible clinical, radiological and serological findings which is only confirmed after a clinical and radiological response to anti-parasite therapy. This article reviews the laboratory techniques available for investigating toxoplasmosis with particular reference to patients with AIDS. These techniques include serology, histology, culture and specific nucleic acid detection by DNA probe and the polymerase chain reaction (PCR). Clinical and radiological findings will not be discussed further.