Experimental concurrent infection with two or more parasite species in mammalian host models may result in heterologous antagonistic and synergistic interactions ranging in magnitude from reduced/enhanced growth and fecundity to blockage/enhancement of establishment/expulsion. With some exceptions only, there is a reasonable correlation between the levels of interaction monitored by parasitological and by clinico-pathological parameters. Heterologous antagonistic interactions mediated by functional and specific immunological cross-reactivity occur between closely related parasite species exhibiting a marked immunobiological similarity. In contrast, antagonistic interactions between antigenetically more remote species of helminths, protozoan-induced resistance to helminth infection and helminth-induced suppression of concurrent protozoan infection generally appear mediated by immunologically non-specific factors like macrophage activation and inflammatory reactions. Synergistic heterologous interactions between helminths, helminth-induced enhancement of concurrent protozoan infection and interference with the development and maintenance of resistance to helminth infection in response to concurrent protozoan infection are generally thought to be mediated by non-specific parasite-induced immunosuppression. Concurrent experimental infection is very complex. There are problems and limitations in extrapolating from experimental studies on concurrent infection in laboratory animals to natural polyparasitism. This fact, coupled with the complex influence of ecological factors on the pattern and frequency of concurrent natural infection means that major consequences of natural concurrent parasite infection have not been definitively demonstrated. Appropriately planned and controlled field studies and further laboratory experiments on primate and domestic animal models are imperative for elucidation of the importance of heterologous interactions in concurrent parasite infection for the disease pattern in man and domestic stock. Experimental studies hitherto conducted on concurrent parasite infection pointing to natural heterologous interactions may be a valuable starting point for further studies.

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Chinchilla x New Zealand white cross breed rabbits (N=24)were challenged with strain of T. congolense. The infections were characterized by intermittent pyrexia, undulating parasitaemia, anorexia and emaciation. The major haematological changes observed were anaemia that was macrocytic normochromic at the first week of the infection and later became normochromic normocytic till the end of the experiment and leucopaenia that is characterized by neutropaenia, eosinopaenia and lymphocytosis. Plasma biochemical changes include hypoglycaemia, elevated total protein and plasma cholesterol. There were significant (p<0.05) elevation of Alkaline phospatase (ALP), Aspertate aminotransferase (AST), total bilirubin and fluctuating changes in the levels of plasma Alanine aminotransferase (ALT) and urea. Gross pathological changes include congested and oedematous lungs, mucoid enteritis, hepatomegaly and splenomegaly. Histopathological changes include mild congestion of the splenic pulp, mild venous congestion of the liver, pulmonary congestion, acute bronchopneumonia, severe emphysema of the lung, and focal centrilobular necrosis and periportal mononuclear cell aggregation in the kidney. This study shed light on the dynamics of haematological alteration and distortion of architectural frame work of various tissues of rabbits experimentally infected with T. congolense and suggested that rabbit is susceptible to T. congolense and could act as reservoir for trypanosomosis of ruminants and domesticated dogs used for hunting.

This study was carried out to validate the efficacy of Spondias mombin, used locally as an anthelmintic, and to standardize the effective dose of the plant extract required for worm control in livestock. In vitro and in vivo studies were conducted to determine the direct anthelmintic effect of ethanolic and aqueous extracts of S. mombin towards different ovine gastrointestinal nematodes. A larval development assay (LDA) was used to investigate the in vitro effect of extracts on strongyle larvae. Another study was conducted in vivo to evaluate the therapeutic efficacy of the extracts administered orally at dose rates of 125, 250, 500 mg/kg to sheep naturally infected with gastrointestinal nematodes. Twenty sheep were selected on the basis of positive faecal egg counts (750 epg). The sheep were allocated randomly to a non-medicated control group (A) or to groups given 125 mg/kg (B), 250 mg/kg (C) or 500 mg/kg (D) of extract, respectively. Sheep in groups B-D were given extracts orally on two days. Individual faecal egg counts were performed on days 0, 3, 6, 9 and 12. The presence of S. mombin extracts in in vitro cultures of larvae decreased the survival of L3 larvae. The LC50 of the aqueous extract of S. mombin was 0.907 mg/ml, while the LC50 of the ethanolic extract was 0.456 mg/ml. This difference in LC50 was statistically significant (p > 0.05). The mean percentage faecal egg reduction of sheep drenched with 500 mg/kg S. mombin extracts was 15.0%, 27.5%, 65.0%, 65.0%, 100.0% against Haenmonchus spp., Trichostrongylus spp., Oesophagostomunm spp., Strongyloides spp. and Trichuris spp. respectively, on day 12. Extracts of S. mombin could find application in the control of helminths in livestock.