New insights into insect vectors causing histomoniasis in poultry

The researchers identified the potential vectors of H. meleagridis based on their prevalence inside and outside houses. Photo: Koos Groenewold

Research on the insectome in and around broiler breeder pullet farms has identified new potential vectors of Histomonas meleagridis, the protozoa that cause histomoniasis. The study results, which were published in the journal Parasites & Vectors, show the presence of new potential arthropods transmitting histomoniasis between farms.

The protozoan parasite Histomonas meleagridis (H. meleagridis) commonly infects chickens and turkeys, causing histomoniasis. It uses the eggs of the caecal worm Heterakis gallinarum (H. gallinarum) as a vector and reservoir. Reports show that in chickens, besides causing lesions in the caeca and liver, the parasite can cause a significant drop in egg production. In turkeys, the protozoan parasite can cause severe inflammation of the caeca and liver with high mortality of up to 90%.

Earthworms have long been identified as paratenic hosts of H. meleagridis and H. gallinarum, but according to the researchers, they are less likely to be responsible for transmission between farms because of their limited mobility. On the other hand, litter beetles (Alphitobius diaperinus) and other arthropod species have been recently implicated as potential vectors.

Researchers working on the current study also highlighted the fact that no drugs for prevention are currently available; they mention that paromomycin, which is available in a few countries, can be given as a therapeutic drug, but the success of the treatment is uncertain. Due to this situation, prevention of the introduction of the disease into poultry flocks is paramount, yet transmission routes between flocks are still unclear.

“More knowledge of litter beetle behaviour and a broader unbiased analysis of all potential flying arthropod vectors responsible for transmission between farms would facilitate targeted control programmes to reduce the transmission of H. meleagridis among unexposed flocks or farms.”

Screening vectors of H. meleagridis

In this study, researchers analysed arthropod populations in and around broiler breeder pullet farms to identify new potential vectors of H. meleagridis. On each farm, 3 types of traps were set inside and outside 2 houses to collect, identify and characterise arthropod species (a vast majority being insects, hence the word ‘insectome’).

The collection was conducted during 4 visiting periods (in autumn, spring, summer and again in the autumn season). The 3 different trap types were placed in 3 locations inside the houses, as well as on the outside wall of the house and at the feed silo. The researchers then identified the potential vectors of H. meleagridis based on their prevalence inside and outside the houses and conducted a quantitative polymerase chain reaction (qPCR) analysis for the DNA of both H. meleagridis and H. gallinarum.

Potential vector arthropods

In this study, researchers trapped a total of 4,743 arthropod specimens. The 3 most frequently encountered orders were Diptera (38%) (represented by flies), Coleoptera (17%) (represented by beetles), and Hymenoptera (7%) (represented by ants and wasps). Up to 50% of litter beetles tested positive for H. meleagridis DNA 4 months after an observed histomoniasis outbreak and, when tested sporadically, the litter beetles were found to be positive for H. gallinarum DNA. Specimens of 4 Dipteran families tested positive for either one or both parasites.

More arthropods were trapped outside than inside: 67% of all arthropod types were collected outside the houses and 11% were collected inside and outside the houses. The composition of the arthropod populations, including the insectome, varied significantly between trap locations and seasons. Coleoptera was more frequently encountered inside the houses than outside. Of the 714 litter beetles trapped, only 3 were found outside the houses. Musca was the most common genus identified, the other genera found were Ahasverus, Condylostylus, Bradysia, Megaselia, Drosophila, Capitophorus, and Psychoda.

The insect species that were positive for either H. gallinarum or H. meleagridis were Condylostylus (long-legged flies), Musca (house flies), Bradysia (fungus gnats), and Sphaeroceridae (small dung flies). “Bradysia and Condylostylus have not been previously described at chicken farms nor have they ever been shown to test positive for Histomonas DNA until this study, to our knowledge,” said the researchers.

Seasonal differences

The total of 4,743 arthropods were trapped and counted during the 3 visits in the spring, summer and autumn. Of these, 1,284 were trapped inside and 3,459 outside the houses. The different seasons influenced the differences in the number of specimens trapped inside and outside the houses (Figure 1). Inside, seasonal differences were minor with a maximum in summer, while outside differences between the seasons were more noticeable with a maximum in the autumn.

Commenting on the seasonal differences, the researchers said: “Seasonal differences in the arthropod population not only outside the houses but more surprisingly also inside the houses might further limit the risk of arthropod-vectored transmission to certain times of the year. However, the types we identified as of interest based on their detection inside and outside the houses and their ability to fly, were mostly detected on all farms and during several seasons.”

They added: “In Europe, the number of histomoniasis cases has a small peak during the warmer months. The outbreak we observed occurred during a colder period in spring with average temperatures of about 4°C. The impact of weather conditions on the insectome is clear, but further studies need to investigate whether there is a direct or indirect connection with histomoniasis outbreaks.”

Transmission between farms

The results showed a limited but present potential of arthropods, especially flies, to transmit histomoniasis between farms. The species that were positive for either H. gallinarum or H. meleagridis were Condylostylus (long-legged flies), Musca (house flies), Bradysia (fungus gnats), and Sphaeroceridae (small dung flies).

The researchers concluded that “the study is important because the quantitative polymerase chain reaction identification of Histomonas in arthropods that fly inside and outside of pullet barns is the first step toward identifying vectors. However, limitations could be that some relevant arthropod species could require different types of traps than the ones we used and that populations in other regions might differ from the region we collected from for this study.”

This article is based on the publication ‘Mapping the poultry insectome in and around broiler breeder pullet farms identifies new potential Dipteran vectors of Histomonas meleagridis’ in Parasites & Vectors.