Trichinella zimbabwensis in a naturally infected mammal

From wildlife to humans: The global distribution of Trichinella species and genotypes in wildlife and wildlife-associated human trichinellosis

Zoonotic nematodes of the genus Trichinella are foodborne parasites that have a global distribution in wild carnivores and omnivores, with spillover and spillback into domestic livestock and people, with concomitant trade and health consequences. Historically, most human cases were linked to domestic pigs infected with Trichinella spiralis, but under current high biosecurity swine production in many countries, wildlife have become a more important source of human trichinellosis. The aim of this review is to update the global distribution of Trichinella species and genotypes reported in wildlife, as well as reported human outbreaks from the consumption of wildlife. Using several online databases and by "snowballing" references, hundreds of reports of Trichinella spp. in wildlife published between January 1991 and December 2023 provide an important update to the host and geographic range for each of the recognized 13 species/genotypes, grouped by continent. Surveillance effort was highest in Europe and North America, while Africa, Asia, Oceania, Central and South America have had limited surveillance, in some instances with human cases serving as sentinels of transmission in a region. Dozens of human outbreaks are described, with wild boars (Sus scrofa) being the most frequently implicated wildlife species in human outbreaks globally. Bears are an important source of infection in North America, for wildlife tourism, and importation of bear meat has also been implicated in multicountry outbreaks. The largest study limitation was the dearth of molecular identification of larvae in both wildlife surveillance studies and human outbreaks, particulary in under-studied regions. We highlight the need for enhanced molecular epidemiological approaches to outbreaks of this important foodborne parasite, and emphasize the need for a One Health approach to manage Trichinella spp. which transmit among terrestrial and marine wildlife (including migratory birds), pigs, horses, and people, often across large geographic scales and borders.

Epidemiology and hypothetical transmission cycles of Trichinella infections in the Greater Kruger National Park of South Africa: an example of host-parasite interactions in an environment with minimal human interactions

Knowledge on the epidemiology, host range and transmission of Trichinella spp. infections in different ecological zones in southern Africa including areas of wildlife-human interface is limited. The majority of reports on Trichinella infections in sub-Saharan Africa were from wildlife resident in protected areas. Elucidation of the epidemiology of the infections and the prediction of hosts involved in the sylvatic cycles within specific ecological niches is critical. Of recent, there have been reports of Trichinella infections in several wildlife species within the Greater Kruger National Park (GKNP) of South Africa, which has prompted the revision and update of published hypothetical transmission cycles including the hypothetical options based previously on the biology and feeding behaviour of wildlife hosts confined to the GKNP. Using data gathered from surveillance studies and reports spanning the period 1964-2019, confirmed transmission cycles and revised hypothesized transmission cycles of three known Trichinella species (T. zimbabwensis, Trichinella T8 and T. nelsoni) are presented. These were formulated based on the epidemiological factors, feeding habits of hosts and prevalence data gathered from the GKNP. We presume that the formulated sylvatic cycles may be extrapolated to similar national parks and wildlife protected areas in sub-Saharan Africa where the same host and parasite species are known to occur. The anecdotal nature of some of the presented data confirms the need for more intense epidemiological surveillance in national parks and wildlife protected areas in the rest of sub-Saharan Africa to unravel the epidemiology of Trichinella infections in these unique and diverse protected landscapes.

Prevalence and molecular identification of Trichinella species isolated from wildlife originating from Limpopo and Mpumalanga provinces of South Africa

Trichinella species are widely distributed on all continents with the exception of Antarctica, although the full spectrum of Trichinella species found in sub-Saharan African countries, and their hosts, has not been fully documented. This study was conducted to determine the prevalence of Trichinella in wildlife from the Greater Kruger National Park (GKNP) and adjacent areas located in the Limpopo and Mpumalanga provinces of South Africa, and to identify the species and/or genotypes of Trichinella larvae isolated from muscle tissues, using molecular techniques. A review of Trichinella spp. and their wildlife hosts reported during 1964-2011 was also conducted and the results were compared with our current study. Ninety samples representing 15 mammalian, two bird and three reptile species were screened for Trichinella infection during 2012-2016, using artificial digestion. Isolates detected were identified using a multiplex polymerase chain reaction (PCR) amplification of the internal transcriber spacers ITS1 and ITS2, and expansion segment V (ESV) regions of ribosomal DNA, followed by molecular analysis of the sequences. Twenty samples from seven wildlife species were positive for Trichinella spp. larvae, with an overall prevalence of 21.1% (20/90). The prevalence was higher in carnivores (18.9%, 18/90) than in omnivores (2.2%, 2/90). Analysis of sequences showed that eight of the isolates - two from spotted hyaena (Crocuta crocuta) (2/8), three from lion (Panthera leo) (3/13), one from leopard (Panthera pardus) (1/6), one from small spotted genet (Genetta genetta) (1/2) and one Nile monitor lizard (Varanus niloticus) (1/2) - conformed to Trichinella zimbabwensis. One isolate from a hyaena was grouped under the encapsulated species clade comprising T. nelsoni and genotype Trichinella T8 reported to be present in South Africa. This is the first report confirming natural infection by T. zimbabwensis in hyaena, leopard, genet and Nile monitor lizard, adding to the body of knowledge on the epidemiology of Trichinella infections in the Greater Kruger National Park of South Africa. Ten Trichinella-like larval isolates recovered after digestion from four wildlife species in this study (2012-2016) revealed inconclusive results due to DNA degradation resulting from poor storage or too few larvae for analysis, in comparison to 20 unidentified isolates from five wildlife species during the 1964-2011 period.

First Report of the Occurrence of Trichinella-Specific Antibodies in Domestic Pigs in Central and Eastern Uganda

Previous research on trichinellosis in Africa focused on isolating Trichinella from wildlife while the role of domestic pigs has remained highly under-researched. Pig keeping in Uganda is historically recent, and evidence on zoonotic pig diseases, including infection with Trichinella species, is scarce. A cross-sectional survey on Trichinella seroprevalence in pigs was conducted in three districts in Central and Eastern Uganda from April 2013 to January 2015. Serum from a random sample of 1125 pigs from 22 villages in Eastern and Central Uganda was examined to detect immunoglobulin G (IgG) against any Trichinella spp. using a commercially available ELISA based on excretory-secretory antigen. ELISA positive samples were confirmed using Western Blot based on somatic antigen of Trichinella spiralis as recommended in previous validation studies. Diaphragm pillar muscle samples (at least 5 g each) of 499 pigs from areas with high ELISA positivity were examined using the artificial digestion method. Overall, 78 of all 1125 animals (6.9%, 95% CI: 5.6-8.6%) tested positive for antibodies against Trichinella spp. in the ELISA at significantly higher levels in Kamuli district compared to Masaka and Mukono districts. Thirty-one percent of the ELISA positive samples were confirmed IgG positive by the Western Blot leading to an overall seroprevalence of 2.1% (95% CI: 1.4-3.2%). The large proportion of ELISA positive samples that could not be confirmed using Western blot may be the result of cross-reactivity with other gastrointestinal helminth infections or unknown host-specific immune response mechanisms in local pig breeds in Uganda. Attempts to isolate muscle larvae for species determination using the artificial digestion method were unsuccessful. Due to the large number of muscle samples examined we are confident that even if pigs are infected, the larval burden in pork is too low to pose a major risk to consumers of developing trichinellosis. This was the first large systematic field investigation of Trichinella infection in domestic pigs in Uganda and its results imply that further studies are needed to identify the Trichinella species involved, and to identify potential sources of infection for humans.

Metabolic and adaptive immune responses induced in mice infected with tissue-dwelling nematode Trichinella zimbabwensis

Tissue-dwelling helminths are known to induce intestinal and systemic inflammation accompanied with host compensatory mechanisms to counter balance nutritional and metabolic deficiencies. The metabolic and immune responses of the host depend on parasite species and tissues affected by the parasite. This study investigated metabolic and immuno-inflammatory responses of mice infected with tissue-dwelling larvae of Trichinella zimbabwensis and explored the relationship between infection, metabolic parameters and Th1/Th17 immune responses. Sixty (60) female BALB/c mice aged between 6 to 8 weeks old were randomly assigned into T. zimbabwensis-infected and control groups. Levels of Th1 (interferon-γ) and Th17 (interleukin-17) cytokines, insulin and blood glucose were determined as well as measurements of body weight, food and water intake. Results showed that during the enteric phase of infection, insulin and IFN-γ levels were significantly higher in the Trichinella infected group accompanied with a reduction in the trends of food intake and weight loss compared with the control group. During systemic larval migration, trends in food and water intake were significantly altered and this was attributed to compensatory feeding resulting in weight gain, reduced insulin levels and increased IL-17 levels. Larval migration also induced a Th1/Th17 derived inflammatory response. It was concluded that T. zimbabwensis alters metabolic parameters by instigating host compensatory feeding. Furthermore, we showed for the first time that non-encapsulated T. zimbabwensis parasite plays a role in immunomodulating host Th1/Th17 type responses during chronic infection.

Malaria endemicity and co-infection with tissue-dwelling parasites in Sub-Saharan Africa: a review

Mechanisms and outcomes of host-parasite interactions during malaria co-infections with gastrointestinal helminths are reasonably understood. In contrast, very little is known about such mechanisms in cases of malaria co-infections with tissue-dwelling parasites. This is lack of knowledge is exacerbated by misdiagnosis, lack of pathognomonic clinical signs and the chronic nature of tissue-dwelling helminthic infections. A good understanding of the implications of tissue-dwelling parasitic co-infections with malaria will contribute towards the improvement of the control and management of such co-infections in endemic areas. This review summarises and discusses current information available and gaps in research on malaria co-infection with gastro-intestinal helminths and tissue-dwelling parasites with emphasis on helminthic infections, in terms of the effects of migrating larval stages and intra and extracellular localisations of protozoan parasites and helminths in organs, tissues, and vascular and lymphatic circulations.

Differential immune responses in mice infected with the tissue-dwelling nematode Trichinella zimbabwensis

To improve diagnostic tools, immunotherapies and vaccine development for trichinellosis surveillance and control there is a need to understand the host immune responses induced during infection with Trichinella zimbabwensis, a tissue-dwelling nematode. In this study, we sought to determine immune responses induced in mice during T. zimbabwensis infection. The parasite strain used (Code ISS1209) was derived from a naturally infected crocodile (Crocodylus niloticus) and is the main Trichinella species prevalent in southern Africa. Sixty 6- to 8-week-old female BALB/c mice were randomly assigned to two equal groups: T. zimbabwensis-infected (n= 30) and the non-infected control group (n= 30). Levels of serum tumour necrosis factor-alpha (TNF-α), interleukin-10 (IL-10), interleukin-4 (IL-4) as well as parasite-specific IgM, IgG, IgG1, IgG2a, IgG2b and IgG3 antibody responses were determined using enzyme-linked immunosorbent assay (ELISA). The cytokines and antibodies provided information on T-helper 1 (Th1)- and Th2-type, T-regulatory and antibody responses. Results showed that during the intestinal stage of infection, higher levels of parasite-specific IgM, IgG, IgG1 (P <  0.05) and IL-10 and TNF-α (P <  0.001) were observed in the Trichinella-infected group compared with the non-infected control group. In the parasite establishment and tissue migration phases, levels of IgG1 and IgG3 were elevated (P <  0.001), while those of IgM (P <  0.01) declined on days 21 and 35 post infection (pi) compared to the enteric phase. Our findings show that distinct differences in Th1- and Th2-type and T-regulatory responses are induced during the intestinal, tissue migration and larval establishment stages of T. zimbabwensis infection.

First report of a mixed infection of Trichinella nelsoni and Trichinella T8 in a leopard (Panthera pardus) from the Greater Kruger National Park, South Africa

At least three Trichinella species, namely Trichinella nelsoni, Trichinella britovi and Trichinella zimbabwensis, and one genotype (Trichinella T8), have been isolated from sylvatic carnivores on the African continent. With the exception of T. britovi, the other species are known to circulate in wildlife of the Kruger National Park (KNP), South Africa, and KNP neighbouring game reserves (collectively known as the greater KNP area). Lions (Panthera leo) and spotted hyenas (Crocuta crocuta) appear to be the most important reservoirs of T. nelsoni and Trichinella T8 in the KNP and surrounding areas. Interspecies predation between lions and hyenas has been implicated as a primary mode of maintaining the life cycles of these two Trichinella species. This is the first report of a mixed natural infection of T. nelsoni and Trichinella T8 in a leopard (Panthera pardus) from South Africa. Trichinella muscle larvae were identified to species level by multiplex polymerase chain reaction (PCR). Probable sources of infection, based on the known dietary preference and prey species' range of leopards, are also discussed. The described occurrence of Trichinella species in a leopard from the greater KNP area raises the question of possible sources of infection for this predator species.

New pieces of the Trichinella puzzle

Contrary to our understanding of just a few decades ago, the genus Trichinella now consists of a complex assemblage of no less than nine different species and three additional genotypes whose taxonomic status remains in flux. New data and methodologies have allowed advancements in detection and differentiation at the population level which in turn have demonstrably advanced epidemiological, immunological and genetic investigations. In like manner, molecular and genetic studies have permitted us to hypothesise biohistorical events leading to the worldwide dissemination of this genus, and to begin crystalising the evolution of Trichinella on a macro scale. The identification of species in countries and continents otherwise considered Trichinella-free has raised questions regarding host adaptation and associations, and advanced important findings on the biogeographical histories of its members. Using past reviews as a backdrop, we have ventured to present an up-to-date assessment of the taxonomy, phylogenetic relationships and epidemiology of the genus Trichinella with additional insights on host species, survival strategies in nature and the shortcomings of our current understanding of the epidemiology of the genus. In addition, we have begun compiling information available to date on genomics, proteomics, transcriptomics and population studies of consequence in the hope we can build on this in years to come.

Development of an ELISA to detect the humoral immune response to Trichinella zimbabwensis in Nile crocodiles (Crocodylus niloticus)

Crocodiles are known reservoir hosts of Trichinella papuae and Trichinella zimbabwensis, two zoonotic parasites that also infect mammals. Since commercial crocodile farming represents a key source of income in several countries, it is important to monitor this nematode infection in both farmed crocodiles and in breeding stocks which are frequently introduced from the wild. For this purpose, an indirect ELISA was developed to detect the anti-Trichinella immune response in crocodile sera. New Zealand rabbits were immunized with pooled sera from non-infected farmed crocodiles in the presence of Freund's complete adjuvant. The anti-crocodile serum was then conjugated with horseradish peroxidase. Serum samples from four Nile crocodiles (Crocodylus niloticus) experimentally infected with T. zimbabwensis and from four uninfected crocodiles were used to set up the ELISA. The larval burden per gram of muscle tissue was determined by muscle biopsy. The test was performed on serum samples from an additional 15 experimentally infected crocodiles as well as eight wild Nile crocodiles. Among the 19 experimentally infected crocodiles, seroconversion was observed in 11 animals. The highest antibody response was observed six weeks post infection (p.i.), but in most of these animals, antibodies were not detectable after six weeks p.i. even though live larvae were present in the muscles up to six months p.i.

Trichinella infections in animals and humans in sub-Saharan Africa: a review

The aim of this review is to provide information on Trichinella infection in humans, livestock and wildlife in sub-Saharan Africa mainly focusing on geographical distribution of species/genotypes, biology, host range, life cycles and to identify research gaps. Trichinella britovi, Trichinella nelsoni and Trichinella zimbabwensis and one genotype (Trichinella T8) are known to occur in sub-Saharan Africa. Distinct geographic ranges with overlapping of some taxa in some areas have been observed. Genetic variants of T. nelsoni has been reported to occur among parasites originating from Eastern and Southern Africa and sequence heterogeneity also occurs among T. zimbabwensis isolates originating from different regions of Zimbabwe and South Africa. Field observations so far indicate that sylvatic Trichinella infections in the region are common in carnivores (mammals and reptiles) and to a lesser extent in omnivores. Cannibalism, scavenging and predation appear to be the most important routes of transmission and maintenance of the sylvatic cycles of the Trichinella taxa. To date, human trichinellosis has been documented in only four sub-Saharan countries (8.7%, 4/46). Bushpigs and warthogs have been the source of human infection with T. britovi and T. nelsoni being the aetiological agents. An increase in bushmeat trade and the creation of Transfrontier Conservation Areas (TFCAs) may have increased the risk of human trichinellosis in the region. With the creation of TFCAs in the region, sampling of wildlife hosts from protected areas of most sub-Sahara African countries is required to fully map the distribution of Trichinella species/genotypes in this region. More structured field surveys are still needed to determine the sylvatic host distribution of the different Trichinella taxa. Biological data of the Trichinella taxa in both wild and domestic animals of sub-Saharan Africa is very limited and further research is required.

The occurrence of Trichinella zimbabwensis in naturally infected wild crocodiles (Crocodylus niloticus) from the Kruger National Park, South Africa

Trichinella zimbabwensis has been found naturally infecting crocodiles (Crocodylus niloticus) in Zimbabwe, Mozambique, Ethiopia and South Africa, as well as monitor lizards (Varanus niloticus) in Zimbabwe. The reports on natural infections were mostly accidental rather than structured surveys and involved very few animals. Previous surveillance studies in South Africa reported a 38.5% prevalence of T. zimbabwensis among wild crocodiles tested from the Mpumalanga province and Kruger National Park (KNP). No studies have been conducted to date on the geographical distribution and occurrence of T. zimbabwensis in wild crocodiles and varans in countries in southern Africa. Recent outbreaks of pansteatitis in crocodile populations of the KNP, South Africa, provided an opportunity to conduct a more structured survey aimed at elucidating the occurrence and distribution of T. zimbabwensis in culled wild crocodile populations within the KNP. Results from this study showed that T. zimbabwensis occurred in 10 out of 12 culled crocodiles form the KNP. The results also showed that the natural distribution of T. zimbabwensis in crocodiles includes all the major river systems in the KNP. The predilection sites of larvae in muscles followed a different pattern in naturally infected crocodiles compared to observations in experimentally infected mammalian hosts.