Trichostrongylus tenuis

Trichostrongylus tenuis, also known as the strongyle worm, is a gut nematode found in the United Kingdom,^[1] sensitive to Pyrantel pamoate. Larvae have a short migration inside the mucosa of the intestine and return quickly to the digestive tract.

This endoparasite causes a condition often called strongylosis or 'grouse disease' and which can be the cause of regular crashes in grouse populations. When the adult worm burrows into the caeca walls it causes a lot of damage and internal bleeding which in itself is harmful to the grouse. The worms ultimately reduce the digestive efficiency thus affecting the condition of the grouse. The eggs of the strongyle worm found in the caecal droppings of red grouse, hatch out into a microscopic larvae stage. Living within the dropping they feed off bacteria and organic matter they develop through two moults reaching their third larval stage, 'L3'. The 'L3' larvae is now at its infective stage so it 'swims' up heather stalks, on a thin film of liquid, to the young shoots where it is most likely to be ingested by a host. Once in the body of a host they enter one of the two caecum in the intestine of the host where the larvae develop into adult worms. The adults mate within the caeca and the female can lay over 100 eggs a day which are passed out in the caecal dropping.

The development of the parasite can vary greatly depending on the conditions, namely temperature and humidity. The eggs and larvae can both tolerate periods of low temperature but neither can survive in hot and dry weather. In perfect conditions with high humidity and temperatures exceeding 15 °C most eggs will hatch within 24 hours, compared with at 5 °C where the eggs may take several days to hatch out. The larvae can remain active above 6 °C with relative humidity but when it is colder they halt their development but stay alive. With the climate changing as it is the worms find tolerable conditions for much of the year but they are picked up by grouse at two main periods of the year. The grouse may ingest infective larvae from the heather tips in the summer and early spring. During autumn and winter not all ingested 'L3' larvae will develop into adults, the larvae may begin to moult and temporarily halt their development and attach themselves to the caecum wall, they are said to be in an 'arrested' state. The larvae over-winter in the bird's intestine and in March/April when conditions are more favourable they resume their development.

Con la edad, la carga parasitaria en los urogallos generalmente aumenta, ya que cada gusano en el intestino debe ser ingerido individualmente y las aves adultas parecen tener poca resistencia al parásito. ^{[ 2 ]} La carga parasitaria se produce cuando las aves se alimentan de brezo y las aves jóvenes se alimentan principalmente de insectos; por esta razón, este parásito tiene poca influencia directa en la supervivencia de los polluelos, pero puede afectar gravemente a los adultos y a las aves inmaduras.

Una alta carga parasitaria en los urogallos puede provocar mortalidad y un mal estado general, lo que facilita la depredación y dificulta la reproducción. El gusano estrongílido se introduce en la mucosa cecal, lo que afecta la capacidad del ave para controlar el olor que emite, haciéndola particularmente vulnerable a los depredadores mamíferos que cazan guiándose por el olfato. Una alta carga parasitaria que deteriora el estado general de la hembra de urogallo puede no afectar el tamaño de la nidada, pero sí su capacidad para incubar los huevos o criar a los polluelos, por lo que este parásito puede afectar indirectamente la mortalidad de los polluelos.