Equine parasites are a complex subject, especially with the recent changes in the guidelines for keeping them under control. 

There are around 20 or more internal parasites that can affect your horse. Some of these feed on the lining of your horse’s intestines, some attach themselves to the gut and cause irritation, and some travel through tissues, blood vessels, the liver and under the skin. 

The main worms of importance are: 

• Ascarids

• Small strongyles

• Large strongyles

• Pinworms

• Bots

• Lungworms

• Threadworms

• Tapeworms (less concerning). 

Ascarids

Ascarids are also known as large roundworms. We are most concerned about these in foals, around three to six months old. A second wave may occur in yearlings, but the count at this age is usually lower. 

These parasites can cause signs of ill thrift, diarrhoea, or a cough.  

Ascarids are large worms, so a high ascarid burden can, in worst case scenarios, cause an impaction in the small intestine.  

The small intestine is much narrower than the large intestine, especially in foals. If an impaction occurs it can cause a serious type of impaction colic. This can be fatal or require surgery to remove the worms. 

Small strongyles  

We are mainly concerned about small strongyles in young horses, and then again as geriatrics.  

The main life cycle stage of small strongyles that is of concern is when they become encysted in the wall of the intestine and hibernate for some time. At this stage in their life cycle, they cannot be tested for, because they are not producing any eggs.  

There is a condition known as larval cyathostominosis. This is when, if a horse has a large burden of encysted parasites which all decide to unencyst at the same time, a lot of inflammation may be caused. This can result in severe watery diarrhoea and protein loss, which can present as colic. Luckily, this condition is rare. 

Large strongyles 

Fortunately, mature horses gain immunity to large strongyles, and they are less commonly seen now.  

However, we are concerned about these worms because they migrate through the bloodstream as part of their life cycle.  

In severe cases, they can cause intestinal infarction (death of tissue), where a blood clot caused by the presence of the worms cuts off the blood supply to a section of the intestine. This can also result in bacterial leakage leading to peritonitis.  

Horses with an intestinal infarction can present with acute and severe colic. 

Tapeworms 

Tapeworms are common. If there is grass, there are tapeworms, because their intermediate host is a mite that lives in the grass.  

The prevalence of tapeworms is variable within a herd. Some horses will have a lot, while some will not have any. Horses do not acquire immunity to tapeworms.  

Tapeworms are most commonly found at a part of the intestine called the ileocecal junction, which is where the small intestine meets the large intestine. This area is a narrow part of the intestine, so if you have a lot of these worms in that area it can cause inflammation, an impaction, or an intussusception (where the inflamed intestine telescopes inside itself).  

All these would present as signs of colic.  

We cannot currently test for this parasite in New Zealand because it does not produce eggs that can be detected in our standard faecal egg count test. Overseas, they do have saliva antibody tests, but they are not yet available here. 

Pinworms 

We are not as worried about pinworms, in terms of the disease that they cause. 

Pinworms lay eggs around the anus of the horse, which may become itchy. As pinworms do not lay eggs in faeces, we will not detect them in a faecal egg count test. 

Instead, we use a sticky tape or scrape test.  

Sometimes, you may see dead females in faeces, because they die after laying their eggs. 

Bots  

Bots are fly larvae, not worms. We do not see a lot of these in Southland, because it is often a bit too cold for these flies down here.  

In rare cases they can cause some damage to the jawbone of the horse, as part of their life cycle involves burrowing into the gums.  

They are not known to cause issues when present in the horse's stomach. 

Lungworms 

Lungworms are also not common, and donkeys are their natural host. If they are present in horses that graze with donkeys, they may cause a chronic cough. 

Threadworm  

Threadworms are only relevant in young foals. They don’t cause too many issues - sometimes mild diarrhoea - however, there are a number of other causes of diarrhoea in young foals, including viruses and bacteria.