Pinworm sellotape tests
If pinworm is confirmed, either with a sellotape test or by seeing them in the horse's droppings, treat with one of the more old fashioned wormers like pyrantel (Strongid P) or fenbendazole (Panacur 5 day guard) which are more effective at treating pinworm than ivermectin or moxidectin becuase of the way these are metabolised in the horse's gut.
Using neem oil on the horse's skin is also sometimes beneficial to prevent the eggs sticking once they're laid. Couple this with good stable hygiene and scrub down areas where the horse may rub his tail to help reinfection.
Occasionally we may see pinworm eggs in worm egg count samples but this is not common because the eggs are generally laid outside the horse around the anus area. Nevertheless they will be reported in your result if they are present in the sample.
Negative Test Results
A negative result simply means that no eggs were detected at the time of the test. If you still suspect a problem you may want to test again, keeping an eye out for other symptoms in the meantime such as tail rubbing, beansprout type worms in droppings or signs of he sticky substance the eggs are laid in around the anus.
Collect the sample in the morning, ideally before 9am. Pinworm are most active at night so this will give the best chance of detecting any activity that is present.