Respiratory disease in cows is something that seems to be on the increase over the last five to ten years.
Depending on the cause it can occur during the grazing season or while cows are housed.
The causes of respiratory disease include viruses like IBR, RSV, PI3; bacteria like Mannheimia, Pasteurella, Mycoplasma; and Lungworm.
In this article, I will concentrate on lungworm, which is often referred to as hoose.
Adult cattle after two grazing seasons generally have built up strong immunity to lungworm and, therefore, reinfection should not be a problem.
For this reason, herd owners can often be surprised to hear that the cause of coughing in their cows could be attributed to lungworm.
To understand why adult cows become infected with lungworm, it is important to understand the life cycle of the lungworm, and the immunity that develops.
There are five stages of larvae (L1-L5) which develop during the life cycle.
Here is a brief description of the life cycle.
L3 ingested off grass, penetrate the intestines and travel to lungs.
L3 develop to L4 and L5 in lungs which develop to adult lungworm in the airways.
Adult worms lay eggs which hatch to L1 and are coughed up and swallowed back into intestines.
25 days later L1 passed in dung.
L1 develop to L3 (1 week) in dung pat and in wet weather travel up grass and are available to be eaten.
Back to L3.
There are two main types of immunity to lungworm.
The first is immunity to the ingested larvae in the gut.
This immunity is strong, and minimises the amount of stage three larvae leaving the gut and reaching the lungs.
However, this gut immunity is not long acting, and lasts at most six months, without leaving memory and must be re-established once lost.
The second type of immunity is in the lungs, and is established by the presence of adult lungworm in the airways.
This immunity prevents larvae maturing to adult lungworm in the lungs.
Its strength is dependent on the level of past challenge, and once established, it persists for a long time.
If gut immunity to lungworm larvae has waned, and cows are exposed to heavily contaminated pasture, large numbers of larvae can be ingested and escape the gut to the lungs, leading to large numbers of larvae arriving in the lungs.
The strong and long acting immunity in the lungs can lead to a severe inflammatory response, leading to coughing in cows, which can be severe.
This scenario in adult cows is known as re-infection syndrome.
Treatment of lungworm in dairy cows during lactation is easily achieved, using eprinomectin.
It has been shown that around 10% of cows in any herd will be shedding lungworm larvae at any time.
This shedding by a small proportion of cows serves to keep the gut immunity of the whole herd ‘topped up’.
Inappropriate use of wormers in cows can interfere with this, leaving cows more exposed.
This can lead to a situation where cows have lost immunity, leading to more dosing being administered to try and keep up with the problem, which further exacerbates the immunity deficit.
This is one reason why it is important to have a worming protocol based on milk and faecal analysis, that allows careful use of anthelmintics for all parasites.
Finally, where immunity to lungworm has been lost, there is an oral lungworm vaccine available, which has been traditionally used in young stock, but has been successfully used in cows before turn-out, to re-establish immunity in the herd.
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