Lungworm the lifecycle (2 / 4)
In the first article and video, I talked about the background as to why lungworm has become such a problem in some herds. https://youtu.be/2ZHuKITmnF0
In this article, I’m going to look at the lifecycle of this parasite (Dictyocaulus Viviparous).
So we know that cattle become infected when they ingest lungworm larvae (immature lungworm) at pasture. Where do these larvae come from and how do these parasites come back and cause problems each year.
The source of the parasite is carrier adult cows, which carry larvae overwinter in their lungs. Another reason seems to be mild winters where lungworm larvae seem to hibernate or survive over winter.
This means at the start of the grazing season there can be small numbers of larvae for animals to ingest. This begins the cycle for a new year.
The lifecycle
The problem occurs when the larvae are ingested and conditions are right for larval levels to build up over the grazing period. Then often within a short period, we can start seeing the clinical signs we associate with lungworm like coughing.
Inside the animal, the larvae travel from the gut to the lungs. They move through the lung tissue and develop into adult lungworms in the large airways of the lung. They can do damage as they migrate through the small airways to eventually the big airways.
Not every time this happens you get coughing either. This also means that the local immune system in the lung comes under pressure. This means other viral and bacterial infections can compound the problems we see.
This can mean that lungworm on its own cause’s problems but also potentially opens the door for other agents. We should remember this when treating animals for lungworm and not getting the desired response. I will be discussing control and treatments in the final article in the series.
When the adult lungworm produces an egg, it quickly hatches to an immature larvae which is coughed up and passed out in the dung. Remember the adult lungworm can produce thousands of eggs. This is why we see the build of infective larvae (numbers) in the pasture if the conditions are right.
The conditions are important because the larvae must get from the dung to grass to be ingested. Mild wet summers and weather can really help this process. Watch the video above to help understand this process better.
Why we need to understand the lifecycle
If we understand the lifecycle then we can understand a few things. That in certain weather conditions and years the risk of lungworm may be increased. We also can understand that if farms have lungworm issues they will be ongoing from year to year.
We must also understand that these young larvae traveling through the lungs can do tremendous damage.
Immunity
The host animal (cattle) can develop immunity to lungworm over time but this requires low levels of exposure, over a sustained period. This is why lungworm vaccination works well in the control programs. This is also why overdosing young animals can mean poor immunity developing in later grazing seasons. Lungworm control really requires planning
Reinfection syndrome
While this immunity develops it can be complicated in some situations. In essence, there are two levels of immunity that occur. One at the gut and lymph system as the larvae enter the cattle’s body, the second is in the lungs.
What seems to happen on some farms is this immunity only lasts 6-8 months at the gut level and in the lungs itself this immunity is much longer. This means animal’s particularly older cows will show symptoms of lungworm without having the full development of adult lungworms. Basically the larvae can get to the lungs and still cause symptoms as they develop while not completing their lifecycle to adults.
This is where diagnosis gets tricky because there will be no evidence of larvae in dung (no adults producing eggs) but all the symptoms. This is why vets are now using lung washes in these trickier cases to make a diagnosis.
Watch the video above for a detailed look at the lungworm lifecycle https://youtu.be/cUtkmw7SrMc
In the next article and video 3, I’m going to look at the clinical signs and making a diagnosis in more detail.